What Darwin didn’t know…. ◦ How traits were inherited ◦ What caused variations
Mutations – change in the DNA Sexual reproduction ◦ “gene shuffling” ◦ crossing-over Sexual reproduction by itself does not cause evolution to occur…..
◦ Gene pool - All the genes (alleles) in a given population. Population - A group of individuals of the same species that interbreed. ◦ Relative Frequency - The number of times an allele appears in a population relative to other alleles.
Sample Population 48% heterozygous black 36% homozygous brown 16% homozygous black Frequency of Alleles allele for brown fur allele for black fur
Evolution happens when the relative frequency of alleles in a population changes. ◦ What can cause relative frequencies of alleles to change?
Single-gene traits have 2 or 3 phenotypes. ◦ Natural selection can lead to changes in allele frequencies.
Frequency of Phenotype (%) 100 80 60 40 20 0 Widow’s peakNo widow’s peak Phenotype
Polygenic traits are more complicated; there are many phenotypes. ◦ Natural selection affects the distributions of phenotypes.
Frequency of Phenotype Phenotype (height) Generic Bell Curve for Polygenic Trait “normal distribution”
Directional Selection Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness
Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection
Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Low mortality, high fitness High mortality, low fitness
Natural selection is not the only source of evolutionary change. ◦ Small populations migrating to a new habitat can quickly cause a change in allele frequencies. This results in a small number of individuals having a profound effect on gene frequencies. The is called genetic drift or the “founder effect” Darwin’s finches are an example of genetic drift.
Sample of Original Population Founding Population A Founding Population B Descendants Genetic Drift
Five conditions for genetic equilibrium (Hardy- Weinberg principle): 1. Random mating 2. Very large population 3. No migration in or out 4. No mutations 5. No natural selection All five conditions can’t exist at once, so evolution must happen.
Formation of a new species. 1.Isolating mechanisms separate a founding population Geographic isolation Behavioral isolation Temporal isolation 2.Natural selection and genetic drift causes changes in the gene pool 3.Members of the two populations can no longer interbreed and are reproductively isolated Ex: Darwin’s finches
Concept Map results from which include produced by which result in Reproductive Isolation Isolating mechanisms Behavioral isolationTemporal isolation Geographic isolation Behavioral differencesDifferent mating times Physical separation Independently evolving populations Formation of new species
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