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Genetics Chapter 18
Population and Evolutionary Genetics
| Term | Definition |
|---|---|
| genetic rescue | the introduction of new genetic variation into an inbred population |
| What is the effect of genetic rescue? | it often dramatically improves the health of the population and can better ensure its long-term survival |
| population genetics | branch of genetics that studies the genetic makeup of groups of individuals and how a group's genetic composition changes with time |
| Mendelian population | group of interbreeding, sexually reproducing individuals that have a common set of genes, the gene pool. |
| a population evolves through changes in its... | gene pool |
| _________ and _________ frequencies are used to describe the gene pool of a population | genotypic and allelic |
| how to calculate genotypic frequency? | add up the number of individuals possessing the genotype and divide by the total number of indiviuals in the sample |
| the sum of all genotypic frequencies always equals ___ | one |
| allelic frequencies can be calculated from what two things? | the number or the frequencies of the genotypes |
| how to calculate the allelic frequency from the numbers of genotypes | count the number of all copies of a particular allele present in a sample and divide by the total number of all alleles in the sample |
| how to calculate the allelic frequency from the genotypic frequencies | add the frequency of the homozygote for each allele to half the frequency of the heterozygote |
| What does the Hardy-Weinberg Law describe? | the effect of reproduction on genotypic and allelic frequencies of a population |
| For an autosomal locus with two alleles, the Hardy-Weinberg Law can make two predictions based on what assumptions of a population? (5) | if a population is large, randomly mating, and not affected by mutation, migration, or natural selection |
| Hardy-Weinberg Prediction 1 | the allelic frequencies of a population do not change |
| Hardy-Weinberg Prediction 2 | the genotypic frequencies stabilize after one generation in the proportions (AA)p^2, (Aa) 2pq, and (aa) q^2, where p= freq of allele A and q= freq of allele a |
| H-W: implication 1: populations and evolution | population cannot evolve if it meets the H-W assumption because evolution consists of change in the allelic frequencies of a population. (reproduction alone will not bring about evolution) |
| H-W: implication 2: population in H-W equilibrium | when a population is in H-W equil., the genotypic frequencies are determined by the allelic frequencies and the heterozygote frequency never exceeds .50 |
| nonrandom mating: inbreeding | preferential mating between related individulats |
| nonrandom mating: inbreeding depression | decreased fitnress arising from inbreeding (often due to the increased expression of lethal or deleterious recessive alleles when inbreeding takes place |
| effect of recurrent mutation on allelic frequencies | eventually leads to an equilibrium, with the allelic frequencies being determined by the relative rates of forward and reverse mutation |
| effect of migration on allelic frequencies | the movement of genes between populations increases the amount of genetic variation within populations and decreases the number of differences between populations |
| Genetic drift | a change in allelic frequencies due to chance factors |
| how does genetic drift arise? (3) | when a population consists of a small number of individuals, is established by a small number of founders (founder effect), or undergoes a major reduction in size (genetic bottleneck) |
| effects of genetic drift (3) | changes allelic frequencies, reduces genetic variation within populations, and causes genetic divergence among populations |
| natural selection | differential reproduction of genotypes |
| how is natural selection measured? | by the relative reproductive successes (fitnesses) of genotypes |
| evolution | genetic change taking place within a group of organisms |
| Evolution is a two step process: | (1) genetic variation arises (2) genetic variants change in frequency |
| anagenesis | change within a single lineage |
| cladogenesis | splitting of one lineage into two |
| the effect of natural selection on the gene pool of a population depends on the ________ values of the genotypes in the population | fitness |
| fitness | reproductive success of on genotype compared with the reproductive successes of other genotypes in the population. |
| fitness (W) ranges from _ to _ | 0 to 1 |
| how do you calculate fitness (W) for each genotype? | divide the mean number of offspring produced by a genotype by the mean number of offspring produced by the most prolific gene (most occurring gene) |
| a related variable is the selection coefficient: | relative intensity of selection against a genotype |
| the selection coefficient = | 1-W |
| directional selection | form of selection in which one allele or trait is favored over another |
| what two types of selection are special because they lead to equilibrium (no further change in allelic frequency)? | overdominance and underdominance |
| overdominance (or heterozygote advantage) | both alleles are favored in the heterozygote, and neither allele is eliminated from the population (W11 < W12 > W22) |
| how does overdominance affect allelic frequencies? | the allelic frequencies change until a stable equilibrium is reached, at which point there is no further change. |
