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Species concepts Darwin’s other stumbling block How do new species arise? i.e., speciation The nature of species The concept of species must account for two phenomena: The distinctiveness of species that occur together at a single locality The connection that exists among different populations belonging to the same species Speciation The process by which new species arise, either by transformation of one species into another by the splitting of one ancestral species into two descendant species Sympatric speciation The differentiation of populations within a common geographic area into species Species that occur together: Are distinctive entities Are phenotypically different Utilize different parts of the habitat Behave separately Subspecies Within a single species, individuals in populations that occur in different areas may be distinct from one another Overlap of subspecies The Biological Species Concept Ernst Mayr’s biological species concept defines species as… “…groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” In short, members of a population mate with each other and produce fertile offspring Reproductive isolation is therefore key to the BSC Gene pools of biological species are isolated Any factor that impedes two species from producing viable, fertile offspring contributes to reproductive isolation Various barriers classified by whether they function before or after zygote formation: Pre-zygotic barriers Post-zygotic barriers Geographic isolation Deer mice Ecological isolation Two species living in different habitats may not encounter each other: Two species of garter snake (Thamnophis) occur in the same area but one species lives in water and the other is terrestrial Lions and tigers are ecologically isolated In captivity lions and tigers can mate and reproduce offspring that survive Behavioral isolation Species-specific signals and elaborate behavior to attract mates Sympatric species avoid mating with members of the wrong species in a variety of ways, including differences in: Visual signals Sound production Chemical signals: pheromones Electrical signals: electroreception Behavioral isolation Many animals recognize mates by sensing pheromones: Female gypsy moths emit a volatile compound to which olfactory organs of male gypsy moths are specifically tuned Behavioral isolation Eastern and western meadowlarks only recognize songs of the same species Specific courtship rituals Temporal isolation Temporal isolation: Brown trout breed in the autumn whereas rainbow trout living in the same streams breed in the spring Mechanical isolation Clasping appendages in dragonflies Floral anatomy corresponding to specific pollinator Genital arch in Drosophila Gametic isolation Sperm of one species may not survive internal environment of female reproductive tract in another species Conspecific sperm precedence Conspecific sperm wins in multiple matings Post-zygotic isolating mechanisms Reduced hybrid viability: Genetic incompatibility may abort development at embryonic stage Reduced hybrid fertility: Species mate and hybrid is viable but sterile Sterile hybrids Post-zygotic isolating mechanisms Hybrid breakdown: First generation hybrids are fertile but subsequent generations are defective Shortcomings of Biological Species Concept 50% of California plant species, in one study, were not well defined by genetic isolation Shortcomings of Biological Species Concept Hybridization is not uncommon in animals 10% of bird species have hybridized in nature Hybrid offspring of Galápagos finches appeared to be at no disadvantage for survival or reproduction Reproductive isolation may not be the only force for maintaining the integrity of species Ecological Species Concept Theories & criticisms Hybridization has little effect because alleles introduced into one species’ gene pool from other species are quickly eliminated by natural selection Difficult to apply biological species concept to populations that are geographically separated in nature Many organisms are asexual and do not mate Other species concepts