The Outer Hebrides: Where Genes and Geography Meet

Proceedings oflhe Royal Society of Edinburgh, 77B, 21^43, 1979

The Outer Hebrides: where genes and geography meet

R. J. Berry Royal Free Hospital School of Medicine, Londonf

SYNOPSIS The Outer Hebridean fauna is almost entirely the result of the chance characteristics of opportunist colonizers modified by subsequent adaptation and sometimes later immigrants. A few primitive relicts may persist (e.g. possibly, the moth Nyssia zonarid), but the fauna is certainly not largely composed of early post-glacial invaders. These, traditional theory argues, are likely to have been eliminated on the mainland by later arrivals but to have been able to persist on the islands by the timely breaking of hypothetical land-bridges. The influence of the original island colonizers is discernible through the persistence of similar traits in both ancestral and descendant populations despite massive genetical differences produced by stochastic sam- pling, and often by an apparently random pattern of differentiation between islands (e.g. field mice, cats, men, wrens). Adaptation is difficult to detect in the presence of unmeasurable founder effects, but is shown by large-scale clines (such as bridling in guillemots or colour phase frequencies in arctic skuas), by the existence of similar local races in different species (notably by melanism in Lepidoptera, especially Triphaena comes; but also possibly in bumble-bees and dragonflies), and by evidence of local selection (e.g. Cochlicella acuta). In the light of these many pressures, simple equilibrium theories of island biogeography are woefully inadequate; every species has to be considered on its merits. This can be done for terrestrial vertebrates, and virtually all but the Pygmy Shrew prove to have been introduced by man.

The Zoology of Archipelagoes will be well worth examination. Charles Darwin In recent times, we are looking in Scotland upon Evolution in its course. James Ritchie

INTRODUCTION The distribution and differentiation of certain groups of animals on the Outer Hebrides (and, indeed, on many other island groups) have led to past generations of zoologists speculating riotously from negligible evidence about rates of evolution; land-bridges; Pleistocene refuges; waves of colonization, competition and extinction; and multitudinous other bulwarks of biogeography. As far as the Outer Hebridean fauna is concerned, the favourite hypotheses involve a short-lived post-glacial land link with the Scottish mainland which allowed early immigrants onto the islands, but which was broken before more successful competitors arrived (Barrett-Hamilton and Hinton 1910-21)—a bridge formed, perhaps, by isostatic recovery of land which had lain under a heavy ice sheet in the glacial phases of the Pleistocene but was soon breached by eustatic movements (Harrison 1948); and (or) survival of some species through the later stages of the Pleistocene, perhaps on low-lying land now submerged 'off the west coast of Scotland, including the present Western Isles and therefore the northern end of the land-connection with Ireland' (Beirne 1943-44; Ford 1955a). Refugia in the middle t Present address: Department of Genetics and Biometry, University College, London.

Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 24 Sep 2021 at 12:03:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0269727000012616 22 R- J. Berry of Pleistocene-like conditions have been described on Kodiak Island off the Alaskan coast, and on the borders of the Skaftafell glacier in south-east Iceland (Lindroth 1970). Certainly St Kilda and possibly parts of Lewis and Harris were never fully glaciated (Wager 1953). Some long-term isolation of at least part of the Hebridean fauna is needed by orthodox theory because of the time required to reach sub-specific rank (Huxley 1942, believed this to be at least 50,000 years; Beirne 1952, put the period at 'not less than about 80,000 years'). Unfortunately, the last retreat of the ice was comparatively recent—probably only about 12,000 years ago. Notwithstanding, as long ago as 1880 Alston was inventing land-bridges in post-glacial times. He suggested that these broke off Ireland, then the Outer Hebrides, and finally Orkney: 'such a hypothesis of the dispersal of English mammals through Scotland and Ireland appears to me to be the only one which explains the peculiarities of their present distribution, and is likewise in accord (sic) with the facts of physical geography'. Harrison (1943) believed there must have been a link between Ireland, the Tiree-Coll group, the Small Isles, and the Outer Hebrides but not with Scotland, 'at some fairly recent time, possibly in early post- glacial times or in some inter-glacial period'. These speculations have become hallowed by non-contradiction, although there are sound ecological, geological, and genetical reasons for believing them wrong for most species. In the first place, there is a deep channel of 50-80 fathoms running along the eastern coast of the Outer Hebrides (Ting 1937; Steers 1953), probably the valley of a former river draining south-westwards into the Atlantic. The Minch coast of the Hebrides is much as it was in pre-glacial times (Ritchie 1966; Sissons 1967). Beirne (1943-44) has argued that the deep channel-is fairly narrow and that much of the Minch would have been dry land up to 10,000 years ago, allowing (for example) non-migratory Lepidoptera to colonize the islands. Nevertheless, there can be little doubt that the Outer Hebrides have never been linked to mainland Scotland since the Tertiary. A worse problem for the 'classical' theorists is that in the tundra-like conditions that followed the final retreat of the ice, only the hardiest of animals would have been able to survive as far north as northern Scotland, and logically the islands ought to have been colonized by land animals which are now found only in the Arctic and near-Arctic of northern Eurasia. Species that do not fit into this category must be either later incomers or extremely fortunate relicts of a Pleistocene spa. It is virtually impossible to disprove the last possibility. Ford (1955a) has argued that the Belted Beauty moth (Nyssia zonaria) is one such species: this is found along the sandy coasts of North Wales and South Lancashire, in coastal areas in Ireland, and on a number of the Hebrides. Other candidates are the Transparent Burnet moth (Zygaena purpuralis), the Marsh Fritillary butterfly (Euphydras aurinia), and a bumble-bee (Bombus smithianus). As far as freshwater fish are concerned, there can be no doubt that they arrived in the Hebrides by swimming, following the northern coastline as it became free from ice and penetrating the river and loch systems. There are no established populations of exotic fish at all in the Western Isles (Campbell and Williamson 1979*). The most convincing arguments for the occurrence of Pleistocene survivors or early post-Pleistocene colonizers have been built intuitively upon the mice and voles of * This paper appears in this Symposium Collection.

Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 24 Sep 2021 at 12:03:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0269727000012616 The Outer Hebrides: where genes and geography meet 23 Ireland and the islands, since small mammals are land-bound and non-adventurous. For example, Ford (1955a) accepted that there must have been a land-bridge between Scotland and Shetland subsequent to the severest part of the Ice Age because of the occurrence in Shetland field mice (Apodemus sylvaticus), and he proceeded to date the antiquity and evolution of the Lepidopteran fauna from the time that the mice colonized. Similarly Matthews (1952) has set out the evidence for both voles (Microtus agrestis neglectus) and field mice (Apodemus sylvaticus hebridensis) existing in the Hebrides as distinct forms on the grounds that they were early immigrants which have been replaced on the mainland by more successful invading forms, and that they have been able to survive in isolation following the rising of the sea-level. Richards (1935) uses the two same species as his basis for suggesting that the bee Bombus smithianus survives as a relict in Shetland and the Hebrides, having 'retreated .. . before a later invading form (B. muscorum) (which) has been able to penetrate to the Orkneys and to most of the Inner Hebrides'. Corbet (1961) has shown the emptiness of the evidence on which this theory relies, and undermined the whole argument for wholesale persistence of isolated relicts. He pointed out that not only are the geological facts against it (as we have seen), but also that the uneven distribution of small mammals (especially the absence of Microtus on Lewis and Harris, and the existence of Apodemus but not Microtus on Shetland) suggests that they are most likely to have been introduced fortuitously (probably by man) after the different islands became separated from each other and the mainland. Furthermore, certain fossils used as evidence of the widespread occurrence in mainland Britain of putative vole relicts (notably Microtus corneri and M. oeconomus) have now been shown to be wrongly identified (Sutcliffe and Kowalski 1976). The voles of Orkney (M. arvalis orcadensis) which were traditionally believed to be the earliest post-glacial colonizers of northern Britain (i.e. prior to the microtines which were to be isolated on the Hebrides) are certainly not relicts of a glacial phase survivor, but can be best interpreted as an eastern Mediterranean race brought directly to Orkney by some of the earliest human settlers (Berry and Rose 1975). Corbet (loc. cit.) does not argue that the whole of the mammal fauna of the Northern and Western Isles was introduced by man. For example, he suggests that the Pygmy Shrew (Sorex minutus) could have been an early post-glacial colonizer, or that it might have survived the last glacial phase in the south of Ireland; and he accepts that the vole of the Highlands north of the Great Glen (Microtus agrestis neglectus) is a relict form which has been replaced further south—although he regards the Outer Hebridean race as being 'clearly introduced' (largely since it is otherwise inexplicably absent from a number of large Inner Hebridean islands: Raasay, Rhum, Tiree, Coll, and Colonsay). The importance of his analysis (and subsequent supporting studies, especially Delany 1970; Berry 1969, 1973; Berry and Rose 1975; see also Fairley 1975) is that it presents an alternative source of the island faunas to the land-bridge/relict explanation. It is the purpose of this paper to examine the likelihood of introduction as opposed to survival being a major source of the Outer Hebridean fauna, and to consider the implications of this.

