08_Speciation - SPECIATION Reading: Chapter 24; Chapter 25,...

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SPECIATION Reading: Chapter 24; Chapter 25, pp. 523-525 A. HOW ARE SPECIES DEFINED AND DISTINGUISHED? Wherever possible, biologists mark species boundaries by the biological species concept : "Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." – Ernst Mayr This definition does not cover all cases (fossils, asexual organisms), but is useful when it does apply. Its chief advantage is that it implicitly links species definition to evolution; interbreeding organisms would experience gene flow between populations, and chart an evolutionary course distinct from that of species with which it did not share genes. In other words, by this definition, species are genetically integrated, independent evolutionary units. B. REPRODUCTIVE BARRIERS BETWEEN SPECIES 1. The biological species concept, and its relationship to evolution, implies that it is important to understand barriers to reproductive between species and how they come about. 2. We define successful reproduction not only as successful mating, but the production of viable, fertile offspring. In other words, successful reproduction is open-ended; the lineage can continue indefinitely. 3. Some barriers to successful reproduction act before the point where a zygote is formed. These are called, logically, pre-zygotic barriers . Some types of barriers: ± geography – two populations might be separated by large distances or geographic barriers. ± habitat differences – two populations might exist in the same area, but occupy different habitats in that area and seldom or never encounter each other. ± behavioural differences – some species have elaborate behavioural traits that exclude outsiders; this is especially true in mating behaviour. ± temporal differences – some species might be kept apart by temporal considerations, such as two plants that flower in different seasons.
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Speciation - 2 ± anatomical / mechanical differences – often there must be a physical match for mating to proceed; a match between animal genitalia, for example, or between a flower and its pollinator. ± failure of gamete recognition – gametes require molecular handshakes at their surfaces for fertilization to proceed; incompatible surface glycoproteins will stop the process of cell fusion. 4. Other barriers to successful reproduction act after zygote formation. These are post-zygotic barriers . ± inviability of hybrids – an interspecific hybrid may form, but be too weak to survive in nature. ± infertility of hybrids – an interspecific hybrid can be viable but sterile; usually, it is because the homologues from each parent are too distantly related to pair in prophase I of meiosis, and hence unbalanced chromosome numbers in gametes result. C. ALLOPATRIC SPECIATION
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08_Speciation - SPECIATION Reading: Chapter 24; Chapter 25,...

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