Gene Pool The combined genetic material for all the members of a population. (all the genes in a population)

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1 POPULATION GENETICS NOTES Gene Pool The combined genetic material for all the members of a population. (all the genes in a population) Allele Frequency The number of times a specific allele occurs in a population divided by the total number of alleles present (for a gene) in that population. Ex. A population of 500 people each have two alleles for blood type (1000 alleles) Alleles There are 300 I A, 100 I B, and 600 i Allele frequency I A = 30%, I B = 10%, i = 60% Speciation The formation of a new species. Genetic Equilibrium - A condition in which the allele frequencies of a population remain constant from generation to generation no evolution would be occurring. Hardy-Weinberg Principle - Describes the specific conditions required for genetic equilibrium to remain unchanged in a population (conditions necessary for no evolution). The conditions for the population are: no natural selection, no migration, no mutations, mating must be random (no preferential selection of mates), and it must be a large population. p = frequency of the dominant allele (A) q = frequency of the recessive allele (a) p + q = 1 (100%) This principle can also be used to predict the probability of genotypes: p 2 = frequency of the homozygous dominant genotype (AA) 2pq = frequency of the heterozygous genotype (Aa) q 2 = frequency of the homozygous recessive genotype (aa) p 2 + 2pq + q 2 = 1 Ex. Predict the allele frequencies for the gene associated with PKU in the United States. 1 in 10,000 babies are born with this recessive disorder.

2 SOURCES OF GENETIC VARIATION (Section 16-1, Miller and Levine) Mutations Mutations are the source of new genetic information. They can form new alleles. May occur as a result a mistake during replication or environmental factors (e.g. radiation, certain chemicals). Mutations do not always affect the expressed phenotype of the organism, but when they do, they can affect the fitness of that organism and therefore the subsequent allele frequencies of future generations. Sexual Reproduction and Gene Shuffling - Sexual reproduction is an important source of genetic variation because it combines genetic information from two sources to produce a new combination of genes (this does not the change the relative allele frequencies of the population, although nonrandom mating would.) - Gene shuffling occurs during the production of gametes. It includes crossing-over and the independent assortment of chromosomes, both of which increase the number of possible genotypes (formation of gametes). (analogous to shuffling a deck of cards you could be dealt many different hands, but it does not the change number (frequency) of aces, kings, queens, etc. in the deck). (this also does not the change the relative allele frequencies of the population).

3 SPECIATION AND ISOLATING MECHANISMS (Section 16-3, Miller and Levine) - Individuals of the same population interbreed and share a common gene pool. - When two populations of the same species become reproductively isolated from each other, they cannot interbreed. These populations start to have separate gene pools and respond to natural selection as separate units. - Reproductive isolation can develop in a variety of ways including the following: Behavioral Isolation Occurs when two populations are capable of interbreeding, but have differences in behavior, such as courtship rituals. Ex. Eastern and Western meadowlarks use different mating songs Geographic Isolation (allopatric speciation) Occurs when two populations are separated by geographic barriers such as mountains, oceans, or rivers. Ex. Subspecies Abert squirrel and Kaibab squirrel (separated by the Colorado River about 10,000 years ago) Temporal Isolation Occurs when two (or more) species reproduce at different times. Ex. Three similar species of orchids all live in the same rainforest. Each species only releases pollen on a single day because each species releases pollen on a different day, they cannot pollinate one another.

4 Genetic Drift Random changes in the allele frequencies of a population due to chance happenings (events). This generally occurs in small populations (large populations tend to be able to withstand these events without significant effect on their allele frequencies). Unlike natural selection, an individual s fitness generally does not have great influence on whether or not it is removed from the gene pool during genetic drift (it is a non-selective event). The small population is not representative of the larger parent population (certain alleles may be lost, over-represented, or underrepresented). There tends to be less genetic diversity. Examples: Bottleneck effect occurs when catastrophic events (volcanic eruption, earthquake, fire, flood, over-hunting) decimate a population so that only a small percentage of the population survives and are left to repopulate the community (or the world, in severe instances) Ex. Cheetahs and the ice age, Northern Elephant Seals and over-hunting Founder effect A small number of individual s from a large population migrate and colonize a new habitat. Ex. Mutiny on the Bounty and Pitcairn Island, Darwin s finches and the Galapagos Islands.

5 EVOLUTIONARY PATTERNS OF DESCENT Speciation - Formation of one or more new species from a single ancestral species. Process by which a new species is formed from an existing one. allopatric evolution - geographically isolated sympatric evolution - reproductively isolated Divergent Evolution (similar populations or species evolving in different environments) - Process in which closely related populations evolve independently (because of isolation) to Become more and more dissimilar. The presence of homologous structures in different species is an indication of divergent evolution. Ex.. polar bear and brown bear, red fox and kit fox Adaptive Radiation - Relatively rapid evolution of many diversely adapted species from a common ancestor. Type of divergent evolution in which ancestral species develop into an array of species, each specialized to fit into a different niche. Ex. Darwin's finches, Hawaiian honeycreepers Convergent Evolution (different populations or species evolving in similar environments) - Process by which unrelated organisms independently evolve similarities. Evolution of similar traits in distantly related organisms. It leads to production of analogous structures. Ex. wings of birds and butterflies, insect mimicry, streamlined shape of fish and small whales Coevolution - Process in which two species evolve in response to changes in each other. A change in one species acts as a selective force on another species. Ex. Flowers (shape, scent, or color of flowers) and their Pollinators (feeding structures of insects, birds, bats), Predators and Prey Genetic Variation - mutations and gene shuffling (sexual reproduction) Changes in allele frequency - selection, genetic drift, mutation