Natural Selection

Natural selection is the primary mechanism of adaptive evolution. It acts on the phenotypic variation within a population, favouring individuals whose traits confer a survival or reproductive advantage. Over successive generations, natural selection changes allele frequencies in the population, driving adaptation to the environment.

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Darwin's Theory of Natural Selection

Charles Darwin's theory of evolution by natural selection can be summarised as follows:

1. Overproduction: Organisms produce more offspring than the environment can support.
2. Variation: Individuals within a population vary in their phenotypes (much of this variation is heritable).
3. Struggle for existence: Resources (food, space, mates) are limited, leading to competition.
4. Survival of the fittest: Individuals with phenotypes better suited to the environment are more likely to survive and reproduce.
5. Differential reproduction: Those individuals that survive pass their advantageous alleles to the next generation.
6. Accumulation: Over many generations, advantageous alleles increase in frequency, and the population becomes better adapted to its environment.

Key Definition: Fitness in biology refers to an organism's ability to survive and reproduce in a particular environment. An organism with high fitness produces more viable offspring that themselves survive to reproduce. Fitness is always relative to the environment — a trait that is advantageous in one environment may be disadvantageous in another.

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Types of Natural Selection

Natural selection can act on a population in three main ways, depending on which phenotypes are favoured.

1. Directional Selection

Directional selection occurs when individuals at one extreme of the phenotypic range have higher fitness than the rest of the population. Over time, the mean phenotype shifts in the direction of the favoured extreme.

Characteristics:

• The frequency distribution curve shifts to one side.
• The mean value of the trait changes.
• Genetic variation may decrease as alleles at the disfavoured extreme are removed from the population.

Example — Antibiotic Resistance in Bacteria:

1. A population of bacteria shows variation in their susceptibility to an antibiotic. Most are susceptible, but a few carry a mutation conferring resistance.
2. When the antibiotic is applied, susceptible bacteria are killed (selection pressure).
3. Resistant bacteria survive and reproduce, passing the resistance allele to their offspring.
4. Over many generations, the frequency of the resistance allele increases until most of the population is resistant.
5. The mean level of resistance has shifted — this is directional selection.

Key Point: Antibiotic resistance is not caused by the antibiotic — the antibiotic does not induce mutations. The mutations arise randomly. The antibiotic acts as a selection pressure that favours individuals that already possess the resistance allele.

2. Stabilising Selection

Stabilising selection occurs when individuals with the intermediate (average) phenotype have the highest fitness, and those at both extremes are selected against.

Characteristics:

• The mean phenotype does not change.
• The range (variance) of the phenotype decreases.
• The frequency distribution curve becomes narrower and taller.

Example — Human Birth Weight:

• Babies with a very low birth weight have reduced survival due to poor thermoregulation and underdeveloped organs.
• Babies with a very high birth weight have an increased risk of complications during delivery.
• Babies of intermediate birth weight (~3.0–3.5 kg) have the highest survival rate.
• Stabilising selection maintains the average birth weight over time, reducing variation.

3. Disruptive Selection

Disruptive selection occurs when individuals at both extremes of the phenotypic range have higher fitness than those with the intermediate phenotype.

Characteristics:

• The frequency distribution curve develops two peaks (bimodal distribution).
• The population may eventually split into two distinct groups.
• Disruptive selection can be a precursor to speciation if the two groups become reproductively isolated.

Example — Beak Size in African Seed-Crackers (Pyrenestes ostrinus):