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CHAPTER 16
POPULATION GENETICS AND SPECIATION
SECTION 1
GENETIC EQUILIBRIUM
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Population genetics-is the study of
evolution from a genetic point of view.
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Evolution at the genetic level is
called microevolution
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a change in the genetic material
of a population
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Genetic material of organisms
consists of many alleles-or
variations-of many genes that code
for various traits
Do you remember that a population
consists of a group of individuals of
the same species that routinely
interbreed?
Populations are the smallest unit in
which evolution occurs
CAUSES OF VARIATION
• What causes variation in traits?
• Environment
• Heredity
• Some occur as a range of phenotypes
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Range of body sizes
Two colors of flowers
• What causes genes to vary?
• Mutation- random change in genes that is passed on
• Recombination-reshuffling of genes in a diploid offspring
• Random pairing-organisms produce large quantities of
gametes. This ensures the union of gametes is partly a
matter of chance.
THE GENE POOL
Population genetics uses the
term gene pool to describe
the total genetic information
available in a population
HARDY-WEINBERG
GENETIC EQUILIBRIUM
• Unless acted on by an outside influence, genotype
frequencies will stay the same.
• If the next 5 items occur, there will be no genetic variation
1. No net mutations occur; alleles remain the same
2. Individuals neither enter or leave the population
3. The population is large; infinitely large
4. Individuals mater randomly
5. Selection does not occur
SECTION 2
DISRUPTION OF GENETIC EQUILIBRIUM
Remember—Evolution is a change in a populations genetic
material over generations!!
MUTATIONS
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The first requirement for genetic equilibrium is that allele
frequencies don’t change due to mutations
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Spontaneous mutations occur constantly, but at very low rates
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• This is at normal conditions
When mutagens are introduced, this will affect genetic
equilibrium, by producing totally new alleles for a new trait
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• Mutagens increase the rates of mutations
Slow changes in alleles take long periods of time to remove
harmful mutations
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In the long run, beneficial mutations with arise and be beneficial
to evolution
NOT THESE MUTANTS
GENE FLOW
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Second requirement for genetic
equilibrium, is that the size of the
population remains constant.
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Immigration- the movement of
individuals into a population
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Emigration- the movement of
individuals out of a population
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The constant movement of genes
rom one population to another
Gene flow
Gene flow can occur through various
mechanisms, such as migration of
individuals, or dispersal of seeds or
spores
GENETIC DRIFT
• Third requirement of genetic equilibrium is the presence
of a large population
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The Hardy-Weinberg principle is based on probability
• Genetic drift- in the phenomenon by which allele
frequencies in a population change as a result of random
events.
• this requires a small to medium sized population
NON RANDOM MATING
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The fourth requirement of genetic equilibrium is random
matings, without care of genetic makeup.
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Many species do not mate randomly
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Mating influenced by geographic
Many species chose mates with similar traits
The selection of a mate based on similar traits is called
assortative mating
Nonrandom mating affects what alleles will be combined within
individuals, but does not affect the overall frequencies within a
population
SEXUAL SELECTION
The tendency to choose a mate based on certain traits
NATURAL SELECTION
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The fifth and final requirement of genetic equilibrium is the
absence of natural selection.
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Natural selction is ongoing, and therefore disrupts genetic
equilibrium.
• Remember!!! Natural selection means some members of a
population are more likely than others to survive and reproduce,
passing their genes to the next generation.
Remember that natural selection works off 4 different variables
When natural selection is working, the distribution of traits shift
between 3 types of bell curves
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Stabilizing selection
Disruptive selection
Directional selection
STABILIZING SELECTION
Individuals with the average form of a trait have the highest
fitness
DISRUPTIVE SELECTION
Individuals with either extreme variation of a trait have
greater fitness that individuals with the average form of a
trait
DIRECTIONAL SELECTION
Individuals that display a more extreme form of a trait have
greater fitness than individuals with an average form of the
trait.