Populations, Genes and Evolution Ch. 16.1 and Ch.17

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 p2 + 2pq + q2 = 1 p2 = DD frequency 2pq = Dd frequency q2 = dd frequency

HW only applies to populations under 5 conditions… 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… 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: The distance between populations The ability of individuals to move 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… 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… 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… 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.

Speciation The splitting of one species into 2 or more different species. What is a species? There are 3 species concepts Morphological species concept Evolutionary Species Concept Phylogenetic Species Concept

Morphological Species Concept Each species is defined by one or more distinct physical characteristics (diagnostic traits) This is the concept Linnaeus used. Drawbacks: bacteria and other microorganisms doesn’t have many measurable traits Cryptic species- look almost identical but have different behaviors (habitat or courtship)

Evolutionary Species Concept Proposed to explain speciation in the fossil records. Again, uses diagnostic traits but also requires the members to share the same evolutionary pathway. This means, abrupt changes in traits classifies a new species.

Phylogenetic Species Concept A phylogeny or “family tree” is used to identify species based on a common ancestor Species is considered the smallest set of interbreeding organisms

Reproductive Isolating Mechanisms For two species to remain, there cannot be any gene flow (mating) between the two. Reproductive barriers that prevent reproduction are called isolating mechanisms Prezygotic- before fertilization Postzygotic- after fertilization

Modes of Speciation Principal modes of speciation Allopatric Speciation Sympatric Speciation Adaptive Radiation

Allopatric Speciation Speciation mode in which populations are segregated by geographical barriers.

Sympatric Speciation Daughter species arises from a group of individuals within an existing population. The geographical distributions of these species overlap.

Adaptive Radiation Speciation that occurs when a single ancestor rapidly gives rise to new species that adapt to specific environments