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Population genetics
Vocabulary
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Microevolution
Average heterozygosity
Geographic variation
cline
Genetic drift
• Chance events cause allele frequencies to
fluctuate unpredictably from one generation
to the next , especially in small populations
In each generation, some individuals may, just by
chance, leave behind a few more descendents (and
genes, of course!) than other individuals. The genes of
the next generation will be the genes of the “lucky”
individuals, not necessarily the healthier or “better”
individuals. That, in a nutshell, is genetic drift. It
happens to ALL populations—there’s no avoiding the
vagaries of chance.
Genetic drift affects the genetic makeup of
the population but, unlike natural
selection, through an entirely random
process. So although genetic drift is a
mechanism of evolution, it doesn’t work
to produce adaptations.
Bottleneck effect
A sudden change in the environment
drastically reduces the pop. size
By chance alone certain alleles survive, while
others are eliminated
Therefore some alleles may be
overrepresented among the survivors; some
may be underrepresented; some may be
absent altogether
Founder Effect
• When a few individuals colonize a new
area, they may establish new pop.
• The gene pool of this group may differ from
the gene pool of the original pop.
Genetic Drift and the Founder Effect
Polydactyly -- extra fingers or sometimes toes -- is one symptom
of Ellis-van Creveld syndrome. The syndrome is commonly found
among the Old Order Amish of Pennsylvania, a population that
experiences the "founder effect." Genetically inherited diseases
like Ellis-van Creveld are more concentrated among the Amish
because they marry within their own community, which prevents
new genetic variation from entering the population. Children are
therefore more likely to inherit two copies of the particular
recessive genes that lead to genetic disease.
Northern elephant seals have reduced
genetic variation probably because of a
population bottleneck humans inflicted on
them in the 1890s. Hunting reduced their
population size to as few as 20 individuals
at the end of the 19th century. Their
population has since rebounded to over
30,000—but their genes still carry the
marks of this bottleneck: they have much
less genetic variation than a population of
southern elephant seals that was not so
intensely hunted.
Gene flow
• The transfer of alleles into or out of a pop.
Due to movement of fertile individuals or
the gametes
Directional selection
Disruptive selection
Stabilizing selection
Sexual selection
• Individuals with certain inherited
characteristics are more likely than other
individuals to obtain mates
• Can result in sexual dimorphism- obvious
differences between the two sexes
Runaway sexual selection posits that extreme male
traits (such as the male peacock's tail, or the huge
antlers of the now-extinct Irish Elk) can evolve
through a process in which the male trait and the
female preference for that trait become genetically
linked. The male trait does not necessarily have to be
beneficial to the male in any way, all it has to do is to
attract females.
Why did the Irish elk
go extinct?
Diploidy and heterozygote
protection
• Recessive alleles may be hidden
• Heterozygote protection maintains a huge
pool of alleles that might be favorable under
different environmental conditions
Heterozygote advantage
• Heterozygotes have a greater fitness
• Ex: sickle cell
Frequency-dependent selection
• The fitness of a phenotype declines if it
becomes too common in a pop.
Neutral variation
• Nucleotide differences that confer no
selective advantage or disadvantage
Why are there no perfect
organisms?
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Selection can only act on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection and the
environment interact