Mechanisms of Evolution

Last week.... Heredity• How traits are passed from one generation to the next• Considered individual organisms & families

Today.... Mechanisms of Evolution• Changes in the frequency of traits over time• Causes of differences among populations in traits• Scaling up to populations

Origin of species & diversity of formSpecies are fixed• ~2.500 years ago• Aristotle & the ancient Greeks• Judeo-Christian culture• Earth is ~6,000 years old (very young!)

Life evolves, descent from common ancestors• Buffon, mid-1700’s, earth older than 6,000 years• Lamarck, early 1800’s, Inheritance of acquired characteristics

• Darwin & Wallace, 1858• Descent with modification, or, natural selection

Lamarck

Wallace

1. Individuals compete for resources2. There is phenotypic variation among individuals3. Some of this variation is heritable4. Those individuals with traits best suited to current conditions

leave more progeny than others (natural selection)*Leads to changes in allele frequencies over time, or, EVOLUTION

Evolution by Natural SelectionThe Origin of Species - Darwin 1859

“I have called this principle, by which each slight variation, if

useful, is preserved, by the term Natural Selection.”

• Overproduction & competition

• Phenotypic variation

• Some variation is heritable

• Differential survival & reproduction

*Leads to changes in allele frequencies over time

The Modern Synthesis• Mid-1900’s• Fusion of the work of Mendel & Darwin

Mendelian genetics can help us understand the patterns of phenotypic

variation observed by Darwin.

Gregor Mendel Charles Darwin

TerminologyEvolution – changes in allele frequencies over time

Natural selection – a mechanism for evolution, differential fitness among phenotypes present in a given environment

Fitness – survival & reproduction, the contribution an individual makes to the gene pool of the next generation

Gene pool – all gametes produced by a population in a generation

Population – a group of individuals of the same species living in the same area at the same time

Allele frequency – # of particular allele/total # of alleles in a population

Genotype frequency – # of particular genotype/total # of individuals in a population

The Equilibrium Population:Assumptions of Hardy-Weinberg

• No mutation (no new genetic variation)

• No genetic drift (infinitely large population)

• No migration (no individuals entering or leaving the pop)

• No selection (genotypes have equal fitness)

• Random mating (dealing with a single population)

p2 +2pq + q2 = 1

Hardy-Weinberg Equilibrium

p + q = 1

For a single locus with two alleles:• p = frequency of the recessive allele (a)• q = frequency of the dominant allele (A)• Allele & genotype frequencies sum to 1 within a population

Genotypic and allelic frequencies are stable & predictable freq. aa = p2, freq. AA = q2, freq. Aa = 2pq