What Drives Evolution?1. Mutations

2. Genetic Drift

3. Gene Flow

4. Natural Selection

Gene Pool1. Gene pool: collectively, all of the alleles of

the population’s genes

2. Allelic frequency: the percentage of any specific allele in the gene pool

3. Genes can enter and leave a population’s gene pool for many reasons

4. Populations in which the gene pool is not changing are said to be in genetic equilibrium

1. In a population, there are 100 individuals. Since each of them would have two alleles for a particular character (trait), the total number of genes in this population is

2. 200 (=100 x 2). 3. 30 homozygous for the dominant trait (AA)4. 50 are heterozygous (Aa)5. 20 are homozygous for the recessive trait (aa). 6. The total number of dominant genes in the population is 7. (30 x 2) + (50 x1) = 110. 8. The frequency of the dominant trait (A) is 9. (110/200) = 0.55 or 55%. 10. The total number of recessive genes in the population is 11. (20 x 2) + (50 x 1) = 90. 12. The frequency of the recessive trait (a) is 13. (90/200) = 0.45 or 45%.

Example – Allelic Frequency

Mutations1. New genes can be added to the

population by errors in DNA copying called mutations

2. Most mutations are BAD!

Mutations1. Very very rarely a mutation might come

along that improves an organism’s chance at survival or reproduction

2. These mutant genes will be passed along to offspring and become more common in the gene pool over time

3. If the mutation offers a large advantage, eventually (after many generations) all members of the population will have the beneficial mutation

Genetic Drift1. Genetic drift: when allelic

frequencies are changed by random events in a isolated population

Gene Flow1. Gene flow: the transport of genes by

traveling individualsa) When an individual leaves a

population, its genes are lost from the gene pool - emigration

b) When an individual enters a population, its genes are added to the gene pool - immigration

Speciation1. A certain population may become

isolated and evolve to fit new or different environmental conditions

2. The isolated population my change so much that it can no longer mate with the original population

3. Speciation – the evolution of a new species

1. There are several reasons why similar populations no longer breed, creating new species:

a) Geographic Isolation leads to:b) Reproductive Isolation

Speciation

1. Geographic isolation: when a physical barrier divides a population preventing them from mating

a) E.g. Canyon, lava flow, continental drift

2. Ecological Isolation: populations occupy different habitats and never encounter each other to mate

a) E.g. Population of monkeys that lives on the ground does not mate with tree dwelling monkeys

3. Temporal Isolation – Populations reproduce at different times of the year

a) E.g. Population of frogs that mate in March do not reproduce with frogs that mate in June

Reproductive Isolation

North rim of GrandCanyon

South rim of GrandCanyon

1. Divergent evolution: species that once were similar become increasingly different

a) Happens when populations adapt to different environments

Two Types of Evolution

Two Types of Evolution1. Convergent evolution: distantly

related organisms evolve to become more similar

a) Happens when unrelated species adapt to similar environments, possibly in different parts of the world