Lecture 5 Natural selection – theory and definitions

 

Charles Darwin Alfred Russel Wallace

Charles Darwin (1809 - 1882)

Evolution by Natural Selection

1.  Variation among individuals

2.  Different survival and/or reproduction

3.  Change in genetic composition of population

4.  Evolution

Natural selection: Facts and inferences

Fact 1. Natural populations have large excess reproductive capacities. Fact 2. Population sizes generally remain stable. Fact 3. Resources are limiting. Inference 1. A severe struggle for existence must occur.

Robert Thomas Malthus (1766-1834)

An Essay on the Principle of Population, 1798

Natural selection: Facts and inferences

Fact 1. Natural populations have large excess reproductive capacities. Fact 2. Population sizes generally remain stable. Fact 3. Resources are limiting. Inference 1. A severe struggle for existence must occur.

Fact 4. An abundance of variation exists among individuals of a species. Fact 5. Some of this variation is heritable.

Gregor Mendel (1822-1884)

Experiments with Plant Hybrids (1865)

Natural selection: Facts and inferences

Fact 1. Natural populations have large excess reproductive capacities. Fact 2. Population sizes generally remain stable. Fact 3. Resources are limiting. Inference 1. A severe struggle for existence must occur. Fact 4. An abundance of variation exists among individuals of a species. Fact 5. Some of this variation is heritable.

Inference 2. Genetically superior individuals outsurvive and outreproduce others. Inference 3. Over many generations, evolutionary change must occur in the population.

A definition of natural selection:

“changes in the relative frequencies of different genotypes (genes) in a population because of differences in the survivorship and/or reproduction of their phenotypes”.

At what level does natural selection act?

- organisms may be decomposed into two components - the genotype and the phenotype.

- genotype is the hereditary material, or set of genetic instructions, that determine an organism’s structural, physiological, and behavioral characteristics.

- the phenotype represents the physical expression of a particular genotype.

- it results from an interaction between genotype and environment.

- a genotype may thus produce a number of different phenotypes depending on the environmental conditions.

Some important principles of natural selection

Some important principles of natural selection

1. Natural selection (usually) acts at the level of individuals, not populations.

Some important principles of natural selection

1. Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve.

Some important principles of natural selection

1. Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. 3. Natural selection is retrospective and cannot predict the future.

Some important principles of natural selection

1. Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. 3. Natural selection is retrospective and cannot predict the future. 4. Natural selection is not necessarily progressive.

Darwinian fitness = the number of gene copies a phenotype places into the next generation. Relative fitness = a phenotype’s Darwinian fitness relative to other phenotypes.

- relative fitness is simply a standardized form of Darwinian fitness - we are usually interested in quantifying how well a genotype does compared, or relative to, other genotypes.

- the relative fitness of a genotype simply summarizes how well a genotypes does in transmitting genes to the next generation compared to other genotypes. - as an example, consider three genotypes A, B, and C.

Genotype Generation 0 Generation 1 Relative fitness ! !No. freq !No. freq!

A ! !10 0.33 !5 0.17 !5/15 = 0.33!B ! !10 0.33 !10 0.33 !10/15 = 0.67!C ! !10 0.33 !15 0.50 !15/15 = 1.0!

! !30 ! !30!

What is the measure of the biological success of a genotype?

What is fitness?

What is fitness? 1. Fitness is a description not an explanation

What is fitness? 1. Fitness is a description not an explanation - the concept of fitness is not circular!

What is fitness? 1. Fitness is a description not an explanation 2. Fitness is an average property

What is fitness? 1. Fitness is a description not an explanation. 2. Fitness is an average property. 3. Total fitness is comprised of several individual components:

What is fitness? 1. Fitness is a description not an explanation. 2. Fitness is an average property. 3. Total fitness is comprised of several individual components:

• viability • fecundity • longevity • mating success

Darwin’s finches

The vampire finch, Geospiza difficilis septentrionalis

Photo by Greg Lasley

Example: Medium ground finches on Daphne Major

Is the finch population variable?

