Physical Isolation as a Barrier to Gene Flow.docx

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Department
Biomedical Engineering
Course Code
BMEN 515
Professor
William Huddleston

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Description
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 divergence  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 areas 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: o Small populations that become isolated start out as a non-random sample of the ancestral population. As drift continues to occur in the small, derived population, it leads to a random loss of allele and the random fixation of existing and new alleles. As a result, the isolated population should undergo rapid genetic divergence from the ancestral population.  Bottleneck- a large-scale but short-term reduction in population size followed by an increase in population size o Has shown that when a population is reduced to a small size for a short period of time only very rare alleles tend to be lost due to drift  For drift to change allele frequencies dramatically, the founding population has to be extremely small and remain small for a significant period of time Natura
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