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Home , Evolution of reptiles, Parapatric speciation, Peripatric speciation

Introduction

Speciation is a burning issue in evolutionary , but it is both fascinating and frustrating. Defining depends on one’s concept viz., typological, biological, evolutionary, recognition etc. In its simplest form, speciation is lineage splitting (ancestor-descendent sequence of populations); the resulting lineages are genetically isolated and ecologically distinct.

Speciation is the process of evolutionary mechanism by which new biological species (or taxa) arise. There are two ways of new species (or taxa) origin from the pre-existing one:- i. by splitting of the parent species into two or more species (by the splitting of phylogenetic lineage) and ii. by transformation of the old species into a new one in due course of time. The Biologist O.F. Cook (1906) seems to have been the first to coin the term ‘speciation’ for the splitting of lineages ().The process of evolutionary mechanism by which new biological plant species (or taxa) arise, is known as plant speciation.

General Mechanism of Speciation operating in : The mechanism of speciation is a two- staged process in which reproductive isolating mechanisms (RIM's) arise between groups of populations. Stage 1

• is interrupted between two populations. • absence of gene flow allows two populations to become genetically distinct as a result of their to different local conditions ( plays an important role here). • as populations differentiate, RIMs appear because different gene pools are not mutually coadapted. • appears primarily in the form of postzygotic RIMs: failure. • these early RIMs are a byproduct of genetic differentiation, not directly promoted by .

Stage 2

• completion of genetic isolation • reproductive isolation develops mostly in the forms of prezygotic RIMs.

● development of prezygotic RIMs is directly promoted by natural selection: Formation of new species.

There are four geographic modes of speciation operate in nature, based on the extent to which speciating populations are geographically isolated from one another, viz., Allopatric, Peripatric, Parapatric and Sympatric which are briefly discussed here.

Fig 1. Four Geographic modes of Speciation models (Source:http://www/en.wikipedia.org).

Allopatric speciation (figs 2 & 5): Greek allos means ‘other’ and patra means ‘fatherland’. It is also often called as Geographic speciation. It occurs when biological populations of the same species become isolated (i.e. vicariant) from each other to an extent that prevents or interferes with genetic interchange, for example, by fragmentation due to geographical change such as mountain building. The isolated (vicariant) populations then undergo genotypic or phenotypic changes as: (a) they become subjected to dissimilar selective pressures, (b) they independently undergo genetic drift, and (c) different may arise in the gene pools of two isolated populations. Two separate populations over time may evolve distinctly different characteristics. If the geographical barriers are later removed, members of the two populations may be unable to successfully mate with each other, at which

point, the genetically isolated groups have emerged as different species. Allopatric isolation is a key factor in speciation and a common process by which new species arise.

Examples observed: Darwin’s Finches () in different of Galapagos originated due to allopatric isolation.

Fig 2. Explanation of (Source:http://www.geo.arizona.edu)

Peripatric speciation or Mayr’s Peripheral isolate model: When allopatric speciation occurs in the peripheral populations, then it is called as . (1940) showed this speciation model based on his work on birds. Mayr (1940) argued that peripheral populations have greater divergences and differences than central populations of a species range. It is a subform of allopatric speciation. In this speciation, new species are originated in isolated smaller peripheral populations which are prevented from exchanging genes with the

main population. It occurs in a few members of the population, which exhibit a different appearance than the majority. Genetic drift plays a major role in this speciation. It is also similar to the concept of Founder Effect, as small populations undergo bottlenecks.

Portions of a populations that exist along the edges of the parent population's geographic territory have higher likelihood of developing reproductive isolation. Such peripheral populations are likely to possess genes that are different from the parental population. After isolation, the founding population is less likely to represent the of the parent population. In addition, peripheral isolates are likely to represent a small number of individuals, meaning their gene pool is more susceptible to the effects of genetic drift (random chance). Furthermore, it is likely that the peripheral population will inhabit an environment different from its ancestral gene pool, likely causing it to be subjected to different selective pressures as it colonizes new areas. Example observed: Origin of the Australian , Petroica multicolour

Fig 3. Successive stages in the process of peripatric speciation. A small 'daughter' population on the periphery of a more widespread 'parental' population evolves reproductive isolation. Both

species remain distinct if the peripherally isolated species invades the geographic range of the parental species. Under this model, the peripherally isolated population diverges from the parental population such that the latter remains unchanged (Source: http://www.trinitygreenconsultancy.com/population dynamics-2/variants-on-the-basic- model.html-2012).