RELICT V INTRODUCED FAUNAS One of the major discoveries in biology in recent years has been the enormous amount of inherited variation present in virtually every animal species: an average

Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 24 Sep 2021 at 12:03:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0269727000012616 24 R. J. Berry individual is likely to be heterozygous at 5-15% of its gene loci, while a population may have more than one allele segregating at 20-30% of its loci. A correlate of this immense variability is that any small group of a species will almost inevitably differ from any other group in the frequencies of a great number of alleles: a few colonizers which become isolated may have such drastically different gene frequencies from the population whence they came, that instant sub-speciation takes place (Berry 1967, 1975). A founder event of this nature is the most effective way of changing gene frequencies that there is. If a population successfully colonizes an empty habitat, it is subject to two evolution- ary pressures: 1. In the first few generations it will increase in numbers until it fills the available space, and will then begin to experience selective forces which lead to the establishment of a local race (R. J. Berry 1971). These forces may be very different to the ones acting on the ancestral population (Mayr 1954). Chance ('drift') changes are unlikely to be important, since if the population is small enough for random processes to be effective, it will be very susceptible to extinction (Berry 1977). 2. The genes that were carried by the original founders will be the same ones (in approximately the same frequencies) as those in the expanded population unless there is early massive immigration. Once the initial founders have filled the space, further incomers are unlikely to have a marked genetical influence (apart from possibly contributing a new allele or gene combination) (Cook 1961; Grant 1970; Lidicker 1976). In other words, a population founded by a small number of individuals is likely to continue to show the genetical constitution of its founders, and reveal a close (albeit variable) relationship with its origins. Populations on a group of islands will differ from each other because each will result from a unique event. They will be distinguish- able from a group of relict populations since these will have begun by being continuous and hence relatively like each other; differences between them will be fairly small, due either to an initial heterogeneity in the distribution of genes or differentiation after isolation (Berry and Warwick 1974). Although 'founded' populations might be expected to be intrinsically less variable than relict ones, this is a difficult property to evaluate, since any marginal population (which could easily become a relict) is likely to carry less variation than a central population, and a typical individual carries so much variation that a founding event may not seriously decrease the total amount (Carson 1958; Berry 1971—although see also Bonnell and Selander 1974).

Hebridean Field-mice and Voles The characteristics of founded populations are particularly well shown by the field mice (Apodemus sylvaticus) of the Hebrides. This species is remarkably constant throughout most of its range from China westwards to the Atlantic coast, but in the British Isles it has distinct forms on St Kilda, on 11 of the Hebrides, and on three of the Shetland islands. In 1895 de Winton described 'a sharply differentiated local form' of the field mouse from Lewis, having a larger size but smaller ears than mainland mice, and being rather dark. He called this A. hebridensis. Four years later Barrett-Hamilton described

Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 24 Sep 2021 at 12:03:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0269727000012616 The Outer Hebrides: where genes and geography meet 25 another species {A. hirtensis) from Hirta of St Kilda. This was 'closely allied to Apodemus hebridensis from which it differs in its slightly larger size, and also in the greater amount of buff or yellowish-brown coloration on the underside'. A year later Barrett-Hamilton (1900) surveyed all the specimens of Apodemus in the British Museum (Natural History) and relegated A. hebridensis and A. hirtensis to sub-species of A. sylvaticus. In 1906 Kinnear acquired six Apodemus specimens from Fair Isle. These had a 'longer and narrower brain-case than typical A. sylvaticus', and were put into a new species, A.fridariensis. By now the splitting and naming fever was in full flow. Hinton (1914, 1919) added another seven sub-species, Montagu (1922) four, and finally Warwick (1940) one (nesiticus, from Mingulay and Berneray). Study of the island mice was carried forward by Delany (summarized 1970) taking classical taxonomic methods as far as they can probably go. Excluding St Kilda, he found 'the mice from Rhum are the largest with those from North Uist, Barra and Colonsay next in size. Tail length is variable, being longest in mice from Rhum and shortest in those from Raasay. . .. The animals from the Outer Hebrides (except Lewis), Colonsay and Rhum are lighter coloured with those from North Uist particularly pale. The more richly coloured mice come from Raasay and Rhum, and those with least colour from North and South Uist. . . . The Barra and Uist mice . . . are of a rather stockier build than the mice from Rhum—which appear of rather similar proportions to the mice from Applecross (on the mainland)' (Delany 1964). And so on. It is obviously possible to continue this sort of descriptive comparison almost indefinitely. Every population differs to a greater or lesser extent from every other one. Delany concluded that of all the island populations he examined, only the Rhum population was distinct enough to retain a sub-specific name {A. sylvaticus hamiltoni). Sense of the many island races was finally achieved by genetically characterizing each on the basis of non-metrical variants of the skull—multifactorially determined traits whose frequencies reflect segregation at a large number of gene loci (Berry and Berry 1971). Using this method, it is possible to calculate between population samples a multivariate 'distance' which is really a measure of genetical relationship since the characters involved are largely genetically controlled. When the Rhum population was compared with the Scottish mainland and with samples from the Outer Hebrides, it turned out to be very distinct on the skeletal statistic, just as Delany had found. However, when compared with mice from all the main neighbouring islands, the Rhum mice were much more like those from Eigg than elsewhere—as also were population samples from Muck and Canna. More surprisingly, the Eigg mice are more like Norwegian ones than Scottish mainland samples (even though samples from as far apart as Applecross, Ardnamurchan, Loch Tay, and Angus are virtually inseparable). This makes more sense in terms of history than of traditional biology: Eigg was a political centre with Scandinavian links from the early days of human organization in northern Britain. In the 8th century the Vikings raided there; by the 13th century the island was a seat of the Lord of the Isles: the Vikings must have had considerable dealings with Eigg in their exploratory journeys before they turned to conquest. In contrast Rhum never had a permanent settlement before the 17th century. If we assume that there were mice on Eigg from the early days of human settlement,

Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 24 Sep 2021 at 12:03:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0269727000012616 26 R. J. Berry it is not difficult to picture a few animals being introduced when someone landed—perhaps taking over livestock in a boat with mice hiding in the straw or foodstuff. If this is what happened, any new population established would be instantly different from the one whence it came: it is ironical that the Rhum population is so distinct yet probably young in occupation of the island. Eigg is important in considering the inter-relationships of the Outer Hebridean mouse populations, because the latter show a complicated pattern of affinities, all of them being closer to either Eigg or Norway than to the Scottish mainland (Fig. 1). Clearly there has been no simple introduction and spread of field-micei n the Hebrides: Lewis/Harris is distant from every sample except South Uist, whose closest affinities are directly with Norway; North Uist is not at all like South Uist, and is closer to Eigg than anywhere; Barra and Mingulay are closely related to each other, and to Eigg; the Shetland populations are more closely related to Norway than elsewhere (Berry 1969). The Uists were only separated from each other relatively recently, yet their mouse populations are genetically very distinct. The relationships fit closely with the expec- tations of separately founded populations, and not at all with the possibility that the mice are relicts. Less work has been done on the voles (Microtus agrestis). Miller (1908) described the Outer Hebridean form as M. a. exsul on the basis of an extra 'loop' on the upper first molar. (The island and mainland races are all variable in coat colour: the ventral pelage tends to be pale in the Outer Hebridean forms—but also in Isle of Wight voles: G. B. Corbet, pers. comm.). Barrett-Hamilton and Hinton (1910-21) interpreted the tooth character as a primitive feature, and regarded exsul as a relict form of M. a. neglectus of the Highlands which is itself a relict form of M. a. hirtus, the sub-species found in England. There are two reasons for rejecting this hypothesis: 1. It is based on the 'progressive wave' idea which gave rise first to M. orcadensis and then the Hebridean forms (q.v. Matthews 1952), but the notion that the Orkney vole is a relict is completely wrong (Berry and Rose 1975). 2. The tooth variation is better regarded as quantitative rather than qualitative (Montagu 1922; Corbett 1975), and there is an uneven cline in the mean size of the extra molar loop from southern England (c. 50 ^m) to north-west Scotland (c. 150 pm). The average size of the loop on North Uist is 236 fim, on Benbecula 133 fim, and on South Uist it is 171 /im; it is 142 fan on Skye (Corbet, pers. comm.). In other words, there is a similar haphazard distribution of traits (and therefore presumptive relationships) to that in Apodemus.

