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Lecture 6

BIOLOGY 1M03 Lecture Notes - Lecture 6: Continental Drift, Plate Tectonics, Sympatry

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Jon Stone

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BIOLOGY 1M03- Chapter 26: Speciation Part 2
26.2 Isolation and Divergence in Allopatry
Genetic isolation happens routinely when populations become separated physically
Physical isolation, in turn, occurs by dispersal or vicariance
Dispersal occurs when a population moves to a new habitat, colonizes it, and forms a
new population
Vicariance occurs when a physical barrier splits a widespread population into subgroups
that are physically isolated from each other
Speciation that begins with physical isolation via either dispersal or vicariance is known
as allopatric speciation
Populations that live in different areas are said to be in allopatry
Biogeography a research field in which geographic distributions for species and
populations are analyzed describes how colonization and range-splitting events occur.
Dispersal and colonization isolate populations
Colonization events often cause speciation because the characteristics for the
colonization population are likely to be different from the characteristics for the source
Physical separation reduces gene flow, and genetic drift via the founder effect which
causes old and new populations to diverge rapidly
Natural selection may cause divergence if the newly colonized environment differs from
the original habitat
Vicariance isolates populations
Vicariance events during the last ice age, when glaciers served as physical barriers, are
thorough to have been responsible for the origin of many modern species
Over the longer term, continental drift (continental plate movements explained by the
theory of plate tectonics) separated species physically
Physical isolation of populations via dispersal or vicariance produces genetic isolation,
the first requirement for speciation
When genetic isolation is accompanied by genetic divergence due to natural selection,
genetic drift, mutation, and possibly biased mating; speciation results
26.3 Isolation And Divergence In Sympatry
Populations or species that live in the same geographic region (close enough to make
interbreeding possible) live in sympatry
Researchers used to think that speciation involving sympatric populations was
impossible because gene flow is possible
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