Why Dark Moths Took Over Soot-Covered Forests

Knowing that life has evolved raises a deeper question: what drives the change? Theories from use and disuse to natural selection, combined with genetics and population mathematics, now explain how species change — and how humans themselves arose.

This part covers Lamarck, Darwin and the modern synthesis, variation and types of selection, the Hardy-Weinberg principle, and human evolution.

Lamarck proposed that organs change through use and disuse and that these acquired changes are inherited, whereas Darwin proposed that heritable variation exists in every population and nature selects individuals that survive and reproduce better; the modern synthetic theory adds mutation and recombination as the sources of that variation.

Lamarck — use and disuse:

- Organs used constantly grow stronger; unused organs weaken
- Acquired characters are inherited
- Rejected because acquired characters, such as strong muscles from exercise, are not inherited

Darwin — natural selection:

- Populations show variation, some of it heritable
- Organisms produce more offspring than can survive, leading to a struggle for existence
- Individuals better suited to their surroundings survive and reproduce more — fitness means reproductive fitness
- Over many generations, favourable traits spread and new species can arise

Modern synthetic theory:

- Combines natural selection with genetics
- Variation comes from mutation and genetic recombination in sexual reproduction
- Natural selection, gene flow, genetic drift and isolation then act on that variation

An everyday example. A wrestler's powerful muscles are not passed on to the children, which is why Lamarck's idea of inheriting acquired characters fails.

The substance. Natural selection does not create variation — it only selects among variations already produced by mutation and recombination.

Natural selection favours whichever heritable variants survive and reproduce best in a given environment — dark moths spread on soot-darkened trees, and resistant microbes and insects spread after heavy use of antibiotics and pesticides — and it can act in stabilising, directional or disruptive ways.

Moths and industrial pollution:

- Where tree trunks were covered with pale lichens, white-winged moths blended in and outnumbered dark moths
- In industrial areas, soot darkened trunks and killed the lichens, so dark-winged moths blended in while white moths were easily picked off by birds
- No new variant appeared — the existing dark variant was selected

Resistance:

- Antibiotics kill susceptible bacteria, leaving any resistant variants to multiply
- Pesticides similarly select resistant insects

Types of natural selection, for a trait with a bell-shaped distribution:

- Stabilising — favours the average; extremes are removed and the curve narrows
- Directional — favours one extreme; the average shifts that way
- Disruptive — favours both extremes; two peaks develop

An everyday example. Stopping an antibiotic course halfway can leave the toughest bacteria alive to multiply — selection happening inside your own body.

The substance. Resistance is selected, not caused — the antibiotic does not make bacteria resistant; it removes those that are not.

The Hardy-Weinberg principle states that allele and genotype frequencies in a population stay constant across generations unless something disturbs them, so for two alleles with frequencies p and q, p + q = 1 and p² + 2pq + q² = 1; gene flow, genetic drift, mutation, recombination and natural selection disturb this equilibrium and bring about evolution.

The equation:

$$p^2+2pq+q^2=1$$

- $p^2$ — frequency of homozygous dominant AA
- $2pq$ — frequency of heterozygous Aa
- $q^2$ — frequency of homozygous recessive aa

Factors that disturb equilibrium:

- Gene migration or gene flow — alleles enter or leave with moving individuals
- Genetic drift — chance changes in frequency, strongest in small populations; the founder effect arises when a few individuals start a new population with a different gene pool
- Mutation — creates new alleles
- Genetic recombination — produces new allele combinations during gametogenesis
- Natural selection — changes frequencies through differential reproduction

Worked example. In a population, $16$ percent of individuals show a recessive trait, so $q^2=0.16$:

$$q=\sqrt{0.16}=0.4,p=1-0.4=0.6,2pq=2\times 0.6\times 0.4=0.48$$

So $48$ percent are heterozygous carriers and $p^2=0.36$, or $36$ percent, are homozygous dominant.

An everyday example. Small, isolated communities that marry mostly within the group can show unusually high frequencies of certain rare disorders — an effect of genetic drift and the founder effect.

The substance. Hardy-Weinberg describes a population that is not evolving — its real value is as a baseline for measuring change.

Human evolution passed from ape-like Dryopithecus and more man-like Ramapithecus to the upright-walking Australopithecines, then to Homo habilis, Homo erectus and Neanderthal man, and finally to Homo sapiens, with brain capacity, tool use and culture increasing along the way.

The landmarks:

- Dryopithecus — hairy, walked like gorillas and chimpanzees; more ape-like
- Ramapithecus — also hairy, but more man-like
- Australopithecines — lived in East African grasslands, hunted with stone weapons and ate mainly fruit
- Homo habilis — brain capacity about 650 to 800 cc; probably did not eat meat
- Homo erectus — brain capacity about 900 cc; probably ate meat
- Neanderthal man — brain size about 1400 cc; lived in the Near East and Central Asia, used hides to protect the body and buried the dead
- Homo sapiens — arose in Africa and spread across continents, later developing cave art, agriculture and settlements

An everyday example. Prehistoric rock paintings at Bhimbetka in Madhya Pradesh preserve the art and culture of early modern humans in India.

The substance. Humans did not evolve from modern chimpanzees — humans and chimpanzees share a common ancestor.