Populations, Genes and Evolution Ch. 16.1

Population Genetics  Study of diversity in a population at the genetic level.  Alleles  1 individual will provide 2 alleles for a given phenotype  DD- homozygous Dominant  Dd- heterozygous  dd- homozygous recessive  In a population, we would expect a mixture of D and d alleles in the gene pool  The allele frequency is the proportion of each allele in the gene pool.

Hardy-Weinberg Equilibrium  When there is no change in allele frequencies over time, the population is said to be in Hardy-Weinberg Equilibrium  Allele frequencies can be determined by p 2 + 2pq + q 2 = 1 p 2 = DD frequency 2pq = Dd frequency q 2 = dd frequency

HW only applies to populations under 5 conditions… 1. No mutations-No new alleles arise by mutation  Mutations are changes in DNA sequence and provide genetic variation.  Most mutations come from errors during DNA replication, but some can occur through exposure to physical or chemical agents called mutagents.

HW only applies to populations under 5 conditions…  Mutations only affect the gene pool if they can and are passed on to next generation.  Though mutations are rare, they are essential to evolution.

HW only applies to populations under 5 conditions… 2. No Migration- no new members moving in and no existing members moving out  When plants and animals migrate between populations, we get gene flow- movement of alleles between populations.  Gene flow is determined by: 1. The distance between populations 2. The ability of individuals to move 3. The behavior that determines if they will move

HW only applies to populations under 5 conditions…  If migration does not occur, two populations become genetically different and therefore reproductively isolated. This is the first step towards new species.

HW only applies to populations under 5 conditions… 3. Large Gene Pool- Large population size  A small population size is affected more by genetic drift (changes in allele frequency by chance)  Bottleneck affect- when large populations suddenly become very small due to natural disaster.  Founder affect- when a few members of a population split off to form a new population.

HW only applies to populations under 5 conditions…  In small populations, the chance of inbreeding increases greatly after a few generations. While this does not lead to a change in allele frequency, it can increase the appearance of rare recessive disorders.

HW only applies to populations under 5 conditions… 4. Random Mating- Individuals mate at random with no bias for genotype or phenotype  Random mating will not change the allele frequencies  Non-random mating will change frequencies because mates are chosen by specific phenotypes and therefore alleles assort according to mating behavior (Assortive Mating)

HW only applies to populations under 5 conditions…  Non-random mating, in itself, does not cause population evolution, it does play a role towards it.

HW only applies to populations under 5 conditions… 5. No selection- natural selection does not favor one genotype over another  In HW equilibrium, every genotype has an equal chance of survival and reproduction.  In nature, one genotype may provide advantages to the individual leading to natural selection.