Mayr’s explanation: Mayr (1940) hypothesized that founder populations, as they are small, may have reduced and low fitness due to genetic drift. Drift may increase the frequency of that were rare in the ancestral population. In such a situation, selection for new combinations of alleles that are compatible with the newly fixed alleles may occur and allow increased fitness in the new conditions. A possible result is a reorganization of the genome that makes it incompatible with the ancestral population.

Parapatric speciation (Partially geographically isolated populations): This speciation is an example of continuous variation within a single connected habitat acting as a source of natural selection rather than the effects of isolation of found in peripatric and allopatric speciations. This type of speciation occurs when the speciating populations are contiguous and only partially geographically isolated. Individuals of each different population of the same species are able to meet across a common boundary during the speciation process, but due to reduced fitness of the heterozygote leading to the selection for behaviours that prevent their interbreeding. Ecologists refer to Parapatric and Peripatric speciations in terms of ecological niches. A niche must be available in order for a new species to be successful.

Example observed: i. formation by the Herring Sea Gull (Larus argentatus) around the North Arctic Ocean. ii. The Grass, Anthoxanthum odoratum has been known to undergo in mine-contaminated areas.

Fig 4. The first steps of parapatric speciation observed in the grass species, Anthoxanthum odoratum. Some of these plants live near mines where the soil has become contaminated with heavy metals. The plants around the mines have experienced natural selection for genotypes that are tolerant of heavy metals. Meanwhile, neighboring plants that don't live in polluted soil have not undergone selection for this trait. The two types of plants are close enough that tolerant and non-tolerant individuals could potentially fertilize each other — so they seem to meet the first requirement of parapatric speciation, that of a continuous population. However, the two types of plants have evolved different flowering times. This change could be the first step in cutting off gene flow entirely between the two groups (Source:http://www.berkeley.edu/evosite/evo101/VC1dParapatric.shtml).

Sympatric speciation (fig 1 & fig 5): In this type, the formation of two or more descendant species happened from a single ancestral species all occupying the same geographical area. In this type, species diverge while inhabiting the same place.This speciation requires a change in host, food and habitat preferences in order to prevent the new species being swamped by gene flow. In theory, it may occur where there is a in the population conferring to two different habitats/niches. Reproductive isolation could then arise if the two morphs had a preference for their habitat.

A common example of occurs in plants through . For example, Sand Dune Grass (Spartina townsendii) is a sympatric polyploidy originated from its parent, S. anglica due to polymorphism.

More Examples observed: i. Diploid (2n=14), Tetraploid (2n=28) and Hexaploid (2n=42) wheat plants were originated by sympatric speciation through polyploidy. ii. melanogaster (2n=8) and D. virilis (2n=12) were originated by sympatric speciation.

Fig 5. Allopatric (left) and sympatric (right) models on speciation (Source: http://www.globalchange.umich.edu/speciation.html)

Brief idea about other modes of speciation operating in nature:

Phyletic speciation: It occurs through transformation of an ancestral species into a new one in time without ever splitting. It is a process of gradual change in a single population. The modern form of the differs from the original form. In this type of speciation, only one species exists at a time, as for example, species A evolves into species B and B into C and so on. Therefore, phyletic speciation could be drawn as a line. The problem with phyletic speciation is that it would only occur if there were a gradual change in the selective regime that progressively favored the modern form. It is presumed to lead to the origin of new genera and families.

