Chapter 16 notes: Evolution of Population PDF

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Chapter 16 notes: Evolution of Population...

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Chapter 16: Evolution of Population 16.1 Genes, Populations, and Evolution In order for traits to evolve, they must have the ability to be passed on to subsequent generations. Evolution is about change in a trait within a population over many generations. Population: A group of organisms of a single species living together in the same geographic area. Microevolution: Change in gene(allele) frequencies between population of a species over time. Population genetics: The study of gene frequencies and their changes within a population. Gene pool: Total of the alleles of all the individuals in a population. Allele frequency: Relative proportion of each allele for a gene in the gene pool of a population.

Hardy-Weinberg equilibrium (HWE): •

Genetic equilibrium, a stable, non evolving state. Derived from the work of Godfrey H. Hardy and Wilhelm Weinberg. Five conditions for Hardy-Weinberg equilibrium to be considered: 1. No mutation: No new alleles arise. 2. No migration: No new members can join the population, nor anybody leaves. 3. Large gene pool. 4. Random mating. 5. No selection: The process of natural selection does not favor one gene over another.

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The H-W principle does not describe natural populations because these 5 conditions can not be met at the same time in real world.

Hardy-Weinberg principle: •

Proposes that the genotype frequencies of a non evolving population can be described by the expression p 2 + 2p q + q 2 .

- “p” represents the frequency of D allele. “q” represents the frequency of d allele. Causes of microevolution Mutation •

A change to the nucleotide(DNA) sequence. This can change a population’s gene pool by providing new alleles. It occurs randomly and some mutations might be more adaptive than others.

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Mutagens: Chemical or physical agents that cause mutation.

Gene flow (migration): •

The movement of alleles between populations. Occurs when plants or animals migrate.

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Continual gene flow reduces genetic divergence between populations.

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Without migration, gene pools become more different over time.

Genetic drift: •

Changes in the allele frequencies of a population due to change rather than selection by the environment. It removes individuals and their genes, from a population at random — without regard for genotype or phenotype.

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This has a stronger effect in small population.

- Bottleneck effect: A type of genetic drift in which the loss of genetic diversity is due to natural disasters (hurricane, earthquake, fires), disease, overhunting, over harvesting or habitat loss.

- Founder effect: A type of genetic drift in which the genetic variation is lost when a few individuals break away from a large population to found a new population. •

The greater the reduction in population size, the greater the effects on allele frequencies.

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Formerly rare alleles will either:Occur at a higher frequency in the new population, or be absent in new population

Nonrandom (assortative) mating: •

Mating among individuals on the basis of their phenotypic similarities or differences, rather than mating on a random basis.

- This will cause Individuals reject mates with differing phenotype. - Results in certain genotypes are more frequent than others. •

Sexual selection: A type of nonrandom mating and also of natural selection. Adaptive changes in males and females lead to an increased ability to secure a mate.

- Females choose to select a male with the best fitness. Natural selection: •

It results in :

- Change in allele frequencies. - Improved fitness of the population. 2/4

16.2 Natural Selection Polygenic trait: They are controlled by multiple genes. Natural selection favors the variant that is most adaptive to the present environment conditions. Types of natural selection Stabilizing selection: •

Occurs when an intermediate phenotype is the most adaptive for the given environmental conditions. (The extreme phenotypes are selected against)

Directional selection: •

Occurs when an extreme phenotype is favored. Curve shifts to one direction.

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Over time, directional selection changes the frequency of a phenotype within a population.

Disruptive selection: •

Two or more extreme phenotypes are favored over the intermediate phenotype. The curve has two peaks.

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Sexual selection •

Changes in males and females, often due to male competition and female selectivity, leading to increased fitness(ability to reproduce a surviving offspring).

Sexual dimorphism: Species that have distinct difference between the sexes, resulting in male and female different size and other traits.

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Cost-benefit analysis: To determine whether the benefit of access to mating is worth the cost of competition among males. Dominance hierarchy: Organization of animals in a group that determines the order in which the animals have access to resources. Territoriality: Includes the type of defensive behavior needed to defend a territory.

16.3 Maintenance of Diversity Natural selection The process of natural selection itself causes imperfect adaption to the environment, and the imperfections are common because of necessary compromises.

Heterozygote advantage: •

Occurs when the heterozygote is favored over the two homozygotes. It assists the maintenance of genetic, and therefore phenotypic, diversity in future. It’s an example of stabilizing selection(H b A H b s is favored over two extreme genotypes, H b A H b A and H b S H b S ).

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Sickle-cell disease is an example.

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