7 Formation of New Population and Species

Mr. Vijit Deepani and Prof. A.K. Kapoor

epgp books

 

Contents:

  • Population and Species: conceptual framework
  • Speciation: Process and typology
  • Isolating Mechanism influencing Speciation
  • Macroevolution: Phyletic Gradualism and Punctuated Equilibrium
  • Summary

 

Learning Objectives:

  • To understand the concept of Speciation.
  • To study the isolating mechanism driving speciation.
  • To gain insight into macro- and micro-evolution.

 

Population and Species: Conceptual Framework

 

Evolution is central to development of life-forms (including mankind) on earth. New life forms develop from pre-existing ones by incorporating modifications (or variations) and thus evolution is referred to as descent with modifications. In order to understand the evolutionary change, which leads to formation of new life forms, basic unit of evolution must be studied.

 

A population constitutes community of potentially interbreeding individuals in a specific area at a certain time. Dobzhansky (1955) emphasized that Mendelian population relates to a reproductive community of sexual and cross-fertilizing individuals sharing common gene pool. An important characteristic about Mendelian population is that they are breeding isolates, although they are isolated to varying degrees. All individuals of the population are not equal in their ability to survive and reproduce.

 

Species can be viewed as largest Mendelian population. Species – a term coined by John Ray in 1693 – forms basic unit of biological classification. With reference to taxonomy, species refer to morphologically distinct and reproductively isolated natural populations or group of populations wherein members of one species interbreed freely among themselves (not with members of other species) and form a genetically closed system (share common gene pool). There would no longer be a distinctive kind of organism if species interbreed among themselves. Phylogenetically, a species caters to a lineage, distinct from other such lineages, perpetuated ancestor to descendant over time.

 

Biologists and taxonomists have tried to define species from several scientific avenues starting from morphology and moving towards genetics. The following table provides glimpse various species concepts.

 

Table I: Glimpse of Species concept –

 

 

Process of Speciation

 

Speciation, term coined by Orator F. Cook (1906), is an evolutionary process related to formation of new (biological) species from the pre-existing ones through genetic modifications. This evolutionary process initiates when the members of a particular species become isolated from each other and start to evolve independently. White (1978) emphasized that speciation is a crucial evolutionary mechanism by which organisms adapt so as to exploit the diverse environmental conditions available to them.

 

Charles Darwin mainly concentrated to demonstrate evolutionary changes by the action of natural selection and discussed how populations adapt to their environment through the process of natural selection, but not how this adaption paves way to formation of new species.

 

Most critics of Darwinian Theory of evolution emphasize that genetic variation can occur within a species as the result of selection. They maintain that such variation is bounded by the genetic limits of that particular species. Thus critics of Darwin theory of evolution view variation within species as a difference in degree and variation between species is perceived as a difference in kind (Pawson, 1977)

 

Breeding isolates are developed in situation where a breeding population does not share a common gene pool and if these isolates are subjected to different types of selection, speciation takes place. If mating barriers prevail, the resulting breeding units (or breeding isolates) will initiate to formulate their own distinct gene pools (Pawson, 1977). Hence the degree of genetic dissimilarity between breeding isolates is intensified by the prevalence of different environmental conditions and different processes of selection. Thus speciation will be more likely to occur.

 

The populations will evolve local differences in gene frequency as they are isolated by a reproductive barrier. Also the process of speciation is greatly sped up if genetic isolation of population subgroups is complete. Pawson (1977) highlighted that processes such as geographical separation, differential selection, or new genetic material from migrations tend to increase genetic variation within a species and favour the formation of breeding isolates, while processes such as panmictic or random mating decrease genetic variation within a species by promoting gene exchange between all segments of the species. Both the fossil record and the degree of genetic similarity between two species can aid in assessing how long ago speciation occurred.


Fig I: Cladogenesis and Anagenesis

 

(Source: Pawson, 1977)

 

Two major processes (related to speciation) – Anagenesis and Cladogenesis– are observed in the course of human evolution. Anagenesis or phyletic transformation, as an evolutionary process, implies that time plays a crucial role as an isolating factor between members of the lineage. In other words, evolutionary transformations can occur in a species over time to a sufficient degree such that later organisms could eventually be treated as a different species. This type of speciation relates to progressive adaptation to a fairly stable environment. In contrast to anagenesis, Cladogenesis relates to branching evolution i.e. it occurs when an ancestral species, subjected to changing environment conditions, splits or branches into several distinct (daughter) species, each occupying its own habitat. This speciation process is quite common in the fossil records.

