A Comparison of Populations of the Polymorphic Land Snail Cepaea Nemoralis (L.) Living in a Lowland District in France with Those in a Similar District in England

A COMPARISON OF POPULATIONS OF THE POLYMORPHIC LAND SNAIL CEPAEA NEMORALIS (L.) LIVING IN A LOWLAND DISTRICT IN FRANCE WITH THOSE IN A SIMILAR DISTRICT IN ENGLAND

RICHARD ARNOLD*

Department of Zoology, Oxford, England Received December 3, 1968

AIN and SHEPPARD(1950, 1954) considered that natural selection, in particular visual selection exerted by predators, was an important agent influencing shell color and banding-morph frequencies in populations of Cepaea nemoralis in England. LAMOTTE( 195 1) on the other hand, working with French populations, concluded that selection was of little significance in most colonies, and that random evolutionary effects exerted an overriding effect. Subsequent research has modified some of the early conclusions, but the differences in opinions are still trenchant (e.g., CAIN and SHEPPARD1961; CAIN and CURREY1963; LAMOTTE1959, 1966). Work in England has tended to confirm CAINand SHEPPARD’Sconclusions. At least in lowland and wooded districts in England, further circumstantial evidence has been forthcoming both that visual selection is an important factor, and that there is little reason to believe that random effects have operated to an appreciable extent in most colonies (CURREY, ARNOLDand CARTER1964; CARTER1968; CAIN and CURREY1963). LAMOTTE (1959, 1966) describes variation both of color and banding-morph frequencies with habitat in populations in some parts of France. He is more inclined, however, to attribute the phenomenon to climatic selection (favoring yellow and unbanded snails in open habitats and pink and banded ones in woods land shady habitats) than to the effects of visual selection. He also considers that the weak- ness of the correlation of morph frequencies with habitat is mainly due to random evolutionary events. GUERRUCCI-HENRION(1966) has found that the frequencies of the color morphs in populations in Brittany are correlated with the amount of precipitation, and suggested that drier climates favor the yellow morph. She concluded, however, that most of the intercolony variation in banding-morph frequencies in Breton populations was best explained by random effects. The methods employed by both groups of workers differed. LAMOTEsampled extensively and irrespective of habitat type, and considered color and banding-morph frequencies separately. CAIN and SHEPPARDchose well defined habitats, selected largely within a small radius around Oxford, and considered color and banding- morph frequencies together in any one colony. Direct comparison and interpretation of results has been somewhat difficult,

* Present address Department of Zoology, Umversity College of North Wales, Bangor, Caerns., U.K

Genetics 64: 589-604 March/Apnl 1970 590 RICHARD ARNOLD because of different methodology, and because the ecological situations in Eng- land and France (with respect to factors influencing the balance of the polymorphism) may well be fundamentally different, as LAMOTTE(1959) has emphasized. The purpose of this work is to examine morph frequencies in populations sampled in a district in France broadly similar to CAIN and SHEPPARD’Snear Oxford, using as closely as feasible the methods which Cain and Sheppard employed, in order to make a comparison and to explain why different conclusions were reached by the different workers.

METHODS Choice of district: Most of northern, north-central, and western France is low lying and well wooded, like the country near Oxford, and C. nemoralis is known to be widespread (LAMOTTE 1951, 1954a). The choice of Touraine as a region in which to work was arbitrary, for apart from LAMOTTE’S(1954a) data on his Centre region, I knew nothing about morph frequencies before sampling commenced. Chaice of the Oxford district was similarly arbitrary for CAIN and SHEPPARD.The survey was centered on VendBme, 56 km northeast of Tours on the River Loir (Figure 1) and not on Tours, as originally intended, simply because the country looked compar- able with that near Oxford and because it offered a suitable range of habitats from which to sample populations. Hence it may stand as a sample district in north-central France for comparison of morph frequencies there with those near Oxford. Collecting and scoring: Random samples were taken from populations of Cepaea living in well defined and homogeneous habitats wherever the species could be found. It was late in the season (12-21 October, 1963), and live snails were seldom encountered. Collecting was not easy,

