Lectures 22-24.Speciation Study Questions.2013

Incompatibility even if mating succeeds gamete

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Unformatted text preview: mnetic forms of 3- spine sticklebacks c) Temporal Isolation: species that breed at different times (daily, seasonally, or yearly) have limited opportunity to mate 1) Example: periodic cicadas that breed in different prime- numbered years d) Behavioral Isolation: species- specific mate recognition systems (courtship behaviors, songs/acoustical signals, chemical signals/pheromones, visual signals) may effectively limit mating between species 1) Example: species- specific flashing and flight patterns in fireflies e) Mechanical Isolation: anatomical mismatches between males and females may make the physical act of copulation impossible (or close enough) 1) Example: mismatched genitalia in robber flies f) Gametic Incompatibility: even if mating succeeds, gamete recognition systems prevent fertilization between gametes from different species 1) Example: Gamete recognition in purple vs. red sea urchins 2. Postzygotic Mechanisms: Act after fertilization has occurred a) Hybrid Inviability: zygotes are formed, but die before developing into adults 1) Example: Rana sylvatica x Rana pipiens b) Hybrid Sterility: hybrid zygotes develop into adults, but the adults are sterile (genes “flow” into hybrids but remain trapped there) 1) Example: Zebra (male) x Horse (mare) ⇒ Zorse (Sterile) c) Hybrid Breakdown: fertile hybrids are formed but they have very low mating success (or backcrosses produce offspring with low fitness) 1) Example: Mallard x China Duck ⇒ Challard (or Muck) IV. THE EVOLUTION OF REPRODUCTIVE ISOLATION 1. How can a trait that almost by definition reduces the fitness of its bearer increase in frequency? 2. The Problem: Gene flow (even a little bit) prevents population differentiation. a) On average, the exchange of one (1) migrant per generation between populations is sufficient to prevent those populations from diverging. b) There must be some EXTRINSIC barriers that initially limit gene flow. 3. The importance of phylogenies: a) A phylogeny is a graphical depiction of the history of relationships among a group of organisms. b) Key terms & concepts: 1) Sister species: a pair of species that share a single most recent common ancestor – they are each other’s closest relatives 2) Node: a branching point on a phylogeny – nodes are the result of a speciation event in the past 3) Monophyletic clade: a taxonomic group that includes...
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