Download TODAY. . . Selection Directional Stabilizing Disruptive More HW

TODAY. . .
Selection
Directional
Stabilizing
Disruptive
More HW
Genetic Drift
Allele fixation
Selection
• Natural: Increase in the frequency of alleles that
increase reproductive success
• Sexual: A specific type of natural selection;
increase in the frequency of alleles that enhance
one’s ability to obtain mates
Something to keep in mind. . .
We have simplified what is really going on with respect
to genetic diversity! There may be many alleles for the
same gene in any given population.
Homework question
1. Construct a version of the Hardy-Weinberg
equations that could be applied to a gene with three
alleles.
Genetic variation
• The number and relative frequency of alleles
that are present in a population
• Some types of selection increase variation,
other types reduce it.
• To the extent that phenotype is determined by
genotype, a reduction in phenotypic diversity will
reduce genetic diversity
4. In a population of leatherback sea turtles, two
alleles that are incompletely dominant exist for a
gene that affects shell thickness and weight (S –
thick, heavy shell; s – thin, light shell). You sample
100 individuals from the population: 37 are SS, 8
are ss, and 55 are Ss.
A)Is this population in Hardy-Weinberg equilibrium?
Why or why not?
B) Create a hypothesis to explain why this population
is, or is not, as the case may be, in Hardy-Weinberg
equilibrium. Which Hardy-Weinberg assumptions
could be violated, if any?
Heterozygote advantage
When heterozygotes have a fitness advantage
over homozygotes, keeping genetic variation in
a population
More on Selection. . .
Directional Selection
• Allele frequencies
change in one direction
• Variation is reduced
• Mean value of trait
increases or decreases
Great Plains Cliff Swallows
• Population endures
extended cold weather
• Food source dies
• Swallows die
• Who’s left? Why?
• Body fat stores
helped carry them
through thin times
Stabilizing Selection
• Extreme phenotypes are
lost
(tails of distribution reduced)
– Reduces variation
• Mean value of trait remains
unchanged
Stabilizing Selection
• Birth weight in
humans
• High mortality rates
for very large and for
very small*
• Nutrition influenced?
Disruptive Selection
• Extreme phenotypes
are favored;
Phenotypes near mean
value are eliminated
• Mean does not change
• Distributions becomes
bimodal
– Variation increased
Disruptive Selection
• Black-billed
seedcrackers
• Large-billed individuals
specialize on large seeds,
small-billed on small
seeds
• Intermediate sizes
handle seeds
inefficiently
Genetic Drift
• Undirected change in allele frequencies
– due to sampling error (random mating)
• In very large populations, sampling error plays a
small role;
in small populations, it matters a lot!
Consequences of 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 (freq = 0)
– Over time drift can produce random loss or
fixation (freq = 1) of alleles
Consequences of Drift
Depends on. . .
• Population size!
– founder effect (colonization of new habitat by few
individuals with a random and reduced sample of
alleles from the source population)
– bottlenecks (rapid, large reduction in population
size)
• Probability that an allele will become fixed in the
population = frequency of that allele in the population
– @ each generation, probability of fixation changes!
Importance in natural
populations?
• Small: zoos, endangered species
• Large: pseudogenes and silent mutations
have no effect on fitness (invisible, for now, to
selection), but are an important source of raw
material for future selection events; Drift can
eliminate these from populations.
• Reduces genetic diversity
After allele fixation, the only way to increase
genetic diversity is to introduce new alleles!
How?
Gene Flow
(immigration and emigration of new alleles)
Effect on average fitness
– Genetic drift reduces allelic diversity, so the
arrival of new alleles might increase fitness
– BUT, If populations are well adapted to their
environment, then new (non-adaptive) alleles might
reduce average fitness
Mutation
1. The only creator of novelty (new alleles)
2. Restores genetic diversity
3. Fitness of mutations is random
a. Most mutations are deleterious (negative
effect on fitness)
b. Sometimes they’re beneficial, and may
spread through the population