Download Mechanisms of Evolution

Mechanisms of Evolution
Population Genetics & Evolution
• Darwin didn’t know about genes.
• Genetic information used to explain
variation within a population.
• Population genetics developed – studies
the behavior of genes in a population.
Populations Evolve – Not
Individuals
• Genes determine individual’s features.
• If feature – phenotype – is poorly adapted,
individual won’t survive to reproduce.
• Organism can’t evolve new phenotype in
response.
• Natural selection acts on ranges of
phenotypes.
• Population = consists of all members of a
species that live in an area.
Gene Pool
• Gene pool = the sum of all the alleles of a
population.
Populations Evolve – Not
Individuals
• Evolution occurs as population’s genes
and frequencies change over time.
• Allelic frequency = percentage of any
specific allele in the gene pool.
• Genetic equilibrium = a population in
which frequency of alleles remains the
same over generations.
• A population in genetic equilibrium is not
evolving.
Evolutionary Mechanisms
• Allele frequency = how often an allele appears
in a population.
– Remains relatively stable, particularly if the
population is large and well-adapted to its
environment.
– Many factors can change allele frequency:
• 1. Mutation
• 2. Migration
• 3. Random change
• 4. Artificial or Natural Selection
• 5. Non-random mating of individuals
Factors Affecting Allele Frequency
1.) Mutation :
• Results in the introduction of a new allele
and causes an immediate, small shift in
equilibrium.
• Affects populations of organisms that
produce a large number of offspring in a
short time period (ex. Bacteria) more so
than organisms that produce a small
number of offspring in a relatively long
period of time (ex. Mammals)..
Factors Affecting Allele Frequency
2.) Migration:
• The movement of organisms into or out of
the population and therefore the gene
pool.
• If organisms entering or leaving the
population have a genotype different from
that of the rest of the population, a change
in equilibrium occurs.
• Gene flow = the movement of genes into
or out of a population because of
migration.
Factors Affecting Allele Frequency
3.) Random change in allele frequencies may
result in a population with distinct
characteristics.
• Usually occurs in small populations.
• Genetic drift = Changes in the gene pool of
a small population due to chance.
– Can occur if a storm or attack by predators
kills a large proportion of the population
and the survivors may have a different
allele frequency than that found in the
original group.
Genetic Drift = Changes in the gene pool of a
small population due to chance
Factors Affecting Allele Frequency
• 4.) Artificial and Natural Selection:
• Determines which individuals in a
population will reproduce and pass on
their genes.
• Individuals with characteristics that helped
them survive in their environment could
produce more offspring than individuals
that did not have such characteristics.
Factors Affecting Allele Frequency
5.) Non random mating of individuals in a
population.
• If certain individuals in a population show
a preference for mating with individuals
that have a particular phenotype, the
mating pattern ceases to be random.
Sexual Selection
Natural Selection Acts on
Variations
• Some variations increase or decrease an
organism’s chance of survival.
• Variations can be inherited and controlled
by alleles.
• Allelic frequencies in a population’s gene
pool will change over generations due to
natural selection of variations.
Natural Selection Acts on
Variations
• Three types of natural selection that
act on variation:
• Stabilizing selection
• Directional selection
• Disruptive selection
Selection Animation
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/chp23/2302001.html
Stabilizing Selection
• Natural selection that favors
average individuals in a
population.
• Ex. Spiders of average size
favored.
– Larger spiders easier to see
and be caught by predators.
– Small spiders find it difficult
to find food.
– Average are more likely to
survive – have a “selected
advantage”
Directional Selection
• Occurs when natural selection
favors one of the extreme
variations of a trait.
• Ex. Woodpeckers with long
beaks are able to reach
insects that live deep in the
tree bark. Long-beaked
woodpeckers have selected
advantage over woodpeckers
with short or average beak
size.
Disruptive Selection
• Individuals with either extreme of a trait’s
variation are selected for.
• Ex. Clams with shell colors ranging from
light (white) to dark brown. On light-colored
rocks – white shells blend in. On darkcolored rocks – brown shells blend in. Tan
colored shells stick out and are eaten by
predators.