Evolution of Populations
2010
The Modern Synthesis
• Population genetics integrates Darwinian evolution and
Mendelian Genetics
• Important terms in population genetics:
Population: group of individuals of same species
Species: groups of populations that are capable of
interbreeding and producing fertile offspring in nature
Gene pool: total aggregate of genes in a population
Hardy – Weinberg Theorem
• Gene pool of non-evolving population
•States that the frequency of alleles and genotypes in a population’s
gene pool remain constant unless acted upon by agents other than
Mendelian segregation and recombination of alleles. p+q = 1
Hardy Weinberg Equilibrium:
p2
+ 2pq +
q2
=1
P2 = Frequency of RR phenotype
2pq -= Frequency of Rr phenotype
q2= Frequency of rr phenotype
Conditions that must be met:
1.
2.
3.
4.
5.
Very large population size.
No migration
No net mutations
Random mating
No natural selection
Do you that any natural populations are in HardyWeinberg equilibrium?
Microevolution
• Generation to generation change in the
population’s frequency of alleles
• 2 main causes: genetic drift – change due to
chance, small populations & natural selection
• Other causes -
Bottleneck Effect
• Drastically reduced population sizes
• Small population size may not be
representative of original gene pool
• Reduction in genetic variablility
The Founder Effect
Occurs when a few individuals colonize an isolated habitat
from a larger population
Gene Flow
• Genetic exchange through migration – gain or
loss of alleles
Mutation
Substitutes one allele for another
Selection Types
Sexual Selection
Speciation – Biological Species
Concept
• Prezygotic Barriers: habitat isolation,
behavioral isolation, temporal isolation,
mechanical isolation, gametic isolation
• Postzygotic Barriers: reduced hybrid viability,
reduced hybrid fertility, hybrid breakdown
Types of Speciation
• Allopatric: geographic barriers, disruption of
gene flow
• Sympatric: genetic causes – polyploid (plants),
other genetic factors
Tempo of Evolution
• Darwin – gradual continual rate
• Punctuated equilibrium model: spurts of rapid
change followed by periods of relatively little
change (Ex. Species exists for 5 million years
but most morphological changes in 50,000 yrs
– just 1%).
Phylogenies
• Many lines of evidence compiled
• Biogeographical, fossil, morphological
similarities (homologous not analogous
structures), genetic evidence
Categories of Evolution
• Convergent evolution - acquisition of the
same biological trait in unrelated lineages.
• Leads to analogous structures
Parallel evolution is the development of a similar trait in related,
but distinct, species descending from the same ancestor, but
from different clades
Divergent evolution - accumulation of differences between groups which can
lead to the formation of new species, usually a result of diffusion of the same species
adapting to different environments
Major Lineages of Life
3 domains: Bacteria,
Eukaryota, Arachae