FIGURE1.-Map showing the positions of the samples. Wood samples: black circles; open- habitat samples: open circles. The dotted line is the Paris-Bordeaux Road. CEPAEA IN FRANCE AND ENGLAND 591 and attention was centered on promising localities for sampling, particularly recently felled deciduous woods, burnt grass verges, and recently trimmed hedgerows and banks, where shells of dead or preyed-upon snails could be found. These situations were not common, and this is the reason why the sampling district was more extensive than the Oxford district (within 16 km of Oxford, CAIN ad~CURREY 1963). The district and the position of the samples are shown inFigure 1. Samples were scored according to the methods of CAINand SHEPPARD(1950, 1954). Details are given in Table 1. The gene nomenclature used is that in CAIN, SHEPPARDand KING (1968). One sample (No. 2) was unscorable, because individuals of C. nemoralis and Cepaea hortensis (Mull.) could not with certainty be distinguished from one another. Thirty scorable samples of mean size 35 (shells) were made in Touraine, compared with 67 samples of mean size 167 made near Oxford. Description of the district: All scorable samples except No. 1 were taken within 36 km of Vendbme, and this radius approximately defines the district (No. 1 was taken 56 km from VendGme, in country like that described). In most respects the country is not dissimilar from that around Oxford, differing in its larger scale-more extensive woods and forests, and more expansive fields. The River Loir divides the district into a northern and southern section, and interrupts the plateau nature of the terrain, as do its tributaries and those of the River Loire. The country is between 225 and 150 metres in altitude, of undulating chalk with loess deposits. In places, particularly near the periphery of the district, the ground may be marshy and the soil probably acid (Heaths, Erica and Calluna spp., and Bracken, Pteridium aquilinum L., were abundant in such localities). The countryside is well wooded, with Oak (Quercus spp.) either dominant in the woods sampled, or (more often) in association with such species as Sweet Chest- nut (Castanea sativa Mill.), Ash (Praxinus excelsior L.) and Hornbeam (Carpinus betulus L.) Most of the land is otherwise given over to cereal and root crops, and vines are grown in sheltered places.

RESULTS Variation with habitat in Touraine: CAIN and SHEPPARD( 1954) based their conclusions on a demonstrable correlation between morph frequency and the backgound nature of the habitat (the most cryptic morphs predominating against a given background type). Figure 2b (from CAIN and CURREY1963) illustrates their result. Each point on the diagram presents morph frequencies in a population on a visual basis in relation to the habitat of that population. The frequency of yellow (versus dark, pink and brown) shells is plotted against the frequency of shells which, to a visual predator, might appear unbanded (“effectively unbanded,” CAIN and SHEPPARD1950, morphs with at least the upper two bands absent), versus those which appear banded, mostly shells with five bands of formula 12345. Figure 2a shows Touraine samples displayed in the same way for comparison. There is, as in Oxford populations, significant variation with habitat of the color- morph frequencies (xI2 = 19.6, P < 0.001) in the direction expected on the hypothesis of visual selection-wood populations have higher frequencies of the dark-color morphs. Further, effectively unbanded shells are frequent in the woodland populations, as near Oxford. There is no evidence of variation with habitat of banding-morph frequencies in Touraine (xIz = 0.1, P > 0.7). Although the open-habitat samples agree with the action of visual selection €or color, they mostly have high frequencies of ef- TABLE 1

