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BMEN 515 Lecture Notes - Genetic Drift, Reproductive Isolation, Allele Frequency

Biomedical Engineering
Course Code
BMEN 515
William Huddleston

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Physical Isolation as a Barrier to Gene Flow
Geographic isolation produces reproductive isolation, and thus genetic isolation
Allopatric Model- the hypothesis that speciation occurs when populations become geographically isolated and diverge
because selection and drift act on them independently
Essence of allopatric speciation is that physical isolation creates an effective barrier to gene flow
Geographic isolation has been an important trigger for the second stage in the speciation process: genetic & ecological
Geographic isolation can come about through dispersal and colonization of new habitats or through vicariance events,
where an existing range is split by a new physical barrier
Dispersal is when a population in one area splits into two and one moves to another isolated area like from mainland to
an island
Vicariance is when a large area is populated and some natural cause occurs and divides the area into two separate
lands, dividing one from the other by nature
Look at page 612 for better understanding.
The dispersal-and-colonization hypothesis makes two predictions based on these facts:
o Closely related species should almost always be found on adjacent islands
o At least some sequences of branching events should correspond to the sequence in which islands were formed
Dispersal to novel environments has proven to be a general mechanism for initiating speciation
Populations can become geographically isolated when individuals colonize a new habitat
Vicariance events split a species’ distribution into two or more isolated ranges and discourage or prevent gene flow
between them
Many possible mechanisms of vicariance, ranging from slow processes such as the rise of a mountain range or a long-
term drying trend that fragments forests, to rapid events such as a mile-wide lava flow that bisects a snail population
Populations can also become geographically isolated when a species’ former range is split into two or more distinct
A Role for Mutation: Polyploidy and Other Chromosome Changes as a Barrier to Gene Flow
Changes in chromosome number isolate populations genetically
Theory predicts that populations may speciate after becoming physically isolated due to dispersal or vicariance, and
data have confirmed that these events are common triggers for speciation
It is entirely possible for speciation to occur in the absence of physical isolation between populations
o Mutations that result in polyploidization can produce instant reproductive isolation between parental and
daughter populations
Mechanisms of Divergence
Dispersal, vicariance, and polyploidization only create the conditions for speciation
o For speciation to take place, genetic drift & natural selection have to act on mutations in a way that creates
divergence in the isolated populations
Genetic Drift
Effects of genetic drifts are random fixation of alleles and random loss of alleles
Drift can produce rapid genetic divergence in small, isolated populations
o Its effects are most pronounced in small populations
Normally, only tiny numbers of individuals are involved in colonization events; vicariance events fragment large
populations into two or more smaller ones; and polyploidization initially produces only a handful of individuals
o Due to this matter, genetic drift has long been hypothesized as the key to speciation’s second stage
A general model of speciation:
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