Quantum speciation: It also occurs through transformation of an ancestral species into a new one in time without ever splitting. It involves rapid shift or sudden chages in the organization of population to a new equilibrium, distinctly different from the ancestral forms and adapted to occupy new conditions. It is also called as macro- and megaevolutions operating above species

level (mainly higher taxonomic groups such as orders, classes etc. Quantum speciation, also known as Saltational speciation, is the process by which a small population of a species rapidly diverges into more than one species that is reproductively isolated from the original population. Scientists theorize that quantum speciation occurs because of genetic drift that results from the founder effect. Genetic drift refers to random genetic changes within a population rather than natural selection. The founder effect occurs because a few individuals from a population colonize a new area and become isolated from the rest of the species. Almost 40% of angiosperms (flowering plants) are polyploids that evolved by this mechanism. For example, scientists theorize that the polyploid Sequoia evolved from a diploid ancestor, and another example in plants is the genus Scalesia (Asteraceae) originated by this type of speciation in Galapagos Islands.

Catastrophic speciation: It is the process of rapid and abrupt speciation, leading to genetic isolation with little or no morphological differentiation, but without polyploidy. It occurs due to drastic chromosomal rearrangements in some extreme range of populations of a species may be due to mutations and environmental stress. The new species differs from its parent by chromosomal rearrangements, presumably fixed by a series of population crashes. For example, the derivation of lingulata from C. biloba within section Sympherica of the family Onagraceae (Gottlieb 1973, 1981).

Fig 6. Catastrophic speciation in Clarkia (adapted from Gottlieb, 1973).

Local speciation: When the phenomenon of speciation occurs in local populations or metapopulations (clusters of several local populations connected by occasional gene flow), and new species evolve within local populations which do not require extensive gene flow and uniform selection over long distances. Levin (1993) called ‘local speciation’ as the peripheral isolation model of speciation. For example, Menges (1990) worked and showed ‘local speciation’ in Pedicularis furbishiae (Scrophulariaceae) populations, while Ouborg (1993) showed in the populations of Plantago media (Plantaginaceae).

Summary

Populations peripheral to main range of a species are likely to exhibit features that promote (i.e., population-level processes are at work)

§ Small populations § subject to drift § Large fluctuations in population size § experience bottlenecks

§ Occupy ecologically marginal habitats § subjected to different selection pressures than central populations Local adaptation equips marginal populations with different capabilities than central populations

Leads to selection for reproductive isolation

Abrupt speciation: It is also called as Polyploid speciation. In this type, the formation of a new species occurs over a relatively short time span. A good example showed recently by Hull & Norris (2009) in Globorotalia plesiotumida-G. tumida lineage of planktonik Foraminifera. They experimentally showed the first population of the descendant, G. tumida, evolves abruptly within a 44,000-year interval.

Hybrid speciation: In this type, hybridization between two closely related species occurs and as a result, a new species arises due to reproductive isolation. A hybrid may have a distinct phenotypic trait which may in rare cases be better fitted to the local environment than the parental lineage and as such natural selection may favour these individuals. If reproductive isolation subsequently is achieved, it will lead to a separate species. The reproductive isolation may be genetic, ecological, behavioural, or spatial, or a combination of these. If reproductive isolation fails to establish, the hybrid population may breed back and finally merge with either or both parent species. This will lead to an influx of foreign genes in the parent population, a situation called an introgression. Introgression is a source of genetic variation, and can in itself facilitate speciation.

Examples observed: It is more common in plants than in . Estimates indicate as much as 2–4% of all flowering plants and 7% of all fern species are the results of polyploid hybridization. A common example is the hybridization between two diploid sunflower species, Helianthus annuus and H. petiolaris (Asteraceae) which were hybridized to produce other three diploid hybrid species: H. anomalus, H. deserticola and H. paradoxus (Rieseberg & Wendel, 1993).

Fig 7. Parent and hybrid species of Helianthus (Asteraceae)-- (Rieseberg & Wendel, 1993).

Fig 8. Hybrid speciation in Iris spp. in Southern Louisiana (adapted from Arnold & Bennett, 1993).

Apomictic speciation: When new species arise by the process of parthenogenesis or apomixis (asexual means) through polyploidy, which often called as apomictic species and the phenomenon of the origin of apomictic species is called apomictic speciation. Reproductive of successful polyploid species is sometimes asexual, and for unknown reasons, many asexual are polyploid. It is more common in ferns, but it is very rare in angiosperms.