Details of samples from Touruine. The samples' numbers cross-reference with Figure 1 cn (0 Number Yellow* Pink* M of sample Locality Total Remark4 Habitat: 0 3 5 EU5 0 3 5 EU5 Brown 1. La Flkche-Tours Road . 24 24 . .13. .. 52 H 2. Nazelles ...... 92 Species confusion MDW 3. Reugny 21 8 13 10 1221 1'0' 59 +C.hort. H 4. VendBme 2 4 513 ..I3 1'3' 29 +C.hort. H 5. VendBme 1671 2 814 3 2'0' M +C.hort. MDW 6. VendBme 1411 1 19 17 4 5'0' 53 +C.hort. MDW 7. Mor6e ..23 8 4 210 .. 29 MDW 8. Haie de Champ 6 912 1 ,341 .. 36 RH 9. Galette 717 8 1 4951 .. 52 G IO. Courtoze 1432 . 7 412 1'0' 34 +C.hort. MDW 11. Masange 4514 ,225 .. 23 H 12. &puisay 127. 1813, .. 32 H dE 13. Rpuisay ,491 .66. .. 26 H 14. For6t de VendBme 112. 413 8 5 2'0' 36 MDW UE 15. La Jouwl-inikre ,152 4454 .. 25 G 16. South of Dan& .2.. 2531 1'0' 14 From 4 adjacent woods OW 17. Sarg6-sur-bra ye 12432 3651 .. 36 +C.hort. H 18. Villaria-Varennes 2113 3494 .. 27 MDW 19. West of Marcilly 115. 4592 5'0' 32 MDW 20. North of Houssay .31 319 4 1 .. 31 MDW 21. St. Rimay 10 3 16 4 1412 3 .. 53 G 22. St. Arnoult 151. 117 4 1 2'0' 32 +C.hort. MDW 23. St. Arnoult ,431 3551 .. 22 H 24. North of Villethiou . . 11 12 163 .. 33 G 25. St. Arnoult ,111 2688 1'0' 28 +C.hort. ow 26. Bois la Barbe 1521 .IO 5 2 .. 26 ow 27. Ste.Anne 218 6 8 ,832 .. 47 G 28. Villeromain .22 3 5 ,622 .. 41) G 29. South of Villethiou .I1 5 2 1244 .. 29 G 30. East of Artins 25 4 7 11 21.2 .. 52 RH 31. Northwest of Marc6 2.. 15 4 2 1'0' 24 ow * '0,"3,' '5,' 'EU5': Phenotypes of band formulae OOOOO, 00300, 12345, and effectively unbanded 00345 (and minor variations), respectively. $ G = grass, RH = rough herbage, H = hedgerow, MDW = mixed deciduous wood, OW = oakwood. t The presence of C.hortensis is noted. CEPAEA IN FRANCE AND ENGLAND 593

100 1

I ,,,,,I..... 0 50 100 0 50 100 % QffQCtiValyunbanded %effectiveiy unbanded FIGURE2.-Scatter diagrams for (a) Touraine samples and (b) Oxford district samples (from CAINand CURREY1963). Habitat symbols: vertical bar, long grass; horizontal bar, rough herbage; cross, hedgerow; horizontal black hemisphere, mixed deciduous wood; vertical black hemisphere, oakwood; dotted circle, sample under 20 shells. fectively unbanded shells, not agreeing with the predominantly grassy, stripey, nature of the backgrounds, in contrast to those from the Oxford district. Figure 1 shows that wood and open-habitat samples from Touraine are well intermingled geographically. The difference in color-morph frequencies between habitat classes is not due to geographical localization of particular color-morph frequencies in different parts of the district. Wood populations in Touraine agree therefore with CAIN and SHEPPARD'S hypothesis. Open-habitat populations, though, have color-morph frequencies that agree, but it appears that visual selection is ineffective on banding in such habitats, and that other factors override its effects. Comparison between the Touraine and Oxford samples: Particular attention is drawn firstly to those moi-phs about which there is an accumulated body of circumstantial evidence concerning the action of climate on them (unbanded, and brown), and to which this stndy can contribute; and then to the main contrast between Touraine and Oxford populations, the higher overall 'frequency of effectively unbanded morphs, especially in the open habitats in the former district. (1) The frequency of the unbanded phenotype: LAMOTTE (1951) found that unbanded snails tended to be slightly more frequent than banded ones in in- solated, open habitats in France than in woodland, and his later data (1959,1966) are consistent with this. He emphasized that the observed differences were slight, and that in no case were they significant. He also demonstrated (1 959) a positive correlation between the frequency of unbanded and mean July temperature, and suggested that climatic factors were of some slight importance in selectively influencing morph frequencies. More recently, evidence from the Pyrenees (ARNOLD1968, 1969) indicates that at least in climates relatively extreme for the species with respect to insolation, dryness and high temperature, unbanded individuals are favored, with five-banded ones advantageous in milder climatic 5 94 RICHARD ARNOLD rkgimes. CAIN and SHEPPARD(1961) agreed that climate might play a stronger r81e than visual selection in France, as part of their explanation why the latter agent appeared to be less effective in France in affecting morph frequencies. Touraine is approximately 450 km south of Oxford, and sunnier (directly com- parable figures are not available, for insolation is measured by sunshine-hours in Britain and by nebdosit6 in France). It is also warmer from March to October. The mean daily maximum temperature in July, for example, for Orlkans (64 km east of Vend8me) is 24.0"C. with a figure of 21.7"C for Oxford (Meteorologi- cal Office 1958). It might be expected that unbanded, therefore, might be more frequent in Touraine populations. To test this, frequencies of unbanded-not-in-brown in the two districts have been compared. The brown morph is nearly always unbanded (CAIN,KING and SHEPPARD1960; Table 1 in this paper). The comparisons are shown as a series of histograms in Figure 3. The means that are given are unweighted, and are derived simply by dividing the sums of the frequencies per colony by the total number of colonies (as LAMOTTE( 195 1,1959) appears to have derived them). Surprisingly perhaps, there is little evidence in these data that unbanded is more frequent in Touraine. The mean frequencies for the open-habitat samples are the same (15.2%) in both Touraine and Oxfordshire, and the difference in frequency distribution is not significant (xI2= 1.4, P > 0.20), although it is in the direction of more colonies with higher frequencies in Touraine. Touraine wood populations have significantly lower frequencies than Oxford ones ( xI2 = 4.5, 0.02 < P < 0.05). mean frequencies being 10.5% and 21.6%, respectively. The distribution of frequencies of unbanded-not-in-brown in the habitat types is different in the two districts. Where Oxford wood populations tend to have higher frequencies than the open-habitat ones, the reverse situation obtains in Touraine: mean frequencies there are 10.5% and 15.2%, in wood and open- habitat colonies, respectively (Figure 3). This observation agrees with LAMOTTE'S results in France, and helps to strengthen his hypothesis about the effects of microclimatic differences on banding. The contrasting situation in the Oxford district might suggest that climate plays a lesser r6le in influencing the frequen- cie; of at least the banding morphs there. In a mild climate, like that of southern Britain, the action of the complex of other selective factors might well override the action of the syndrome of climatic ones. (2) The frequency of the brown phenotype: Climatic selection has also been invoked to explain the distribution of the brown color morph (especially the dark- brown morph) ,in respect both to its northerly distribution ( CAINand SHEPPARD 1954; LAMOTTE1959). and to its distribution in some places in England with respect to cooler microclimates (CAIN and CURREY1963; CAIN 1968; CARTER 1968). CAIN and CURREY1963, p. 42 summarize the position concerning the first line of evidence thus: ". . . it is known that dark browns are rare in France (LAMOTTE,personal communication; examination of the Locard collection, . . .) but common in northern Europe including the British Isles. This distribution seems to indicate that dark browns are better able to withstand cooler summers." I have found no published data on its actual frequencies in French populations, CEPAEA IN FRANCE AND ENGLAND 595