Birds, Cats, Fleas and Men Although field-mice give the clearest information about the origins and differentiation (which was called 'segregation' by the older naturalists) of the Hebridean fauna, other groups show similar evidences once the characteristics of colonization are recognized. The description of the St Kildan wren as a distinct species by Seebohm in 1884 brought to the fore the whole question of local differentiation within the British Isles, and produced fierce controversy (Barrington 1884; Dresser 1886). Notwithstand- ing, later work confirmed that local races of birds certainly exist in the Western Isles,

NORWAY

St.'Kilda

\ \ Vfa\

Mingulay

FIG. 1.—Multivariate 'distances' (100 x) between Field Mouse (Apodemus sylvaticus) populations, calculated from non-metrical skeletal variant frequencies. The values are an expression of genetical similarity. Based on Berry (1969).

although some of the taxonomic claims of ornithologists were as optimistic as those of the mammal workers described above. There is a tendency among many Hebridean birds to be darker than their mainland relatives (Meinertzhagen 1934; see also Hartert 1907; Bird 1936), and this has led to the recognition of at least five Hebridean sub-species: twite (Acanthis flavirostris bensonorum—this is no longer accepted as distinct), hedge-sparrow {Prunella modularis hebridium—now regarded as indistinguishable from Irish birds), stonechat (Saxicola torquata theresae), song thrush (Turdus philomelos hebridensis—said to occur in rural areas, but to be replaced by the typical form in Stornoway), and wren {Troglodytes troglodytes hebridensis). The wren is the most illuminating, illustrating the apparently random nature of the Hebridean races. A Hebridean form distinct from the previously described St Kildan form was named by Meinertzhagen (1924) on the grounds of showing 'characters intermediate between Scotland and Shetland birds, but as the Outer Hebrides cannot be said to be geographically intermediate between the Shetland Islands and Scotland, the race is worthy of a name'. But the situation is more complicated: hebridensis tends towards the

Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 24 Sep 2021 at 12:03:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0269727000012616 28 R. J. Berry Shetland zetlandicus in appearance, whilst the race on St Kilda (hirtensis) is a greyer more barred form similar to that (fridariensis) on the Shetland outlier of Fair Isle (Williamson 1951). While it could be maintained that the convergence between the St Kilda and Fair Isle races is adaptation towards life on a small, exposed island (e.g. Armstrong 1953; Handford and Pernetta 1974; Bourne 1974; Hawthorne et al. 1976), it seems more plausible to argue that the distinguishing characteristics of the four sub-species arose from genes fortuitously carried by the original founders. Domestic cats (Felis catus) form a stark contrast to birds. There can be no doubt that cats were introduced consciously by man, nor that some attempt is made by owners to determine the colours of the animals they allow to breed. Notwithstanding, the actual mating of cats is certainly at random; and there is good reason to believe that human selection has a comparatively small effect on the characteristics of the breeding population—even in high 'cat-caring' areas (Clark 1975). However the gene-pool of cats in towns has changed markedly in recent decades towards producing darker phenotypes, with a high correlation between human density and cat phenotype (Clark 1975). The pressures producing this change are absent in rural areas, and so by extrapolating back the probable rate of gene frequency change in urban centres (which are also the historical areas of population spread), it is often possible to deduce the time and source of origin of a cat colonization (Todd et al. 1975). Clark (1976) has determined the colour of cat coats—and thence the frequencies of six colour-determining alleles—in cats from 27 Scottish localities, including north-west Lewis, Harris, North Uist, and Barra. All four Hebridean samples are similar (except for th, which ranges in frequency from 35% to 61%), and have been pooled (Table 1). When this is done, it is clear that Hebridean cats are more like the Orkney and Shetland populations than mainland Britain, particularly in the frequencies of orange (O) and dominant white (W). There is no obvious cline in the frequency of these alleles (as there is, for example, in blotched tabby, th), and it is plausible to regard the island populations as relicts of the genomes introduced by the Vikings (N. B. Todd, pers. comm.). Similar considerations about origins apply to fleas, albeit the variation in distribution here is between and not within species. George (1975) has argued that all the fleas found on the Western Isles were introduced by man, even Megabothris rectangulatus which may be an early post-glacial relict in the Highlands proper. A particularly interesting distribution is that of Typhloceras poppei, which is monoxenous on Apo- demus sylvaticus. This flea occurs on the islands at 59°N and in southern Norway and Sweden, but no further north than 54°N on the British mainland. It is thus more likely that it was introduced to the islands with mice from a warm area, rather than surviving there as a relict. In other words, the fleas support the idea that Apodemus also was introduced. Another species, Malaraeus pencilliger mustelae, occurs on Microtus in North Uist, but is not found on Lewis and Harris, where there are no voles. This implies (although does not prove) that there never have been voles on the northern island, as expected on the introduction but not the relict hypothesis (S. Angus, pers. comm.). Relationship to its source population is one consequence of a 'founded' population; local differentiation between isolates is another. The origin of the human population of the Hebrides from continental Europe is discussed below; on a rather trivial level, local differentiation is also found. Morton et al. (1976) have studied the population of Barra in great detail. Using birth, marriage, and death records, they have shown that both

TABLE 1 Percentage gene frequencies in cats (after Toddet al. 1975, Clark 1976)

Orange Blotched No. of (tortoiseshell) Non-agouti tabby Dilute Spotting White Coefficient samples N O a th d S W of darkness England 3 16-7 79-0 810 22-3 360 0-8 _ Scotland (mainland) 19 1501 18-7 71-9 601 30-8 27-5 1-8 0-50 Kirkwall 1 120 33-8 72-5 57-4 35-5 25-3 3-9 0-39 Hebrides 4 63 32-0 63-7 48-6 32-4 32-2 5-2 0-33 North-west Lewis 1 21 33-3 651 61-4 29-9 39-5 3-5 0-39 Harris 1 18 33-6 64-7 46-3 31-4 32-2 5-3 0-32 North Uist 1 15 310 600 350 31-4 29-8 5-2 0-27 Barra 1 9 30-3 65-2 51-6 36-9 27-2 6-8 0-35 Shetland mainland 3 146 351 73-6 63-7 29-8 44-9 2-1 0-43 Yell 1 72 28-4 63-2 44-7 41 5 47-4 9-3 0 31 Iceland (rural) 99 20-2 63-2 30-4 34-2 44-1 6-2

* Frequency for males controlled by alleles at the O, a, t and rfloci.

HLA antigen B7 occurs at 42% on Barra, but 27% in both Stornoway and Glasgow (Izatt 1973; Dick and Izatt 1978); while the frequency of the rhesus negative gene (d) varies between 40% in rural Lewis and 57% in North Uist (E. J. Clegg, pers. comm.). Even within a single island, allele frequencies vary considerably and randomly—again indicating chance assortment of groups of individuals in establishing small communities (Table 2). No differences between Hebridean and mainland forms of earthworms and of spiders have been reported (Boyd 1956; Bristowe 1927). This could mean: (a) That these are truly relict groups on the islands. (b) That genetical differences in external morphology were not involved in the establishment of the island populations; or (c) that earthworms and spiders carry little genetical variation, and hence are not subject to founder differentiation.

