Population Genetics A Population is: a group of same species organisms living in an area An allele is: one of a number of alternative forms of the same gene that may occur at a given site on a chromosome. The gene pool is: All of the alleles of all individuals in a population Population genetics: study of genetic variation within a population Modern Synthesis Theory Combines Darwinian selection and Mendelian inheritance 1940s: comprehensive theory of evolution (Modern Synthesis Theory) Introduced by: Fisher & Wright GENES are responsible for the inheritance of characteristics POPULATIONS, not individuals, evolve due to natural selection & genetic drift SPECIATION usually is due to the gradual accumulation of small genetic changes The Hardy-Weinberg Theorem Used to describe a non-evolving population Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium Deviation from Hardy-Weinberg equilibrium usually results in evolution Understanding a non-evolving population, helps us to understand how evolution occurs Assumptions of the H-W Theorem Large population size - small populations can have chance fluctuations in allele frequencies (e.g., fire, storm). No migration - immigrants can change the frequency of an allele by bringing in new alleles. No net mutations - if alleles change from one to another, this will change the frequency of those alleles
Random mating - if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles. No natural selection - if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation. Hardy-Weinberg Equilibrium The gene pool of a non-evolving population remains constant over multiple generations In other words, the allele frequency does not change over generations of time Hardy-Weinberg Equations If two alleles (A and a) for a specific gene in a population are represented by p(a) and q(a), then we can represent the ratio of the alleles in the population: p + q = 1 The allele frequency in a population will determine the genotype( and hence phenotype) distribution in the population according to the equation: p 2 + 2pq + q 2 = 1 where p2 = frequency of AA genotype; 2pq = frequency of Aa plus aa genotype; q2 = frequency of aa genotype Causes of microevolution 1. Genetic drift a) Bottleneck effect may lead to reduced genetic variability following some large disturbance that removes a large portion of the population.
b) Founder effect may lead to reduced variability when a few individuals from a large population colonize an isolated habitat. 2. Natural Selection As previously stated, differential success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring 3. Gene flow Genetic exchange due to the migration of fertile individuals or gametes between populations 4. Mutation o Mutation is a change in an organism s DNA and is represented by changing alleles. o Mutations can be transmitted in gametes to offspring, and immediately affect the composition of the gene pool. o The original source of variation. Mutation and sexual recombination generate genetic variation a. New alleles originate only by mutations In stable environments, mutations often result in little or no benefit to an organism, or are often harmful. Mutations are more beneficial (rare) in changing environments. (Example: HIV resistance to antiviral drugs.) b. Sexual recombination is the source of most genetic differences between individuals in a population. Vast numbers of recombination possibilities result in varying genetic make-up Diploidy and balanced polymorphism preserve variation a. Diploidy often hides genetic variation from selection in the form of recessive alleles. b. Balanced polymorphism is the ability of natural selection to maintain stable frequencies of at least two phenotypes. Heterozygote advantage is one example of a balanced polymorphism, where the heterozygote has greater survival and reproductive success than either homozygote (Example: Sickle cell anemia where heterozygotes are resistant to malaria). Patterns of Selection Directional selection favors individuals at one end of the phenotypic range. Most common during times of environmental change or when moving to new habitats.
Diversifying selection (or Disruptive selection) favors extreme over intermediate phenotypes. Occurs when environmental change favors an extreme phenotype. Stabilizing selection favors intermediate over extreme phenotypes. Reduces variation and maintains the current average. o Example = human birth weights. Sexual selection leads to differences between sexes Sexual dimorphism is the difference in appearance between males and females of a species. Intrasexual selection is the direct competition between members of the same sex for mates of the opposite sex. This gives rise to males most often having secondary sexual equipment such as antlers that are used in competing for females. In intersexual selection (mate choice), one sex is choosy when selecting a mate of the opposite sex. This gives rise to sophisticated secondary sexual characteristics; e.g., peacock feathers. Natural selection does not produce perfect organisms a) Evolution is limited by historical constraints (e.g., humans have back problems because our ancestors were 4-legged). b) Adaptations are compromises. (Humans are athletic due to flexible limbs, which often dislocate or suffer torn ligaments.) c) Not all evolution is adaptive. Chance probably plays a huge role in evolution and not all changes are for the best. d) Selection edits existing variations. New alleles cannot arise as needed, but most develop from what already is present. Species and Speciation Morphological concept of species: Classifying species based upon appearance and structure of the organism - Was used for many years Biological species concept: Proposed by Ernst Mayr, a species is a population of organisms that can successfully interbreed amongst themselves but not with others.
Isolation and Speciation Geographic Isolation: Populations are prevented from interbreeding by geographic isolation, rivers change course, mountains rise, continents drift and organisms migrate. Allopatric Speciation: In this mode of speciation, something extrinsic to the organisms prevents two or more groups from mating with each other regularly, eventually causing that lineage to speciate. Isolation and Speciation Reproductive Isolation: Merely exploiting a new niche may automatically reduce gene flow with individuals exploiting the other niche. Sympatric Speciation In this mode of speciation, the evolution of reproductive isolating mechanisms occurs within the range and habitat of the parent species.