4.1B - 4.3 - Natural selection and resistance

4.1B — Darwin, Wallace and modern biology

In the nineteenth century, naturalists had evidence that living things varied and that species had changed over long periods, but they needed a convincing mechanism to explain how such change could happen. Charles Darwin and Alfred Russel Wallace each developed the same central answer: natural selection.

Darwin gathered specimens and observations during the voyage of HMS Beagle from 1831 to 1836. He combined this evidence with many years of further study to develop an explanation in which inherited variation affects which organisms leave offspring. Wallace collected and studied organisms in the Amazon and the Malay Archipelago. Working independently, he reached a closely related explanation and sent Darwin an essay in 1858.

Writings from both scientists were presented together at the Linnean Society in 1858. Darwin then published On the Origin of Species in 1859, setting out natural selection and a large body of supporting evidence. Wallace was therefore not Darwin's assistant: he was an independent co-originator of the idea, even though Darwin's name later became more widely associated with it.

Their work changed biology by supplying a natural, testable mechanism for evolution. Modern genetics later explained how inherited variation can arise and be passed on, something Darwin and Wallace could not explain using DNA because genes and DNA were not yet understood. Evolution by natural selection is now a central framework for explaining adaptation and population change, and it helps biologists understand modern problems such as antibiotic resistance.

4.2 — How natural selection works

Natural selection acts on a population, a group of organisms of the same species living in an area. Individuals in the population are not identical. Some of their variation is caused by differences in inherited genetic information, and mutation can introduce a new genetic variant. The variation exists before a particular environmental pressure selects among the individuals; organisms do not produce a useful mutation because they need it.

The mechanism can be followed in a stable order:

1. Inherited variation is present. Individuals have different alleles and may show different characteristics.
2. There is a selection pressure. More offspring are produced than can survive, so factors such as limited food, predators, disease or a drug affect survival and reproduction.
3. Reproductive success differs. Individuals with a characteristic that gives an advantage in that environment are more likely to survive and reproduce. Advantageous is more precise than strongest or fittest.
4. The characteristic is inherited. Those individuals pass the advantageous allele to some of their offspring.
5. The population changes. Over many generations, the allele and its characteristic become more common. This change in inherited characteristics of a population across generations is evolution.

Selection within a cohort is not the whole generational explanation. If susceptible individuals die, the advantageous variant can immediately form a larger fraction of the survivors, even before any reproduction. To explain its inheritance and spread into later generations, you must also describe reproduction and the passing on of alleles. A changed survivor fraction alone does not show that any offspring inherited the variant.

An advantage is always linked to an environment. A characteristic that is useful under one selection pressure may be neutral or disadvantageous if conditions change. Natural selection is therefore not a ladder towards a perfect organism; it is differential survival and reproduction in the current conditions.