Download Chapter 16 How Populations Evolve

Title
Chapter 16
How
Populations
Evolve
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Population Genetics
• A population is all of the members of a single species
occupying a certain area at the same time.
• Population genetics studies the variation in alleles in a
gene pool.
Microevolution
• Evolution that occurs within a population is called
microevolution.
• The gene pool is the total of all the alleles in a
population; it is described in terms of gene
frequencies.
Sexual
reproduction
alone cannot
bring about a
change in
genotype and
allele frequencies
of a population.
• Also, the dominant allele need not increase from one
generation to the next. Dominance does not cause an
allele to become a common allele.
• The potential constancy, or equilibrium state, of gene
pool frequencies was independently recognized by G.
H. Hardy and W. Weinberg.
• The Hardy-Weinberg principle states that an
equilibrium of gene pool frequencies, calculated using
the binomial expression, will remain in effect in each
succeeding generation of a sexually reproducing
population, as long as five conditions are met
– No mutation: no allelic changes occur, or changes in one
direction are balanced by changes in the other direction.
– No gene flow: migration of alleles into or out of the
population does not occur.
– Random mating: individuals pair by chance and not
according to their genotypes or phenotypes.
– No genetic drift: the population is large so changes in
allele frequencies due to chance are insignificant.
– No selection: no selective force favors one genotype over
another.
• In real life, conditions of the Hardy-Weinberg law are
rarely if ever met, and allele frequencies in the gene
pool of a population do change from one generation to
the next, resulting in evolution.
• Microevolution can be detected by noting any deviation
from a Hardy-Weinberg equilibrium of allele
frequencies in the gene pool of a population.
• A change of allele frequencies is expected to result in a
change of phenotype frequencies.
Industrial Melanism
• The case of the peppered moths provides a case study
in a shift in phenotype frequencies under selection.
• Before trees became coated with soot from air
pollution, the percentage of dark-colored moths was
10%.
• With birds acting as a selective agent, the light colored
moths were reduced while dark-colored moths were
better adapted to survive on the darkened trees.
• The last generation observed has 80% dark-colored
moths.
Causes of Microevolution
• Mutation
–
–
–
–
Mutations are permanent genetic changes.
Without mutations, there could be no inheritable
phenotypic variations among members of a population.
Mutations are the primary source of genetic differences
among prokaryotes that produce asexually.
In sexual reproducing organisms, both mutations and
sexual recombination are important in generating
phenotypic differences.
• Nonrandom Mating and Gene Flow
– Nonrandom mating occurs when certain genotypes or
phenotypes mate with one another.
– Gene flow (gene migration) is the movement of alleles
among populations by migration of breeding individuals.
•When animals move between populations, or when pollen is
distributed between species, gene flow has occurred.
•Continued gene flow decreases diversity among populations,
causing gene pools to become similar.
•Gene flow among populations can prevent speciation from
occurring.
• Genetic Drift
– Genetic drift refers to changes in allele frequencies of a
gene pool due to chance rather than selection by the
environment. Therefore, genetic drift does not necessarily
result in adaptation to the environment, as does natural
selection.
– Genetic drift occurs in both large and small populations;
small populations are more likely to show the effects of
drift.
– Genetic drift occurs when founders start a new population,
or after a genetic bottleneck with interbreeding.
– The bottleneck effect prevents most genotypes from
participating in production of the next generation.
•The bottleneck effect is caused by a severe reduction in
population size due to a natural disaster, predation, or habitat
reduction.
•The bottleneck effect causes a severe reduction in the total
genetic diversity of the original gene pool.
Fig. 16.6
– The founder effect is an example of genetic drift where
rare alleles or combinations occur in higher frequency in a
population isolated from the general population.
•Founding individuals could contain only a fraction of the total
genetic diversity of the original gene pool.
•Which alleles the founders carry is dictated by chance alone.
•As an example, dwarfism is much higher in a Pennsylvania
Amish community due to a few German founders.
Maintenance of Diversity
• The environment includes specific selecting agents that
help maintain diversity. We have already seen how
insectivorous birds can help maintain the frequencies
of both the light-colored and dark-colored moths,
depending on the color of background vegetation.
• Heterozygote Advantage
– Heterozygote advantage occurs when the heterozygote is
favored over the two homozygotes. In this way,
heterozygote advantage assists the maintenance of genetic,
and therefore phenotypic, diversity in future generations.
– Sickle-Cell Disease
•Heterozygotes are more fit in malaria areas because the sicklecell trait does not express unless the oxygen content of the
environment is low; but the malaria agent causes red blood cells
to die when it infects them (loss of potassium).
•Some homozygous dominants are maintained in the population
but they die at an early age from sickle-cell disease.
•Some homozygotes are maintained in the population for normal
red blood cells, but they are vulnerable to malaria.
– Cystic Fibrosis
•In cystic fibrosis the recessive allele causes the person to have a
defective plasma membrane protein.
•The agent that causes typhoid fever can use the normal version
of this protein, but not the defective one, to enter cells.
•Heterozygote superiority caused the recessive allele to be
maintained in the population.
Fig. 16.16