Isolating mechanisms: Isolation is the phenomenon of segregation of a particular population of a species into smaller units or the segregation of individuals of different closely related species by some mechanisms, so as to prevent gene flow among them. Therefore, isolation helps in

splitting of the parental species into separate groups of individuals and their into distinct species. The term ‘isolating mechanism’ was introduced by T. Dobzhansky (1937).

Fig 9.Different types of barriers in Isolating mechanism (Source:http://www.bio.utexas.edu/originspp.html)

Recent authors have pointed out that the word "mechanism" is particularly misleading as pre-mating and post-mating isolation are likely to evolve as a by-product of natural selection or genetic drift within species, rather than as a direct result of their utility as barriers to fertilization and gene mixing between species (a process known as reinforcement). Dobzhansky (1937) was the first evolutionary biologist who broadly separated isolation phenomenon into Geographic and reproductive isolation, the former operating in geographically isolated populations or allopatric species and the latter representing genotypic species which do not permit hybrid formation. Based on whether isolating mechanisms work before or after sexual fusion, two broad kinds of mechanisms between species are typically distinguished: Premating and Postmating. Postmating again divided into Prezygotic and Postzygotic mechanisms (after Levin, 1971, 2000).

I. Premating Isolating Mechanisms (operating before sexual fusion) Prevent interspecific crosses

i. Geographical isolation: Two populations of a species are separated by some physical barriers. These barriers may be mountain ranges, dense forests, land bridges, deserts, water connections etc. These barriers prevent migration and intermingling of the individuals of two populations and thus greatly reduce the exchange of genes between them, so that new mutations, genetic drifts and operation of natural selection occur independently in isolated populations. Platanus orientalis (Mediterranean region) and P. occidentalis (North America) are considered as two separate species but readily interbreed when brought into the same area.

Fig 10. Isolation model (Source: http://www.tutorvista.com/content/biology/biology-iii/organic- evolution/isolation.php) ii. Ecological/Habitat isolation: Two populations of a species or two closely related species occupying the same territory but live in different habitats, and thus do not meet. It occurs in two closely related species viz., Silene alba grows in light soils in open places, while S. dioica in heavy soils in shade. Their habitats rarely overlap, but when they meet forming fertile hybrids.

Fig 11. Habitat isolation in Ceanothus americanus (Rhamnaceae) on Catalina (Source: http://www.botany.wisc.edu/courses/botany-400/lecture/11speciation) iii. Temporal isolation: It is again 2 types- a. Seasonal: When two species occur in the same region but flower at different seasons. Flowering at different seasons of the year is common among many closely related species, such as several Phlox spp. and Quercus spp. that grow together. Another example of two closely related species is Sambucus racemosa and S. nigra which flower nearly 7 weeks interval.

b. Diurnal: When two closely related species flower during the same period/season but at different times of the same day. Flowering at different times of the day may also effectively isolate species that would otherwise hybridize. Example observed in Agrostis tenuis flowers in the afternoon, while A. stolonifera flowers in the morning of the same day.

Fig 12. Showing Temporal isolation in tropical forest (Source: http://www.botany.wisc.edu/courses/botany-400/lecture/11speciation) iv. Floral isolation: Different modes of adapatations of flowers to attract different pollinators limit or prevent gene exchange between many species. These adaptations may work through floral structure or through effects on pollinator behavior (Judd et al. 2008). It is again 2 types:- a. Behavioural/Ethological: It reflects the capacity of pollinators to distinguish floral signals, such as colour, shape and scent. For example, some closely related orchid species in the genus Ophrys, growing primarily in the Mediterranean region, produce different floral fragrances that attract males of different species of bees and wasps. b. Structural: A common example observed in two closely related species of Aquilegia (Ranunculaceae). A. formosa flowers are red, nodding with short nectar spurs and pollinated by Hummingbirds, while another species, A. pubescens flowers are pale yellow to white, upright with long nectar spurs and pollinated by Hawkmoths. These two species grow mostly at different elevations, and when they do come together, some hybridization occurs. However, even though genes are exchanged between two species, the structural differences of flowers largely persist. These observations suggest that it is important for the species to maintain their floral syndromes and that floral isolation is the primary barrier to gene flow between them. v. Reproductive mode of isolation: The shift from outcrossing to uniparental reproduction, either by self-fertilization or by agamospermy, has occurred in many plant species and creates a barrier to mating.

