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BIOL 2400 (32)
Chapter 13.1

Chapter 13.1 to 13.4.docx

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University of Guelph
BIOL 2400
Cortland Griswold

Chapter 13: The Origin of Species 13.1 What is a Species?  Within a species, alleles flow among populations, evolutionary forces affecting one population also affect other populations  Parts of the same whole, evolving as one linage; no cohesion between species  Phylogenetic species concept – species are the smallest possible groups whose members are descended from a common ancestor and who all possess defining or derived characteristics that distinguish them from other such groups  Biological species concept – Species are groups of actually (or potentially) interbreeding natural populations that are reproductively isolated from other such groups  Metapopulation – a group of spatially separated populations of the same species that interact at some level (e.g., exchange alleles)  General lineage species concept – species are metapopulations of organisms that exchange alleles frequently enough that they comprise the same gene pool, and therefore, the same evolutionary lineage 13.2 Barriers to Gene Flow: Keeping Species Apart  Isolating barrier – an aspect of the environment, genetics, behaviour, physiology, or ecology of a species that reduces or impedes gene flow from individuals of other species. Isolating barriers can be geographic or reproductive  Geographic barriers to gene flow are features of the environment that physically separate populations from each other  Most common barrier to gene flow  Populations do not interbreed simply because they are geographically isolated from each other (i.e., they exist in allopatry)  Allopatry – occurs when populations are in separate, non-overlapping geographic areas  Sympatry – occurs when populations are in the same geographic area  Separation of populations in space or in time reduces the likelihood they will exchange gametes  Divergent behaviour (e.g., habitat preference, spawning time) acts as a reproductive barrier to gene flow  Reproductive barriers to gene flow are intrinsic properties of organisms that reduce the likelihood of interbreeding between individuals of different populations  May also play important roles in keeping some species apart  Reproductive isolation – occurs when reproductive barriers prevent or strongly limit reproduction between populations. The result is that few or no genes are exchanged between the populations  Floral traits can act as reproductive barriers to gene flow when their divergence causes them to attract different pollinators  Molecular incompatibilities between eggs and sperm can be important barriers to gene flow between species, causing conspecific sperm to be more likely to fertilize eggs than heterospecific sperm  Gametic incompatibility – occurs when sperm or pollen from one species fails to penetrate and fertilize the egg of another species  Genetic incompatibilities can be important barriers to gene flow between populations if they cause hybrid offspring to be sterile or to perform poorly  Prezygotic reproductive barriers – aspects of the genetics, behaviour, physiology, or ecology of a species
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