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Life Science 4 Sem Paper: Genetics and Evolutionary Biology

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Teacher: Dr Gouri Course: Life Science 4th sem Paper: Genetics and Evolutionary Biology Unit 9 – Processes of Evolutionary Change Part -2 Biological Species Concept According to Biological Species Concept, a species is a group of populations whose members have the potential to interbreed in nature and produce fertile offspring but not produce fertile offspring with members of other species. The mechanisms that prevent interbreeding in nature and help in maintaining their hereditary integrity are called Isolating Mechanisms. These are also called as Isolating Barriers or Reproductive Barriers as they prevent gene flow between biological species. Darwin and Wallace – proposed geographical isolation Moritz Wagner 1868, 1889 – emphasized the role of geographic isolation in formation of new species and to maintain its genetic integrity. Dobzhansky 1937 coined the term Isolating Mechanisms in his book “genetics and the origin of species” JBS Haldane 1922 explained the pattern of postzygotic isolation that When in the F1 offspring of the two different animal races one sex is absent, rare, or sterile(had lower fitness), that sex is the heterozygous one. Muller 1930s- proposed along with Dobzhansky that postzygotic isolation could evolve without difficulty if it was controlled by interaction among more than one genetic locus. This is known aDs obzhansky–Muller theory . Mayr also contributed significantly to understand the operating isolating mechanisms. Ref: M Ridley 3rd edition Most modern evolutionists such as Mayr (1948, 1970), Stebbins (1966, 1971) etc. have classified the reproductive isolating mechanisms into 2 classes namely Premating or Prezygotic isolating Mechanisms and Postmating or Postzygotic Isolating Mechanisms. 1. Premating or Prezygotic isolating Mechanisms – prevents mating between the members of closely related species and prevent the wastage of germ cells. 2. Postmating or Postzygotic Isolating Mechanisms prevent the formation of fertile hybrids between the species because the gene pool of these species have accumulated so many genetic differences that they have become genetically incompatible. These isolating mechanisms operate after the zygote is formed by fusion of gametes and do not prevent the wastage of gametes. Premating or Prezygotic Isolating Mechanisms- 1. Geographic Isolation – two populations of same species are seperayted by some physical or geographical barriers or they occupy different geographic areas. These barriers may be mountains ranges, desert, thick forest, land bridges, water bodies etc. Because of the above barrier, individuals of seperated populations donot have chance to meet and interbreed, greatky reducing the exchange of genes between them and a single gene pool is splitted into two or more gene pools. These gene pools keep accumulating new mutation, genetic drifts and action of natural selection occur independently and become adapted to local conditions. This leads to genetic divergence and in due course of time segerated po[yultaion become independebt species Geographical isoslation provides conditipons for Effectiveness of physical barriers and duration of seperation. Eg. Chattam Islands are 400 miles east of New Zealand. About one million years ago, these were connected to New Zealand by a land bridge but not now. The flora and fauna of these islands are similar to the New Zealand but there are slight differences. For example wood pigeon of New Zealand Cisa rpophaga novaezealandise while that of Chattam IslandsC i.sc hathmensis . Similarly, New Zealand lizaLrydg, osoma moco and Chittam lizLa.rdde ndyl , are similar but different slightly. These species are unable to interbreed because of the ocean separating the two lands. 2. Ecological Isolation or Habitat Isolation - is seen in populations or closely related species that occupy different habitats in the same geographical area of their duistribution. Because they live, feed and mate in different habitats, they rarely venture out and hardly interbreed. This also called Environmental Isolation. However the hybrids can be obtained in an laboratory condition between these species. Thus their gene pools are isolated physically but not physiologically. Habitat isolation is most common in plants due to their sedentary nature. In plants it operates in two ways a. The species may live in same area but may have different habitat preferences, that they are rarely close enough to cross fertilize each other frequently. b. If hybrids are formed occasionally, they may be either unable to grow to maturity or may produce fewer progeny under natural conditions. Eg. CH Muller in 1952 studied 4 species of oaks occurring in Texas area,Q uercus mohriana, Q. havardi, Q.grisea, and Q.stellate. These 4 species grow in distinct soil types and few hybrids between them are confined to the zones where the Sympatric species are related and share a portion of their ecological ranges but remain isolated from one another not by space but through the physiologic expression of genetic differences. Eg . The scarlet oak –Q uercus coccinea and black oakQ .velutina are symatric throughout Eastern United States and are distinguishable by the shapes of their leaves and acorn. Habitat isolation is not a very effective isolating mechanism in mobile animals. 