Chapter 18
EVOLUTIONARY CHANGE IN POPULATIONS
A population consists of individuals of the same species that live in a particular place at the same time.
Each population possesses a gene pool, which is the total genetic material of all the individuals that make
up that population and includes all the alleles of all the genes present in the population.
Variation among individuals is a sign that members of the population have different combinations of
alleles.
Evolution of populations uses allele frequencies in order to understand the rate and direction of
evolutionary change.
Allele frequency is the percentage of a specific allele of a given gene locus in the population.
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If the allele frequency remains constant form generation to generation, that population is not
undergoing evolutionary change.
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The population is said to be in genetic equilibrium.
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Changes in allele frequency are a sign that evolution has occurred.
HARDY-WEINBERG PRINCIPLE.
The principles states that a population at genetic equilibrium, allele and genotype frequencies do not
change from generation to generation.
The following conditions have to be met for a population to remain in genetic equilibrium.
1. Random mating. Each individual of the population has equal chance of mating.
2. No net mutations. The frequencies of genes must not change due to mutations.
3. Large population size in order to avoid frequency changes due to random fluctuations.
4. No migration. There can be no exchange of genes with other populations that might have different
allele frequencies.
5. No natural selection in order to avoid that some genotypes be favored over others.
The principle shows that the process of inheritance by itself does not cause changes in allele frequency.
It also explains why dominant alleles are not necessarily more common that recessive ones.
The required conditions for the Hardy-Weinberg principle to work probably do not occur in the real
world.
Example: p2 + 2pq + q2 = 1
where p is the frequency of the dominant allele B and q the frequency of
the recessive allele b.
p2 is the frequency of AA. 2pq is the frequency of Aa. q2 is the frequency of aa. 1 is the total population.
Departure from genetic equilibrium (Hardy-Weinberg principle) indicates the amount of evolutionary
change.
Change from generation to generation is sometimes called microevolution.
CHANGES IN ALLELE FREQUENCY.
1. In non-random mating, individuals select their mates on the basis of phenotype or genotype.
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Inbreeding is the mating of genetically similar individuals that are more closely related than if they
had been chosen at random from the entire population.
Homozygosity increases with each successive generation of inbreeding.
Self-fertilization is an extreme example of inbreeding.
Inbreeding causes inbreeding depression in the population, where individuals have lower fitness
than those that are noninbred, e.g. high juvenile mortality, lower fertility.
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In assortive mating, individuals select their mates by their phenotypes.
2. Mutations increase variations within the population.
Mutations occur unpredictably and spontaneously.
The rate of mutations appear to be relatively constant for a particular gene.
Somatic mutations are not inheritable.
Mutations do not determine the direction of evolutionary change.
3. Genetic drift refers to the production of random evolutionary changes in small breeding populations.
Alleles present in low frequency could be lost completely by pure chance, e.g. predators, by chance, kill
individuals with the low frequency allele.
Alleles are lost regardless of being beneficial, harmful or neutral.
Genetic drift decreases the genetic variation within the population but it tends to increase the differences
among different populations.
Genetic drift becomes a major evolutionary force when the population goes through a genetic bottleneck.
In genetic bottlenecks, a small population survives a catastrophe and it determines the gene frequency of
the succeeding generation.
As the population increases in size, the allele frequency might be very different from that of the previous
generation before the catastrophe.
The founder effect occurs when a few individuals, founders, establish a new colony.
The founders bring only a small fractions of the genetic variation of the original population.
4. Gene flow is caused by the migration of individuals between populations.
Populations that are isolated tend to have gene pools showing different frequencies of alleles.
Gene flow tends to decrease the amount of variation between the populations.
NATURAL SELECTION
The mechanism of natural selection leads to changes in allele frequency and to adaptation.
Natural selection operates on the phenotype.
It changes the genetic composition of a population in a favorable direction for a particular environment.
1. Stabilizing selection favors the mean at the expense of phenotypic extremes.
2. Directional selection favors one phenotypic extreme over another, causing a shift in the
phenotypic mean.
3. Disruptive selection favors two or more extremes.
GENETIC VARIABILITY.
Most populations have abundant genetic variability.
Genetic polymorphism is the presence in a population of two or more alleles for a given locus.
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Some alleles may be present in low frequencies.
Many alleles do not produce a distinct phenotype.
Heterozygote advantage occurs when the heterozygote exhibits greater fitness than either homozygote.
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Both alleles, dominant and recessive, are maintained in the population.
Frequency-dependent selection acts to decrease the frequency of the more common phenotypes and
increase the frequency of the less common types.
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It maintains genetic variation in prey species.
Neutral variations do not confer any selective advantage.
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Its presence does alter the ability to survive.
An allele that is neutral in one environment may be beneficial or harmful in another environment.
A cline is a gradual geographical change in a species phenotype and genotype frequencies.
Geographical variation is the genetic variation that exists between among populations within the same
species.