The Evolution of Populations

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Gene flow consists of the movement of alleles among populations.

Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen).

Gene flow tends to reduce differences between populations over time.

Gene flow is more likely than mutation to alter allele frequencies directly.

Slide 23

Gene Flow

Slide 24

Only natural selection consistently results in adaptive evolution.

Natural selection brings about adaptive evolution by acting on an organism’s phenotype.

Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

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Natural Selection: Relative Fitness

The natural selection phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals.

Reproductive success is generally more subtle and depends on many factors.

Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.

Selection favors certain genotypes by acting on the phenotypes of certain organisms.

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Directional, Disruptive, and Stabilizing Selection

Three modes of natural selection:

Directional selection favors individuals at one end of the phenotypic range.

Disruptive selection favors individuals at both extremes of the phenotypic range.

Stabilizing selection favors intermediate variants and acts against extreme phenotypes.

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Natural Selection

Original population

(c) Stabilizing selection

(b) Disruptive selection

(a) Directional selection

Phenotypes (fur color)

Frequency of individuals

Original

population

Evolved

population

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The Key Role of Natural Selection in Adaptive Evolution

Natural selection increases the frequencies of alleles that enhance survival and reproduction.

Adaptive evolution = the match between an organism and its environment.

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Natural Selection - Adaptive Evolution

(a) Color-changing ability in cuttlefish

(b) Movable jaw

bones in

snakes

Movable bones

Slide 30

Because environments change, adaptive evolution is a continuous process.