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 phenotype2pq -= Frequency of Rr phenotypeq2= Frequency of rr phenotype

Conditions that must be met:

1. Very large population size.2. No migration3. No net mutations4. Random mating5. No natural selection

Do you that any natural populations are in Hardy-Weinberg 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 EffectOccurs when a few individuals colonize an isolated habitat from a larger population

Gene Flow• Genetic exchange through migration – gain or

loss of alleles

MutationSubstitutes 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