SURVIVAL, SELECTION AND ADAPTATION Populations inevitably show traits determined by the genes carried by their fore- bears. But the test for a population is not just arriving at an uncolonized locality: it is surviving in it—and survival will almost always mean adjustment and adaptation. Since every new population is almost certainly genetically unique, this means that any

TABLE 2 Percentage gene frequencies for ABO and Rhesus blood groups* (E. J. Clegg, unpublished)

Frequency of alleles ABO system Rh system N A B d Lewis rural 571 17-6 7-0 39-9 Lewis (north-west) 118 15-7 4-8 34-4 Lewis (north-east) 136 160 5-3 47-7 Lewis (south-west) 124 32-1 9-2 43-1 Lewis (south-east) 75 16-9 7-7 25-8 Lewis (east) 118 16-2 8-4 39-1 Stornoway Burgh 240 15-8 6-4 43-2 Harris 99 18-2 5-1 50-2 North Uist 37 195 8-5 56-9 Benbecula 141 17-9 70 43-8 South Uist 283 19-4 7-5 411 Barra 98 18.6 8-5 41-6

• Determined on pregnant women.

suggested that cloud cover and humidity might be important. W. R. P. Bourne (pers. comm.) has pointed out that starlings (Sturnus vulgaris) have been present in large numbers on the Atlantic islands for far longer than on the central mainland of Scotland (Harvie-Brown 1895; Clarke 1919), and the Hebridean-Shetland form (zetlandicus) described by Hartert (1918) could be the result of many generations of isolation and adaptation. Similar explanations have been put forward for the very much more marked melanism which exists in the Lepidoptera. This occurs in races found both on north- west Scotland and the islands (such as the Meadow Brown, Maniola jurtina splendida; Large Heath, Coenonympha tullia; Marbled Coronet, Hadena conspersa; Oak Eggar, Lasiocampa quercus callunae; etc.), and in forms occurring in the Hebrides only (Dark Green and Pearl-bordered Fritillaries, Argynnis aglaia and A. selene; Common Blue, Polyommatus icarus; Grey Mountain Carpet, Entephria caesiata; Twin-spot Carpet, Colostygia didymata; Mottled Beauty, Cleora repandata). Speculations about their origins go back a long way. For example, Weir (1881) wrote 'The Hebrides have certainly had the coloration of the Geometrae and other moths, which rest on the rocks in the daytime, very much affected by the grey colour of the gneiss. Those that most assimilated in colour to that of the rock would be less easily perceived by birds, and in

https://doi.org/10.1017/S0269727000012616 . https://www.cambridge.org/core/terms at available

, subject to the Cambridge Core terms of use, of terms Core Cambridge the to subject , 12:03:07 at 2021 Sep 24 on , 170.106.34.90 address: IP . https://www.cambridge.org/core from Downloaded

uoijBidBpB JBOOJ inoqB qonui pajBaAaj SBq SJIBUS puBj uo ipjBasaj 'SJBSK luaoaj UI (9Z.6I MOISJBJ pun sauajjaf ;e/.6l -ÎP^n) uoipaps Aq psjjojiuoo 3JB ai ui sapuanbajj aua§ aqi iBq} iqnop apjij aq UBO ajaqj }nq '(suuaqjosi jajBA\-aOEJJTlS MOJJOJ Ol SJBaddB II qgnoq^B) UMOUÛtl SI 3Uip Siqj JO tguiUB3Ul, aqx 'sapniiaBi DIPJB ui %o§ 'J o} 'aSuBJ ^sapads aqa jo JTUIIJ ujaqjnos aq} }B %J UBqj ssaj UIOJJ 'S9SB3J0UI qoiqM IIBJI jaqiouB '(£ siq^X) .apBpads, Suiipuq pajuaqm aqi jo %si 3 3ABq (a5/»» ow/j) jouianmQ 11011x11103 aqj jo suoi^Bindod aqi XjiBjiuiis (0/.6I si^â PUB Airaa '£f61 UJsqjnos) Sui^Bi-SSa jo amp JIBJJ iBjnoiABqaq B JO uoijBjsajiueui ]BUJ3JX9 aqj ^luiByao JSOUIJB SI UOIJBUBA P-ii3 OIPJV 3V Qiqji^ %00I 8uiqoB3J 'suoijBjndod ujsqjjou SJOUI ui jo uoiyodojd aqj_ "3SBqd jnojoo 3jBd paûaqui aqj jo Abuanbsjj JSSAVOJ aqj OSJB puB 'sapuqaj-i sqi ui 3§UBJ SJI JO JIUIIJ ujaqjnos sqj saqoB3J (snoijisDuod snuvuojudis) Bnifs Dnojy sqi 'sjdraBxa JOJ sauip 3JBOS-3SJBI in aoBjd ajBudoaddB sjBnpiATpui uBapuqajj Jsjno UIOJJ pajjajul aq UBD uopBjdBpB jo sajduiBxa SJ3UipOUI JO SJ9S JU3J3JJIp Xq p3A3iq3B U33q SBq EJJBg UI pUB X3U5(iO m. US1JM3 JO 33UBUIUI0p 'spjOM J3I(1O UJ 'SUIJOJ JBUIJOU 3qj OJUI UOpB§3J§3S OU SBA\ 3J3\\\ —jq§H o; >}JBp UIOJJ pauBA SSSSOJO aqj jo XuaSojd aijx "siq^qsmSupsip XjiBoidAjouaqd 3J3M SUIJOJ 5[JBp aqj JOU jq8;i aqj Jaqjpu pUB 'jaqjo qDB3 UIOJJ puB jJBdB sajiui 001 ->9AO 3J3A\ 3UIB3 Xaqj qoiqM UIOJJ suonBjndod BJJBg ui nspmo jo uoiinqujsip aqa jo spus OMI aqj UIOJJ sqjoui jqgii puB >[jBp passojo (qgg6l) Pjod dBpaAO ajoSXzouioq jaqio aqi puB aioSXzojapq sqj jo 3DUBJB9ddB aijl jnq \s3uioo JBUIJOU) aaoSXzouioq 3AISS303J aq; UIOJJ sjqBqsinSuiisip S^BMJB 3JB saioS^zojaaaq :IUBUIUIOP Xpiaiduioo JSOUIJB SI qoiqM 3\3\\e ajSuis B Xq si nsiimo 'j 'puBjjoos jo qjjou aqi ui uoijnqujsip 3piA\ B SUIABIJ ajidssp 'SSUBJ SJI JO saJBd luajsjjip ui XpaBJBdss p3AjOA9 SBq (sdiuoo vuavydijx) qjoui SuiAviapun ôiPA jassaq aqj jo (usjuny) uuoj ^JBp aqi jBqi uA\oqs SBq pjo j 'Bjaidopidaq oj ujnpj ox (§§61 Jsupjô) SSUBJ ui JBISBOO puB ujsqijou 'sapads pooS B SB paidaooB MOU SI JI 'JSASMOH 'puBjsi BjppBjAj uo punoj X[uo si qDiijM 'wniDjouis s jo UIJOJ B SB papjBSsj 8uoi jnq z 161 u! A"BA\OUJOIS UIOJJ paquosap ^HBUISUO 'suaosauSiu iumiddvu£s AJJUOSBJP pajnojoD-^JBp sv^\ si ajduiBxa jB^nDBpads SJOUI v .'' ' SBll UIJOJ siqj SSUBJ jBqM A\OU5[ 01 sapuqafj aqa ui spuBjsi jaipo UIOJJ jBuaaBui 3ABq oi AJOPBJSIIBS 3JOUI sq pjnoM 11 'japBJBip siqj ui aiBipauijajui jaqjBJ si UIJOJ •sdp psj yoqs qjiM 5j3Bjq JO ÔBjq ApgjB] SJIBIJ JBjnoiqjOD aqj SuiABq ui SSOBJ aqjUIOJJSuijsjjip(snjpuofgjo)SSOBJmaqiJouJO jaquinu Baq o] JBaddBajaqx, '{\i6\ PITAV °1 'mi ui) spJBqDi-y ^ -o ^q uoiaduosap JBUISIJO aqj ssauiiA\ SB 'saAJasap \\ uBqj UMOU5[ XppiM ajoui XjqBqojd si siqx sisuapuqatj snjpuof snqwog 'aaq-ajquinq B SI auo •soiuBpui UBapuqsH aABq SJBUIIUB jaqjo jo jaquinu B 'sqjoui puB saiujajjnq sapisag '(Z.Z.61 AJâ -eZ.61 H3M31W3-JI) S33JOJ aAipaps jo aSuBJ B Xq papajjB XjuiBijao st ajaqi uisiuBpui puB 'puBpaqs ui punoj sauo UBqj auiajjxa ssa| anq JBJIUIIS 3JB SUIJOJ uBapuqan aqi l^qj si piBS aq UBD jBqi asoiu aqx (§f6l PJ0dl ^tÊHl 3«-ipa OSIB 33S -8t76I UOSIJJBH) uoiiBidBpB JO sasnBO aqi uBqj SUISIJO ui pajsaja;ui ajoui uaaq aABq Xaqi inq (BUUB^ JO jpqduiB3 q f qiiM jaqpSoa) sauapms siq puB UOSIJJBH dojsaH jo Suipaijoo aqa UIOJJ sSuuds a§pajA\ou>[ ujapoui pajiBpp :sDiuBpui jo uoijBJauaouoo aqj JOJ uosBaj aqa uiBjjao JOJ SA\OU>[ auo n 33S ou 'jaAaMOH (1681 W l 'âiâj AjjBa UB JOJ) tsuoiiipuoo iBoiSojojoapui ui JOJ pa5[ooj aq o\ SBM uisiuBpui JO asnBO Suiapxa aqa, jBqj paouiAuoo SBM (^8 aaiqM 3INM \uoqBAjasajd jo aouBqo jajpq aqj aABq pjnoM aouaasixa JOJ ajSgrujs aqj \ £ jddtu t(i{dvu8o98 puv sauaS 3uaijM :sapuq3ff 32 R. J. Berry The most informative species has been Cepaea nemoralis, but this reaches its northern limit well south of the Hebrides (Jones 1973a). The closely related species C. hortensis is scarce in the Outer Hebrides, and only three samples (all from Barra) have been described. These were monomorphic for yellow ground colour, contained no un- banded individuals, and showed a high frequency (59%) of band fusions. The species has also been collected on Skye and at Applecross, where again all the snails were yellow, although unbandeds were present (P. Rawlings, pers. comm.). In the Sand- wood Bay area of north-west Sutherland, up to half the individuals of C. hortensis are brown, a morph which has been associated in C. nemoralis with increased tolerance to cold (Cain et al. 1969; Jones 1973b).

