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Lecture 17

Biology 1001A Lecture 17.docx

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Biology 1001A
Tom Haffie

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Biology 1000A Lecture 17: Phylogeny Recall from Lecture 16: Speciation  Achieving Reproductive Isolation  Usually occurs in allopatry  Isolation  Divergence  Important process because biodiversity is formed through this  We would not achieve a ‘tree of life’  Speciation is when 2 populations become separated from each other that diverge apart so much that they cannot interbreed with each other. o This prevents gene flow between 2 populations  How is this separation achieved? o One is geological separation, the populations are separated and become allopatric  i.e. Glacier, river, etc…  Geographic ranges are separated, do NOT overlap o Allotropy can happen if a founder effect happens  i.e. if a small population from the main land swim, fly, and etc, across the ocean to an island.  Ocean is the barrier  Selection can cause populations to diverge, different from each other o Genetic Drift can also cause populations to differ  Isolation and Divergence combination can cause populations to be vary enough, so that they can not interbreed  Sometimes Secondary Contact and Reinforcement can occur o Does NOT need to happen o Biological species concept allows us to test to see if these 2 populations can SUCCESSFULLY interbreed  Glacier melts, no physical barrier separating populations, and can see if populations can successfully interbreed Islands: Hotbeds of Speciation  Isolation, small populations, founder effects, different selection pressures  Animals colonize new islands from mainland o Physically isolated from other lands because of water o Small Islands, populations small  Genetic Drifts are very strong  Cause populations to diverge from one another o Could be Founder effects  Individuals on island will have different allele frequencies compared to ancestors on the mainland  Genetically different o Different selection pressures on islands compared to mainland pressures  Causes DIVERGENCE Secondary Contact  If populations haven’t diverged much, 2 Possible outcomes: o 1) Populations resume interbreeding  Divergence and speciation stops, and population collapses (hybrid swarm)  They go from 2 populations back into one. o 2) If populations have been apart for a long time, there could a lot of divergence between 2 populations, then it’s likely genetic differences may have risen due to chromosomal incompatibility (i.e. diff. in chromosome numbers)  Results in post-zygotic isolating mechanisms, if populations with diff. chromosome numbers interbreed, the hybrid offspring are going to be sterile (low fitness)  If this diverged population begins to interbreed with members with another population, they are quickly going to be selected against, since their hybrid offspring will have very low fitness, because of post-zygotic isolation mechanisms  When this happens, we see reinforcement  Reinforcement describes a situation where selection tends to favour the devolvement of PRE-zygotic isolating mechanisms o Post-zygotic isolating mechanisms are much more COSTLY, in terms of fitness of parent individuals.  Controversial whether post-zygotic or pre-zygotic isolating mechanisms usually evolve first Speciation without Allopatry?  Not all speciation happens in allopatry  Sometime populations may be right next to each other (geographically), when speciation occurs o Parapatric Speciation  A situation where population A is right next to population B, not divided like in Allopatry  Sometimes speciation occurs in parapatry  Not OVERLAPPING, but neighboring, therefore, could be SOME gene flow across the border, therefore, parapatry less powerful than allopatry  Not easy for parapatry to happen. Gene flow tends to keep populations together  Parapatry happens in a very patchy environment, intermediate forms (hybrids) selected against  I.e. Grass growing in a polluted/unpolluted areas (patchy area)  Again, selects for pre-zygotic isolation o Sympatric Speciation (overlapping ranges)  Rare in Vertebrates
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