| the allelic frequency (q hat) depends on the relative fitnesses of two homozygotes. What is the equation? | qhat= f(A^2)= s11/ (s11 + s22) ;s11= sel coef of A1A1 homo, s22= sel coef of A2A2 homo |
| underdominance | the heterozygote has lower fitness than both homozygotes (W11 > W12 < W22) |
| how does underdominance affect allelic frequencies? | it leads to an unstable equilibrium; allelic frequencies will not change as long as they are at equilibrium but, if they are disturbed from equil by some other evolutionary force, they will move away from equil until on allele becomes fixed |
| biological species concept (not all biologists adhere to this concept) | defines a species as a group of organisms whose members are capable of interbreeding with one another but are reproductively isolated from the members of other species. Because different species do not exchange genes, each species evolves independently |
| the key to species differences under the biological species concept is ___________ __________ | reproductive isolation |
| reproductive isolation | biological characteristics that precent genes from being exchanged between different species |
| reproductive isolating mechanism | any biological factor that prevents gene exchange |
| prezygotic reproductive isolating mechanism | prevent gametes from two different species from fusing and forming a hybrid zygote |
| what are 5 types of prezygotic reproductive isolating mechanisms? | ecological, temporal (reproduction takes place at different times), mechanical, behavioral, gametic |
| postzygotic reproductive isolating mechanism | gametes of two species fuse and form a zygote, but there is no gene flow between the two species, either because the resulting hybrids are inviable or sterile or because reproduction breaks down in subsequent generations |
| 3 types of postzygotic reproductive isolating mechanisms | hybrid inviability (zygote does not survive), hybrid sterility, hybrid breakdown (F1s are viable and fertile, but F2s are inviable and sterile) |
| speciation | process by which new species arise |
| new species arise in two principle ways: | allopatric speciation and sympatric speciation |
| allopatric speciation | arises when a geographic barrier first splits a population into two groups and blocks the exchange of genes between them. the interruption of gene flow then leads to the evolution of genetic differences that result in reproductive isolation |
| sympatric speciation | arises in the absence of any external barrier to gene flow; reproductive isolating mechanisms evolve within a single population. |
| evolutionary relations (phylogeny) can be represented by a phylogenetic tree. What does this consist of? | nodes that represent organisms and branches that represent their evolutionary connections |
| when one internal node represents a common ancestor to all other nodes on the tree, the tree is said to be _______ | rooted |
| What are two different approaches to constructing phylogenetic trees? | distance approach and parsimony approach |
| distance approach | evolutionary relationships are inferred on the basis of the overall degree of similarity between organisms |
| parsimony approach | infers phylogenetic relationships on the basis of the minimum number of evolutionary changes that ust have taken place since the organisms last had an ancestor in common |
| Rates of molecular evolution: findings from molecular studies of numerous genes have demonstrated that different genes and different parts of the same gene often evolve at different _____. | rates |
| rates of evolutionary change in nucleotide sequences are usually measured as the rate of nucleotide ___________ | substitution |
| rate of nucleotide substitution | the number of substitutions taking place per nucleotide site per year. |
| nonsynonymous substitutions | nucleotide changes in a gene that altar the amino acid sequence of a protein |
| synonymous sunstitutions | nucleotide changes, particularly those at the third position of a codon, that do not alter the amino acid seuquence |
| There is a relationship between the function of a sequence and its rate of evolution: | higher rates are found where they have the least effect on function |
| if the rate at which a protein evolves is roughly constant with the passage of time, the amount of molecular change that a protein has undergone can be used as a _________ ______ to date evolutionary events | molecular clock |
| What does the molecular clock hypothesis propose? | a constant ate of nucleotide substitution, providing a means of dating evolutionary events by looking at nucleotide difference between organisms |
| Genome evolution takes place through the what three things? | the duplication of genes to form gene families, whole-genome duplication, and the horizontal transfer of genes between organisms |
| new genes have evolved through the duplication of whole genes and their subsequent divergence. This process creates _________ __________ | multigene families |
| multigene families | sets of genes that are similar in sequence but encode different products |
| horizontal gene transfer | genes can be passed among distantly related organisms |
| new genes can evolve through the duplication of _____ and the duplication of _____ _________ | genes, whole genomes |
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cmccartney2
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