3. Seasonal Isolation (Temporal Isolation) - Difference in the breeding season prevent mating and interbreeding among individuals of different population, races ot of different species that occupy similar habitat. Seasonal isolation is very common in plants and occur frequently in insects and certain other invertebrates and aquatic animals. Eg 3 species of frogR ana clamitans, R.pipens and R.sylvatica breed in same pond of northeastern America but do not interbreed because the reproductive seasons of these species are differentR. .sylvatica breeds in 44 degree FR, pipens in 55 degreea Fn d R. clamitans in 60 degree F. 4. Ethological Isolation or Behavioral Isolation – even when populations of sympatric species are in contact and breed at the same time, the individuals of one species may reject or fail to recognize individuals of other species as mating partners due to incompatibilities in behavior determined genetically. The species specific stimuli which helps the two sexes for the simultaneous recognition of each other for courtship forms the basis of ethological isolation. The courtship involves a continuing exchange of visual, auditory, tactile, olfactory or chemical (pheromones) stimuli between the male and female until both have reached a state of physiological readiness in which successful copulation can occur. These species specific behaviors of males towards female are called species-specific recognition patterns. Eg. More than 500 species ofD rosophila in Hawaiian Island maintain their distinctiveness by displaying different mating pattern behavior. These include distinct courtship actions like orientation of body, display of wings and licking with tongue. 4. Mechanical Isolation – is provided due to difference in the floral structure of the related species of plants or in the genital organs of different species in animals with internal fertilization, so that cross-pollination or copulation can be prevented. According to Leon Durfour (1844) the genitalia in insects are developed on Lock and Key principle to exactly fit into each other and slight deviation in structure renders copulation impossible. Eg. Interspecific crosses inD rosophila and Glossina may cause injury or even death of the female. Postmating or Postzygotic Isolating Mechanisms 1. Gametic Mortality/ Gametic Isolation/ Gametic Incompatibility – even if the mating occurs, the sperms of one species fail to fuse with the eggs of other species to form the zygote. This can be observed in both external and internal fertilization. The sperm may fail to fertilize the egg and both of them perish. The other species sperm may not be physiologically compatible and can have antigenic reaction in genital tract of female of different species and are killed. Eg. Patterson reported “insemination recation” in which insemiantion between related species o fDrosophila americana, D.virilis, D.montana and D. lacicola leads to loss of sperm mobility rapidly in the seminal receptacles of females of other species. 2. Zygotic Mortality- the gametes of 2 different species may fuse but the zygote may not survive or the development of hybrid zygote is irregular and ends at any stage during development without reaching adulthood. Eg. In a cross between sea urchinP aracentrurus lividus andP sammechinus microtuberculatus, the embryos die before gastrula stage. 3. Hybrid Inviability – in some natural interspecific breeding, the zygote develops normally but the hybrids have lower survival than the parental species due to genetic incompatibility. In this case the hybrid fails to survive long and reach reproductive stage. Eg. Warwick and Berry showed that the cross between goat and sheep produces normal embryos but they die much before birth due to gene incompatibilities. 4. Hybrid Sterility/ Hybrid Infertility – hybrids of few interspecific crosses are found to be sterile or semisterile that is unable to produce normal sperms or eggs. This might occur due to failing of chromosomes to pair at meiosis or abnormalities in the formation of spindle fibers or failure of cell division, usually rendering the male individuals sterile. The reduced fertility is caused by structural differences in the chromosomes of closely related species due to aneuploidy or due to differences between genes from two parents.
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