Fig 13. Reproductive isolation due to F2 breakdown in Gossypium sp. (Source: http://www.botany.wisc.edu/courses/botany-400/lecture/11speciation)

II. Postmating Prezygotic Isolating Mechanisms (operate after sexual fusion) i. Pollen-Style incompatibility: it is 2 types. a. Gametophytic isolation: This is a common isolating mechanism in which cross-pollination occurs but the pollen tube fails to germinate or if germinated, it is unable to reach and penetrate the embryo-sac. b. Gametic isolation: It often occurs in crop plants. In this isolation, the pollen tube releases the male gametes into the embryo-sac, but gametic and endospermic fusion does not occur.

Fig 14. Gametic incompatibility in Heliconia sp. of Musaceae (Source: http://www.botany.wisc.edu/courses/botany-400/lecture/11speciation)

III. Postmating Postzygotic Isolating Mechanisms (operate after sexual fusion) i. Seed incompatibility: Although a hybrid embryo forms, it may not develop into a viable seed because of incompatibility between the parental genomes within the embryo or between the hybrid embryo and maternal endosperm. The hybrid embryo resulting from the cross of Primula elatior and P. veris (Primulaceae) is a common example of seed incompatibility. ii. Hybrid inviability: It refers to the failure of hybrids to develop normally and reach reproductive maturity, as observed in the case for crosses between Papaver dubium and P. Rhoeas. iii. Hybrid Floral Isolation: It refers to the absence of effective pollinators for a hybrid of two parental species that are adapted to very different pollinators. iv. Hybrid sterility: It refers to the nonfunctional hybrid formation due to their fail to pair during meiosis because the chromosomes of the parental species differ in number or have diverged sufficiently to hinder pairing. A common example observed between the cross of Brassica oleracea and Raphanus sativus produceing Raphanobrassica. The F1 hybrids are vigorous, but the chromosomes from two parental species do not pair with one another in meiosis, so functional gametes do not form. v. Hybrid Breakdown/F2 hybrid sterility: The progenies of hybrids (F2 or backcross generations) have reduced viability or fertility. When F1 generation may be viable and fertile, but backcrosses or later generation individuals may be sterile. For example, the hybrid of two grasses, Festuca rubra and Vulpia fasciculata, produces new offsprings, and the F2 plants are weak and do not flower.

Concept of the Genus: The genus is the principal fundamental category in the taxonomic hierarchy above the species. Although, the International Code of Nomenclature for algae, fungi and plants (Mc Neill et al. 2012 in Melbourne Code) allows several other categories (viz., subgenus, section and series) in between the genus and species categories, but these are not fundamental to the hierarchy and are not always used in classification within a particular group. Actually, closely related species are grouped together to form the genus. The genus is more difficult to define than the species. As Robinson (1906) defined long back “a genus is the group of species which from likeness appear to be more nearly related to each other than they are to other species...”. Mayr (1957) defined “the genus as a taxonomic category which contains either one species or a monophyletic group of species, and is separable from other genera by a decided discontinuity gap”. Till date two symposia have been held to discuss the generic concept in detail with reference to plants (Bartlett, 1940; Verdoorn, 1953), although another symposium held in respect to the generic concept in Compositae only (Lane & Turner, 1985).

History of Generic concepts: Tournefort (1700), regarded as the father of Generic concept, placed the concept of genus on sound footing. Although he did not invent the generic category, but he did place all the plants in his book, Institutiones Rei Herbariae into genera, and he

believed that of the six parts of a plant (roots, stems, leaves, flowers, fruits, seeds), five should be considered for the purpose of generic delimitation. However, in 1694, Tournefort provided the clear guidelines for describing genera. Linnaeus (1737) based his generic concepts clearly on those of Tournefort (1694, 1700) and Plumier (1703), although his approach to generic delimitation was outlined in detail in his Philosophia Botanica (1751). He emphasized that features of flowers & fruits should be used to distinguish genera what Tournefort provided in the primary generic delimitation. Today’s genera are ultimately built on a Linnean foundation. However, modern taxonomists (mostly the supporters of Angiosperm Phylogeny Groups) did not believe that there was a rank of genus (or family, for example) and even the species in nature. Recently different types of data viz., morphology, anatomy, geography, cytology & cytogenetics, crossing studies, phytochemistry, pollen morphology etc. and even DNA information are often used to delimit a particular genus.

