Download 17-2 Mechanisms of Genetic Change

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Transcript
Sources/Mechanisms of Evolutionary Change
 1. Natural Selection
 2. Mutations
 3. Gene Flow (migration)
 4. Genetic Drift (bottleneck & founders effect)

All of these mechanisms can cause changes in the frequencies of genes in populations

Natural selection and genetic drift cannot operate unless there is genetic variation
(mutations, gene flow and sexual reproduction)

If the population of beetles were 100% green, selection and drift would not have any
effect because their genetic make-up could not change.
 Imagine that green beetles are easier for birds to spot
(and hence, eat). Brown beetles are a little more likely
to survive to produce offspring. They pass their genes
for brown coloration on to their offspring. So in the
next generation, brown beetles are more common than
in the previous generation.
 A mutation could cause parents with genes for bright
green coloration to have offspring with a gene for
brown coloration. That would make genes for brown
coloration more frequent in the population than they
were before the mutation.
 Exchange of genes due to mixing of populations
 Some individuals from a population of brown beetles
might have joined a population of green beetles. That
would make genes for brown coloration more frequent
in the green beetle population than they were before
the brown beetles migrated into it.
 Random change in allele frequencies
 Some individuals may, by chance, leave more
descendants
 Next generation “lucky” genes not necessarily better
adapted
 Unlike natural selection  b/c it is random & doesn’t
work to produce adaptations
 Occurs when populations size is reduced for at least 1
generation
 Can reduce populations genetic variation a lot
 May not be able to adapt to new selection pressures
 Northern elephant seals
 have reduced genetic variation
 Hunting reduced their population size in 1890’s to as few as
20 individuals
 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
 when a new colony is started by a few members of the
original population
 small population size means that the new colony may
have reduced genetic variation from the original
population
 Founding individuals may carry alleles that differ in
frequency from original population
 G. equilibrium- allele frequencies remain same; not
evolving (this is hypothetical)
 Hardy-Weinberg Principle- allele frequencies of
population should remain constant unless :
 1. non-random mating occurs (sexual selection- choose




mate with certain trait) (or)
2. small population size (or)
3. Immigration or emigration (or)
4. Mutations (or)
5. Natural selection (if different genotypes have different
fitness)
 One or more of these usually hold true for any
population therefore, evolution will occur!