PACKET #59CHAPTER #23

Microevolution

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Microevolution

Change in a population’s allele, or genotype, frequencies over successive generations Occurs when a population does not meet all of the

assumptions of the Hardy-Weinberg principle The small changes are referred to as microevolution.

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Microevolution II

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There are five micro-evolutionary forces: - Non-random mating Mutations Genetic drift

Bottleneck effect Founder effect

Gene flow Natural selection

Stabilizing selection Directional selection Disruptive selection

NON-RANDOM MATING

Microevolution

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Non-random Mating

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MUTATIONS

Microevolution

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Mutations

Unpredictable changes in DNA resulting in the production of new alleles Introduces variation Cause small deviations from Hardy-Weinberg

equilibrium Mutations in somatic cells are not heritable

Mutations in alleles found in sex cells allow those changes to be passed to the offspring

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GENETIC DRIFT

Microevolution

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Genetic Drift

Random events that change allele frequencies in small populations Small populations are

more prone to lose alleles present in low frequencies

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Bottleneck Effect

Rapid and severe declines in population size due to an adverse environmental factor

Results in an increase in different allele frequencies

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Bottleneck EffectExample

Examples Northern elephant

seals Cheetah American bison Wollemi Pine

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Founder Effect

Occurs when a small population colonizes a new area.

Common in island populations Finnish population exhibits much less allelic variation

than the general European population Amish population in Pennsylvania has a significant

number of individuals with the allele composition for a form of dwarfism.

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GENE FLOW

Microevolution

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Gene Flow

The movement of alleles caused by a migration of individuals between populations Migration of breeding individuals introduces new

allelic frequencies to a population Tends to counteract natural selection and genetic drift

Causes populations to become more genetically similar Humans have experienced an increase in gene flow in

the last few hundred years.

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NATURAL SELECTION

Microevolution

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Natural Selection

Natural selection results in changes in allele composition that lead to adaptation and is based on differential reproduction

Natural selection does not act directly on an organism’s genotype but acts on the phenotype. The phenotype represents an interaction between the

environment and all the alleles in the organism’s genotype.

Natural selection weeds out those individuals whose phenotypes are less adapted to environmental changes Allowing the better adapted organisms to survive and pass

their alleles to future generations Natural selection acts indirectly on the genotype.

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Natural Selection

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There are three types of natural selection: - Stabilizing selection Directional selection Disruptive selection

Natural SelectionStabilizing Selection

Favors intermediate phenotypes

Results in reduced variation in a population Classic example is

human birth weight

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Natural SelectionDirectional Selection

Favors one phenotype over another

Favors one extreme of the normal distribution over the other phenotypes.

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Natural SelectionDisruptive Selection

Favors phenotypic extremes

Selects for two or more different phenotypes May result in splitting

of a population into 2 or more separate species.

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Necessities for Natural Selection

Populations contain genetic variation, a necessity for natural selection, introduced because of Mutations Crossing Over Independent Assortment Random Fertilization

In order to investigate genetic variation scientists examine genetic polymorphism The presence of two or more

alleles at a given locus. Locus is the location of a gene on

the chromosome. Scientists compares different

forms of a protein using gel electrophoresis Resulting from slight changes in

the gene’s DNA nucleotide sequence

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