Assignment Chapter 7 Life History + Evolution Bring outline to class!

This lecture: parts of Ch 6 + 13: Population: Evolutionary Unit

Darwin’s finches…

Population Ecology

Genetics Evolution

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)

Cyanide experienced by an insect population

Cyan-resistantindividuals

Non-Cyan-resistant

TIMEUse these words to explain evolution of cyanide resistance.

• 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. 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.

2. 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.

3. The results support the hypothesis.4. The population is strongly affected by natural selection;

gene flow is not strong because the insect has limited mobility.

5. 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.

Then …subject to genetic drift

Reducedvariation

Population bottlenecks often result in reduced or no genetic variation (e.g. cheetahs).

Genetic drift:5 of 10 plants 2 of 10 plantsleave offspring 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.

Allelebecomesfixed = novariation.

+ genetic drift

***Summarize the results. What is the potential consequence for small populations?

Population size

% polymorphicgenes

***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