Mader: Biology 8 th Ed. Process of Evolution Chapter 18.

Mader: Biology 8th Ed.

Process of Evolution

Chapter 18


Outline

• Evolution in a Genetic Context - Microevolution

• Hardy-Weinberg• Causes of Microevolution• Natural Selection

– Types of Selection• Maintenance of Variations• Speciation

– Modes of Speciation


Evolution in a Genetic Context

• Microevolution– In population genetics, the various alleles

at all the gene loci in all individuals make up the gene pool of the population.

To determine the frequency of each allele, calculate its percentage from the total number of alleles in the population.


Hardy-Weinberg

• The Hardy-Weinberg principle states an equilibrium of allele frequencies in a gene pool, p2 + 2pq + q2, will remain in effect in each succeeding generation of a sexually reproducing population, assuming:– No Mutations– No Gene Flow– Random Mating– No Genetic Drift– No Selection


Hardy-Weinberg

• Under real conditions, these conditions are rarely, if ever, met, and allele frequencies in the gene pool of a population change between generations.– Evolution has occurred.

Accumulation of small changes is referred to as microevolution.


Industrial Melanism


Causes of Microevolution

• Genetic Mutations– Once alleles have mutated, certain

combinations of alleles might be more adaptive than others in a particular environment.

• Gene Flow– Movement of alleles between populations

by migration of breeding individuals.Continual gene flow reduces variability

between populations.


Gene Flow



• Nonrandom Mating– Individuals do not choose mates randomly.

Assortative mating - Individuals tend to mate with those with the same phenotype.

Sexual selection - Males compete for the right to reproduce and females choose to mate with males possessing a particular phenotype.



• Genetic Drift– Changes in allele frequencies of a gene

pool due to chance.Larger effect in small populations.


Genetic Drift


Genetic Drift• Bottleneck Effect

– An event prevents a majority of genotypes from entering the next generation.

• Founder Effect– Rare alleles occur at a higher frequency in

a population isolated from the general population.

The alleles carried by population founders are dictated by chance.


Natural Selection

• Natural Selection is the process that results in adaptation of a population to the biotic and abiotic environment. Requires:– Variation– Inheritance– Differential Adaptiveness– Differential Reproduction

Relative Fitness


Types of Selection

• Directional Selection– An extreme phenotype is favored and the

distribution curve shifts in that direction.Can occur when a population is

adapting to a changing environment.


Directional Selection


Types of Selection

• Stabilizing Selection– Occurs when an intermediate phenotype

is favored.Can improve adaptation of the

population to constant conditions.


Stabilizing Selection


Types of Selection

• Disruptive Selection– Two or more extreme phenotypes are

favored over any intermediate phenotype.Two distinctly different phenotypes are

found in the population.


Disruptive Selection


Maintenance of Variations

• Maintenance of variation is beneficial because populations with limited variation may not be able to adapt to new conditions.– Only exposed alleles are subject to natural

selection.Sickle-Cell Disease

Homozygote remains in equilibrium in some regions of Africa because the heterozygote is protected from sickle-cell and malaria.


Sickle-Cell Disease


Speciation

• Speciation is the splitting of one species into two or more species, or the transformation of one species into a new species over time.– Species Definition

MorphologicalBiological

Reproductive IsolationPhylogenetic


Reproductive Isolating Mechanisms

• Prezygotic Isolating Mechanisms– Prevent reproduction attempts, and make

it unlikely fertilization will be successful.Habitat IsolationTemporal IsolationBehavioral IsolationMechanical IsolationGamete Isolation


Reproductive Isolating Mechanisms

• Postzygotic Isolating Mechanisms– Prevent hybrid offspring from developing

or breeding.Zygote MortalityHybrid SterilityF2 Fitness


Modes of Speciation

• Allopatric Speciation– Occurs when one population is

geographically isolated from other populations.

• Sympatric Speciation– A population develops into two or more

reproductively isolated groups without prior geographic isolation.


Adaptive Radiation

• Adaptive Radiation is an example of allopatric speciation.– Many new species evolve from a single

ancestral species when members of the species become adapted to different environments.


Review

• Evolution in a Genetic Context - Microevolution

• Hardy-Weinberg• Causes of Microevolution• Natural Selection

– Types of Selection• Maintenance of Variations• Speciation

– Modes of Speciation

Mader: Biology 8 th Ed. Process of Evolution Chapter 18.

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Transcript of Mader: Biology 8 th Ed. Process of Evolution Chapter 18.