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.

Examples in context

Example 1. Industrial melanism in Australian peppered moths. Although first studied in 1850s England, peppered moth (Biston betularia) variants in Australia show the same selection mechanism. In Melbourne suburbs with cleaner air, light-coloured moths blend with pale lichen and avoid bird predation. Near the Yallourn brown-coal power stations of the 1970s, soot darkened tree trunks and dark-form moths survived better; allele frequency for the dark allele rose to 80 percent locally. After 1990 emission controls, lichen returned and the light form recovered. The case demonstrates Darwin and Wallace's mechanism: variation existed in the population (the rare dark allele), selection pressure (predation) differed by environment, and differential reproductive success changed allele frequencies over generations.

Example 2. Antibiotic resistance in Staphylococcus aureus at Doherty Institute. Doherty Institute infectious disease researchers track methicillin-resistant Staphylococcus aureus (MRSA) in Victorian hospitals. Each year a few resistant bacteria arise spontaneously by mutation in the mecA gene. In hospitals using methicillin, sensitive strains die and the rare resistant ones survive and reproduce. Within months, resistance allele frequency in the hospital population rises from less than 1 percent to over 50 percent. This is natural selection in real time: variation (mutation), selection pressure (antibiotics), differential reproductive success (resistant strains grow), and fitness measured as offspring count. Hospital antibiotic stewardship programs aim to reduce selection pressure by restricting antibiotic use.

Try this

Q1. State the four conditions required for natural selection to operate, as proposed by Darwin and Wallace. [4 marks]

• Cue. Heritable variation; more offspring than the environment can support; differential survival and reproduction; reproduction passes traits to the next generation.

Q2. In a wheat field, 80 percent of plants carry rust-resistance allele R. Calculate (a) allele frequency of R, (b) expected genotype frequencies under Hardy-Weinberg, and (c) the new allele frequency if 10 percent of resistant plants and 50 percent of susceptible plants die from rust. [3 marks]

• Cue. (a) p = 0.8, q = 0.2. (b) $p^2 = 0.64$ RR, $2pq = 0.32$ Rr, $q^2 = 0.04$ rr. (c) After selection, allele frequency for R rises - solve from differential survival.

Q3. Refer to MRSA in hospitals. (a) Explain how variation in resistance arose initially. (b) Identify the selection pressure. (c) Suggest two strategies hospitals use to slow the rise of resistance. [2+2+2 marks]

• Cue. (a) Spontaneous mutation in mecA gene; existed at low frequency before antibiotic exposure. (b) Methicillin (or other beta-lactams). (c) Antibiotic stewardship (reduce unnecessary prescribing); strict hygiene to prevent transmission of resistant strains.