Why Some Moths Turned Dark When Soot Blackened the Trees

Individuals in a population are never exactly alike. When some variations help their owners survive and reproduce, those traits become more common over generations, and the population slowly changes. Genetics later explained how that variation arises and how it can be measured.

This lesson covers Darwinism and natural selection, Neo-Darwinism with the Hardy-Weinberg principle, and the stages of human evolution.

Darwinism states that organisms produce more offspring than can survive, that individuals vary, that those with favourable variations survive and reproduce more in the struggle for existence, and that this natural selection gradually produces new species.

Salient features of Darwinism:

- Overproduction — populations can grow very fast, but resources are limited
- Struggle for existence — individuals compete for food, space and mates
- Variation — no two individuals are identical, and some variations are inherited
- Survival of the fittest — better-suited individuals leave more offspring; fitness means reproductive success
- Origin of species — favourable variations build up over many generations, forming new species

Examples of natural selection:

- Industrial melanism — in industrial areas where soot darkened tree trunks, dark peppered moths were better hidden from birds and became more common, while in clean areas the light form stayed common
- Antibiotic resistance — bacteria carrying resistance genes survive treatment and multiply
- Pesticide resistance — insects resistant to DDT survive spraying and pass on that resistance

An everyday example. Doctors in India urge patients to finish the full course of antibiotics, because stopping early lets partly resistant bacteria survive and spread.

The substance. Natural selection does not create variation — it only acts on variation that already exists in a population.

Neo-Darwinism combines natural selection with genetics, explaining variation through mutation and recombination, and the Hardy-Weinberg principle states that allele frequencies stay constant in a large, randomly mating population unless evolutionary forces act on it.

Neo-Darwinism (the modern synthetic theory):

- Gene mutations and chromosomal changes create new variation
- Genetic recombination during sexual reproduction shuffles alleles
- Natural selection favours some variants over others
- Reproductive isolation separates populations into new species

Hardy-Weinberg principle:

- For a gene with two alleles, the frequencies $p$ and $q$ satisfy $p+q=1$
- Genotype frequencies follow $p^2+2pq+q^2=1$, where $p^2$ is AA, $2pq$ is Aa and $q^2$ is aa
- The balance is disturbed by gene flow, genetic drift, mutation, genetic recombination and natural selection

Worked example. In a population, 16 per cent of individuals show a recessive trait (aa).

$$q^2=0.16\Rightarrow q=0.4,p=1-0.4=0.6$$

$$p^2=0.36,2pq=2\times 0.6\times 0.4=0.48$$

So 36 per cent are homozygous dominant and 48 per cent are carriers.

An everyday example. Thalassaemia carrier screening in India uses this kind of calculation to estimate how many people in a community may carry the allele.

The substance. Genetic drift matters most in small populations — chance alone can change allele frequencies sharply, as when a few individuals found a new population in what is called the founder effect.

Human evolution passed through ape-like forms such as Dryopithecus and Ramapithecus, the Australopithecines, and then Homo habilis, Homo erectus, Neanderthal man and Homo sapiens, with a steady increase in brain size and a shift to upright walking.

Stages:

- Dryopithecus — more ape-like; hairy and walked like gorillas and chimpanzees
- Ramapithecus — more human-like in its teeth and jaws
- Australopithecines — lived in East African grasslands; probably hunted with stone weapons but ate mainly fruit
- Homo habilis — the earliest species placed in the genus Homo; brain capacity 650 to 800 cc; probably did not eat meat
- Homo erectus — brain capacity about 900 cc; probably ate meat
- Neanderthal man — brain capacity about 1400 cc; lived in East and Central Asia; used hides for protection and buried their dead
- Homo sapiens — modern humans, who spread across continents, created cave art and later developed agriculture and settled life

Trends in human evolution:

- Increasing brain size and intelligence
- Upright posture and walking on two legs
- Smaller jaws and changes in the teeth
- Development of language, tools and culture

An everyday example. Rock paintings in the Bhimbetka shelters of Madhya Pradesh are among the traces left by early modern humans in India.

The substance. Humans did not evolve from today's chimpanzees — humans and chimpanzees share a common ancestor, and both lineages have evolved since they split.