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Home , Little greenbul, Peripatric speciation

SUBJECT: ZOOLOGY

EVOLUTION

SPECIATION TYPES

CONTENT

Introduction  characteristics  Formation of speciation  Modes of speciation  Types of speciation   Peripatric speciation  INTRODUCTION; The term speciation is coined by American botanist Orator F .cook(in 1906 )Speciation – it is an evolutionary process by which population Evolves to become distinct species.That are reproductively isolated from one another.  is the successive modification in inherited traits over a huge span of time, usually over generation. (1859) English biologist, proposed the theory of evolution he mentioned about evolution in his book the origin of species.  Charles Darwin noted that living change their physical and Anatomical structures over a long period of time for better to the changing environment. The change is by natural process and these organisms which do not adjust to it, find it difficult to survive. Please put forward the concept of And Darwin called it survival of the fittest.

Species Definition: species is a group of organisms or individuals with similar characteristics, where they can interbreed to produce fertile offspring. All the organisms of a species share in the same gene pool.

Characteristics of a species:

1. Members of a species exhibit distinctive features in common, in which they different from all other such groups. 2. Intermediate or transitional forms between the species are usually not found i.e., species are reproductively isolated. 3. Members of separate species do not usually interbreed, so that natural hybrids of the species are either totally absent or are very rare. Even if hybrids are produce artificially, these are usually sterile 4. Allied species usually have separate but adjoining territories. 5. Species occupies a specific , unoccupied or unutilised by another closely related species. 6. Species interacts with the environment and with other species present in a given environment. 7. Species have fully effective reproductive isolating mechanisms. 8. A species is a genetic unit consisting of a large intercommunicating gene pool.

Origin of species (speciation)

Formation of new species from the parental species is called speciation or Origin of species. recognised speciation depends on two factors:

1.Isolation of gene pool of populations ()

2. of their gene pool

Isolation of Gene Pools of Related Populations (Reproductive Isolation): The central event in speciation is the separation of gene pool of ancestral species into two or more isolated gene pools. When isolated gene pools become so much different that interbreeding and exchange of genes between them is almost impossible, the populations with these gene pools become independent species establishing reproductive isolation. If there is free in the gene pools of these populations, the two gene pools exchange genetic changes also and the two gene pools tend to be similar.

Genetic Divergence: The isolated gene pools need to accumulate large number of genetic differences so that their gene pools do not permit free gene flow. If isolated populations are reunited they either fail to interbreed or do not produce fertile offspring. Their gene pools diverge due to different environmental Pressures and natural selection or due to genetic drifts. MODES OF Several possible mechanisms have been proposed to account for the occurrence of speciation. Based on different modes, following types of speciation have been recognised 1. phyletic Transformation or Autogenous speciation : phyletic speciation is gradual transformation of one species A into another species B over a long period by accumulation of slow changes in the gene pool. Neither there is splitting of the parental gene pool nor evolution of reproductive isolation. The gene pool of parental species modifies due to changes in gene frequencies, chromosomal inversions, other structural changes in the . These changes accumulate in response to:  to a shifting environment  increasing specialisation for a particular environment  improved adaptations in a constant environment In Phyletic speciation, evolving species present a line of succession in which one species is replaced by other 2 Speciation through Fusion of Species : A new species may arise by the fusion of two already existing species through the breakdown of their reproductive isolation. Fusion of gene pools is possible though hybridisation. The new species formed has combination of characters from both parent species but is reproductively isolated from either of them. Several new plant species have arisen in nature by hybridisation.

3.True Speciation :True speciation is the origin of two or more species from one parental Species, it requires splitting of parental gene pool or origin of two or more new gene pools from the gene pool of single species and establishment of reproductive isolation between them. The true speciation may be of two types: 1. Sudden speciation or Quantum speciation 2. Gradual speciation Sudden Speciation or Quantum Speciation or Instataneous speciation: In this type of speciation, chromosomal rearrangements produce reproductive isolation and result in speciation. These evolutionary events occur suddenly and intermittently. Quantum speciation may occur due to chromosomal aberrations (inversion and translocation) or change in number (, autopolyploidy and amphidiploidy). According to Simpson, the represents accelerated pace of because of accumulation of genetic changes in relatively Rapid succession. in Quantum speciation biological barrier to gene exchange arise within an initial randomly mating population without any spatial segregation of the incipient species. it means quantum or sudden speciation due to disruptive selection.

Gradual Speciation: Gradual speciation is a microevolutionary event. The new species originate from daughter populations of the same species by a Gradual accumulation of many minute gene differences over a long period of time under the influence of natural selection.in this process one of the species gives rise to one or more new species. The Gradual speciation may be of following types: —  Allopatric speciation  Peripatric speciation  Parapatric speciation  Sympatric speciation.

ALLOPATRIC SPECIATION: (Greek: allos, "other"; patri, "Fatherland")

Definition: Allopatric speciation is the divergence of physically separate populations of a species, occupying separate geographical areas into new species. This is also called Speciation by geographic isolation.

