Chapter 23- Evolutionary Processes
- Evolution is now understood to be driven by four processes:
- Niαtural selection increases the frequency of certain alleles, the ones that contribute to reproductive success in a particular environment
- Genetic drift causes allele frequencies to change randomly. In some cases, drift may cause alleles that decrease fitness to increase in frequency
- Gene flow occurs when individuals leave one population, join another, and breed.
- Mutαtion modifies allele frequencies by continually introducing new alleles.
23.1 Null Hypothesis: The Hardy-Weinberg Principle
- They imagined a hypothetical scenario in which all of the alleles from all the gametes produced in each generation go into a single group called the gene pool
- Hardy-Weinberg principle which serves as a mathematical null hypothesis for the study of evolutionary processes.
- If biologists observe frequencies that conform to 1e Hardy-Weinberg predictions, the frequencies are said to be in Hardy-Weinberg equilibrium.
- The mathematical model underlying the Hardy-Weinberg principle is based on five important assumptions about how populations and alleles behave:
- Random mating
- No natural selection
- No genetic drift(random allele frequency changes)
- No gene flow
- No mutation
23.2 Nonrandom Mating
- The most intensively studied form of nonrandom mating is called inbreeding, the mating between relatives.
- Deleterious alleles are alleles that lower fitness
- Inbreeding depression is the decline in average fitness that may take place when homozygosity increases and heterozygosity decreases in a population.
- Sexual selection is fundamentally different from inbreeding because it does lead to changes in allele frequencies in 1e population and increases fitness, and us it is a form of natural selection
23.3 Natural Selection
- Traits that enable organisms to survive in their physical and biological environments, are sometimes called ecological ( or environmental) selection.
- Genetic variation is the number and relative frequency of alleles that are present in a particular population.
- Natural selection occurs in four main patterns, or modes, each affecting genetic variation in different ways:
- Directionαl selection changes the average value of a trait.
- Stabilizing selection reduces variation in a trait.
- Disruptive selection increases variation in a trait.
- Bαlαncing selection maintains variation in a trait.
- When directional selection occurs, the average phenotype of a population changes in one direction.
- When deleterious alleles decline in frequency, purifying selection is said to occur.
- Disruptive selection eliminates phenotypes near the average value and favors extreme phenotypes
- Stabilizing selection favors phenotypes near the average value and eliminates extreme phenotypes
- Balancing selection occurs when no phenotype has a distinct advantage.
- Of the various mechanisms of balancing selection, the following are the most common:
- Heterozygote advantage occurs when heterozygous individuals have higher fitness than homozygous individuals do.
- Frequency-dependent selection occurs when certain alleles are favored when they are rare, but not when they are common
- Intersexual selection is the selection of an individual of one sex for mating by an individual of the other sex.
- Individuals of the same sex sometimes compete with one another to obtain mates is referred to as intrasexual selection.
- A territory is an actively defended area where the owner has exclusive or semi“ exclusive use.
- Both intersexual and intrasexual selection help to explain why some aits differ between the sexes of the same species, a difference called sexual dimorphism
- ==Natural selection is the only evolutionary process that results in adaptation, but it is not the only evolutionary process that violates the Hardy-Weinberg assumptions.==
23.4 Genetic Drift
- Genetic drift is defined as a change in allele frequencies in a population that is due to chance.
- Sampling error occurs when the allele frequencies of a chosen subset of a population (the sample) are different from those in the total population, by chance
- Key information to remember about genetic drift are:
- Genetic drift is random with respect to fitness
- Genetic drift is most pronounced in small populations
- Over time, genetic drift can lead to the random loss or fixation of alleles
- A genetic bottleneck is a sudden reduction in the diversity of alleles in a population
23.5 Gene Flow
- Gene flow is the movement of alleles between populations
- If a population has lost alleles due to genetic drift, then the arrival of new alleles via gene flow should increase genetic diversity.
- If increased genetic diversity results in better resistance to infections by bacteria or viruses or other parasites, for example, gene flow would increase the average fitness of individuals.
23.6 Mutation
- ==Natural selection often favors certain alleles and leads to a decrease in overall genetic variation in a population.==
- ==Genetic drift tends to decrease genetic diversity over time, as alleles are randomly lost or fixed in a population.==
- ==Gene flow increases genetic diversity in a recipient population if new alleles arrive with immigrating individuals.== But gene flow may decrease genetic variation in the source population if alleles leave with emigrating individuals.
- Mutations can occur in a number of ways:
- Point mutation: A change in a single base pair in DNA.
- Chromosome-level mutation: A change in the number or composition of chromosomes.
- Lateral gene transfer: The transfer of genetic information from one species to another, rather than from parent to offspring.
- Beneficial allele is an allele that allows individuals to produce more surviving offspring, increasing fitness.
- Neutral allele is an allele with no effect on fitness, as occurs when a point mutation is silent
- Key information about mutations are that
- Mutation is the ultimate source of genetic variation
- Mutations are random with respect to fitness
- If mutation did not occur, evolution would eventually stop.
- Mutation alone is usually inconsequential in changing allele frequencies at a particular gene.