 

The number of available ecological niches influences the rate of speciation. In certain cases, a population of one species disperses throughout a geographical domain, and each acquires a distinct ecological niche and over time, the varied demands of their new lifestyles leads to subsequent branching of the single ancestral species. This process is termed as adaptive radiation. As a sign of adaptive radiation, the fossil records exhibit the relative sudden proliferation of related species.

 

Fig II: The honeycreeper birds exhibit adaptive radiation as from one original species of bird, multiple others evolved, each with its own distinctive characteristics. (Source: https://s3-us-west 2.amazonaws.com/courses- images/ wpcontent/ uploads/ sites/ 198/ 2016/ 11/ 28194406/Figure_18_02_05-768×768.jpg)

 

Darwin’s finches offer typical example of adaptive radiation as at present there are fourteen distinct species of finches on the Galapagos Islands and all of them have evolved from one ancestral species, which migrated to the islands only a few million years ago. Thus from ancestral seed eating species, several different species with altered beaks arose, facilitating them to become insectivorous, seed eaters and cactus eaters.

 

Modes of Speciation

 

Evolutionary biologists have outlined several modes of speciation. J. S. Huxley (1942) emphasized three crucial types of speciation, namely geographical, ecological and genetic.

 

M. J. D. White (1978) suggested that three main sets of variables are engaged in the speciation process:

 

(i)  Genetic mechanisms generating genetic variability

 

(ii) Genetic isolating mechanisms leading to the origin of reproductive isolation, and

 

(iii) Geographic component ranging from complete (allopatry) to absent (sympatry). He outlined following seven models to illustrate the mechanisms of speciation: (White, 1978)

 

•  Strict allopatry without a narrow population bottleneck.

 

•  Strict allopatry with a narrow population bottleneck (founder principle).

 

•  Extinction of intermediate populations in a chain of races.

 

•  Clinal speciation.

 

•  Area-effect speciation (primarily genic).

 

•  Stasipatric speciation (primarily chromosomal).

 

•  Sympatric speciation.

 

The first three models represent allopatric (or geographic) speciation and the next three relate to semi-geographic speciation.

 

Allopatric, sympatric, parapatric and stasipatric are the prominent modes of speciation (to illustrate the process in animal species).

 

Fig III: Modes of Speciation

(Source: https://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Speciation_modes.svg/350px-

Speciation_modes.svg.png)

Allopatric mode of speciation

Fig IV: Allopatric speciation model

(Source: Jenkins, 1990)

 

Allopatric (allo- = “other”; -patric = “homeland”) speciation is also referred as geographic speciation because it requires physical separation (due to distance or geographical barrier) at initial stage. It takes place when a main ancestral population is divided by some sort of barrier and separated segments of population adapt to different environments. These separated segments of population experience genetic drift (as barrier inhibits gene flow), acquire mutations and formulate a distinct gene pool separate from ancestral population. The genetic difference between the newly separated populations intensifies, the longer they remain isolated. Even if there is some gene flow after the members of the two populations initially come back into contact, character displacement and reinforcement may amplify the initial differences between the populations and lead to formation of two new species (Boyd and Silk, 2006).

 

There are two main sub-types of allopatric speciation – dichopatric and peripatric. The former model did not assume founder principle but in peripatric speciation model, there is involvement of the founder principle.

 

Allopatric speciation is exhibited by several cases such as evolution of Darwin’s finches on the Galapagos Islands, evolution of land snails of the genus Partula in the Society Islands, and locusts and mammals on various islands (Singh, 2012).

 

Sympatric mode of speciation

 

Fig V: Sympatric speciation model

(Source: Jenkins, 1990)

 

Bush (1975) defined sympatric (sym- = “same”; -patric = “homeland”) speciation as the origin of a new species characterized by reproductive isolation within the dispersal area of the parental species. Sympatric speciation is evolution of reproductive isolation “without geographic isolation” (Mayr, 1963).

 

Singh (2012) stated that sympatric speciation involves instantaneous appearance of reproductive isolation between the segments of the same population and new species originate in the same geographical area. Four criteria have been outlined to infer that a particular case is best illustrated by sympatric speciation: (Coyne & Orr, 2004)

 

1 Species thought to have arisen via sympatric speciation must have largely overlapping geographical ranges. In principle, sympatrically derived species could become allopatric over time, but it is not clear how to demonstrate such secondary allopatry.