Oxford open habitats woods mean =15.2% mean = 21 -6Ol0

0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 o/o unbanded - not - in - brown

Tour aine Tour aine open habitats woods U mean =15.2% mean =10-5%

0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70

O/O unbandsd - not -in -brown FIGURE3.-Frequency diagrams for unhanded-not-in-brown in the main habitat classes 'woods and open habitats) for Oxford and Touraine. and the data reported here permit as direct a comparison as can probably be obtained of frequencies in southern Britain and northern France. Touraine frequencies are compared with those near Oxford in Figure 4. In both habitat classes, brown tends to reach higher frequencies in the Oxford district. As the sizes of samples made from the two districts are very different, it is not valid to compare directly the frequency of occurrence of brown in samples between the two districts. However, the 17 smallest open-habitat samples from Oxford are very similar in mean size to the Touraine open-habitat samples (4.0, compared with 38), and if the frequency of occurrence of brown in them is compared, it is found that it occurs in 8 of the Oxford, and 2 of the Touraine open- habitat samples. The difference approaches significance (xle = 3.5, 0.05 < P < 0.10). These data are in accordance therefore with observations that brown is more frequent in the north. When the occurrence of b'rown is compared between habitat classes within the two districts, it is found that although there is no evidence for a difference be- 596 RICHARD ARNOLD

Oxford Oxford open habitats woods imean =4;?o/o mean =7.9"/0

0 5 10 15 20 25 30 35 w, 09 5 10 15 20 25 30 35 40 45 50 O/O brown "lo brown

Tour aine Tour aine open habitats woods . mean=0.30/~ mean = 4.6°/0

0 '5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 40 45 50

Ol0 brown o/o brown FIGURE4.-Frequency diagrams for brown in the main habitat classes (woods and open habitats) for Oxford and Touraine. tween Oxford wood and open habitats (xI2= 1.9, 0.20 < P < 0.30), there is a significant difference in occurrence between Touraine wood and open habitats (exact test, P = 0.01). Its occurrence in the different habitats in the two districts is shown in Table 2. These differences in distribution are in line with current ideas about the action of climate on the brown morph. CAINand CURREY(1963) found that area effects in brown occurred mainly in reentrants and other places where cold air tends to CEPAEA IN FRANCE AND ENGLAND 597

