The Evolution of Populations

The Smallest Unit of EvolutionMicroevolution – the smallest unit of

evolutionary change in populations. The changes in allele frequencies in a population over generations.

There are three main mechanisms that can cause allele frequency change:Natural selectionGene flowGenetic drift

Genetic VariationDarwin provided evidence on how life changed over

time. He knew that natural selection was the primary mechanism for change in a species population.

Darwin could not explain how organisms pass heritable traits to their offspring.

Gregor Mendel – proposed that organisms transmit discrete heritable units (genes) to their offspring.Worked with pea plants to show inheritance of genes.

Genetic differences are produced by:Mutation Sexual reproduction

Variation Within a PopulationCharacters that vary within a population may

be discrete or quantitative.Discrete characters – can be classified on

an either-or basis, and are determined by a single gene locus with different alleles that produce distinct phenotypes.

Quantitative characters – vary along a continuum within a population. Usually results from the influence of two or more genes on a single phenotypic character.

Variation within a PopulationAverage heterozygosity – the average

percent of loci that are heterozygous.Average heterozygosity is usually estimated

by surveying the protein products of genes using gel electrophoresis.

Variation Between PopulationsBesides observing variation in a population,

species also exhibit geographic variation.Geographic variation- the differences in the

genetic composition of separate populations.When isolated from one another, populations

can evolve independently from the other population resulting in genetic variations from chance events (drift) rather than natural selection.

Variation Between PopulationsCline – a graded change in a character along

a geographic axis.Some clines are produced by a gradation in

an environmental variable, as illustrated by the impact of temperature on the frequency of cold-adaptive allele in mummichog fish.

Clines usually result from natural selection.

MutationMutations are the ultimate source of new alleles.Mutation – a change in the nucleotide sequence of

an organism’s DNA.It cannot be predicted which segments of DNA will be

altered.In multicellular organisms, only mutations in cell lines

that produce gametes can be passed to offspring.Most mutations occur in somatic cells and are lost

when the organism dies.Point mutations – a change of as little as one base

in a gene. Can have significant impact on phenotypes.Sickle-cell disease

Mutations that alter Gene Number or SequenceChromosomal changes that delete, disrupt, or

rearrange many loci at once are almost certain to be harmful.

When these large-scale mutations leave genes intact, their effects on organisms may be neutral.

Sometimes these mutations can be beneficial.A translocation of part of one chromosome to a

different chromosome could link DNA segments in a way that results in a positive effect.

Mutation RatesMutation rates are really low in plants and

animals and even lower in prokaryotes.One every 100,000 genes per generation.However, prokaryotes have short generation

spans, so mutations can quickly generate genetic variation in a population.

Sexual ReproductionSexual reproduction produces the most

genetic variation in a population that reproduces sexually.

Sexual reproduction shuffles the existing alleles and deals them at random to determine individual genotypes.

Shuffling is due to three mechanisms:Crossing overIndependent assortment of chromosomesFertilization

Hardy-Weinberg and Evolving PopulationsThe individuals in a population must differ

genetically for evolution to occur.Population Population – a group of individuals of the

same species that live in the same area and interbreed, producing fertile offspring.

Gene pool – a populations genetic makeup that consists of all the alleles for all the loci in all individuals of the population.

If there are two or more alleles for a particular locus in a population, individuals may be either homozygous or heterozygous.

Hardy-Weinberg PrincipleThe gene pool of a population that is not

evolving can be described by the Hardy-Weinberg principle.

This principle state that the frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.

This type of gene pool is in Hardy-Weinberg equilibrium.

Conditions for Hardy-Weinberg Equilibrium1. No mutations2. Random mating3. No natural selection4. Extremely large population size5. No gene flow

Natural selectionNatural selection can alter allele frequencies in a population.

Natural selection can cause adaptive evolution (evolution that results in a better match between organisms and their environment.

Genetic DriftGenetic drift can alter allele frequencies in a

population.Genetic drift - Chance events that cause

allele frequencies to fluctuate unpredictable from one generation to the next, especially in small populations.

The Founder EffectThe founder effect can also cause a fluctuation

in the allele frequencies of a population.Founder Effect – when a few individuals

become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population.

The Founder Effect probably accounts for the relatively high frequency of certain inherited disorders among isolated human populations.

The Bottleneck EffectA certain change in the environment, such as a

fire or flood, may drastically reduce the size of a population.

A severe drop in population size can cause the bottleneck effect.

The bottleneck effect is named so because the population has passed through a restrictive “bottleneck” in size.

Some alleles may be overrepresented and some will be underrepresented. Some populations may even cease to exist.

Effects of Genetic Drift: a summary1. Genetic drift is significant in small

populations2. Genetic drift can cause allele

frequencies to change at random.3. Genetic drift can lead to a loss of

genetic variation within a population.4. Genetic drift can cause harmful

alleles to become fixed.

Gene FlowGene flow can also cause the alleles

frequencies in a population to fluctuate.Gene flow – the transfer of alleles into or

out of a population due to the movement of fertile individuals or their gametes.

Because alleles are exchanged among populations, gene flow tends to reduce the genetic differences between populations.

A closer look at natural selectionRelative fitness – the contribution

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

The terms “survival of the fittest” and “struggle for existence” are sometimes misleading if taken to mean direct competition.

Directional, Disruptive, and Stabilizing SelectionNatural selection can alter the

frequency distribution of heritable traits in three ways:Directional SelectionDisruptive SelectionStabilizing Selection

Directional SelectionDirectional Selection – occurs when

conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting the frequency curve for the phenotypic character in one direction or the other.

Directional selection is common when a population’s environment changes or when members of a population migrate to a new habitat.

Disruptive SelectionDisruptive selection – occurs when

conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes.Ex. Members of a population of birds whose

members display different beak sizes.

Stabilizing SelectionStabilizing Selection – acts against both

extreme phenotypes and favors intermediate variants.

This mode of selection reduces variation and tends to maintain the status quo for a particular phenotypic character.

The Role of Natural Selection in Adaptive RadiationSexual Selection – a form of natural

selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.Sexual selection can result in sexual

dimorphism.Sexual dimorphism – marked differences

between the two sexes in secondary sexual characteristics which are not directly associated with reproduction or survival.

How does Sexual Selection Work?Intrasexual Selection – meaning selection within

the same sex, individuals of one sex compete directly for mates of the opposite sex.In many species, intrasexual selection occurs

among males.Intersexual Selection – also called mate choice,

individuals of one sex (usually the females) are choosy in selecting their mates from the other sex. In many cases, the female’s choice depends on

the showiness of the male’s appearance or behavior.

The Preservation of Genetic VariationBecause most eukaryotes are diploid, a

considerable amount of genetic variation is hidden from selection in the form of recessive alleles.

Balancing Selection – occurs when natural selection maintains two or more forms in a population.This type of selection includes heterozygote

advantage and frequency-dependent selection.Heterozygote advantage – exhibited when

individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes.

Preservation of Genetic Variation cont…Frequency-Dependent Selection – the

fitness of a phenotype declines if it becomes too common in the population.

Neutral Variation –It happens when, in humans, many of the nucleotide differences in noncoding sequences appear to confer no selective advantage or disadvantage.