Biology Lecture 14: Evolutionary Forces; why sex?
o Mutations are not directed towards the needs of an organism they are just mistakes in DNA
o Mutations are an evolutionary force, because they produce new alleles, and are ultimately
the only source for new alleles
o They are the source of all new genetic variation and tend to change allele frequencies over
time, but because mutations are rare, mutation rates tend to be low. Mutation on its own is
not a particularly strong evolutionary force.
Migration: movement of alleles between populations.
o Can occur in anything (Example: plant seeds can be moved into different populations by the
wind or other factors)
o Can result in a change of allele frequency.
o Tends to equalize allele frequency between populations connected by gene flow
o Increases genetic variation within a single population
o Any population can experience genetic drift, but it is more common in small populations
o Genetic drift is unpredictable changes in allele frequencies between generations (decreases
amount of genetic variation)
o Bottlenecks occur when population becomes very small and then regrows
o Founder events occur when a small colony leaves and breeds their own population
o Genetic drift can occur due to founder and bottleneck events
o Removes genetic variation in a population
o When individuals with one particular genotype are more likely to mate with an individual
with one particular genotype
o Assortative mating: individuals prefer mating with others with similar traits
Disassortative mating: individuals prefer mating with others with opposite traits
o Increases proportion of homozygotes and decreases proportion of heterozygotes
Clicker Question: Inbred populations often gave high rates of genetic disorders. Why?
A. Inbreeding increases the frequency of recessive alleles
B. Inbreeding increases the frequency of homozygotes
C. Inbreeding increases the frequency of harmful alleles
D. Inbreeding increases the mutation rate.
Inbreeding does not affect allele frequencies, and doesn't make a recessive allele more common
It changes the distribution of the different genotypes (increases the proportion of homozygotes)
There is more opportunity for the recessive al