13: Natural Selection

Natural selection

“Survival of the fit enough.”

Inevitable consequence of inherited variance in fitness.

The only process that leads to adaptation.

Acts upon the phenotype, not the genotype.

Ribozymes

Relics of an early RNA world, in which RNA acted as both enzyme and hereditary material.

Normally require magnesium ions (Mg²⁺) to facilitate DNA splicing.

Ribozyme experiment

Scientists switched the presence of magnesium ions (Mg²⁺) with calcium ions (Ca²⁺).

Originally, there was no (or very little) DNA spliced in the substrate.

After several generations, the frequency of a particular allele became more prevalent and allowed the substrates to be spliced in the presence of calcium.

Fitness

The number of offspring left after one generation, or after a chosen time interval.

Fitness components

Different organisms have different fitness components.

Infectious disease: R = number of infectious particles × probability of transmission; if R > 1, then the disease spreads.

Human fitness: W = probability of surviving to adulthood × probability of finding a mate × number of offspring.

Types of fitness

Darwinian:

• Fitness on a discrete scale.

• Most animal and plant populations reproduce in generations.

Malthusian:

• Fitness on a continuous scale.

  • Bacteria divide continuously, thus a different parameter has been introduced.

Absolute fitness

Determines the rate of growth or decline of the population.

Equilibrium means that the population is steady (not growing or declining).

For haploid organisms, Darwinian fitness equilibrium = 1.

For diploid organisms, Darwinian fitness equilibrium = 2.

Relative fitness

The ratio (W₁ / W₂) between the absolute fitness of two populations in discrete time; or the difference in growth rates in continuous time.

Compares how one deme is doing compared to another.

Can also compare fitness across time, between the parental (or ancestral) population and its daughter population.

Selection coefficients (s)

Differences in relative fitness.

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Example of selection coefficient

Absolute fitness:     W₁ = 1.25     W₂ = 2.0

Relative fitness:     W₁ = 1     W₂ = W₁ / W₂ = 1.25 / 2 = 0.625

s  =  1 - 0.625  =  0.375

Fisher’s Fundamental Theorem

Determines how selection increases the fitness of a population and the key measure of the overall extent of selection.

ΔŴ = var_a (W) / Ŵ

• ΔŴ = increase in mean fitness due to natural selection.

• var_a = additive genetic variance in fitness.

  • W = mean fitness (average).

Additive genetic variance

Equation that determines genetic variance of a specific allele in a population.

Selection as an evolutionary force

Only process that causes adaptation.

Reduces variation and is not random.

A gradual process that works by increasing the frequency of alleles that are individually favorable; does not just pick the “best.”

Way selection works

The ratios of allele frequencies q:p change in proportion to the fitness of the alleles.

Increases the frequency of alleles that are individually favorable.

Slight differences in fitness eventually become significant.

Frequency-dependent selection

Favors phenotypes that are either common (+fds) or rare (-fds).

Fitness depends on many factors, which can change over time.

Adaptive landscape

The idea of the fitness landscape can be misleading because the landscape is not fixed and depends on many genes.

On the adaptive landscape, populations evolve to peaks.

Models of selection (quantitative traits)

Stabilizing

Directional

Disruptive

Balancing

Stabilizing selection

Favors intermediate trait values (normal distribution).

Acts to reduce variation.

Directional selection

Favors one allele over another.

Normal distribution that shifts over.

Disruptive selection

Favors extreme values; distribution is high on sides and low in middle.

Acts to increase variation.

Balancing selection

Multiple alleles are preserved; distribution is a long, flat top.

Maintains polymorphism.