Transcript Population Genetics and Speciation

Population Genetics and
Speciation
Chapter 16
Evolution
• O. Understand the mechanisms that can cause
change in the genetics of a population.
• J. Explain the 4 steps in the process of natural
selection as explain by Darwin
Evolution and Genetics
• Microevolution: the change in the
collective genetic material of a
population.
• Populations are the smallest unit in
which evolution occurs.
Section 1
• Population biologists study many different
traits in population such as size and color
• Traits vary and
can be mapped
along a bell curve,
which shows that
most individuals
have average
traits, whereas a
few individuals
have extreme
traits.
What causes variation?
• Could be environmental….but most often it is
heredity
• Variations in genotype arise in three different
ways:
– Mutation
– Recombination
– Random pairing of gametes
Gene Pool
the total genetic information available
in a population
This can be used to determine the
frequency of the alleles for a particular
trait in a population
• Assume a gene pool of 10 gametes for a gene
which is controlled by only 2 alleles
– 8 are allele A
– 2 are allele a
– Frequency of A is 8/10 or .8
– Frequency of a is 2/10 or .2
– How many light blue individuals would you expect
in a population of 100? 0.2 X 0.2 = .04 or 4 light
blue individuals.
Hardy-Weinberg: showed allele
frequencies don’t change unless they
are acted on by outside forces.
• So… a population that is not evolving:
– 1.
– 2.
– 3.
– 4.
– 5.
no mutation
no individuals enter or leave the population
the population is very large
individuals mate randomly
selection does not occur
Does this ever happen?
NO!
Disruption of Genetic Equilibrium
• Mutation: affect genetic equilibrium by
producing totally new alleles for a trait.
• Gene flow: the movement of individuals into
or out of a population causes gene flow– the
process of genes moving from one population
to another
• Genetic Drift: the change of allele frequencies
in a population due to random chance.
Happens most often in small populations.
Nonrandom Mating
Mating is nonrandom whenever individuals may
choose partners.
Sexual selection occurs when certain traits increase
an individual success at mating.
Sexual selection explains the development of traits
that improve reproductive success but that may
harm the individual
bright colors = sexy
bright colors = more visible to
predators.
Natural Selection
• Can influence evolution in one of three
general patterns.
– Stabilizing selection favors the formation of
average traits
– Disruptive selection favors extreme traits rather
than average traits.
– Directional selection favors the formation of
more-extreme traits.
Formation of Species
• A species is a population of organisms that can
successfully interbreed but cannot breed with
other groups
• Geographic isolation results from the
separation of population subgroups by
geographic barriers. Geographic isolation may
lead to allopatric speciation.
• Reproductive isolation results from the
separation of population subgroups by barriers to
successful breeding.
• Reproductive isolation may lead to sympatric
speciation.
• In the gradual mode of speciation, species
undergo small changes at a constant rate.
• In punctuated equilibrium model, new species
arise abruptly differ greatly from their ancestors
and then don’t change for long period of time.
Rates of Speciation
Gradualism
Punctuated Equilibrium