Infraspecific taxa: The International Code of Nomenclature for algae, fungi and plants (McNeill et al. 2012 in Melbourne Code) recommends five infraspecific categories for use: subspecies, variety (varietas), subvariety (subvarietas), form (forma) and subform (subforma). Of these, subspecies, variety and form are used commonly in taxonomic literature, and these three categories are discussed here.

Subspecies: Workers like Clausen (1941), Boivin (1962), Davis & Heywood (1963) regarded the first botanical usage of the ‘subspecies’ was in Persoon’s Synopsis Plantarum, vol. 1 (1805). Du Rietz (1930) defined ‘subspecies’ as “a population of several biotypes (groups of individuals from the same parent having similar features) forming more or less a distinct regional facies of a species”. Fosberg (1942) regarded the ‘subspecies’ as subdivisions of ‘an aggregate species’ (). Fosberg’s concept of ‘subspecies’ was supported by many workers like Turner (1956), Northington (1976), Cronquist (1980), Keil & Stuessy (1981) and Rollins (1981). The modern definition of subspecies: “A subspecies is an aggregate of geographically isolated local populations (or demes) of a species, which inhabit in different geographic subdivision of the range of species and differ genetically as well as morphologically from other populations of the species, but still interbreed and produce fertile hybrids”.

Variety: The ‘variety’ was the first category below the species level category to be used for plants. Linnaeus in his Species Plantarum (1753) often used this category, and this was the beginning of its common use in plant . Linnaeus stated clearly and defined ‘variety’ in his Philosophia Botanica of 1751 (pp. 239-249) as “A plant changed by accidental causes due to climate, soil, heat, winds etc...... Species and genera are regarded as always the work of Nature, but varieties are more usually owing to culture”. Therefore, according to Linnaeus, ‘variety’ was primarily an environmentally induced variation, not genetically controlled what modern taxonomists are mostly agreed.

Du Rietz (1930) defined variety as “a population of several biotypes, forming more or less a local facies of a species”. Modern definition: “A variety is a group of individuals of a population of a species having some variations which are mostly due to environmental factors, not usually due to genetic causes, sometimes these variations are unstable”. Actually, the term ‘variety’ is commonly used for morphologically distinct populations occupying a restricted geographical area.

Form (forma): The first usage of the term ‘forma’ in botany was by Miquel (1843). Asa Gray (1856) stated ‘forma’ as “lesser varieties”. Some eminent plant taxonomists like Davis & Heywood (1963) never used the category ‘forma’ in a formal scheme of classification on the grounds that such minor morphological variations are not a useful function of biological classification. Modern definition: “a form (forma) is a group of individuals of a population of a species having a single or a few sporadic variations like corolla colour, leaf size & shape etc.”.

Causes of variation in plant populations: and genetic recombination are the important causes of variation within plant populations, and they form the basis for natural selection and random genetic drift (Judd et al. 2008). Role of mutation: Mutation refers to

Fig 15.Genetic variation within species showing in Achillea lanulosa of Asteraceae (Source: http://www.botany.wisc.edu/courses/botany-400/lecture/11speciation).

alterations of DNA sequences. It can change in single base () to insertions, duplications, deletions and inversions of parts of a , to gain or losses of whole chromosomes, and finally to changes in whole genomes. The extent to which a mutation spreads through a population varies greatly, depending on the importance of the affected region of DNA to the organism. The effect of a mutation may be lethal (when essential gene products is disrupted), neutral (having no effect on the survival of the organism), or selectively advantageous (new chromosomal arrangements of genes creating beneficial and coordinated gene expression). One of the important mutation types is the duplication of genes, which creates extra copies of genes that are free to mutate into new genes and thus causing intra- population variation. The largest mutations are aneuploidy and polyploidy. Aneuploidy plays a major role in some plant groups like Carex in Cyperaceae, Poa in Poaceae and Salix in Salicaceae. But polyploidy leads to gene duplication and greater on which natural selection can act.