TABLE 2 The number of samples containing brown in the different habitats in the two districts

Brown present Brown absent Oxford Open habitats 23 23 woods 15 6 Touraine Open habitats 2 15 Woads 9 4 accumulate at night, and suggested that this morph might be favored in cooler microclimates. CAIN (1 968) has developed the hypothesis, and has further shown that brown tends to be more frequent in sand-dune populations where the topogra- phy of the dunes is complex rather than simple, i.e., in places which would tend to hold nocturnal cold air in pockets. That brown occurs more frequently in woods than in open habitats in Touraine, while there is no evidence for a difference in the Oxford district, suggests that greater insolation and summer warmth in Touraine (or perhaps its more continental climate generally; it is possible that brown does not withstand extreme conditions so well) mitigate against it more strongly in open habitats there than near Oxford. CAMERON(1968) has shown that Cepaca hortensis, which ranges further north and occurs less far south than C. nemoralis (TAYLOR1914), differs from C. nemo- raZis ecologically in preferring slightly cooler and damper situations in at least central Britain. In Touraine, it was found on five occasions in woods (omitting sample No. 2), and on three occasions in open habitats (Table 1) . Two of the three open habitats in which it occurs (localities 3 and 17) are hedgerows very close to small rivers, the Brenne and Braye; the third open-habitat locality (4) is a hedgerow within the former flood-plain of the Loir just east of Vend6me (Figure 1 ) . It was not found in open habitats on the plateaux. Brown occurs in two of the three (3 and 4) and in no other open-habitat samples. It occurs in all the woods where C. hortensis was present. There is thus a good correlation between the OC- currence of brown in C. nemoralis and the presence of C. hortensis (xI2= 9.3, 0.001 < P < 0.01) (as CAIN and CURREY1963, found in southern England). Since evidence shows that C. hortensis prefers cooler habitats than C. nemoralis, there is reason for believing i hat the two open-habitat populations exceptional in containing brown may be in unusually cool situations; this seems likely from their topographical positions also, for both are near rivers where cold air is likely to accumulate at night. (3) Other morph frequencies: Neither Touraine wood samples ( x12 = 0.1, P > 0.7) nor Touraine open-habitat samples (xI2= 2.1, 0.10 < P < 0.20) have frequencies of yellow significantly different from Oxford wood and open-habitat samples. In the case of the open habitats, the difference is in the direction of more colonies with a higher frequency of yellow phenotypes in Touraine. The class "effectively unbanded" is significantly more frequent both in Tour- 598 RICHARD ARNOLD aine wood populations (xI2= 8.0, 0.001 < P < O.Ol), and in Touraine open- habitat populations (xI2= 15.9, P < 0.001). These comparisons indicate that a selective factor is acting on all populations in Touraine irrespective of habitat. It can be suggested that, in Touraine wood populations, it acts in the same direction as visual selection, permitting the balance of the polymorphism to be more in favor of effectively unbanded shells (visually) than in wood populations near Oxford. In Touraine open-habitat populations, it would appear to override the effects of visual selection on the banding morphs. It is likely that the effect is selective (and not due, for example, to large population size and common origin). The populations are well isolated by distances that in the majority of cases (Figure 1) are huge compared with the size of the panmictic area of C. nemoralis populations (about 30 m in diameter, MURRAY 1962) , and there are, moreover, physical barriers of various efficacies isolating populations, such as the River Loir. It seemed from the paucity of dead shells at most localities that few of the colonies were either dense or very large. The frequencies of effectively unbanded are not randomly distributed along the abscissae of Figure 2a, as would be expected if random processes were more important than selection in influencing banding-morph frequencies. Mean frequencies per colony of the major banding morphs in the habitat classes in the two districts are shown in Table 3. Interest centers on the frequencies of midbanded and morphs of formula 00345, as it is these two morphs present at higher frequency in Touraine which cause most of the Touraine open-habitat populations to exhibit high frequencies of effectively unbanded shells, in contrast to Oxford open-habitat populations (Figure 2). Three banding morphs, each controlled by an unlinked dominant major gene, contribute to the class ‘effectively unbanded.’ Morphs of banding formula 00345 are controlled by the gene T+j,which modifies the recessive banded 12345 formula produced by BB. Shells of banding formula 00300 are controlled by the modifier U$,which is epistatic to T3J5and BB. Shells of banding formula 00000 are controlled by Bo, dominant to it5 allele causing presence of bands, BE, and epistatic to both TSJ5and Us ( LAMOTTE1954b; CAINand SHEPPARD,195 7; CAIN,SHEPPARD and KING1968). Frequencies of Bo and UJfor Touraine are plotted in Figure 5, for comparison with their frequency distributions in Oxford colonies (CAIN and CURREY1963, Figure 17). The mean gene frequency of Bo in both districts is similar (0.07 in Touraine; 0.09 in Oxford), whereas the mean gene frequency of U3is over twice that of colonies in the Oxford district (0.24, compared with 0.10).