 

2 Speciation must be complete. We cannot declare a case of sympatric speciation if speciation has not occurred. In practice, whether or not speciation is complete is a taxonomic decision that is contingent on the definition of ‘species’ (the biological species concept in Coyne and Orr’s case).

 

3 Clades thought to arise via sympatric speciation must be sister species or monophyletic groups; i.e. we can support sympatric speciation only when the evidence has not been obscured by subsequent non-sympatric diversification.

 

4 According to Coyne and Orr, ‘the bio-geographic and evolutionary history of the groups must make the existence of an allopatric phase very unlikely This is not so much a criterion as a statement that the first three criteria are necessary but not sufficient, in their view, to reject alternatives to sympatric speciation. These criteria clearly relate to the biogeographical concept of sympatric speciation. To infer sympatric speciation under the population genetic concept, an additional condition must be met: Evidence must support panmixia of the ancestral population

 

Parapatric mode of speciation

 

This mode of speciation emphasizes that selection alone is not sufficient to form a new species and new species can be formed if the selection works in association with partial genetic isolation (Boyd and Silk, 2006).There is no specific extrinsic barrier to gene flow. A particular species occupy a wide array of environments, incorporate different morphological and behavioural features in each of these environments and this variation may cause the particular species to vary in different environments. Under such situations of slight distributional overlap such as at habitat boundaries, members of two different species having different characteristics may mate and produce hybrids. Singh (2012) highlighted that parapatric species may lack behavioural isolation and hybridize, but their hybrids are sterile.

 

Endler (1977) suggested that isolating mechanisms develop and intensify in a cline, along an ecological escarpment, until the two adjacent populations are finally reproductively isolated. Selection inhibits mating between members of populations from different habitat and as a result gene will be reduced and finally two new reproductively isolated species will evolve.

 

Stasipatric mode of speciation

 

Stasipatric mode of speciation is also termed as classic chromosomal speciation model (White et al, 1967 and White, 1968) because new species arise sympatrically, initially by chromosomal rearrangement within the geographical range of the parent species and the new population disperses and proliferates within the range of parent species (spreading by parapatric distribution) (Singh, 2012). This model of speciation is characterized two principle features: (Kawakami et al, 2011)

 

(i) Chromosomal rearrangements produce barriers to gene flow between parental and daughter chromosome types due to meiotic abnormalities in chromosomal heterozygotes, and

 

(ii) The spread of new chromosome types from their point of origin into the distribution of a parental chromosome type occurs without geographic isolation, leading to parapatric distributions of chromosomal races.

 

 

Isolating Mechanism influencing Speciation

 

The mechanisms of reproductive isolation refer to an assemblage of evolutionary mechanisms, behaviours and physiological processes that allow isolated groups to evolve into distinct populations or species necessary for speciation event to occur. These reproductive barriers inhibit mating between members of different species or ensure that (if mating occurs) the offspring are non-viable or infertile. Hence reproductive barriers guard the integrity of a species because these barriers restrict and inhibit gene flow between related species.

 

Scientists have classified the mechanisms of reproductive isolation into two major categories: pre-zygotic barrier (or pre-mating isolation) and post-zygotic barrier (post-mating isolation). The prezygotic barrier refers to mechanism that inhibits reproduction from taking place; this includes barriers that prevent fertilization when organisms attempt reproduction and the post-zygotic barrier (occurs after zygote formation) ensures that if the mating occurs, the product of mating does not produce a viable offspring (Bear and Rintoul, 2014).

 

Pre-mating isolation mechanisms include – temporal (Differences in breeding schedules), ecological (difference in ecological or habitat preference), mechanical (difference in genitalia in animals), physiological (adverse physiological reaction such as non-viability of sperms in reproductive tract of female of other species) and behavioural (presence or absence of a specific behaviour) isolation and post-mating isolation mechanisms incorporate – gametic incompatibility (non-fusion of gametes), zygotic mortality (zygote dies soon after formation) and hybrid inviability (hybrid dies before attaining sexual maturity).

 

Macroevolution: Phyletic Gradualism and Punctuated equilibrium

 

Macroevolution is primarily based upon describing evolutionary patterns above the species level. Macroevolutionary studies focus on change that manifests at or above the level of species and in contrast, microevolution relates to evolutionary changes within species. Mutation, genetic drift, Isolation, (Natural, Sexual and Social) Selection and hybridization are prominent micro-evolutionary forces which lead to evolutionary changes in terms of changes in gene frequency.

