CHAPTER 16 POPULATION GENETICS AND SPECIATION. SECTION 1 GENETIC EQUILIBRIUM Population genetics-is...
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CHAPTER 16 POPULATION GENETICS AND SPECIATION. SECTION 1 GENETIC EQUILIBRIUM Population genetics-is the study of evolution from a genetic point of view
SECTION 1 GENETIC EQUILIBRIUM Population genetics-is the study
of evolution from a genetic point of view. Evolution at the genetic
level is called microevolution a change in the genetic material of
a population 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
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CAUSES OF VARIATION What causes variation in traits?
Environment Heredity Some occur as a range of phenotypes 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.
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THE GENE POOL Population genetics uses the term gene pool to
describe the total genetic information available in a
population
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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
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SECTION 2 DISRUPTION OF GENETIC EQUILIBRIUM RememberEvolution
is a change in a populations genetic material over generations!!
MUTATIONS The first requirement for genetic equilibrium is that
allele frequencies dont change due to mutations Spontaneous
mutations occur constantly, but at very low rates This is at normal
conditions When mutagens are introduced, this will affect genetic
equilibrium, by producing totally new alleles for a new trait
Mutagens increase the rates of mutations Slow changes in alleles
take long periods of time to remove harmful mutations In the long
run, beneficial mutations with arise and be beneficial to
evolution
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NOT THESE MUTANTS
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GENE FLOW Second requirement for genetic equilibrium, is that
the size of the population remains constant. Immigration- the
movement of individuals into a population Emigration- the movement
of individuals out of a population 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
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GENETIC DRIFT Third requirement of genetic equilibrium is the
presence of a large population 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
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NON RANDOM MATING The fourth requirement of genetic equilibrium
is random matings, without care of genetic makeup. Many species do
not mate randomly 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
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SEXUAL SELECTION The tendency to choose a mate based on certain
traits
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NATURAL SELECTION The fifth and final requirement of genetic
equilibrium is the absence of natural selection. 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
Stabilizing selection Disruptive selection Directional
selection
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STABILIZING SELECTION Individuals with the average form of a
trait have the highest fitness
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DISRUPTIVE SELECTION Individuals with either extreme variation
of a trait have greater fitness that individuals with the average
form of a trait
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DIRECTIONAL SELECTION Individuals that display a more extreme
form of a trait have greater fitness than individuals with an
average form of the trait.