Role of Genetic recombination: It refers to the rearrangement of genes that occurs primarily during meiosis. In meiosis, homologous pairs of chromosomes line up together and routinely exchange chromosomal segments so that genes are rearranged as a result of crossing over phenomenon which gives a mixture of parental genes and thus causing variation in populations of a species. Recombination is the primary source of variation. It increases if the frequency of crossing over and the number of chromosomes increase, and it is affected mainly by population size, breeding system, seed and pollen dispersal.

Variation is also affected by gene flow which may introduce new genetic material into a population, and by random genetic drift, the chance fixation of genes in small populations.

Fig 16. Genetic variation within species showing on islands in Central Panama (Source: http://www.botany.wisc.edu/courses/botany-400/lecture/11speciation).

Ecotypes and ecads

Ecotypes: It is a biosystematic category equivalent to Linnean category ‘subspecies’. The term ‘ecotype’ was proposed by Turesson (1922) for a genetic variant within a species that is adapted to a particular environment, although yet remains interfertile with all other members of the species.

Gregor etal. (1936) defined ecotype as “a population distinguished by morphological and physiological characters, most frequently of quantitative nature, interfertile with other ecotypes of the ecospecies (equivalent to species of Linnean category), but prevented from freely exchanging genes by ecological barriers”.

Stace (1989) defined “ecotypes are discrete biotypes characteristic of distinct habitats”.

Fig 17. Genetic variation within species and formation of ecotypes (Source: http://www.botany.wisc.edu/courses/botany-400/lecture/11speciation).

Ecads: This is also a biosystematic category equivalent to Linnean category ‘variety’. The term ‘ecad’ was proposed by Turesson (1922) for the individuals in a particular population occupying a particular habitat and adapted to it phenotypically, but never genotypically. It is also often known as ‘ecophene’. Variations produced by a ‘ecad’ are environmentally induced, temporary and reversible.

Evolution and differentiation of species: In order to understand speciation mechanism and its role in evolution, it is useful to know how much genetic changes take place during the course of species development. It is of considerable significance to ascertain whether new species arise by altering only a few genes or whether the process requires drastic changes—a genetic ‘revolution’, as postulated by several evolutionists in the past.

Genetic changes are measured with two parameters: genetic identity (I) which estimates the proportion of genes that are identical in two populations, and genetic distance (D) which

estimates the proportion of gene changes that have occurred in the separate evolution of two populations. The value of I may range between 0 and 1 which correspond to the extreme situations in which no or all genes are identical. The value of D may range from zero to infinity. D can reach beyond 1, because each gene may change more than once in one or both populations as evolution goes on for many generations.

Patterns and rates of species evolution: Evolution often takes place by , in which changes occur within a lineage (ancestor-descendant sequence of populations), or by cladogenesis, in which a lineage splits into two or more separate lines. Anagenetic evolution, for example, has doubled the size of the human cranium over the course of two million years, while in the lineage of the horse it has reduced the number of toes from four to one. Cladogenetic evolution has produced the extraordinary diversity of the living world, with its more than two million species of animals, plants, fungi and microorganisms.

The most essential cladogenetic function is speciation, the process by which one species splits into two or more new species. As species are reproductively isolated from one another, they are independent evolutionary units, i.e., evolutionary changes occurring in one species are not shared with other species. Over time, species diverge more and more from one another as a result of anagenetic evolution. Descendant lineages (recently originated groups from its ancestral lineage) of two related species that existed millions of years ago may now be classified into quite different biological categories, such as different genera or even different families.

Fig 18. Anagenesis and cladogenesis Fig 19. Anagenesis & cladogenesis in birds

Diagrammatic model (source:www.geol.umd.edu/ (only for better explanation)

204ancestors.html.)

