Transcript CB-Evolution of Populations

Evolution of
Populations
I. Genetic Variation
A. Review
1.
2.
3.
4.
Genes control traits
Many genes have more than one version (allele)
Many traits are controlled by more than one gene
A variation is an inherited trait that makes an
individual different from other members of the
species and results from a mutation in the
organism’s genes.
Variation
B. Important Terms
1. Population = Group of individuals of the
same species that can interbreed
2. Gene pool = all the genes, including
alleles, present in a population
3. Relative frequency = number of times an
allele occurs in a gene pool
Sample Population
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
Frequency of Alleles
allele for
brown fur
allele for
black fur
Populations and Gene Pool Movie
4.
Single-gene trait – trait that is controlled by a
single gene that has two alleles
a) As a result, only two phenotypes are
possible in a population for this gene
5.
Polygenic trait – trait that is controlled by two
or more genes
a) As a result, one polygenetic trait can have
many possible genotypes and
phenotypes
C. Important Concepts
1. Populations, not individuals evolve over time
2. Microevolution is a generation-to-generation
change in a population’s frequencies of
alleles and genotype
3. The more variation in a population  the
more likely some members of the species will
survive if the environment changes
Evolution in Action
II.
Sources of Genetic Variation
A. Natural Selection – can affect phenotype in 3 ways
1.
Stabilizing - environment selects against extremes
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
DRAW
2. Directional – environment selects a phenotype at
end of the curve
DRAW
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
3. Disruptive- environment selects against the
average trait, so over time there will be 2 new
phenotypes at the extremes
DRAW
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
B. Genetic drift - over time, a series of chance
occurrences can cause an allele to become more
common in a population
C. Gene flow - Populations gain or lose alleles due to
migration of individuals between populations
D. Non-random mating – Inbreeding or selective
breeding for specific phenotypes (purebred dogs)
E. Mutations
 All of these sources of genetic variation
cause changes in the gene pool
Gene Pool Changes Movie
III.
Speciation “creation of a new
species”
A. Species –organisms that are similar in physical
appearance, have the same number of
chromosomes and can interbreed to produce
fertile offspring.
B. How do you get a new species?
1. You must isolate populations for a long time.
2. Isolation can happen in several ways:
a. Behavioral isolation - 2 populations can
interbreeding but have differences in courtship
or other reproductive strategies that involve
behavior
i. Ex: Western and Eastern meadowlarks are
the same species, but use different songs to
attract mates
b. Geographic isolation - populations are
separated by physical barriers
(i.e. a river splits a valley in two)
c. Temporal isolation– different mating times
i. Ex: 3 similar species of orchid release
pollen on different days
d. Anatomical Isolation
3. With time the two isolated populations change
independently and eventually become different
species.
Speciation Movie
IV. Patterns of Evolution
A. Extinction
1. More than 99 % of all species that have ever lived
have gone extinct
2. Reasons: competition or environment changes
3. Effect of mass extinctions – disappearance of many
species leaves habitats wide open providing
opportunity for organisms that survived
B. Adaptive radiation
1. Process by which a single species evolves into
several different species (ex: Galapagos finches)
Adaptive radiation
C. Convergent evolution
1. Produces unrelated organisms that look similar
2. When organisms face similar environmental changes,
such as moving through water, natural selection
favors adaptations that have similar functions
a. Dissimilar appendages with similar functions:
flippers of a dolphin, wings of a penguin, and fins
of a shark
Convergent Evolution
D. Co-evolution
1. 2 species evolve in response to changes in each
other over time
a. Ex: flower types only being able to reproduce if their
shape, color, and odor attracts the right type of pollinator
E. Punctuated equilibrium
1. Darwin thought evolution was only slow,
gradual change (gradualism), but we have
evidence this is not always the case
2. Punctuated equilibrium = pattern of long,
stable periods interrupted by brief periods of
more rapid change
a. Ex: surviving organisms evolving rapidly after
mass extinctions