Biology: Ecology and Evolution
Biology: Ecology and Evolution 20551
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Date Created: 04/16/14
CHAPTER 23 SPECIATION SPECIES Species a group of organisms that can mate with one another and produce fertile offspring Speciation the divergence of biological lineages and the emergence of reproductive isolation between those lineages Species are the result of speciation Morphological species concept a construct that assumes that a species comprises individuals that look alike and that individuals that do not look alike belong to different species Linneaus used this Reproductive isolation a state in which two groups of organisms can no longer exchange genes Ernst Mayr proposed biological species concept Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups Reproductive isolation is what allows sexual species to evolve independently of one another Actually says that the individuals live in the same area and interbreed with one another Potentially says that even tough the individuals do not live in the same area and therefore do not interbreed other information suggests that they would do so if they were able to get together Lineage species concept species branch on the tree of life In this framework one species splits into two descendant species which thereafter evolve as distinct lineages A lineage species concept allows biologists to consider species over evolutionary time Embraces the idea that sexual species are maintained by reproductive isolation Lineage is an ancestordescendant series of populations followed over time GENETIC BASIS OF SPECIATION DozhanskyMuller model a single ancestral population is subdivided into two separate populations by some barrier to gene ow and these two populations then evolve as independent lineages In one of the populations a new allele A arises and becomes xed In the other population a new allele B becomes xed at a dg erent gene locus Neither new alleles at either locus results in any loss of reproductive compatibility However the two new forms of these two different genes have never occured together in the same individual or population BARRIERS TO GENE FLOW RESULT IN SPECIATION Allopatric speciation speciation that results when a population is divided by a physical barrier The dominant mode of speciation in most groups of organisms SYM PATRIC SPECIATION Sympatric speciation speciation without physical isolation Disruption selection is an example of sympatric speciation POLYPLOI DY Most common means of sympatric speciation is polyploidy which results from the duplication of sets of chromosomes within individuals Polyploidy can arise either from chromosome duplication in a single species autopolyploidy or from the combining of the chromosomes of two different species allopolyploidy An Autopolyplod individual originates when for example two accidentally unreduced diploid gametes each with two sets of chromosomes NEWLY FORMED SPECIES COME INTO CONTACT Reproductive isolation may be incomplete when incipient species come back into contact however in which case some hybridization will occur Selection will result in the strengthening or reinforcement of mechanism that prevent hybridization Mechanisms that prevent hybridization from occurring are called prezygotic isolating mechanisms Mechanisms that reduce the tness oh hybrid offspring are called postzygotic isolating mechanisms which result in selection against hybridization which in turn leads to the reinforcement of prezygotic isolating mechanisms PREZYGOTIC ISOLATING MECHANISMS PREVENT HYBRIDIZATION Mechanical Isolation Differences in the sizes and shapes of reproductive organs may prevent the union of gametes from different species Temporal Isolation If two closely related species breed at different times of the year or a different time of the day they may never have an opportunity to hybridize Behavioral Isolation Individuals may reject or fail to recognize individuals of other species as potential mating partners Habitat Isolation When two closely related species evolve preferences for living or mating in different habitats they may never come into contact during their respective mating periods Gametic Isolation The sperm of one species may not attach to the eggs of another species because the eggs do not release the appropriate attractive chemicals or the sperm may be unable to penetrate the egg because the two gametes are chemically incompatible POSTZYGOTIC ISOLATING MECHANISMS END IN SELECTION AGAINST ISOLATION Genetic differences that accumulate between two diverging lineages may reduce the survival and reproductive rates of hybrid offspring in any of several ways LOW hybrid zygofe Viabiiify Hybrid zygotes may fail to mature normally either dying during development or developing phenotypic abnormalities that prevent them from becoming reproductively capable adults LOW hybrid CICIUII39 vicibiIiIy Hybrid offspring may have lower survivorship than nonhybrid offspring Hybrid infertility Hybrids may mature into infertile adults RATES OF SPECIATION VARY Diet specialization Populations of species that have specialized diets may be more likely to diverge than are those with more generalized diets Pollination Speciation rates are faster in animalpollinated than in windpollinated plants Sexual Selection Mechanisms of sexual selection result in high rates of speciation Sexual dimorphism Male animals have ashier bodies than females of the same species Dispersal Ability Speciation rates are usually higher in groups with poor dispersal abilities than in groups with good dispersal abilities because even narrow barriers can be effective in dividing a species whose memebers are highly sedentary RAPID SPECIATION CAN LEAD TO ADAPTIVE RADIATION Evolutionary radiation the rapid proliferation of a large number of descendant species from a single ancestor species Often occur when a species colonizes a new area Adaptive radiation If a rapid proliferation of species results in an array of species that live in a variety of environments and differ in the characteristics they use to exploit those environments
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