11/14/2014. What is a species? Species and speciation. The biological species concept (BSC) emphasizes reproductive isolation

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1 Species and speciation What is a species? Chapters 17 & 18 The biological species concept (BSC) emphasizes reproductive isolation Gene pools of biological species are isolated by pre- and post-zygotic barriers A biological species is defined as a population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring but cannot do so with members of other species The species is the largest unit of population in which gene flow is possible 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 Pre-zygotic barriers 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 1

2 Behavioral isolation Behavioral isolation Species-specific signals and elaborate behavior to attract mates 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 Mechanical isolation 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 2

3 Conspecific sperm precedence Post-zygotic isolating mechanisms Conspecific sperm wins in multiple matings Reduced hybrid viability: Genetic incompatibility may abort development at embryonic stage Reduced hybrid fertility: Species mate and hybrid is viable but sterile Post-zygotic isolating mechanisms Hybrid breakdown: First generation hybrids are fertile but subsequent generations are defective 3

4 How species are diagnosed Premating isolation is a stronger barrier to gene exchange than postzygotic isolation in sympatric taxa Morphology Phenotypic characters Molecular characters Genes cause reproductive isolation Epistatic interactions cause hybrid sterility Genes have been identified that have an effect on sperm motility in hybrids Chromosome differences 4

5 Speciation Mechanisms of speciation Allopatric speciation Vicariance Peripatric speciation or founder effect speciation Sympatric speciation Parapatric speciation Allopatric speciation Populations are first geographically isolated, then become reproductively isolated Species separated by geographic barriers will diverge genetically If divergence is great enough it will prevent breeding even if the barrier later disappears Effectiveness of barrier varies with species Isolation is correlated with genetic distance Historic isolation 5

6 Reproductive isolation may result from adaptive divergence of two populations Apple maggot fly Rhagoletis pomonella Rhegoletis pompella Native to Midwest and northeastern US Relied on native hawthorne fruit trees Adults mate on the fruit, females lay eggs in the fruit Flies began using apple trees sometime after they were introduced from Europe ~ 300 years ago Selection has favored distinct habitat preferences in apple and hawthorn flies There appears to be an increase in fitness from switching to apple trees: Escape from parasitoids: The average level of wasp parasitism is 70% less on apple than on hawthorne Escape from intraspecific competition due to the large size of apple fruit Escape from interspecific competition Sexual selection drives speciation More species in sister group with promiscuous mating systems 6

7 1 Peripatric speciation Peripatric or founder effect speciation 1 2 Genetic drift in founder population Sympatric speciation by polyploidy Change in chromosome number (3n, 4n, etc.) In autopolyploids, offspring have a doubling of chromosome number from parents Allopolyploids are interspecific hybrids Offspring with altered chromosome number cannot breed with parent population Common mechanism of speciation in flowering plants 7

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