 

It has been emphasized that both the evolutionary processes are related to each other as they do not imply any difference in the underlying agencies and the aggregate effect of microevolution during the course of lifetime in the long term leads to macroevolution.

 

Macroevolutionary studies are centered upon examination of the fossil records. There are two prominent models related to the tempo and mode of macroevolution –

 

  • First emphasizes that microevolutionary processes alone can sufficiently explain grand patterns and radical changes on the tree of life. This model suggests that speciation may occur through gradual change within the lineage, such that over time species `a’ is transformed into species `b.’
  • In contrast to first model, the second model proposes that grand patterns in the history of life cannot be explained solely on the basis of change in allele frequencies over time, even rapid ones. Instead scientists proposed that large changes on the tree of life were preceded by events that decoupled the tempo and mode of evolutionary change from predictable microevolutionary processes.

 

Thus, in the first model, change is slow; in the second, speciation events are rapid and interspersed among periods of no change or stasis. The first model has been termed `gradualism’ and the second, is referred as `punctuated equilibrium’ (Eldridge and Gould, 1972). Gradualism and punctuated equilibrium are two ways in which the evolution of a species takes place. Punctuated equilibrium and phyletic gradualism are contrasting patterns of evolution among a spectrum of patterns observed in the fossil record. In punctuated equilibrium, species tend to exhibit morphological stasis between abrupt speciation events; while in phyletic gradualism species undergo more continuous change (Eldredge, 1971).

 

The central theme of punctuated equilibrium includes three concepts – stasis, punctuation and dominant relative frequency. Stasis refers to a long period of relatively unchanged form; punctuation is radical change over a short duration; and dominant relative frequency is the rate these events occur in a particular situation (Eldridge and Gould, 1972 and Gould, 2007).

 

 

In addition, Punctuated Equilibrium emphasizes that: (Saylo et al, 2011)

  • Evolutionary change is connected in speciation events.
  • Most species remain pretty much the same once they have come into being. This lack of substantial change over millions of years is called stasis.
  • Speciation events are normally confined to small populations-peripheral isolates-that have become separated from the bulk of the species. As these isolated populations are small and transient, we should not expect to find them in the fossil record.

Fig VI: Prominent differences between Phyletic Gradualism and Punctuated Equilibrium. Stippling is used to denote different species. The morphological variability within the populations (at any point in time) is represented by the bell curves (Vrba, 1980). (Source: http://homepage.smc.edu/grippo_alessandro/punkeek.jpg)

Table II  provide   account   of   major   differences   between   Phyletic   Gradualism   and    Punctuated Equilibrium.

 

Table II: Major differences between phyletic gradualism and punctuated equilibrium

Summary

 

The concept of evolution is central to several biological concepts and ideas. Evolutionary transformations lead to formation of new life forms from ancestral ones. It relates to the adaptations of organisms to their dynamic environmental conditions and can lead to altered genotypic and phenotypic framework and even to formation of new population and species.

 

Species relate to basic unit of evolution. Evolutionary biologists have made several attempts to provide fundamental understanding of species on the basis of different diagnostic criteria ranging from morphology and moving towards genetics. Several prominent species concepts have been proposed – biological species concept, phylogenetic species concept, evolutionary species concept, ecological species concept and many others that specify how species boundaries can be drawn in practice.

 

As gene exchange between populations is inhibited (owing to evolution of isolating mechanisms), new species eventually arise. This relates to an important evolutionary process called Speciation. Prominent speciation models are – allopatric, sympatric, Stasipatric and parapatric speciation. Evolutionary biologists have emphasized that reproductive isolation (mechanisms that restrict or prevent gene flow) can occur in a variety of ways. Isolation mechanisms have been broadly classified into two groups – pre-zygotic barriers and post-zygotic barriers.

 

Speciation rate is influenced by two prominent model of macroevolution- `gradualism’ and `punctuated equilibrium’. The first model – ‘Gradualism’ – is characterized by slow change that lead to gradual and uniform transformation of one species into new species through anagenesis. In contrast, second model – ‘punctuated equilibrium’ is characterized by rapid speciation events are rapid and interspersed among periods of stasis (or no change).

 

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References

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