AP Biology Chapter 23

How does sexual recombination produce variation?

A population contains a vast number of possible mating combinations, and fertilization brings together the gametes of individuals with different genetic backgrounds. Sexual reproduction reshuffles alleles into fresh combinations every generation.

popular genetics

the study of the distributions and changes of allele frequency in a population, as the population is subject to the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow.

phenotypic polymorphism

variation in the physical structure of a certain species

genetic drift

variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce

relative fitness

way of measuring the reproductive success of organisms within a population

disruptive selection

also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups

fitness

an organism's ability to survive and reproduce in a particular environment

heterozygote advantage

describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. The specific case of heterozygote advantage due to a single locus is known as overdominance

genetic polymorphism

discontinuous genetic variation that results in different forms or types of individuals among the members of a single species; promotes genetic diversity within a population; common example is different allelic forms that make humans have many blood types

genetic variation

diversity in gene frequencies. Genetic variation can refer to differences between individuals or to differences between populations. Mutation is the ultimate source of genetic variation, but mechanisms such as sexual reproduction and genetic drift contribute to it as well

gene pool

the set of all genes, or genetic information, in any population, usually of a particular species. This also proves to be the basic level at which evolution occurs

Which parts of the H-W equation correspond to the frequency of individuals that have at least one PKU allele?

2pq + q2; 2pq represents heterozygotes with one PKU allele and q2 represents homozygotes with two PKU alleles.

Distinguish genetic drift and gene flow in terms of A) how they occur and B) their implications for future genetic variation in a population.

Genetic drift results from chance fluctuations of allele frequencies from generation to generation; it tends to decrease variation over time. Gene flow is the exchange of alleles between populations; it tends to increase variation within a population but decrease allele frequency differences between populations.

Suppose a population of organisms with 500 gene loci is fixed at half of these loci, and has two alleles at each of the other loci. How many alleles are found in it's gene pool? Explain

Half of the loci (250) are fixed, meaning only one allele exists for each locus: 250 x 1 = 250. There are two alleles each for the other loci: 250 x 2 = 500. 250 + 500 = 750.

What did Mendel's findings about genetics add to Darwin's theory of evolution by natural selection?

Mendel showed that inheritance is particulate, and subsequently it was shown that this type of inheritance can preserve the variation on which natural selection acts.

Of all the mutations that occur, why do only a small fraction become widespread in the gene pool?

Most mutations occur in somatic cells that do not produce gametes and so are lost when the organism dies. Of mutations that do occur in cell lines that produce gametes, many do not have a phenotypic effect on which natural selection can act. Others have a harmful effect and are thus unlikely to spread in a population from generation to generation because they decrease the reproductive success of their bearers.

In what sense is natural selection more predictable than genetic drift?

Natural selection is more "predictable" in that it tends to increase or decrease the frequency of alleles that correspond to variations that increase or decrease an organism's reproductive success in its environment. Alleles subject to genetic drift all have the same likelihood of increasing or decreasing frequency

natural selection

the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution

duplication

a DNA segment in a chromosome that is a copy of another segment

modern synthesis

a consolidation of the results of various lines of investigation from the 1920s through the 1950s that supported and reconciled the Darwinian theory of evolution and the Mendelian laws of inheritance in terms of natural selection acting on genetic variation

cline

a gradation in one or more characteristics within a species or other taxon, especially between different populations

average heterozygosity

a measure of genetic diversity at the population scale and indicates the average proportion of individuals that are heterozygous for a given trait

directional selection

a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype

population

a particular section, group, or type of people or animals living in an area or country

Hardy-Weinberg equilibrium

a principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors. When mating is random in a large population with no disruptive circumstances, the law predicts that both genotype and allele frequencies will remain constant because they are in equilibrium

stabilizing selection

a type of natural selection that favors the average individuals in a population. This process selects against the extreme phenotypes and instead favors the majority of the population that is well adapted to the environment. Stabilizing selection is often shown on a graph as a modified bell curve that is narrower and taller than the norm

gene flow(gene migration)

is the transfer of alleles or genes from one population to another. Migration into or out of a population may be responsible for a marked change in allele frequencies (the proportion of members carrying a particular variant of a gene)

bottleneck effect

occur when a population's size is reduced for at least one generation

founder effect

reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors

balancing selection

refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population at frequencies above that of gene mutation

Hardy-Weinberg Theorem

states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences

microevolution

the change in allele frequencies that occur over time within a population. This change is due to four different processes: mutation, selection (natural and artificial), gene flow, and genetic drift

mutations

the changing of the structure of a gene, resulting in a variant form that may be transmitted to subsequent generations, caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes

geographic variation

the differentiation of distinctive subdivisions from geographically isolated parts of a potentially interbreeding population due to restriction of interbreeding between fractions and natural selection of locally valuable mutations