TABLE 3 Frequencies of bridling in Hebridean and Sutherland Common guillemots (after Southern 1962)

1938-39 1948-50 1959-60 N % bridling N % bridling N % bridling Sula Sgeir 370 19-5+1-6 North Rona 3206 13-2±0-6 1875 12-7 + 0-7 Flannans 500 16-4+1-6 5562 16-0 + 0-4 St Kilda 873 16-5 + 1-2 1366 10-5±0-8 1597 14-9+0-9 Heiskeir 1246 12-4 + 0-9 Shiants 1126 11-1+0-7 581 9-8± 10 Mingulay 4004 121+0-4 Canna and Sanday 366 9-8+1-5 I slay 75 8-0±3-l 99 5-1 + 3-0 Bulgach 139 9-3 + 2-9 Handa 4705 100 + 0-3 2200 9-2 + 0-6 3691 10-0±0-5

A high correlation between habitat and variation has been found in the small snail Cochlicella acuta in the Outer Hebrides (Table 4). At all sites where collections have been made from different habitats, ranging from exposed dunes through 'mixed vegetation' to close turf, the frequencies of banding and opacity are lowest on sand, and increase on other backgrounds (Lewis 1977). This species is predated by Rooks (Corvus frugilegus), and they may exercise visual selection. This does not, of course, rule out physiological, climatic, and other forms of selection. Lanyon (1966) has shown that banding in Helicella itala is polymorphic in the Outer Hebrides. Cain et al. (1969) have suggested that variation in this species might be correlated with temperature and rainfall. The littoral snail Nucella lapillus is mainly white in the Hebrides (Berry and Crothers 1974), but local coloured or banded populations exist—in Glen Bay but not Village Bay on Hirta; at Floddaymore (North Uist); Usinish Point (South Uist); Hellisay; Berneray; and Mingulay (J. H. Crothers, pers. comm.). The reason for local high frequencies of pigmented shells in this species is not understood, but it is fairly certain that it is the result of selective processes differing in time or space (Berry 1977). All the Holarctic forms of the Three-spined Stickleback (Gasterosteus aculeatus) seem to occur in the Western Isles, and are certainly unevenly distributed, which may indicate local adaptation (Campbell and Williamson 1979*). Unfortunately little is

ISLAND FAUNAS Is it possible to generalize about the biological diversity of the Outer Hebrides? From the time of Darwin and Wallace, workers have commented on the faunal poverty of islands, and the Hebrides are no exception (Fig. 2) (Alston 1880; Carlquist 1974; Johnson and Simberloff 1974). However, ecologists have now joined forces with

TABLE 4 Percentage phenotype frequencies o/Cochlicella acuta morphs (after Lewis 1977)

Exposed sand Mixed vegetation Close turf Continuously Continuously Continuously N Banded opaque N Banded opaque N Banded opaque Lewis: Cunndal 90 0 0 Gress 78 0 1-3 Coll 35 0 0 Valtos 51 90-2 490 Harris: Starasta 143 72-6 55-0 North Uist: Veilish 73 85-8 35-2 95 90-5 48-4 Benbecula: Nunton 173 52-3 12-4 263 66-8 30-8 196 81-9 56-2 South Nunton 690 89-0 58-2 332 950 74-7 South Uist: Stonybridge 170 36-9 7-6 127 71-2 31-7 174 88-7 610 Kilpheder 91 67-1 4-5 176 69-9 12-5 111 73-9 27-9 Pollachar 101 82-2 59-4 Barra 113 690 6-8 165 80-2 191 693 87-9 30-2

biogeographers in attempts to quantify the number of species that are found on any island (MacArthur and Wilson 1967; MacArthur 1972; Lack 1976). These endeavours have been less successful than the elegance of the various models suggests (e.g. Lynch and Johnson 1974). The main factors involved are immigration (which is related to isolation), establishment correlated with available habitat, and hence with area), and extinction (which depends on competition and availability of resources) (e.g. Ball and Glucksman 1975). Lack (1969) has added a further property, that ecological tolerance can also be significant, so that a species capable of occupying a wide 'niche' may exclude 'specialists'. Lack's contribution is both important and confusing: important because it moves species diversity from the mechanical interactions of numbers (e.g. Johnson and Raven 1973) into the realms of ecological interactions, but confusing because niche 'width' is a genetical property of a population and hence capable of modification. Most colonizers (with the exception of sea birds) are r-selected 'weeds', potentially capable of speciali- zation (Berry 1977). In the context of islands, it might be better to replace the idea of 'width' with 'opportunistic adjustment'. This is a biologically valid concept, but

Rona 49^- Land vertebrates 0 Y- Birds 1^143 ><• Plants Lewis & Harris

FIG. 2.—Numbers of land vertebrates, breeding birds, and plants in the Highlands and some of the Western Isles. Data from Sharrock (1976), Perring and Walter (1962) and R. N. Campbell, unpublished.

mathematically intractable since it will vary with virtually every species and every situation. 'Opportunistic adjustment' can be considered in terms of the terrestrial vertebrate fauna of the Outer Hebrides (Table 5), because there are relatively few species involved and the history of many of them is known. Indeed, perhaps too much is known: omitting the Otter (Lutra lutra) which is virtually a marine species in northern Scotland, only two species (the Pygmy Shrew, Sorex minutus, and the Red Deer, Cervus elephas) can be fairly definitely asserted to have arrived without the intervention of man. Shrews are found on all the larger islands and can tolerate near-arctic conditions, as is shown by their occupation of such sites as the Cairngorm Plateau and the summit of Ben Nevis. No island races have been described (although inter-island differences exist in non-metrical skeletal traits: P. King, pers. comm.). Red deer were present on the Outer Hebrides in prehistoric times, as witnessed by the occurrence of large

Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 24 Sep 2021 at 12:03:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0269727000012616 available at Downloaded from https://www.cambridge.org/core/terms https://www.cambridge.org/core