Methods of Physical Isolation of Populations: Physical isolation of an original large population of a species may occur in three ways:  by physical splitting of a large with a widespread population by some new physical barrier into two or more large isolated areas (Vicariance)  by Divergence of a few members of a widely distributed population to some new habitat (Founder effect)  by the of intermediate links in a chain of interconnected populations The physical splitting of habitat is called vicariance. Speciation that begins with Physical isolation of populations either by dispersal or vicariance is allopatric Speciation,

Allopatric speciation by vicariance : volcanoes, earthquakes, storms, tidal waves, Glacier, floods and formation or destruction of mountains and forests forest and appearance or disappearance of water bodies acts as vicariance agents for speciation in terrestrial population. The geographically separated populations are exposed to variable forces of natura selection, , , random mating that cause their gene pools to diverge. Their gene pools become so different that the free gene flow between isolated populations is totally prevented and reproductive isolation is established. Finally, these reproductively isolated populations become independent species. In case, the geographic barrier is lifted, the members of these species meet but do not interbreed.

Stages in Allopatric Speciation by Vicariance: Allomen (1992) divided the process of allopatric speciation into following three stages:  Fragmentation of a widespread population due to geographical barriers and separation of its gene pool.  Persistence of isolation of populations and their gene pools, so that their gene pools become different from that of parental population, i.e., genetic divergence.  Establishment of reproductive isolation between new populations and formation of new species, i.e., speciation, Examples of Allopatric Speciation by Vicariance: Pupfish of Nevada, California: The blue- gray pupfish which inhabits a warm spring at the base of mountains near Valley, Nevada, has evolved by allopatric speciation. This spring got isolated from other water bodies about 50,000 years ago. The fish trapped in the spring, became geographically isolated from main fish population. In due course of time its gene pool became so different that it became a distinct species.

PERIPATRIC SPECIATION OF MARGINAL POPULATION: (Greek: peri, "around"; patri, "Fatherland")

Definition: It is origin of new species from peripatric populations that bud off from the Periphery of a parental population, Such populations are also Called Marginal isolaters. Mayr (1982) described founder effect speciation as peripatric speciation. His hypothesis was based on the observations that in many and mammal,the isolated marginal populations on the periphery of widely distributed Parent population become so distinct from parental population that these populations are classified as distinct species,

Examples of Peripatric Speciation: Peripatric Speciation in Paradise Kingfishers: In New Guinea, paradise kingfisher, Tanysiptera galatea is widely distributed throughout low-land area.

PARAPATRIC SPECIATION: (Greek: para, "beside"; patri, "Fatherland")

Definition: Speciation is the the development of reproductive isolation among the members of a continuous population or spatially Distinct populations, between which there is Some genflow in the absence of geographical barrier,. it means parapatric populations occupy adjacent regions with different Selective pressures.They coexist Only in in one or more overlapping regions at the peripheries of their geographical distributions and are not completely isolated

Mechanism of Parapatric Spaciation: Parapatric speciation occurs in neighbouring populations of widely distributed species . through separated, border zone .Most individuals mate within their own population, but a few go and mate with individuals present in the border zone. such hybrids formed in the border zones are genetically different from The two original population and form the initial step toword Formation of a parapatric species The population present in the border zone possesses new combination of characters. Natural selection acts on this gene pool and further isolates it from the two original populations. Ernst Mayr called it semigeographic or parapatric speciation.Such parapatric populations are separated by distance but are not completely isolated. According to Russel_Landle (1982) prezygotic isolation arises in populations isolated by long distance due to divergent .

Examples of Parapatric Speciation: Example 1: Parapatric Speciation in : The , little Greenbul (Andropadus virens) in Tropical rain forests of Cameroon, westAfrica.it also lives in neighbouring grassland. The border area where forest and grassland meet, is called ecotone. The birds living in these two are quite different in their beak, legs, body size and body weight. Greenbul from ecotone could still mate with both rain forest and grassland counter parts. But changes Introduced in the gene pools are more than the gene flow between the two species .The divergence in their gene pools overshadows the effect of gene flow between them and each population becomes a new species.

SYMPATRIC SPECIATION: (Greek: patri, "Fatherland"; sym, "together") Definition: Sympatric (same country) speciation refers to the origin of new species due to the appearance of some biological barrier in the individuals of an initially randomly mating population that live in the Same geographic area. Characteristics of Sympatric Speciation:  Sympatric species occur in the same geographic area.  Populations are not Separated by any physical barrier.  There are no physical barriers to Prevent free gene flow (as in allopatric speciation) between members of the populations.  sympathetic populations may be isolated by preferences for different habitats or niches. Mechanisms of Sympatric Speciation: Sympatric speciation occur by genetic divergence in the gene pools and reduced gene flow between members of populations. In sympatric speciation, the gene flow between the members of a single population in the given area may get reduced by:  Ecological isolation or habitat differentiation and disruptive selection.  Polyploidisation or change in chromosome number.  Chromosomal aberration or change in chromosomes structure.  Introgressive hybridisation.

EXAMPLE: The hawthorn fly (Rhagoletis pomonella), also known as the apple maggot fly, appears to be undergoing sympatric speciation REFERENCE BOOKS:

1.Organic evolution, 13th Edition Veer Bala Rastogi.

2. Textbook of Zoology- physiology, Genetics and Evolution-Telugu Academy( 2011)

WEB LINKS:

1. https://en.wikipedia.org/wiki/Speciation 2. https://www.oercommons.org/courseware/lesson/15029/overview

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PREPARED BY

Dr.CH. LALITHA KUMARI LECTURER IN ZOOLOGY D.R.W.COLLEGE(A),GUDUR SPSRNELLORE DIST A.P