Adaptive radiations: Living organisms exhibit plasticity in their organization. is evolution in several specialized directions starting from a common and generalized ancestral type, or the entry of the organisms of the original stock to new adaptive zones. It can be explained as the rapid speciation after new characters arise (e.g. photosynthesis in plants, or flight muscle in animals etc.), a new habitat is occupied (e.g. water to land), a new area is colonized (oceanic islands). Adaptive radiation is often called as Adaptive divergence or or , as this type of evolution results in the production of new adaptive types through a process of population fragmentation and genetic divergence. Adaptive radiations often occur as a result of an organism arising in an environment with unoccupied niches, such as a newly formed lake or isolated island chain. The colonizing population may diversify rapidly to take advantage of all possible niches. In Lake Victoria, an isolated lake which formed recently in the African rift valley, over 300 species of cichlid fish adaptively radiated from one parent species in just 15,000 years.

Adaptive radiations can happen whenever the reproductive success of a population brings it into contact with new environments in which its members are able to adapt and succeed. This adaptation allowed their descendants to radiate into many different niches. No one species could occupy all these specialized niches. But in competition with each other, as well as with other species, populations of the original species quickly

diverged to encompass these adaptations and others. This original adaptive radiation was therefore enabled by the evolution of a particular trait.

Adaptive radiations commonly follow mass : following an , many niches are left vacant. A classic example of this is the replacement of the non-avian with at the end of the , and of brachiopods by bivalves at the Permo- boundary. Four features can be used to identify an adaptive radiation:

1. A common ancestry of component species: specifically a recent ancestry. Note that this is not the same as a in which all descendants of a common ancestor are included. 2. A phenotype-environment correlation: a significant association between environments and the morphological and physiological traits used to exploit those environments. 3. Trait utility: the performance or fitness advantages of trait values in their corresponding environments. 4. Rapid speciation: presence of one or more bursts in the emergence of new species around the time that ecological and phenotypic divergence is underway.

Mechanism of Macroevolution or Adaptive Radiation: It usually operates above species level and results in the establishment of new genera, families and orders. The changes in the organization of populations occur on account of sudden mutations of large size, which are called ‘macromutations’. It occurs in a group of individuals of a population in a species which have entered a new adaptive zone free of competition. In a new adaptive zone, the number of individuals is far less and the opportunities to avail new habitats are more. Thus, the intraspecific struggle is roughly nil. Meanwhile, the new zone will be almost free from competition by other individuals. Therefore, the newly entered population enters all the available habitats of the adaptive zone and start adapting themselves according to the conditions and need. It results that the one population which had acquired the new zone.

Darwin’s finches on the Galapagos islands (14 species)

Fig 20. Adaptive radiation in Galapagos finches (Adapted from Gavrilets & Vose, 2009).

A mainland radiation of columbines (about 70 species)

Fig 21. Showing adaptive radiation in the genus Aquilegia (Ranunculaceae)/Columbine— Adapted from Gavrilets & Vose, 2009.

HAWAIIAN SILVERSWORD ALLIANCE

§ 28 SPECIES DERIVED FROM A SINGLE COMMON ANCESTOR. THESE PLANTS OCCUPY HABITATS RANGING FROM EXPOSED LAVA TO WET FOREST.

§ DIVERSE FORMS INCLUDE, VINES, TREES, ERECT SHRUBS, ROSETTES, AND HERBACEOUS MATS

Fig. 22. Adaptive radiation of Hawaiian Silversword Alliance (the genus of the family Asteraceae)—Adapted from Robichaux et al. 1990 (Ref. Annals of the Missouri Botanical Garden 77(1): 64-72).

gets splitted up into several sub populations, each of which accumulates mutations and evolve independently but simultaneously in different directions. On account of the different environmental conditions, there is different urge of natural selection and adaptive modifications occur in different directions. Adaptive modifications in each subpopulation have a cumulative effect and are therefore, directional.

Researches on Adaptive Radiation or macroevolution were mostly done on groups like evolution of and horses (mammals) representing the best documented examples in the records. Other notable example is Darwin’s Finches (birds) in Galapagos islands. For plants, one example showed by Cody & Overton (1996) in some individuals of Lactuca muralis in Hawaiian islands, which were new colonisers to this islands and were better at dispersing.