Microevolution: Evolutionary
change within a species
ν
Changes in allelic frequencies
over time
When evolution is and is not
occurring?
Causes of microevolution:
υ Genetic DriftDrift- +, - or 0
υ Gene Flow - +, - or 0
υ Mutations - +, - or 0
υ Natural Selection - always
+… the only cause of
“adaptive evolution”
evolution”
ν
ν
Genetic Drift
ν
Consider random chance acting in small and large
populations
υ Sampling error.
Φ Is there a change in allelic frequency?
Φ Is it adaptive evolution?
Genetic Drift: Changes in the gene pool that result from
sampling error
ν
Bottleneck Effect
ν
When natural or
unnatural forces result in
a diminished population
that is nonnonrepresentative, the gene
pool may change as a
result of genetic drift.
NS cannot operate in the
absence of all alleles.
ν
Gene Flow: when a population acquires additional
variation from other populations through migration
Founder Effect
ν
ν
Mutations: The ultimate source
of new alleles in the population.
ν
ν
ν
ν
What happens to the genetic variation of different
populations when gene flow occurs between
them?
Few individuals colonize an island, lake, or some
other isolated habitat
Darwin’
Darwin’s Galapagos Finches, Cichlid fishes of
Lake Malawi, Africa
Initially undetectable
in the population.
Entirely random
υ Spontaneous
υ Induced
The raw material for
natural selection.
Natural Selection: The driving
force in adaptive evolution
ν
ν
ν
Three General Modes of Selection
Results in increases in reproductive success
Acts on existing alleles that arose from mutations
or sexual recombination
Survival of the fittest?
υ Darwinian fitness
Φ Relative fitness
υ Fitness is about survival and reproduction
Φ Longevity does not always equal fitness
Φ More often than not, the product of many
genes
Directional selection for beak size in a Galápagos population
of the medium ground finch
The Preservation of Genetic
Variation
ν
ν
Diploidy: AA, Aa, aa
υ Heterozygous protection
Balancing Selection - when more than a single
phenotype has fitness yielding balanced
polymorphisms. 3 mechanisms:
υ Heterozygous advantage
υ FrequencyFrequency-dependent selection
υ Neutral variations
The Disadvantage of Sex - genetic dilution or
infusion?
Final Comments on Microevolution:
ν
ν
ν
The role of sex and sexual selection…
selection…or why
males aren’
aren’t a total waste in the population…
population…
Sexual dimorphism and mate choice
υ Intrasexual selection
υ Intersexual selection
ν
ν
ν
Evolution is not about
novel inventions.
Adaptations must
represent
compromises.
Only natural selection
is adaptive.
Evolution lacks
purpose or intent,
relates to prevailing
conditions
Hardy-Weinberg Equilibrium
ν
Demonstrates that allele frequencies do not
change over many generations as long as:
υ Very large population size
υ No migration
υ No net mutations
υ Random mating
υ No natural selection