Mechanisms of Evolution

Changes in Gene Frequency of the Gene Pool of a Population

The Gene Pool Definition: the frequency of alleles in a population B/B+b = Freq B b/B+b = Freq b

Genetic Variation in the Gene Pool Polygenic Traits in a Population Fingers and Beans

Genetic Variation in the Gene Pool

Gene Frequency change over time: Small Gene Pool - more variation Selective Mating (Choice or Isolation) Mutation Migration/Emigration (Gene Flow) Natural Selection - adaptation

1. Small Gene Pool – Genetic Drift random fluctuations in gene frequency Genetic drift refers to random fluctuations in the gene frequency of a population over time. This is more likely to occur in a small population. The smaller the population, the more the gene frequencies are likely to fluctuate from generation to generation. In the example here, we start with the assumption that 50% of the genes are one allele and the other allele is 50%. The population size is small – 20 individuals. If we run the simulations on this small population we get a wide variation in the allele frequnecy over time but two observations stand out: 1. one of the alleles disappears from the population and 2. the disappearance is within a few generations. Neither allel is better or more fit than the other but random selection occurs and one of the two alleles is eliminated. This pattern is expected in small populations. In large populations, the alleles persist and remain relativity constant in frequency over many generations.

Large Gene Pool – Slight Genetic Drift With a large population, n=1000; both alleles persist through many generations. We call this population equilibrium. When one observes this equilibrium, one knows that neither allele conveys an advantage and therefore there is no favoring of one allele over the other.

2. Selective Mating If one color or one pattern is more attractive to a mate, that pattern or color will persist and the others will decrease in frequency and may eventually disappear.

3. Mutations

4. Emigration / Immigration:

5. Natural Selection This lab illustrates the

What if natural selection is operating? O r I g I n a l O v e r T i m e Stabilizing Selection Directional Selection Disruptive Selection

Stabilizing Selection Range narrows One phenotype dominates

Directional Selection the mean shifts One end of the distribution gains dominance

Directional Selection One phenotype gains dominance Allele Frequency Generation 1 Generation 8

Directional Selection and back again p2 + 2pq + q2 = 1

Disruptive Selection Splitting the mean Two phenotypes dominate

Speciation barriers to gene flow How new species arise The process of forming new species is called speciation.

Isolation

Geographic Isolation

the Bottle Neck Effect Reduced population size Reduced genetic diversity Founder Population rare alleles increase in frequency

Reproductive Isolation

Adaptive radiation emergence of many species from one original species

Darwin’s Finches

Survival of the Fittest Descent with modification Adaptive Radiation and Beak Shape Survival of the Fittest Descent with modification

regulation of the Bmp4 protein is the principal way in which beak variation occurs in the finches

Gradualism or Punctuated Equilibrium?

Gradualism or Punctuated Equilibrium?

There is no master plan