The hierarchical nature and processes of
different levels of ecological systems: now we
focus on the population…
This lecture: parts of Ch 16/26:
Population: Evolutionary Unit
Population Ecology
Genetics Evolution
Darwin’s finches…
Objectives
• Understand (micro)evolution and its relation to
genetics
• Sources of genetic variation
• Forces causing change in gene frequency in pop
Natural selection
Small population size
Assortative mating
Gene flow
*** A ‘pre-test’ on phenotypic variation,
genetic variation, natural selection,
fitness, evolution, adaptation (pre-adapt)
Non-Cyanresistant
Cyanide experienced by
an insec tpopulation
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?
***How much genetic variation exists?
***Why is genetic variation important?
• In changing environments, the reservoir
of genetic variation may take on positive
survival value.
• Rapid environmental change by humans
may exceed the capacity of a population
to respond by evolution --> extinction
***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.
• Mutations are likely to be 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.
***What forces can cause change in genotype
frequency?
1) Natural selection --> differentiates
subpopulations
2) Effects of small population size
a) Genetic drift
b) Founder effect
c) Population bottlenecks
3) Assortative (non-random) mating
4) Gene flow (= dispersal/migration) --> homogenizes
subpopulations
Evolution by natural selection…
Finches beak size changes in response to
change in seed hardness in drought years.
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
Types of natural selection:
Intermediates
most fit
Most common
in unchanging
environments;
Removes variation
Extremes
most fit
Removes
genetic
variation
Genetic drift:
5 of 10 plants
leave offspring
Generation 1
=.7, q =.3
Generation 2
p = .5, q = .5
2 of 10 plants
leave offspring
Generation 3p
p = 1.0, q = 0
Change in allele frequency due to random variation
in births and deaths.
p
Population Bottleneck: period of small pop. size.
…subject to genetic drift
Population bottlenecks often result in reduced
or no genetic variation (e.g. cheetahs).
Small populations experience genetic drift,
founder events, and population bottlenecks.
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
• 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.
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 animal or
gametes
• Plants: dispersal of pollen and seeds
Summary of forces
• Remove genetic variation:
•
Natural selection
•
Small population size
• Maintain genetic variation:
•
Mutation
•
Gene flow
•
Varying selective pressures in
time and space
Genetic structure (differentiation) of
populations is determined by ecological
factors, e.g. heavy metals from mines causing
natural selection (foreground).
Sample exam question:
• In the previous picture the plants in
the foreground are tolerant of heavy
metals; those in the background are
not. Use the 7 words “to define on
the lecture outline” to write a
scenario whereby the original plant
population got subdivided into two as the one in the foreground evolved
tolerance to heavy metals.
***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.
***Sample exam question…acclimation
3 species grown in both hot + moderate temp;
then PS rate of both groups of plants was
measured at a range of temperatures.
Red: raised in hot T
Blue: raised in moderate T
1. What is the major question being
addressed in this experiment?
2. Describe how Larrea and Tiderstromia
responded relative to the temperatures at
which it was grown.
3. What is the likely mean temperature (high
or moderate) of Larrea and Tiderstromia?
4. What is the likely temperature range
during the year (high or low) of Larrea and
Tiderstromia? Explain.
5. What is the major conclusion of the
experiment?