Download 05 Evolution 2010

Assignment
Chapter 7 Life History + Evolution
Bring outline to class!
This lecture: parts of Ch 6 + 13:
Population: Evolutionary Unit
Population Ecology
Genetics Evolution
Darwin’s finches…
The hierarchical nature and processes of
different levels of ecological systems: now we
focus on the population…
Objectives
• Understand (micro)evolution and its relation to
genetics
• Sources of genetic variation
• Forces causing evolution:
•
(change in allele frequency in population)
Natural selection
Gene flow
Small population size
Assortative mating
• Forces causing populations to:
•
Differentiate
•
Homogenize
*** A ‘pre-test’ on phenotypic variation,
genetic variation, natural selection,
fitness, evolution, adaptation (pre-adapt)
Non-Cyanresistant
Cyanide experienced by
an insect population
Cyan-resistant
individuals
Use these words to explain
evolution of cyanide resistance.
TIME
• What is definition of (micro)evolution?
•
Change in allele frequency in a
population through time…
• What must be present for natural selection to
cause evolutionary change?
•
Genetic (allele) variation
• What is the ultimate source of genetic variation?
•
Mutation, a change in nucleotide in DNA
•
---> change in amino acid it specifies
•
---> change in phenotype of organism
Genetic variation is also produced by
chromosome recombination during 1)meiosis
and by 2) fertilization.
Does this slide show phenotypic or
genotypic variation? What underlies
different patterns of coloration (different
phenotypes) in the population?
***Why is genetic variation important?
• In changing environments, the reservoir
of (neutral) genetic variation may take on
positive survival value.
***Are most mutations beneficial? Are
most mutations dominant? What
happens to harmful mutations?
• Most mutations are harmful and recessive; natural
selection weeds out most deleterious alleles,
leaving those that best suit organisms to their
environments.
• Some mutations are neutral. They may become
beneficial when the relationship of the organism to
its environment changes; organism is pre-adapted
to change.
• Selection for beneficial mutations is the basis for
evolutionary change, enabling organisms to
exploit new environmental conditions.
Predict which morph increases in frequency with:
heavy coal use (soot on bark?)
light coal use? What type of selection occurs?
Types of natural selection:
Natural selection:
• change in the frequency of traits in a
population because of differential survival
and reproduction of individuals with those
traits.
• Individuals with the most offspring are
selected and the proportion of their genes
increases over time.
• Fitness: the genetic contribution by an
individual to future generations.
• Relative fitness: Maximum = 1 = most fit
***What forces can cause change in genotype
frequency (= cause evolution)?
1) Natural selection --> differentiates
subpopulations
2) 2) Gene flow (= dispersal/migration) -->
homogenizes subpopulations
3) Effects of small population size
a) Genetic drift
b) Founder effect
c) Population bottlenecks
4) Assortative (non-random) mating
Gene flow (migration)--> mixes alleles
between subpopulations and homogenizes
them.
• Maintains genetic variation
• ***What represents gene flow in
animals?
plants?
• Animals: dispersal of the adult or larval
animal or gametes
• Plants: dispersal of pollen and seeds
***Sample exam question.
A species of scale insects extracts fluids from
branches of pine trees. They have very limited
movement. In an experiment, these insects were
transplanted 1) between branches of the same pine
tree, and 2) from one pine tree to another pine tree
of the same size.
1. State the hypothesis/prediction that was being
tested as an “If…then…”
2. Summarize the results in one concise sentence.
3. Do the results support the hypothesis?
4. Predict whether gene flow or natural selection
would be a more powerful force affecting the
genetic structure of this insect. Explain your
choice.
5. Predict whether the genetic makeup of
populations of the insect on adjacent trees
would be homogeneous or differentiated.
Explain your choice.
Answer to Exam ?
1.
2.
3.
4.
5.
If the insect is locally adapted, then insect survival is be
lower for transplants to other trees than to a branch of
the ‘home’ tree.
The distribution of the proportions of individuals
surviving to 9 mo is closer to 1.0 for transplants to a
branch of the same tree than to another tree.
The results support the hypothesis.
The population is strongly affected by natural selection;
gene flow is not strong because the insect has limited
mobility.
The subpopulations are differentiated because of
different selection forces on different trees and no gene
flow to homogenize the population.
***What forces can cause change in genotype
frequency (= cause evolution)?
1) Natural selection --> differentiates subpopulations
2) 2) Gene flow (= dispersal/migration) --> homogenizes
subpopulations
3) Effects of small population size
a) Genetic drift
b) Founder effect
c) Population bottlenecks
4) Assortative (non-random) mating
Population Bottleneck: period of small pop. size.
Reduced
variation
Then …subject to genetic drift
Population bottlenecks often result in reduced
or no genetic variation (e.g. cheetahs).
Genetic drift:
5 of 10 plants
leave offspring
2 of 10 plants
leave offspring
Generation 1
Generation 2
Generation 3
p =.7, q =.3
p = .5, q = .5
p = 1.0, q = 0
Change in allele frequency due to random variation in births
and deaths.
Small populations experience founder
events, population bottlenecks and genetic
drift. Each causes a loss in genetic variation.
+ genetic
drift
Allele
becomes
fixed = no
variation.
***Summarize the results. What is the
potential consequence for small populations?
% polymorphic
genes
Population size
***What forces can cause change in genotype
frequency (= cause evolution)?
1) Natural selection --> differentiates subpopulations
2) 2) Gene flow (= dispersal/migration) --> homogenizes
subpopulations
3) Effects of small population size
a) Genetic drift
b) Founder effect
c) Population bottlenecks
4) Assortative (non-random) mating
• Assortative mating: when individuals
choose mates non-randomly with respect
to their own genotypes.
• Negative: mates differ genetically -->
increases proportion of heterozygotes
• Most individuals do this to avoid
inbreeding.
• Positive: like mating with like (includes
inbreeding) --> increases proportion of
homozygotes
Positive assortment
• increases the proportion of homozygotes
• unmasks deleterious recessive alleles -->
inbreeding depression (decrease in fitness)
% homozygosity depends on level of
inbreeding.
Summary of forces
• Remove genetic variation:
•
Natural selection
•
Small population size
•
Inbreeding
• Maintain genetic variation:
•
Mutation
•
Outbreeding
•
Gene flow
•
Varying selective pressures in
time and space