TABLE 5 Occurrence of amphibians, reptiles and mammals (compiled by R. N. Campbell from information collected in 1972)

CQ DC . . IPaddress: https://doi.org/10.1017/S0269727000012616 -a -8 > c ole iled .2 — red ton'

str oo

:er rt-l ibit % "O low > §g 5 1 1 & I 1 1 1 I ! 111 f g * & 11 1§1 11 o o§ 111 §3§! I s 11 f i I ^ 1 I I Fal Ban Rat Pipi <* Sho Mol 170.106.34.90 > -J Q North Rona/Sula Sgeir Flannans Shiants Lewis + +

, on + Harris + + 9

24 Sep2021 at12:03:07 Scarp St Kilda: Hirta + Pabbay + 1 North Uist + + + Benbecula + + a. South Uist * + + Monachs + 1 Barra + + Mingulay

, subjectto theCambridgeCore termsofuse, + Vatersay Rhum + + ++ + ++ + ++ + 1 Skye + + + ++ + + + + +*+ + + + +?+ + + + + + +?+ ++?+ + + + Mull + 4 + ++ + + + + + + ++ + + + + +? +?+++ + + +?+ + Mainland (Wes t +Coast + +) + + + + + +?+ + +?+ + + +?+ + + + + + + + + + + + +?++ + ++ + + + +

* = recently introduced. 36 R. J. Berry quantities of their bones in kitchen middens at least as early as the Iron Age (Lacaille 1954), and by the occasional rinding of antlers in peat (Alston 1880). The only other mammal which might possibly be truly indigenous is the Pine Marten {Martes martes), but this species became extinct in the early 1880s, as a result of human persecution (Harvie-Brown and Buckley 1888). Its niche is being rapidly reoccupied on Lewis and Harris by feral mink (Mustela vison), presumably escapes from the mink farms in Lewis (at Steinish and Dalmore), both of which closed down in 1961 (Cuthbert 1973; S. Angus, pers. comm.). Ferrets (M. putorius furo) were released on Harris in an attempt to control rabbits (Gordon 1944), but they do not seem to have spread significantly. Hewson (1955) states that the Mountain Hare (Lepus timidus) is found 'naturally' in the Outer Hebrides, but there is no evidence for this. The species now occurs only on Lewis and Harris, being introduced at Rodel in Harris about 1859, and spreading rapidly over the whole island (Harvie-Brown and Buckley 1888). Brown Hares (L. europaeus) were also first introduced at Rodel, towards the end of the 18th century: both Pennant in 1777 and Buchanan in the 1780s note the absence of hares in the Long Island, while the Old Statistical Account of 1797 noted that in Barvas 'of late a few have made their appearance from a breed introduced by Lord Seaforth' (q.v. Harvie-Brown and Buckley 1888). They were introduced to Barra about the same time, but were extinct by 1900 (Millais 1906). An introduction to North Uist was also unsuccessful, as were at least three attempts to establish Mountain Hares there (Beveridge 1934). Rabbits had got to the Outer Hebrides by 1677 (Lever 1977); they were stated to be on South Uist prior to 1842 (q.v. New Statistical Account), and spread thence to most of the other islands, although several attempts failed to establish them on Lewis until 1865. The date of colonization by field mice and voles is unknown, but was likely to have been about a thousand years ago. The St Kilda House Mouse (Mus musculus muralis) became extinct in the 1930s, but it was related to the mice of Shetland and the Faroes, and was probably also introduced by the Vikings (Berry and Tricker 1969; Berry et al. 1978). Eagle Clarke (1905) believed that some mice from Lochmaddy in North Uist resembled muralis, while Lewis House Mice have been reported to be indistinguishable from typical specimens (Hinton and Hony 1916). One of the earliest Scottish references to Black Rats (Rattus rattus) was given by Martin Martin in 1703, when he recorded 'a great many rats in the Village Rowdil (Rodel), which have become very troublesome to the Natives . . .'. Martin also records the dying of the entire human population of North Rona soon after 1685, following the landing of rats from a wrecked ship. Brown Rats (R. norvegicus) presumably reached the Western Isles in the 18th century, since they were introduced to the Faroes in 1768 from a ship wrecked on Lewis but which finally ran aground on Faroe, although they were not noted in the Highlands until 1814 (Lever 1977). They have now replaced the Black Rat everywhere in the Outer Hebrides except, apparently, on the Shiants. Hedgehogs and frogs have been introduced to the islands very recently. Nothing seems to be known about the antecedents of the reptiles. Does this catalogue add up to anything, other than the dominant influence of man on the terrestrial vertebrates of the islands? Certainly the Outer Hebrides have a higher proportion of the birds and vascular plants found in the British Isles than of the land animals when compared with the inshore islands and the mainland (although the

HEBRIDEAN MAN Despite the wealth of prehistoric remains in the Outer Hebrides, surprisingly little is known about the relationships of the humans who inhabit the islands and who have had such a marked influence on the natural history (Lacaille 1954; Wainwright 1955; Murray 1966; Brothwell 1974). An anthropometric study of the Benbecula population found that the average Outer Hebridean is tall, and can be 'somewhat inconsequently described as mesocephalic, leptoprosopic and leptorrhine. . . . The large Benbecula head breadth appears to be an extreme for the contempory male inhabitants of the British Isles' (Searight et al. 1944). A. C. Berry (1971, 1974) compared a sample of skulls excavated from an eroding graveyard on Ensay in the Sound of Harris, which served southern Harris until the end of the 18th century (A. E. W. Miles, pers. comm.), with samples from a number of other north-west European samples of different dates, using non-metrical variants as genetical traits. She found that the Ensay population was most like a sample of skulls from south-east Scotland also of mediaeval date (Turner 1914), and one from the early Stone Age of Denmark (before 3000 BC) (Fig. 3). If the Ensay population can be taken as typical of the Outer Hebrides, this implies that the gene-pool of Hebridean man has remained relatively unchanged genetically since the early waves of post-Neolithic colonizers, and that the periods of Viking dominion had a comparatively small impact on the gene-pool (Berry and Muir 1975). However, another set of comparisons can be made using the frequencies of blood group and protein variants in different living samples. Sufficient data exist to calculate 'genetical distances' between Lewis and Barra, and a number of other relevant populations (Table 6). Lewis is, as already noted, very distinct from Barra, and is most like Orkney and Iceland, suggesting a strong Scandinavian element in the island's people.

Iceland Norway

Mediaeval "1 I Denmark 1.8 ^^ Stone Age I

I) Glasgow \ V m)i 2.9o

O (Romano- Bntish)

Maiden Castle / (Iron Age)

FIG. 3.—Multivariate 'distances' (100 x) between human population samples, calculated from the frequencies of non-metrical variants of the skull. Data from A. C. Berry (1971, 1974).

TABLE 6 Genetic distances (Rogers 1972) between human population, based on enzyme and blood group frequencies (data from Morton et al. 1977)

No. of loci classified Faroe Denmark Norway Orkney Lewis Barra Highlands Lowlands England Ireland 14 Iceland 0050 0055 0-055 0051 0033 0-077 0-075 0044 0070 0042 6 Faroe 0086 0085 0090 0068 0-053 0067 0-070 0129 0056 13 Denmark 0022 0055 0041 0-058 0-077 0023 0040 0-040 13 Norway 0060 0034 0064 0090 0033 0044 0040 14 Orkney 0029 0-057 0045 0046 0068 0038 8 Lewis 0066 0068 0031 0074 0-042 7 Barra 0062 0035 0087 0057 10 Highlands 0055 0100 0045 14 Lowlands 0040 0-027 14 England 0054 11 Ireland

The origins of the human population must not be over-emphasized in considering the fauna of the Hebrides. As we have seen, the Vikings (who were the first people with adequate ships for routine voyages in northern waters) were certainly influential in introducing rodents, while other species have an Atlantic coastal or even American source (Balfour-Browne 1938; Harrison 1948). The humans are important in their role as an ecological factor in their own right, and not merely as passive agents for transporting animals and plants (Darling 1955; Darling and Boyd 1964). Fifty years ago, Ritchie (1930) looked on the animal life of Scotland in exactly the same way. Already convinced of the impact of man on natural history (Ritchie 1920), he concluded his Inaugural Lecture as Regius Professor at Aberdeen as follows: 'In the fauna of St Kilda and in the thirty-two distinctive birds and thirty distinctive mammals of Scotland, we are looking upon the modelling from old species of new species and of geographical races, which we regard as the incipient stages of new species. In short, in the changes taking place in the balance of life, in the plasticity of animal form, and in the formation of new races and species, not in the distant past of the geologists, but in recent times, we are looking in Scotland upon Evolution in its course.' Fifty years later, there is no reason to disagree.

