Download Population evolution

A population shares a common gene pool
Populations, not individuals, evolve
Genetic variation in a population increases the chance that some
individuals will survive
.
• Genetic variation leads to phenotypic variation.
• Phenotypic variation is necessary for natural selection.
• Genetic variation is stored in a population’s gene pool.
– made up of all alleles in a population
– allele combinations form when organisms have offspring
• Allele frequencies measure genetic variation.
– measures how common allele is in population
– can be calculated for each allele in gene pool
Genetic variation comes from several sources.
• Mutation is a random change in the DNA of a gene.
– can form new allele
– can be passed on to
offspring if in
reproductive cells
•
Recombination forms new combinations of alleles.
– usually occurs during meiosis
– parents’ alleles
arranged in new
ways in gametes
• Mutations produce the genetic variation needed for
evolution.
Natural selection acts on distributions of traits.
• A normal distribution graphs as a bell-shaped curve.
– highest frequency near
mean value
– frequencies decrease
toward each extreme
value
•
Traits not undergoing natural
selection have a normal distribution.
Natural selection can change the
distribution of a trait in one of three
ways.
Stabilizing selection—favors the
average form
Directional—one extreme is
favored
Disruptive selection-either extreme
variation is favored
Natural selection is not the only
mechanism through which populations
evolve
• Gene flow is the movement of alleles between
populations due to immigration and
emmigration
• Gene flow keeps neighboring populations
similar.
• Low gene flow increases the chance that two
populations will evolve into different species
• Gene flow moves alleles from one population to
another.
Natural selection is not the only
mechanism through which populations
evolve
1. Genetic drift—allele frequency changes due
to random event—much more important in
small populations
• Genetic drift changes allele frequencies due to
chance alone.
Hardy-Weinberg equilibrium describes
populations that are not evolving
1. No mutations occur
2. Individuals do not enter or leave the population
(no immigration or emigration)
3. The population is large (no genetic drift)
4. Individuals mate randomly (no sexual selection)
Sexual selection—in general females choosing
males—peacocks with large feathers, lions with
largest mane—extreme traits attract female
5. All traits are equally successful so natural
selection does not occur
• If these conditions are met, the frequency of
the alleles does not change
However, these conditions rarely occur, so the
gene pool changes (equilibrium is disrupted)
New species can arise when populations are isolated.
• Divergence—the accumulation differences
between groups
• Geographic isolation—condition in which 2
populations of the same species do not breed
with each other due to geographic separation
• As natural selection occurs, members of a
species become so different over time that
they no longer can interbreed and become
separate species
Behavioral barriers can cause isolation.
called behavioral isolation
includes differences in courtship or mating
behaviors
• Hybridization is the crossing of two different
species.
– occurs when individuals of closely related species
mate—rarely occurs in nature
– Example—Mule (cross between mare and
donkey)
– Many are sterile and cannot reproduce
• The effects of natural selection add up over
time.
Patterns of evolution
• Coevolution—species in close association
(predators and prey, parasite and host)
• Convergent evolution—the environment
selects similar phenotypes, even though the
ancestors were very different
• Divergent evolution—species become more
and more different, usually due to differing
habitat
Convergent evolution
• Sharks and porpoises have similar appearance
but very different genetic background
Divergent evolution
• Example--Darwin’s finches—beaks differ by
diet
Factors that drive natural selection
1. All populations have genetic variation-due to
mutations, crossing over, independent
assortment, random fertilization.
2. The environment presents challenges to
successful reproduction
3. Individuals tend to produce more offspring
than the environment can support
4. Individuals better able to cope with the
challenges presented by their environment
tend to leave more offspring
Tempo of evolution
• Gradualism—evolution is a gradual process
that occurs continuously
• Punctuated equilibrium (catastrophism)—
major environmental changes cause evolution
to occur rapidly followed by periods in which
successful species change little