topic 8 - genetic drift Flashcards
describe HW eq - why might a pop not be in it
- Hw equilibrium
- Infinite populations
- Gene frequencies remain constant over generations
- What if pops are finite? Or there is a sampling error?
FOUR major factors alter allele
frequencies and bring about most of the
evolutionary changes?
mutation
○ Natural selection
○ Genetic drift
○ Gene flow
mechanisms - assumptions at hq?
random mating, no immigration, genetic drift, mutation, or NS.
no alteration
drift alteration?
imperfect sampling causes some alleles to be underrepresentative relative to others
natural selection alteration to allele freq
environmental factors are unfavorable for certain alleles
migration/gene flow alteration to allele freq
individuals with a new allele enter the pop
mutation alteration to allele fre
one allele is altered
• Natural selection definition
• Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions
• Genetic drift defintion
when is it more likely? random or?
• Process of random fluctuation in allele frequencies
due to sampling effects in finite populations - dr das defintion
• Genetic drift results from the influence of
chance. When population size is small, chance
events more likely to have a strong effect
what does genetic drift describe and alter in terms of allele freq?
• Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to
the next
• Genetic drift tends to reduce genetic variation
through losses of alleles - some lost, others fixed
when can an allele be fixed quicker
Smaller population sizes cause bigger random changes in allele freq. may lead to quicker fixation of one allele or the other Alleles are lost c more rapidly in small populations
A Magnitude of fluctuations depends on? large vs small?
• A Magnitude of fluctuations depends on population
size
○ Large population: Small fluctuations
○ Small population: Large fluctuations
○ Inverse relationship
• Mean time to fixation or loss also depends on? large vs small
• Mean time to fixation or loss also depends on
population size
○ Large population: Long time
○ Small population: Short time
Coalescence ?
• Coalesce = come together to form one
mass or whole
• Coalescence = Model of distribution of
gene divergence in a genealogy
• Possible history of descent of gene
copies in a population representing two
alleles
describe ex of coalescence
• Gene copies by generation 7 descended from a single copy in generation 0; others went extinct • If failure to leave offspring is random, then gene copies in generation 7 could equally have come from any of the original gene copies • One can trace all alleles of a gene shared by all members to a single ancestral copy - most recent common ancestor
coalescence and geanology?
• We can trace the descendants of a gene just like a haploid organism. • If we look back in time, all the current gene copies shared a single common ancestor.
* * THE GENEALOGY OF THE PRESENT SEQUENCES COALESCES TO A SINGLE COMMON ANCESTOR.
how does coalescence occur
due to the random extinction of lineages.
• Eventually, in the absence of new mutation, coalescence
will result in the fixation of a single allele in the population.
• Derivation of the gene copies in one or more
populations from a single ancestral copy,
viewed retrospectively
• As a result of coalescence, a population will
eventually become monomorphic for one allele
or the other, and that one allele will be fixed
(= reaching a freq of 1, i.e., 100%) rather than
the other, equals the initial frequency of that
allele
• Evolution by genetic drift - how?
• Allele frequencies fluctuate @ random within a population
• Genetic variation @ a locus declines and is eventually lost
• @ any time, an allele’s probability of fixation = its frequency,
and is not affected/predicted by its previous history or change
in frequency
• Populations with same initial allele frequency (p) diverge and
a portion of p is expected to become fixed for that allele
• A new mutation is more likely to be fixed in a small population
than in a large population
• Evolution by genetic drift proceeds faster in small than in
large populations
• Effects of genetic drift can be very strong when compounded
over many generations
• Consequences of genetic drift
• No allele is more fit than any other (no natural selection)
○ Drift is random with respect to fitness
• BUT, some alleles clearly “won” the reproduction lottery
○ They randomly increased their frequency in the population
• In finite populations equally fit alleles are at risk of
disappearing = loss
• Over time drift can produce random loss or fixation of alleles
• Genetic drift produces a steady decline in heterozygosity = loss of variation
•
when does founder affect occur
• Founder effect occurs when a few individuals
become isolated from a larger population
founder effect consequence
Allele frequencies in the small founder
• population can be different from those in the
larger parent population
• Loss of genetic diversity due to colonization of new
habitat by few individuals with a random & reduced
sample of alleles from the source population
where does founder effect typically occur?
○ Islands and island like habitats
○ Caves, ponds, mountain-top forests, alpine meadows
ex silvereye island hopping bird
• Study suggests a gradual decline in allele freq. following the path of the pop.
look at zebra finch ex
ok
bottleneck effect?
• Bottleneck effect is a sudden reduction in
population size due to a change in the
environment
• Resulting gene pool may no longer be reflective
of the original population’s gene pool
• If the population remains small, it may be
further affected by genetic drift
