Major Points
1. Hardy-Weinberg equilibrium: stable allele frequencies
2. Causes of changes in allele frequency (microevolution)
Variation Among Individuals
Differences arise by mutation
They move through a population as a result of sexual reproduction
Meiosis shuffles genes, creating new combinations of mutant alleles
There can be great variation in alleles in a population
Genetic Equilibrium
A population in which the variability doesn't change over generations is
at genetic equilibrium
The proportion of each allele is its "allele frequency"
At equilibrium the proportion of homozygotes and heterozygotes can be predicted
The Hardy-Weinberg Rule
This is an equation showing the ratio of two alleles A and a
at equilibrium
p^2 AA + 2 pq Aa + q^2 aa = 1
Where:
p is the allele frequency of A
q is the allele frequency of a
How is this equation derived?
Deriving Hardy-Weinberg Equilibria
Consider a Punnett square:
Applying Hardy-Weinberg
Frequency of AA = p * p = 0.5 * 0.5 = 0.25
Frequency of Aa = 2 pq = 2(0.5 * 0.5) = 0.5
Frequency of AA = q * q = 0.5 * 0.5 = 0.2
Therefore, the frequency of alleles in the next generation is 1:2:1\
If Allele Frequencies are Unequal
Consider the alternative Punnett square
Using the Hardy-Weinberg Rule
We can apply the Hardy-Weinberg rule:
Assume a population in which p = 0.7 and q = 0.3
(Why must p + q = 1?
The frequency of AA = 0.7 * 0.7 = 0.49
The frequency of Aa = 2(0.7 * 0.3) = 0.42
The frequency of aa = 0.3 * 0.3 = 0.09
Note that (0.49 + 0.42 + 0.09 = 1)
What Hardy-Weinberg Assumes
1. No mutations (changes allele frequency)
2. Population is large (sampling effects)
3. Population is isolated (changes in allele frequency)
4. All members survive & reproduce (changes in allele frequency)
5. Mating is random\par}
Situations in which all these assumptions may never occur!
Use of Hardy-Weinberg
It represents an idealized situation (which may never happen)
Real situations can be compared to the ideal
The divergence from the equilibrium tells you how the population
is changing (note that it doesn't say why
What Causes Divergence from Hardy-Weinberg Equilibrium?
Mutation: "a heritable change in DNA that can change gene expression"
Some are neutral--have no effect on survival or reproduction
Some are beneficial
Some are harmful or even lethal}
Mutation can change the chance of survival or reproduction\par}
What Causes Divergence from Hardy-Weinberg Equilibrium?
Genetic drift = "a random change in allele frequencies owing to chance
events alone"
Especially in small populations
Chance of survival or reproduction can be unrelated to "fitness"
Negates Darwin's dictum "the survival of the fittest
What Causes Divergence from Hardy-Weinberg Equilibrium?
The Founder Effect: A type of genetic drift
Occurs when a new subpopulation forms
The allele frequency can be very different from the parent population
Is essentially a sampling difference
What Causes Divergence from Hardy-Weinberg Equilibrium?
Bottlenecks: Another kind of genetic drift
When population size crashes the allele frequency among the remaining individuals
can be different from the original population
Happens to endangered species (e.g. Nene & cheetah)
Affects the ability of the species to rebound
What Causes Divergence from Hardy-Weinberg Equilibrium?
Gene Flow
Distinct from genetic drift
Movement of alleles between populations
Can be in (immigration) or out (emigration)
Reduces the chance that allele frequency in the two populations will be
very different
What Causes Divergence from Hardy-Weinberg Equilibrium?
Natural Selection
Probably the greatest effect on allele frequencies
Occurs when environmental conditions affect the ability of individuals to
survive or reproduce
Can be any of three types
Types of Natural Selection
Directional Selection
Selection that favors one allele over another
The classic example is the peppered moth
A second example is the rise of antibiotic resistance in bacteria
Types of Natural Selection
Stabilizing Selection
Selection against either extreme stabilizes an intermediate phenotype
The book gives the example of human birth weight
Types of Natural Selection
Destabilizing Selection
Selection that disfavors the intermediate phenotype
Stabilizes a population with two extreme phenotypes
The book's example is a population of finches with large or small beaks,
but no intermediate types
Types of Natural Selection
Balanced Polymorphism
Occurs when selection tends to stabilize a situation with multiple alleles
Sickle-cell trait in tropical Africa is a case
Resistance to malaria of heterozygotes stabilizes the sickle-cell allele
frequency