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CHAPTER 24LECTURE

SLIDES

Prepared by

Brenda LeadyUniversity of Toledo

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Overview

Natural Selection Reproductive Fitness Types of Natural Selection

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Natural selection

Process in which beneficial traits that are heritable become more common in successive generations

Over time, natural selection results in adaptationsChanges in populations of living organisms

that promote their survival and reproduction in a particular environment

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Reproductive successLikelihood of an individual contributing fertile

offspring to the next generationAttributed to 2 categories of traits

Certain characteristics make organisms better adapted to their environment and more likely to survive to reproductive age

Traits that are directly associated with reproduction, such as the ability to find a mate and the ability to produce viable gametes and offspring

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Modern description of natural selection

1. Within a population, allelic variation arises from random mutations that cause differences in DNA sequences

2. Some alleles encode proteins that enhance an individual’s survival or reproductive capability compared to other members of the population

3. Individuals with beneficial alleles are more likely to survive and contribute their alleles to the gene pool of the next generation

4. Over the course of many generations, allele frequencies of many different genes may change through natural selection, thereby significantly altering the characteristics of a population

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Fitness

Relative likelihood that a genotype will contribute to the gene pool of the next generation as compared with other genotypes

Measure of reproductive success Hypothetical gene with alleles A and a

AA, Aa, aa

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Suppose average reproductive successes are…AA produces 5 offspringAa produces 4 offspringaa produces 1 offspring

Fitness is W and maximum is 1.0 for genotype with highest reproductive abilityFitness of AA: WAA = 5/5 = 1.0

Fitness of Aa: WAa = 4/5 = 0.8

Fitness of aa: Waa = 1/5 = 0.2

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Mean fitness of population

Average reproductive success of members of a population

As individuals with higher fitness values become more prevalent, natural selection increases the mean fitness of the population

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Natural selection patterns

Directional selection Stabilizing selection Disruptive/Diversifying selection Balancing selection

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Directional selection

Individuals at one extreme of a phenotypic range have greater reproductive success in a particular environment

InitiatorsNew allele with higher fitness introducedProlonged environmental change

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(a) An example of directional selection (b) Graphical representation of directional selection

Many generations

Light fur Dark fur

Light fur Dark fur

Population of mice in a dimly lit forest

Many generations

Number ofindividuals

Number ofindividuals

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Stabilizing selection

Favors the survival of individuals with intermediate phenotypes

Extreme values of a trait are selected against

Clutch sizeToo many eggs and offspring die due to lack

of care and foodToo few eggs does not contribute enough to

next generation

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Disruptive/Diversifying selection

Favors the survival of two or more different genotypes that produce different phenotypes

Likely to occur in populations that occupy heterogeneous environments

Members of the populations can freely interbreed

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(b) Graphical representation of disruptive selection

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Metal sensitive Metal resistant

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Metal sensitive Metal resistant

Many generations(a) Growth of Agrostis capillaris on contaminated soil

Agrostis capillaris

Contaminated soil

a: © Courtesy Mark McNair/University of Exeter

Agrostis capillaris

Contaminated soil

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Balancing selection

Maintains genetic diversity Balanced polymorphism

Two or more alleles are kept in balance, and therefore are maintained in a population over the course of many generations

2 common waysFor a single gene, heterozygote favored

Heterozygote advantage – HS alleleNegative frequency-dependent selection

Rare individuals have a higher fitness