The Genetic Basis of Evolution

Themes• Population gene pools evolve, not individuals

• Most populations have a large amount of genetic variation

• Genetic variation is the pre-requisite for evolution

• The mechanism of evolution can either be because of selection or because of chance

Sources of Genetic Variation1. Sexual Reproduction

*meiosis and independent assortment *combines genes in different ways

2. Mutation *can create new beneficial genes/traits via deletion, substitution, insertion or inversion of DNA bases

3. Exon Shuffling

• can create new genes/traits by combining mini-genes in new ways

• Exons 1,2,3 code for gene 1 (normal)• Exons 4,5,6 code for gene 2 (normal)• Other combo’s may combine to produce new

genes

1 2 3

4 5 6

DNA

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4. Gene Duplication & Divergence

• multiple copies of the gene can be “played” with. One copy will still function in the “old” way while mutated copies can serve as trials for potentially new beneficial genes

Gene 1 Gene 2

Gene 1 Gene 1 Gene 2

Gene 1 Gene 1A

Agents of EvolutionThere are 5 agents of Evolution – 5 things that can cause gene frequencies to change over time:

1. Natural Selection (and artificial selection)2. Mutation (& exon shuffling & gene duplication/diver.)3. Gene Flow (migration)4. Non-random mating (sexual selection)5. Genetic drift

Natural Selection

Environmental changes can cause gene frequencies to change.

Ex. Polluted forests favoring black moths over white ones

It is the main mechanism of evolution, but not the only mechanism

Mutation (New Variation)Mutation, exon shuffling and gene duplication

& divergence can create new genes

Though rarely, new genes will occasionally offer an adaptive advantage

In this case, the new genes will be passed on to future generations and will accumulate and become more abundant than “old” genes

Gene Flow migration can cause changes in

gene frequencies

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Non-Random Mating Also known as Mate Selection or Sexual

Selection

Organisms with desired characteristics are selected

Skews gene frequencies: over time the desired characteristic increases in number

Genetic Drift

Unexpected random changes in gene frequency that occur because of rapid reduction in population size:

The smaller the population, the greater the role that genetic drift plays

Two mechanisms: Population Bottleneck Founder Affect

Population BottleneckA large portion of the population is wiped out,

leaving behind a very small population (ex. Natural disaster)

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Survival of the Lucky, not the Fittest

Founder EffectA small portion of the population breaks away

from the parent population and becomes isolated from it (ex. Hutterites)

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