TABLE 3 Mean frequencies per colony of the major banding morphs in Touraine and Oxford

Morph 00000 00300 W345 12345 00000 00300 00346 12345 Touraine 10.5% 43.5% 17.1% 28.9% 15.2% 32.2% 18.4% 34.2% Oxford 21.6% 23.0% 6.4% 48.9% 15.2% 12.1% 4.2% 68.5% CEPAEA IN FRANCE AND ENGLAND 599

m 3

\c 0

I I I 1 0.1 0.2 0.3 0.4 frequency of BO FIGURE5.-Gene frequencies of midbanded (US) plotted against gene frequencies of unbanded (Bo) for Touraine. Habitat symbols as in Figure 2.

T345is also often important in Touraine populations in causing high frequencies of effectively unbanded, for phenotypes of formula 00345 (EU5, Table 1) are sometimes very frequent. Calculation of gene frequencies can be undertaken using a method analogous for deriving frequencies of Us;namely, frequency of Ts45= 1 - (frequency of shells with unmodified bands)% (ignoring shells of formula 00000 and 00300). Its mean frequency per colony in Touraine (0.20) is much higher than the mean for Oxford populations (0.04). The presence at higher frequencies of both USand T8h5results in Touraine populations being much more effectively unbanded; and because of the domi- nance and epistatic relations of the genes, a high frequency of effectively unbanded nearly always obtains, even if one of the components is at low frequency or absent in the population. In the open habitats particularly, the proportions of the components vary considerably. It seems to be relatively unimportant how the high frequency is obtained, either 00000. or 00300, or 00345 taking high frequencies. Thus, in the open-habitat samples, 00000 is the most important component in 4 samples (Nos. 3,17,21, and 30) ; in 9 samples it is 00300 (Nos. 1,8,9,12,13, 23, 27, 28 and 29); and in 4 samples (Nos. 4, 11, 15, 'and 20) shells of formula 00345 predominate. This implies that se!ection in Touraine favors shells with at least the upper two bands absent, no matter by what genetic means this be achieved. CAINand 600 RICHARD ARNOLD CURREY(1963), in contrasting the Oxford district where visual selection appears to be important in strongly influencing morph frequencies with one where it is ineffective, suggested that it was visually unimportant how the effectively unbanded class was composed with respect to visual predators. They showed that a high frequency of effectively unbanded morphs could be reached by any frequencies of Bo and U3,or Bo or Up,in the Oxford district, implying visual equivalence of the morphs 00000 and 00300. Murray (1967) showed that visual similarity between populations on the Scilly Islands. Cornwall, was underlain by consider- able genetic diversity, and suggested on similar grounds that it was due to visual selection. However, it seems unlikely that visual selection is responsible for the high frequencies of effectively unhanded in Touraine open habitats, for reasons outlined earlier. There is some suggestive evidence which indicates that shells with reduced banding formulae may absorb less radiant energy than heavily banded shells (LAMOTTE1959, 1966). Also, populations on arid hillsides above the River Segre, south of the Pyrenees, not only have higher frequencies of unhanded, but also have higher frequencies of banded shells with at least the upper two bands absent (mainly 00300 and 00345) than populations living in shadier and more humid habitats beside the river, suggesting that in very warm and dry environ- ments, snails with reduced numbers of bands are favored (ARNOLD1969). The climate of Touraine, slightly more extreme than that of Oxford, might similarly favor morphs which have reduced banding formulae, especially in open habitats.

