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.

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