ACKNOWLEDGMENTS I am most grateful to many who have discussed the Outer Hebridean situation with me, and allowed me to quote unpublished data or observations, especially Professor E. J. Clegg, Drs W. R. P. Bourne, J. C. Cadbury, G. B. Corbet, Heather Dick, R. D. Hill,

REFERENCES The paper carrying an asterisk appears among the Symposium collection contained in Proc. Roy. Soc. Edinb., 77B. ALSTON, E. R., 1880. The Fauna of Scotland, with Special Reference to Clydesdale and the Western District. Mammalia. Glasgow: Nat. Hist. Soc. ARMSTRONG, E. A., 1953. Island wrens: conditions influencing subspeciation and survival. Br. Birds, 46, 418-420. BALFOUR-BROWNE, F., 1938. The aquatic Coleoptera of the Outer Hebrides. Scott. Nat., 1938, 33-46. BALL, E. and GLUCKSMAN, J., 1975. Biological colonization of Motmot, a recently created tropical island. Proc. Roy. Soc. B., 190, 421-442. BARRETT-HAMILTON, G. E. H., 1900. On geographical and individual variation in Mus sylvaticus and its allies. Proc. Zool. Soc. Lond., 1900, 397^28. and HINTON, M. A. C, 1910-21. A History of British Mammals. London: Gurney and Jackson. BARRINGTON, R. M., 1884. The St Kilda wren. Zoologist, 8, 383-385. BEIRNE, B. P., 1943-44. The relationships and origins of the Lepidoptera of the Outer Hebrides, Shetland, Faroes, and Iceland. Proc. Roy. Ir. Acad., 49B, 91-101. , 1952. The Origin and History of the British Fauna. London: Methuen. BERRY, A. C, 1971. The use of minor skeletal variants in the study of human populations. Unpublished Ph.D. thesis, Univ. of London. , 1974. The use of non-metrical variations of the cranium in the study of Scandinavian population movements. Am. J. Phys. Anthrop., 40, 345-358. and BERRY, R. J., 1971. Epigenetic polymorphism in the primate skeleton. In: Comparative Genetics in Monkeys, Apes and Man. B. Chiarelli, Ed., pp. 13-41. London and New York: Academic. BERRY, R. J., 1967. Genetical changes in mice and men. Eugen. Rev., 59, 78-96. , 1969. History in the evolution of Apodemus sylvaticus (Mammalia) at one edge of its range. /. Zool., Lond., 159,311-328. , 1971. Conservation aspects of the genetical constitution of populations. In: The Scientific Management of Animal and Plant Communities for Conservation. E. A. G. Duffey, Ed., pp. 177-206. Oxford: Blackwell. , 1973. Chance and change in British long-tailed field mice (Apodemus sylvaticus). J. Zool., Lond., 170, 351-366. , 1975. On the nature of genetical distance and island races of Apodemus sylvaticus. J. Zool., Lond., 176, 293-296. , 1977. Inheritance and Natural History. London: Collins New Naturalist. and CROTHERS, J. H., 1974. Visible variation in the Dog-whelk, Nucella lapillus. J. Zool., Lond., 174, 123-148. and DAVIS, P. E., 1970. Polymorphism and behaviour in the Arctic Skua (Stercorariusparasiticus (L)). Proc. Roy. Soc. B., 175, 255-267. , JAKOBSON, M. E. and PETERS, J., 1978. The house mice of the Faroe Islands: a study in microdifferentia- tion. J. Zool., Lond., 185, 73-92. and MUIR, V. M. L., 1975. The natural history of man in Shetland. /. Biosoc. Sci., 7, 319-344. , PETERS, J. and VAN AARDE, R. J., 1978. Sub-Antarctic house mice: colonization, survival and selection. J. Zool, Lond., 184, 127-141. and ROSE, F. E. N., 1975. Islands and the evolution of Microtus arvalis (Microtinae). J. Zool., Lond., Ill, 395-407. and TRICKER, B. J. K., 1969. Competition and extinction: the mice of Foula, with notes on those of Fair Isle and St Kilda. J. Zool., Lond., 158, 247-265. and WARWICK, T., 1974. Field mice (Apodemus sylvaticus) on the Castle Rock, Edinburgh: an isolated population. J. Zool., Lond., 174, 325—331. BEVERIDGE, G., 1934. The hare in North Uist. Scott. Nat., 1934, 94. BIRD, E. G., 1936. Two new subspecies from the Outer Hebrides (Emberiza, Anthus).Bull. Br. Orn. Club, 56, 54-56. BONNELL, M. L. and SELANDER, R. K., 1974. Elephant seals: genetic variation and near extinction Science N.Y., 184,908-909. BOURNE, W. R. P., 1974. Geographical variation in Shetland birds. In: The Natural Environment of Shetland. R. Goodier Ed.,' pp. 145-146. Edinburgh: Nature Conservancy Council.

Downloaded from https://www.cambridge.org/core. IP address: 170.106.34.90, on 24 Sep 2021 at 12:03:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0269727000012616 The Outer Hebrides: where genes and geography meet 41 BOYD, J. M., 1956. The Lumbricidae in the Hebrides. II. Geographical distribution. Scott. Nat., 68,165-172. BRISTOWE, W. S., 1927. The spider fauna of the Western Islands of Scotland. Scott. Nat., 1927, 88-94; 117-122. BROTHWELL, D., 1974. Evidence of population change and variability in the British Isles. In: Rassen- geschichte der Menschheit. K. Sailer and I. Schwidetzky Eds, vol. 2, pp. 7-43, Munich and Vienna: Oldenbourg. CAIN, A. J., CAMERON, R. A. D. and PARKIN, D. T., 1969. Ecology and variation of some Helicid snails in northern Scotland. Proc. Malac. Soc. Lond., 38, 269-299. •CAMPBELL, R. N. and WILLIAMSON, R. B., 1979. The fishes of inland waters in the Outer Hebrides. Proc. Roy. Soc. Edinb., 77B, 377-393. CARLQUIST, S., 1974. Island Biology. New York and London: Columbia. CARSON, H. L., 1958. Response to selection under different conditions of recombination in Drosophila. Cold Spring Harb. Symp. Quant. Biol, 23, 291-306. CLARK, J. M., 1975. The effects of selection and human preference on coat colour gene frequencies in urban cats. Heredity, 35, 195-210. , 1976. Variations in coat colour gene frequencies and selection in the cats of Scotland. Genetica, 46, 401-412. CLARKE, W. E., 1905. On the house mouse of the Outer Hebrides. Scott. Nat., 14, 198-199. , 1919. The starling of Shetland, Fair Isle and St Kilda. Scott. Nat., 1919, 183-185. CLEGG, E. J., 1975. Marriages in Lewis and Harris. In: Biosocial Interrelations in Population Adaptation. E. S. Watts, F. E. Johnston and G. W. Lasker, eds, Pp. 147-163. The Hague: Mouton. COOK, L. M., 1961. The edge effect in population genetics. Am. Nat., 95,295-307. CORBET, G. B., 1961. Origin of the British insular races of small mammals and of the 'Lusitanian' fauna. Nature, Lond., 191, 1037-1040. , 1975. Examples of short- and long-term changes of dental pattern in Scottish voles (Rodentia; Microtinae). Mammal Rev., 5, 17-21. CUTHBERT, J. H., 1973. The origin and distribution of feral mink in Scotland. Mammal Rev., 3, 97-103. DARLING, F. F., 1955. West Highland Survey: an Essay in Human Ecology. Oxford: Univ. Press. and BOYD, J. M., 1964. The Highlands and Islands. London: Collins New Naturalist. DELANY, M. J., 1964. Variation in the long-tailed field-mouse (Apodemus sylvaticus (L.)) in north-west Scotland. I. Comparison of individual characters. Proc. Roy. Soc. B., 161, 191-199. , 1970. Variation and ecology of island populations of the long-tailed field-mouse (Apodemus sylvaticus (L.)). In: Variation in Mammalian Populations. R. J. Berry and H. N. Southern, Eds, pp. 283-295. London: Academic. DE WINTON, W. E., 1895. The long-tailed field-mouse of the Outer Hebrides: a proposed new species. Zoologist, 19 (3) 369-371, 426. DICK, H. M. and IZATT, M. M., 1978. HLA antigens and IgG allotypes on the Island of Barra (Outer Hebrides). Ann. Hum. Biol, 5, 441-451. DRESSER, H. E., 1886. On the wren of St Kilda. Ibis, 1886, 43^5. ENDLER, H. A., 1973. Gene flow and population differentiation. Science, N. Y., 179, 243-250. FAIRLEY, J. S., 1975. An Irish Beast Book. Belfast: Blackstaff. FORD, E. B., 1945. Butterflies. London: Collins New Naturalist. , 1955a. Moths. London: Collins New Naturalist. , 1955b. Polymorphism and taxonomy. Heredity, 9, 255-264. GARDNER, A. E., 1955. A study of the genitalia of the two species Sympetrum nigrescens Lucas and 5. nigrifemur (Selys) with notes on their distribution. Entomologist's Gaz., 6, 86-108. GEORGE, R. S., 1975. The possible sources of the British flea fauna. Unpublished paper read at 1st European Conf. on Fleas, Lund. GORDON, S., 1944. A Highland Year. London: Eyre and Spottiswoode. GRANT, P. R., 1970. Colonization of islands by ecologically dissimilar species of mammals. Can. J. Zool. ,48, 545-553. HANDFORD, P. T. and PERNETTA, J. C, 1974. The origin of island races of Apodemus sylvaticus: an alternative hypothesis. J. Zool., Lond., 174, 534-537. HARRISON, J. W. H., 1943. The range of the Greasy Fritillary (Euphydras aurinia) in the Hebrides and some possible deductions therefrom. Entomologist's Rec. J. Var., 55, 27. , 1948. The passing of the Ice Age and its effect upon the plant and animal life of the Scottish Western Isles. New Nat. J., 1, 83-90. HARTERT, E., 1907. Birds represented in the British Isles by peculiar forms. Br. Birds, 1, 208-222. , 1918. Notes on starlings. Novit. Zool, 25, 327-337. HARVIE-BROWN, J. A., 1895. The starling in Scotland, its increase and distribution. Ann. Scott. Nat. Hist., 4, 2-22. and BUCKLEY, T. E., 1888. A Vertebrate Fauna of the Outer Hebrides. Edinburgh: Douglas.