DISCUSSION The correlation between the frequencies of the color morphs and habitat in populations in Touraine, and the nonrandom distribution of the frequencies of effectively unbanded, show that selection is more effective than random evolutionary factors in influencing morph frequencies. Cepaea lives in diverse environ- ments, and populations are undoubtedly subject to a complex of selective forces of different intensities (LAMOTTE1966), among which the few identified with reasonable security are visual selection, selection by some climatic factors, and selection by the Glow-worm (Lampyris noctiluca L.) (O’DONALD1968). It is not surprising, therefore. to find variation in morph frequencies in habitats scored crudely as “oakwood”, etc. No example of sampling drift or founder effect has yet been demonstrated in Cepaea populations, and in the populations studied here, there is no reason to believe that such factors have had any major effect on morph frequencies. LAMOTTE(1959) describes populations in four districts in France where there is variation of the color-morph frequencies with habitat, near Pans, and in Nor- mandy, Provence and Aquitaine. Variation ’of color-morph frequencies with habitat may well be a general pheno&enon io lowland France, for all the above places and Touraine are widely separated geographically. He is inclined to believe, however, that selection by climate rathw than visual selection is the cause. In his 1966 paper, he states that climatic and microclimatic conditions of the CEPAEA IN FRANCE AND ENGLAND 601 environment undoubtedly play a selective r6le in those populations living in stable and well defined habitats, but that although in some populations the action of visual selection could be demons-trated, there is no reason to conclude that it is effective in causing the observed correlation of morph frequencies with habitat. The first part of the conclusion is based on two main lines of evidence. First, yellow snails are generally less frequent in shady localities, like woods; and secondly, yellow snails tend to be more resistant to radiant energy than pink snails. It is not possible to infer whether climatic or visual selection is the more likely factor responsible for variation with habitat in Touraine if only color-morph frequencies are considered. However, when color- and banding-morph frequencies are taken together, the evidence points strongly to visual selection as the main cause, at least for the predominant morph frequencies in Touraine woods. LAMOTTE(1959, 1966) shows that not only are pink snails less resistant to heat- ing from a light source, but that snails with bands (00300 and 12345) are also less resistant than unbanded ones; amongst these, pink five-banded snails are particularly susceptible. Hence, if selection by climate were the more important agent in Touraine woods, where the populations are predominantly pink, a higher frequency of morphs of formula 12346 (which are available at moderate frequencies) might be expected than in the open habitats, as CAINand SHEPPARD pointed out for the Oxford district. Figure 2a shows that this is not so; woodland populations are as effectively unbanded as the open-habitat populations, and not less so, as LAMOTTE’Shypothesis would require. Although in Touraine, as in some other places in France (LAMOTTE1951, 1959) there is evidence that woods tend to have populations with somewhat lower frequencies of unbanded than open- habitat populations, this does not (in Touraine) result in them having higher frequencies of morphs of formula 12345. Twelve of the Touraine samples include snails preyed upon by the Song-thrush (Turdus ericetorum Turton) , probably the most important visual predator of adult Cepaea, and one which exerts strong visual selection ( SHEPPARD195 1 ) . The breeding range of the species extends throughout France (PETERSON,MOUNT- FORT and HOLLOM1966) and although it may not be as common as previously- a result perhaps of indiscriminate shooting-it is by no means rare. I sometimes saw it in Touraine. The presence of at least this species known to exert selection, and the agreement of color and banding-morph frequencies in Touraine woods with those expected with visual selection, sugge;t that at least in woods in some districts in France, visual selection is responsible for populations resembling their backgrounds. Open-habitat color-morph frequencies agree with the action of both kinds of selection, but the high frequency of effectively unbanded shells does not, at least not under circumstances the same as those obtaining in England. It has been shown above that Touraine wood populations also have higher frequencies of effectively unbanded than Oxfordshire populations, and it seems likely that a general effect related to the environment of the region and possibly climatic in origin is operating. The effect appears to be very widespread, for comparison of LAMOTTE’S(1954b) data for 10 departments (his Centre region) with mine from 602 RICHARD ARNOLD 2 departments shows that the correspondence is close. The extent of the effect lends support to a climatic interpretation. Data are compared in Table 4. In Touraine, therefore, visual selection appears to be the main factor controlling morph frequencies in the woods, whereas it may not control color-morph frequencies in the open habitats, and almost certainly has little effect on banding- morph frequencies there. A possible reason for this difference in selection rhgime between habitats can be suggested. Recent work (ARNOLD,in preparation) has shown that, in some years, the amount of visual predation by the Song-thrush may be greatly different in woodland and in some kinds of open habitat, which suggests that the net effect of visual selection may be less in the latter habitat types. The results of some experiments performed in 1966 designed partly to examine such differences are summarized in Table 5. Other factors influencing the polymorphism may more easily override or tend to override the weaker force of visual selection in open habitats, whilst having little effect on wood populations. The more restricted range of morph frequencies in wood populations supports this, for both in Oxfordshire and Touraine, a wider range is taken in open-habitat populations (Figure 2). The less mild climate of north-central France compared with the more atlantic climate of south-cenwal England, as well as perhaps a lower density of Song-thrushes in France, might well result in physiological selection overriding whatever visual selection occurs in the open habitats in Touraine. It is perhaps significant that one of the banding genes favored in Touraine ( U3)is also the one frequently responsible for causing area effects in Cepaea (CAIN and CURREY1963) on high chalkland in south- central England, where the climate is probably less mild than in lowland wooded country and on chalk on the south coast of England (where area effects in mid- bandedappear to be rare) (C~IN1968; ARNOLD1966). Both aspects to which LAMOTTE(1959) drew attention probably contribute to the differences in some of the conclusions reached by him and by CAINand SHEP- PARD. The ecological situation with respect to the factors influencing the polymorphism appears to be different, climate probably playing a stronger r6le in France, as CAIN and SHEPPARD(1954, 1961) suspected. Secondly, the treating

