Chapter 8 Flashcards
- two main features affecting allele frequency
Genetic Drift and Natural Selection
random, nonadaptive evolution
Genetic Drift
Happens through adaptation
Natural Selection
all genes are subject to this
Genetic Drift
all genes are not subject to this
Natural Selection
largely developed by Sewall Wright in the 30’s and Motoo Kimura in the 60’s
Theory of Genetic Drift
the difference between your sample and the real population
Sampling error
more than likely the small population will go to 0
Genetic Drift
small independent population that interbreeds with one another
Deme
set of Demes
Metapopulation
May start out the same say p=0.5
But random changes may push one way or another
One deme goes one way & another may go the other way
Thus originally identical demes may become different
Random Fluctuations
In a finite population, allele frequencies fluctuate even without NS
Genetic Drift
Some alleles fixed, others lost, homozygotes increase
Genetic Drift
Separate populations diverge
Genetic Drift
Genetic Drift
Easier to alter large or small population sizes?
small
most populations will go to..
fixation
even large populations diverge from same…
starting point
leads to homozygosity
-random, it could be good or bad
Fluctuations and population size
example of loss of Heterozygosity
New zealand snapper fishery
- overexploited
- drift taken over
- fewer heterozygotes
- but 3,000,000 individuals!!
- few reproduce
how many bread
effective population size ( 200 in new zealand fishery )
will alter it - only large ones mate for example
Natural Selection
may allow offspring to mate with parents
Overlapping generations
small repeats in DNA
Microsatallites
evolve rapidly, and selectively neutral
Microsatallites
can track population
Microsatallites
formed from slippage-induced mutation
Microsatallites
sea levels down, islands connected
Glaciation
drift is stronger in small populations?
true
history of a particular gene
Gene trees
Looking backwards, the alleles come together (coalesce) to a single individual
Coalescence
Chance: small populations will become monomorphic
Coalescence
Not all individuals have as many offspring
Coalescence
time to coalescence
2N
-N is population size in generations
Cannibalistic bugs – only 1 survives when they meet
When there are a lot, they run into each other a lot
When few, it is less likely
Bugs in a Box ex of coalescence
will quickly coalesce to beneficial mutation
positive selection
no coalescence
balancing selection
neutral drift
arises and slowly replaces the ancestral allele by drift
Multiregional Hypothesis
Multiple origins from H. erectus-like ancestor
Coalescence and Human Origins
Out of Africa – single origin of anatomically modern humans
Coalescence and Human Origins
Equal variation within populations
Africa, Europe, and Asia would all be monophyletic
Mutiregional
Greater variation in Africa
African populations non-monophyletic, some closer to Europe+Asia
Out of Africa- supported
- African lineages
- sequential sister taxa to rest of world
- African lineages with far -more genetic diversity
- Little divergence in rest of humans
Used coalescence to find
mitochondrial Eve
mitochondrial Eve
Ancestor of all human mitochondria
Estimates are about 156,000-250,000 YA
- This ancestor existed prior to the divergence of modern populations
- Values much more recent than if human populations descended separately from archaic Homo sapiens
Mitochondrial Eve
- Support out of Africa Hypothesis
- Variation and mutation rates leads to effective breeding population of 4,600-11,200 people
Mitochondrial Eve
- Must have been a fairly cohesive group in Africa
- Could not have been spread around because too few of them to form cohesive species if also in Europe and Asia
Mitochondrial Eve
-Brief period of small population size
-Only some make it through
Reduce variation
Population bottlenecks
No variation in 62 enzyme-coding loci
Unusually high homozygosity for a natural population
More than in Southern Elephant seal
30,000 individuals now
But 20 in the 1890’s
And even lower effective size because only 20% of males reproduce
example of population bottlenecks
-museum samples from before the bottleneck had greater variation
Change in allele frequencies as a result of the small number of individuals that colonize a region
Founder effect
Retreating glaciers created habitat
Spruce that colonized new habitat from… what?
leading edge
- nuclear DNA
- mitochondrial DNA
- Dispersed by wind, but not far
seeds
- nuclear DNA
- Long distance dispersal by wind
pollen
Gamache et al. did 1000k N-S transect
Leading Edge Founder Effect
Mitochondrial – 1 haplotype in N
Leading Edge Founder Effect
Nuclear same types as in S
Leading Edge Founder Effect
High between population variability throughout N for mitochondria
Leading Edge Founder Effect
Hallmark of drift
Leading Edge Founder Effect
Nuclear not variable between sites
Leading Edge Founder Effect
believed Slightly beneficial allele with fitness of 1+s arises in large population
Haldane
Fixation probability determined to be
2s
population large, effects strong
selection dominates
selection week, population small
drift dominates
for initial mutation, population ________
drops out
s>1/2Ne
drift
s<1/2Ne
selection
PA with highest fitness
Overdominance
introduced a tremendous amount of variation
cross-breeding
drift only important in _____ generation
first
_____ not different in large vs. small treatments as a whole
%P
variation was greater in the
small population
led to discovery of differences of amino acid sequence without phenotypic differences
electrophoresis
Too much variation to explain by NS
Neutral theory of molecular evolution
are neutral, not mutations
substitutions
generally are deleterious and are purged from the genome by NS
mutations
More closely related things had fewer differences
AA clock
all members of clade should be equally distant from outgroup
Genetic equidistance
-So a bird, a lizard, a human, and a frog should have the same number of differences from a fish
Multiple hits at same site – more likely with time
- More likely at 3rd position
- By applying what is known, we get a halfway decent clock
Saturation
Appears to not have an effect
Molecular clock and fossil divergence dates the same in mammals
Generation time