HAWTHORNE, I., CROCKFORD, R., SMITH, R. G. and WESTON, I., 1976. The wren on the Uists, Outer Hebrides. Bird Study, 23, 301-303. HEWSON, R., 1955. The mountain hare in the Scottish islands. Scott. Nat., 67, 52-60. HINTON, M. A. C, 1914. Notes on the British forms of Apodemus. Ann. Mag. Nat. Hist., 14, 117-134. , 1919. The field mouse of FOM\SL. Scott. Nat., 1919, 177-181. and HONY, G. B., 1916. Notes on two collections of mice from Lewis, Outer Hebrides. Scott. Nat., 1916, 221-227. HUXLEY, J. S., 1942. Evolution, the Modem Synthesis. London: Allen and Unwin. IZATT, M. M., 1973. The Gm(l) and Gm(2) factors in Scotland. In: Genetic Variation in Britain. D. F. Roberts and E. Sunderland, Eds, pp. 197-205. London: Taylor and Francis. JEFFERIES, D. J. and PARSLOW, J. L. F., 1976. The genetics of bridling in guillemots from a study of hand-reared birds. J. Zool, Land., 179, 411-420. JOHNSON, M. P. and RAVEN, P. H., 1973. Species number and endemism: the Galapagos revisited. Science, N.Y., 179,893-895. and SIMBERLOFF, D. S., 1974. Environmental determinants of island species numbers in the British Isles. J. Biogeog., 1, 149-154. JONES, J. S., 1973a. Ecological genetics of a population of the snail Cepaea nemoralis at the northern limit of its range. Heredity, 31, 201-211. , 1973b. Ecological genetics and natural selection in molluscs. Science, N. Y., 182, 546-552. KETTLEWELL, H. B. D., 1973. The Evolution of Melanism. Oxford: Clarendon Press. KINNEAR, N. B., 1906. On the mammals of Fair Isle, with a description of a new sub-species of Mus sylvaticus. Ann. Scott. Nat. Hist., 15, 65-68. LACAILLE, A. D., 1954. The Stone Age in Scotland. London: Oxford Univ. Press. LACK, D., 1969. The numbers of bird species on islands. Bird Study, 16, 193-209. , 1976. Island Biology. Oxford: Blackwell. LANYON, P. G. H., 1966. Discontinuous variations in the banding on the shells of the heath snail, Helicella itala. Rep. Marlboro. Coll. Nat. Hist. Soc, 105, 44-49. LEVER, C, 1977. The Naturalized Animals of the British Isles. London: Hutchinson. LEWIS, G., 1977. Polymorphism and selection in Cochlicella acuta. Phil. Trans. Roy. Soc. B., 276, 399-451. LIDICKER, W. Z., 1976. Social behaviour and density regulation in house mice living in large enclosures. J. Anim. Ecol, 45, 677-697. LINDROTH, C. H., 1970. Survival of animals and plants on ice-free refugia during the Pleistocene glaciations. Endeavour, 29, 129-134. LYNCH, J. F. and JOHNSON, N. K., 1974. Turnover and equilibria in insular avifaunas, with special reference to the California Channel Islands. Condor, 76, 370-384. MACARTHUR, R. H., 1972. Geographical Ecology. New York: Harper and Row. and WILSON, E. O. 1967. The Theory of Island Biogeography. Princeton, N.J.: Princeton Univ. Press. MARTIN, M., 1703. A-Description of the Western Islands of Scotland. Glasgow: Morison/London: Hamilton, Adams. MATTHEWS, L. H., 1952. British Mammals. London: Collins New Naturalist. MAYR, E., 1954. Change of genetic environment and evolution. In: Evolution as a Process. J. Huxley, A. C. Hardy and E. B. Ford, Eds, pp. 157-180. London: Allen and Unwin. MEINERTZHAGEN, R., 1924. A note on the Scottish wrens (Troglodytes) with characteristics of a newly defined Hebridean race. Scott. Nat. 1924, 135. , 1934. The relation between plumage and environment, with special reference to the Outer Hebrides. Ibis, 8(4), 52-61. MILLAIS, J. G., 1906. The Mammals of Great Britain and Ireland, vol. 3. London: Longmans. MILLER, G. S., 1908. Eighteen new European voles. Ann. Mag. Nat. Hist., 1, 194-206. MONTAGU, I. G. S., 1922. On a further collection of mammals from the Inner Hebrides. Proc. Zool. Soc. Land., 1922,929-941. MORTON, N. E., DICK, H. M., ALLEN, N., IZATT, M., HILL, R. D. and YEE, S., 1977. Bioassay of kinship in north-western Europe. Ann. Hum. Genet., 41, 249-255. , SMITH, C, HILL, R. D., FRACKIEWICZ, A., LAW, P. and YEE, S., 1976. Population structure of Barra (Outer Hebrides). Ann. Hum. Genet., 39, 339-352. MURRAY, W. H., 1966. The Hebrides. London: Heinemann. PERRING, F. H. and WALTERS, S. M., 1962. Atlas of the British Flora. London: Nelson. POORE, M. E. D. and MCVEAN, D. N., 1957. A new approach to Scottish mountain vegetation. J. Ecol., 45, 401^*39. RICHARDS, O. W., 1935. Bombusmuscorum (Linnaeus) and B. smithianus White (Hym.). Trans. Soc. Br. Ent., 2, 73-85. RITCHIE, J., 1920. The Influence of Man on Animal Life in Scotland. London: Cambridge Univ. Press. , 1930. Scotland's testimony to the march of evolution. Scott. Nat., 1930, 161-169.