TABLE 4 A comparison of banding morph frequencies in samples obtained by LAMOTTEfrom his Centre region (comprised of the departments of Cher, Indre, Indre-et-Loire,Nikvre, Loiret, Ewe-et-Loir,Loir-et-Cher, Vienne, Sarthe and Mayenne) with the summed data in Table I of this paper from the departments of Loir-et-Cher and Indre-et-Loire

~ Momh LAMOTTE ARNOLD 00000 19.1% 16.2% 00300 28.8% 35.5% 00345 17.3% 16.8% 12345 34.1% 31.5% Number 8,031 1,056 CEPAEA IN FRANCE AND ENGLAND 603

TABLE 5

Comparison of the incidence of visual predation by the Song-thrush on artificial colonies of adult C. nemoralis released at the same time in June, 1966, and at the same density in grass and wood habitats in south-central England. In each case, the grass colony was not more than 50 metres from the wood

Kingston grass Kingston woad Size of colony 1078 1148 Number preyed upon 21 (1.9%) 452 (39.4%)

Coombe grass Coonibe wood Size of colony 1001 1121 Number preyed upon 1 (0.1%) 417 (37.2%) of color- and banding-morph frequencies together shows that climatic selection does not adequately explain the phenotypic composition of populations in woods in Touraine, and that visual selection is probably effective in at least some populations in France as it is in England. Little support can be derived from these results for the importance of random evolutionary effects in Touraine. I wish to thank Professor A. J. CAIN,my supervisor during the period in which this work was performed, for advice and for criticism of the paper; Professor MAXIMELAMOTTE, for kind assistance in Paris in 1963; Professor P. R. CROWE,for information; and Dr. R. A. D. CAMERON, Dr. L. M. COOK,J. J. D. GREENWOOOD,Esq., and Professor P. M. SHEPPARD,F. R. S., for discussion of the results. Part of the work was carried out in the department of Professor J. W. S. PRINGLE,F. R. S., to whom I am grateful for the facilities provided. Miss JOAN BAMBRIDGE typed the manuscript. I was supported by a subsistence grant from the States of Guernsey, which I gratefully acknowledge.

SUMMARY Populations were sampled in a lowland district in France using the methods which CAINand SHEPPARDemployed near Oxford. Color-morph frequencies were found to vary with habitat, populations in woods having lower frequencies of the yellow morph than populations in open, essentially grassy, habitats. However, banding-morph frequencies do not vary with habitat in the district chosen for study. It is shown that visual selection by predators is the most likely cause of populations resembling their backgrounds in Touraine woods, but that some other form of selection, probably ultimately climatic, is responsible for the high frequencies of “effectively unbanded” shells in the open habitats. The results are discussed with respect to CAIN and SHEPPARD’Sand also LAMOTTE’Sconclusions.

LITERATURE CITED

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