the contributions of Charles Darwin and Alfred Russel Wallace to the theory of evolution by natural selection; selection pressures, variation, differential reproductive success, fitness, adaptation, and the change in allele frequency over time

What this dot point is asking

VCAA wants the historical contributions of Darwin and Wallace, the mechanism of natural selection (variation, heritability, selection pressure, differential reproductive success), the meaning of fitness and adaptation, and the outcome as a change in allele frequency over time.

The answer

Evolution is heritable change in a population over generations. Natural selection is the main mechanism, proposed independently by Charles Darwin and Alfred Russel Wallace in the 1850s.

Darwin and Wallace

Charles Darwin (1809-1882) developed his ideas during the voyage of HMS Beagle (1831-1836), particularly from observations of finches and tortoises in the Galapagos Islands. He proposed that species change over time through descent with modification, driven by natural selection. He delayed publication for over twenty years while gathering evidence.

Alfred Russel Wallace (1823-1913) independently proposed essentially the same theory while working in the Malay Archipelago. In 1858 he sent a paper to Darwin outlining his ideas. Their work was presented jointly at the Linnean Society in 1858, and Darwin published On the Origin of Species in 1859.

Both scientists drew on:

• Observations of biogeography (similar species in similar habitats, distinct species on different continents).
• Artificial selection (selective breeding by farmers and pigeon fanciers).
• Thomas Malthus's idea that populations grow faster than resources, leading to a struggle for existence.

The conditions for natural selection

Natural selection occurs whenever four conditions are met:

1. Variation. Individuals in a population differ in their traits. Most variation arises from sexual reproduction (meiosis, fertilisation) and mutation.
2. Heritability. The variation must have a genetic basis so it can be passed to offspring. Environmental variation alone is not enough.
3. Selection pressure. Some environmental factor (predation, disease, food availability, climate, mate choice) affects survival or reproduction differently for different phenotypes.
4. Differential reproductive success. Individuals with favourable phenotypes leave more viable, fertile offspring than individuals with less favourable phenotypes.

Over generations, the alleles that contribute to favourable phenotypes become more common, and the alleles for less favourable phenotypes become rarer. The population's gene pool changes. This is evolution by natural selection.

Selection pressures

A selection pressure is any factor that causes differential survival or reproduction. Common examples include:

• Predation. Faster gazelles escape lions more often.
• Disease. Resistant bacteria survive antibiotic treatment.
• Competition. Plants with deeper roots reach water in drought.
• Climate. Mammals with thicker fur survive cold winters.
• Mate choice (sexual selection). Males with brighter plumage attract more mates.

Fitness

Fitness is the relative reproductive success of an individual or genotype: how many viable, fertile offspring it leaves compared with others. It is measured by descendants, not by strength or longevity.

A male peacock with a long tail has high fitness if his tail attracts mates, even though it slows him down. A sterile worker ant has zero direct fitness but high inclusive fitness because it helps relatives reproduce.

Adaptation

An adaptation is a heritable trait that increases an organism's fitness in its environment. Adaptations can be:

• Structural (camouflage colouration, thick fur, sharp teeth).
• Physiological (heat tolerance, ability to digest cellulose, antibiotic resistance).
• Behavioural (migration, courtship displays, tool use).

Adaptations arise because individuals with favourable alleles reproduce more, so those alleles become more common. Adaptation is the outcome of natural selection acting over generations.

Change in allele frequency

Evolution at the population level is measured by a change in allele frequency in the gene pool. If the allele for grey beetle colour rises from 30 per cent to 80 per cent of the population over twenty generations, the population has evolved.

A population that is not evolving meets the conditions of the Hardy-Weinberg equilibrium: no mutation, no migration, no genetic drift, random mating, and no selection. Real populations rarely meet all five conditions, so allele frequencies almost always change to some degree.

Other mechanisms of evolution

Although natural selection is the focus, three other mechanisms also change allele frequency:

• Genetic drift. Random changes in allele frequency, especially in small populations.
• Gene flow. Movement of alleles between populations by migration.
• Mutation. Introduces new alleles at a low rate.

Natural selection is the only mechanism that produces adaptation to the environment.

Worked example

Antibiotic resistance in bacteria is a textbook case of natural selection in action:

1. Variation. A bacterial population contains a few individuals with random mutations that make them resistant to an antibiotic.
2. Selection pressure. A patient takes the antibiotic. Susceptible bacteria are killed.
3. Differential reproductive success. Resistant bacteria survive and continue to reproduce, doubling every 20 minutes.
4. Heritability. Resistance is encoded in DNA and is inherited.
5. Change in allele frequency. After a few days, almost all bacteria in the infection are resistant. The resistance allele has gone from less than 1 per cent to nearly 100 per cent. The population has evolved.

This is why doctors warn against incomplete antibiotic courses.

Common traps

Saying organisms evolve "in their lifetime". Individuals do not evolve. Populations evolve. An individual cannot change its alleles after birth.

Saying organisms evolve "because they need to". Selection is not goal-directed. Variation already exists; the environment then favours some variants over others.

Confusing fitness with strength. Fitness is reproductive success, not physical fitness. A weak individual that leaves many offspring has higher fitness than a strong one that leaves few.

Saying mutation drives evolution alone. Mutation provides variation. Natural selection acts on that variation. Both are needed.

Forgetting heritability. A tan from sunbathing is variation, but it is not heritable, so natural selection cannot act on it.

In one sentence

Natural selection, proposed by Darwin and Wallace, occurs when heritable variation in a population is exposed to a selection pressure that produces differential reproductive success, so that fitter (more reproductively successful) phenotypes pass on their alleles more often than less fit phenotypes, leading to a change in allele frequency over generations and the accumulation of adaptations to the environment.