Is some of the variation heritable?

Is there evidence for fitness differences?

Is there evidence for fitness differences?

Was there selective mortality?

Did the population evolve?

1.  Directional selection

2. Purifying selection 3. Balancing selection

Natural selection at a single locus

1.  Directional selection

a form of selection where an advantageous allele enters a population and displaces the previously existing allele(s).

-  example: AZT resistance in the HIV-1 virus.

Natural selection at a single locus

Natural selection at a single locus

1. Directional selection

• a form of selection acting on advantageous mutations.

• the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0).

Example: Genotype: AA Aa aa

Fitness: wAA wAa waa

1.0 1.005 1.010

• here, the small a allele would reach fixation in about 3,000 generations.

2. Purifying selection

a form of selection acting to eliminate harmful (deleterious) alleles from natural populations.

-  example: human recessive diseases like Tay-Sachs or porphyria.

Natural selection at a single locus

Natural selection at a single locus

2. Purifying selection

• a form of selection acting against deleterious (harmful) alleles.

• the majority of deleterious alleles are recessive.

• purifying selection drives deleterious recessives to low frequencies where they are maintained at mutation-selection balance:

rate of introduction = rate of removal by mutation by selection

e.g., Tay-Sachs disease, cystic fibrosis, etc.

3.  Balancing selection

various forms of natural selection that actively maintain genetic variation in natural populations.

- example: human sickle cell anemia (an example of

overdominance).

Natural selection at a single locus

Natural selection at a single locus

3. Balancing selection

- various forms of selection that lead to the active maintenance of genetic variation in natural populations.

- alleles are said to be “balanced” because a stable equilibrium state is reached.

- if allele frequencies are perturbed from this equilibrium, selection will return them back to that state.

- overdominance occurs when fitness of the heterozygote exceeds either homozygote. Alleles: HbA = normal hemoglobin allele HbS = sickle cell allele Genotypes: Relative fitness HbA HbA: susceptible to malaria 0.88 HbA HbS: resistant to malaria, 1, experiences mild anemia HbS HbS: susceptible to severe anemia 0.12

Forms of Balancing selection 1. Overdominance

Forms of balancing selection 2. Frequency-dependent selection

Forms of balancing selection 2. Frequency-dependent selection

• the relative fitnesses of genotypes are not constant but vary with their frequencies in the population.

Forms of balancing selection 2. Frequency-dependent selection

• the relative fitnesses of genotypes are not constant but vary with their frequencies in the population. Genotype: AA Aa aa Fitness: wAA wAa waa

1-p2 1-2pq 1-q2

Forms of balancing selection 2. Frequency-dependent selection

• the relative fitnesses of genotypes are not constant but vary with their frequencies in the population. Genotype: AA Aa aa Fitness: wAA wAa waa

1-p2 1-2pq 1-q2

Example: Self-incompatibility (S) loci in flowering plants

S loci in flowering plants

S loci in flowering plants

● leads to obligate outcrossing

S loci in flowering plants

● leads to obligate outcrossing ● at equilibrium, all S alleles occur at equal frequencies

Forms of balancing selection

3. Spatially or temporally varying selection

- some genotypes are more fit than others in some habitats, or under some environmental conditions, than others.

Environment A

Genotype: AA Aa aa Fitness: wAA wAa waa

1 0.95 0.91

↑ ↓ gene flow

Environment B

Genotype: AA Aa aa Fitness: wAA wAa waa

0.84 0.93 1

Environment A

Genotype: AA Aa aa Fitness: wAA wAa waa

1 0.95 0.91

↑ ↓ gene flow

Environment B

Genotype: AA Aa aa Fitness: wAA wAa waa

0.84 0.93 1

Example: The Lap locus in the mussel, Mytilus edulis

Lap cline in Mytilus edulis in Long Island Sound