Introduction

• Natural selection explains many of the most fascinating things in nature, from the genetic code to the complexities of the human brain

• To completely understand evolution, one must understand the mechanisms of inheritance, according to Darwin.

• The keystone to understanding evolution is the “genetical theory of natural selection”, according to R.A. Fisher.

Natural Selection and Evolution in Real Time

• Evolution can happen both rapidly and over time.

• Artificial selection is the selective breeding of animals and plants by humans to produce desired traits.

Evolution of Selection and Inheritance

• Selection results when there is a consistent relationship between a phenotype and fitness

• Evolution by selection also requires a second ingredient: inheritance. In the simplest terms:

  • If:

    • there is a correlation between a phenotypic trait and the number of offspring that individuals leave to the next generation, and

    • there is a correlation between the phenotype of a trait in parents and in their offspring

  • Then:

    • the trait will evolve.

• Natural selection and evolution are not the same thing.

• If selection on a trait occurs but the trait is not inherited, then evolution will not happen

Fitness: The Currency of Selection

• An individual’s absolute fitness is the number of zygotes (offspring) produced over its lifetime. The symbol W represents absolute fitness.

• Fitness components are one of several events in the life cycle of many organisms that contributes to the determination of fitness, such as survival to maturity, mating success, and fecundity.

• W = (probability that the individual survives to maturity) * (expected number of offspring if the individual does survive)

• The strength of selection is determined by fitness differences. It is the relative (or proportional) differences that matter.

  • An individual who leaves 2 offspring has a fitness of 2

  • This individual has a huge fitness advantage over those with a fitness of 1 but is at a disadvantage over those with a fitness of 4

• Relative fitness is the absolute fitness divided by a fitness reference that is agreed on

  • The choice of the fitness reference is a matter of convenience, and changes depending on the situation under consideration.

• Relative fitnesses play a critical role in determining the speed and outcome of evolution by natural selection.

Positive Selection: The Spread of Beneficial Mutation

• Positive selection is the selection for an allele that increases fitness.

• The rate at which an allele’s frequency changes – that is, the speed of evolution – is determined by the relative fitness advantage of the favored allele.

• Selective sweeps are situations in which a beneficial mutation spreads through a population.

• Fixed (fixation) is the attainment of a frequency of 1 (i.e., 100 percent) by an allele in a population, which thereby becomes monomorphic for the allele.

• The selection coefficient (s) is a natural measure of the strength of the selection that favors the beneficial allele.

The Rate of Adaptation

• We can predict the course of evolution if we know the current state of the population and the strength of selection.

  • Equation: delta p = sp(1-p)

  • Variables: delta p = rate at which the allele frequency evolves

  • s = strength of selection; when s=0, there is no selection acting and delta p = 0 (there is no evolution)

  • p(1-p) = a natural measure of genetic variation; when p=0, there is no genetic variation at this locus and delta p = 0 (there is no evolution); variation is maximized when p=(1-p)=0.5.

• The key conclusion: the rate of evolution is proportional to the strength of selection and the amount of genetic variation.

  • In the absence of either of those two ingredients, there is no evolution by selection.

• When a beneficial allele spreads by selection, the final outcome is that it becomes fixed (it reaches a frequency of 1).

  • Positive selection ultimately eliminates genetic variation, so other evolutionary factors must be responsible for maintaining all the genetic variation in nature.

• The rate at which positive selection causes an allele frequency to evolve depends on dominance.

  • An allele is dominant if it causes the same phenotypic effect when heterozygous as when homozygous.

• Deleterious mutations are mutations that decrease fitness.

  • The same logic and equations that apply to positive selection also apply to deleterious mutations.

• Many genetic diseases in humans are caused by mutations that are nearly or completely recessive.

  • Because they are at low frequency, almost all copies are in heterozygotes who have fitness close to or equal to that of individuals who do not carry the mutation. Selection is therefore very ineffective at removing these disease-causing mutations from the population.

Chance and adaption: The probability that a beneficial mutation spreads

• Even if an allele increases survival on average, any particular individual who carries the allele might not survive.

