Download Population Genetics

Population Genetics
Reconciling Darwin & Mendel
Darwin
• Darwin’s main idea (evolution), was accepted
• But not the mechanism (natural selection)
– Scientists did not understand Darwin’s mechanism
because there was no understanding of genetics
• Even once scientists grasped Mendel, genetics
was viewed as an either/or
– didn’t understand many traits are polygenic
• So how do you get the variation on which
selection works?
Ideas About Evolution
• Orthogenesis
– 1920’s
– saw evolution as a predictable progression
to more & more elite forms of life
• Population Genetics
– 1930’s
– reconciled Darwin & Mendel
Genetics of Populations
• Population
– a localized group of individuals belonging to same
species
– The definition of a species not always clear
• Gene pool = The total genes in a population
• Evolution on the smallest scale occurs when
the relative frequency of alleles in a
population changes over a succession of
generations = microevolution
Genetics of a Non-evolving Population
• The gene pool is in stasis
• This is described by Hardy-Weinberg Theorem:
• The frequencies of alleles in a population’s gene
pool remain constant over the generations
unless acted on by agents other than sexual
recombination
• i.e. shuffling the deck has no effect on the
overall genetic make-up of the population
The Hardy-Weinberg Theorem
• Example
• In pink flowers (A), is dominant over white
flowers (a)
• 2 alleles for at this locus
•
Sample 500 plants:
•
20 white flowers (aa)
•
480 pink [320 (AA); 160 (Aa)]
• Therefore there are 1000 genes for flower
color in the population
•
Example (Continued)
• The dominant allele accounts for 800 of
these:
– [(320 x 2) + (160 x 1)]
• Therefore:
– the frequency of A in the population = 80%
– the frequency of a = 20%
Predicting Change
• How will genetic recombination during
sexual reproduction affect the frequencies
in the next generation?
• If mating is random:
– the probability of picking 2 AA = (0.8 x 0.8) = .64
– the probability of picking 2 aa = (0.2 x 0.2) = .04
– and of heterozygotes = 2(0.8 x 0.2) = 0.32
• There are 2 heterozygote combinations: aA & Aa
• sperm or egg
Hardy-Weinberg Equilibrium
• This shows that the alleles are present in
the gene pool in the same frequencies as
they were in the previous generation
A: [0.64 + (0.32  2)] = 0.8
a: [0.04 + (0.32  2)] = 0.2
• The gene pool is at equilibrium
• This is called Hardy-Weinberg equilibrium
The Hardy-Weinberg Equation
• This example is the simplest case:
– 2 alleles, one is dominant
• For this case:
– if p = frequency of one allele
– q the frequency of the other
• Then: p + q = 1
probability of AA = p2
probability of aa = q2
probability of Aa = 2pq
• Therefore:
p2 + 2pq + q2 = 1
Uses of Hardy-Weinberg
• Thus you can calculate the frequency of
a gene in a population if you know the
frequency of the genotypes
• This is important in genetic disease
counseling
Relevance to Evolution
• A population at genetic equilibrium does
not evolve
• Hardy-Weinberg tells us what to expect
in non-evolving populations
• Therefore it is a baseline for comparing
actual populations where gene pools
may be changing.
• Can determine if the population is
evolving
Genetic Equilibrium
• Hardy-Weinberg equilibrium is maintained
only if the population meets all 5 of the
following criteria:
– Very large population size
– Isolation from other populations
• migration can effect the gene pool
– No net mutations
– Random matings
– No natural selection
• no difference in reproductive success)
• Describes an ideal that never exists in nature
Altering Genetic Equilibrium
• For evolution to take place something must
upset the genetic equilibrium of the
population:
• Factors that change genetic equilibrium are:
– Genetic drift
– Migration (Gene flow)
– Non-randon mating (Isolation)
– Mutation
– Natural selection
Genetic Drift
• Changes in gene frequency of a very small
population due to chance
• Controlled by the laws of probability &
chance
• Bottleneck effect
– Chance sampling error due to small population
• Founder’s effect
– a few individuals colonize a remote spot
– causes drift
Illustrating Genetic Drift
The Bottleneck Effect
Gene Flow (Migration)
• Movement of organisms into or out of a
population
• Takes their genes out of the gene pool
• Most populations are not completely
closed
– gain & lose alleles
Non-random Mating
• More apt to mate with close neighbors
• Promotes inbreeding
• Assortive mating
– seek mate like self (i.e. size)
Isolation
Mutation
• A change in a gene
• An alteration of
DNA
• The original source
of variation
• Raw material on
which natural
selection works
Natural Selection
• If one type produces more offspring than
another, upsets the balance of equilibrium
• There are three types of natural selection:
– Stabilizing Selection
– Disruptive Selection
– Directional Selection