• When an allele first appears in a population by mutation, it is represented by only a single copy. It may be lost by chance then, or in a later generation while it is still rare.

• The conclusion is that even when a mutation increases fitness, it is not certain that natural selection will cause it to spread to fixation.

Evolutionary Side Effects

• Genetic correlations occur when two traits tend to be inherited together.

  • One cause of genetic correlations is pleiotropy.

• An allele that increases fitness through its effect on one trait sometimes decreases fitness because of its effect on another trait.

  • When there is an evolutionary trade-off, natural selection favors the allele that has the highest fitness overall. As that allele spreads, it will increase some fitness components but have negative effects on others.

Hitchhiking: When one allele goes for a ride with another

• Two loci are in linkage disequilibrium when an allele at one locus is found together with an allele at another locus more often than expected by chance.

  • A consequence of linkage disequilibrium is hitchhiking.

    • This happens when an allele at one locus spreads by natural selection acting on a second locus that is in linkage disequilibrium with the first.

• Hitchhiking is responsible for the evolution of genes that themselves do not impact survival or fecundity, but that do have other effects.

• Standing genetic variation is when an allele that is present in the population is initially not favored, but then suddenly becomes beneficial when conditions change.

  • Before the change, different copies of the mutation will have had time to recombine onto chromosomes with different combinations of alleles at other sites. As a result, when the selected allele reaches fixation, only a very small region of the chromosomes around the selected site shows reduced polymorphism.

When Selection Preserves Variation

• Balancing selection is the selection that maintains genetic variation within a population.

  • Balancing selection is fundamentally different from the selection on beneficial and deleterious alleles, which acts to remove genetic variation.

Overdominance

• Overdominance occurs when the heterozygote has higher fitness than both homozygotes.

  • Through overdominance, the population evolves to a stable polymorphic equilibrium, which means that both alleles are maintained.

Other forms of balancing selection

• Overdominance is one form of balancing selection. A second type can occur with frequency-dependent selection, which occurs when the fitnesses of alleles change depending on their own frequencies.

  • In some cases, an allele gets a fitness advantage when it is rare, a situation called negative frequency dependence.

• Multiple niche polymorphism is when different genotypes specialize in different ecological niches. Each genotype is partly shielded from competition with other genotypes, and so has its own ecological carrying capacity.

Selection That Favors the Most Common

• Balancing selection preserves genetic variation, and in most cases, the population will evolve to the same allele frequency no matter where it begins.

• Historical contingency is the outcome of evolution that is determined by where the population begins.

Underdominance: When heterozygotes suffer

• Heterozygotes for some chromosome rearrangements have lower fertility than either homozygote because their chromosomes fail to pair correctly during meiosis, leading to infertility.

  • When a new rearrangement is still at low frequency, almost all of its copies are in these low fitness heterozygotes. Thus selection acts to eliminate a new chromosome rearrangement when it is still rare.

    • This situation in which heterozygotes have the lowest fitness is called underdominance.

Positive frequency-dependent selection

• When frequency-dependent selection favors the most common allele, this is called positive frequency dependence.

The Evolution of a Population’s Mean Fitness

• The mean fitness of a population is the average fitnesses of the individuals in it.

The fundamental theorem of natural selection and the adaptive landscape

• The increase in mean fitness per generation is equal to the genetic variance for fitness itself. This is called the fundamental theorem of natural selection.

• Natural selection causes populations to evolve so that they become better adapted to their environment: the average survival and reproduction of individuals increase through time.

• The adaptive landscape is a plot created by Wright that tells us how the population will evolve.

  • His key insight was that selection causes the population to evolve uphill on the landscape.

Deleterious Mutations

• The vast majority of mutations that have fitness effects are deleterious.

  • Studies suggest that deleterious mutations are at least ten times more common than beneficial mutations.

A mutation-selection balance

• Purifying selection is the selection that acts to remove deleterious mutations from a population

• Mutation-selection balance is when the flow of new mutations into the population is offset by natural selection that acts to eliminate them.

The mutation load

• The mutation load, represented by L, is the proportion by which the mean fitness of individuals in the population is reduced by deleterious mutations compared with a hypothetical population without mutations.