Evolution by Natural Selection

How did the enormous variety of life on Earth come about, and how do living things come to be so well suited to where they live? The answer is evolution by natural selection — one of the most important ideas in all of biology, first set out in detail by Charles Darwin. This lesson, part of Topic B5 of OCR Gateway Science A, explains the theory step by step, shows it at work in a present-day example you can actually observe — antibiotic-resistant bacteria such as MRSA — and explains why the theory took time to be accepted. It builds directly on variation and mutation from the previous lesson.

By the end of this lesson you should be able to state the theory of evolution by natural selection, set out the logical sequence of steps, explain how antibiotic resistance arises and spreads, and explain why Darwin's theory was only gradually accepted.

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The Theory of Evolution

Evolution is the gradual change in the inherited characteristics of a species over many generations. The mechanism that drives it is natural selection, proposed by Charles Darwin (and independently by Alfred Russel Wallace) and published in Darwin's book On the Origin of Species in 1859.

Over a very long time, natural selection can change a species so much, or split a population into groups that change in different ways, that new species form. The modern theory also recognises that the variation natural selection acts on ultimately comes from mutations in DNA — something Darwin himself did not know about, because genes had not yet been discovered.

Exam Tip: Be careful with the word "evolution". It means a change in the inherited characteristics of a population over many generations, not a change within a single individual's lifetime. An individual organism does not evolve; populations evolve over generations.

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The Logic of Natural Selection

Natural selection follows a clear, logical sequence. Learn these steps in order, because exam questions very often ask you to put them together into a full explanation.

flowchart TD
  A["Variation:<br/>individuals in a species show variation<br/>(caused by differences in alleles)"] --> B["Competition / struggle to survive:<br/>more offspring are produced than can survive;<br/>they compete for food, mates and space"]
  B --> C["Survival of the fittest:<br/>individuals with advantageous characteristics<br/>are more likely to survive and reproduce"]
  C --> D["Inheritance:<br/>survivors pass on the alleles for the<br/>advantageous characteristics to their offspring"]
  D --> E["Over many generations:<br/>the advantageous alleles become more common,<br/>so the species changes (evolves)"]

In words:

1. Variation. The individuals of a species show variation in their characteristics, caused by differences in their alleles (and ultimately by mutation).
2. Competition / the struggle to survive. Organisms produce more offspring than can survive, so there is competition for resources such as food, water, mates and space; many individuals do not survive.
3. Survival of the best-suited. Individuals with characteristics that make them better suited to their environment (for example, faster, better camouflaged, or better at finding food) are more likely to survive the competition and live long enough to reproduce.
4. Inheritance. These survivors pass on the alleles for their advantageous characteristics to their offspring.
5. Change over generations. Over many generations, the advantageous alleles become more common in the population, while less useful ones become rarer, so the characteristics of the species gradually change — the species evolves.

A useful example is a population of rabbits in which a few, by chance, have slightly better hearing. If predators are common, the better-hearing rabbits are more likely to detect danger, survive and breed, passing on the alleles for good hearing. Over many generations, good hearing becomes the norm in the population.

Exam Tip: The phrase "survival of the fittest" does not mean the strongest. "Fittest" means best suited to the environment — which might mean best camouflaged, fastest, most resistant to a disease, or best at getting food. Use "better suited to their environment" or "have a survival advantage" for full marks.

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A Present-Day Example: Antibiotic-Resistant Bacteria

One of the best pieces of evidence that natural selection really happens is that we can watch it occur in bacteria. Bacteria reproduce extremely quickly, so evolution that would take thousands of years in a large animal can happen in a matter of days. The rise of antibiotic-resistant bacteria — such as MRSA (methicillin-resistant Staphylococcus aureus) — is natural selection in action.

Antibiotics are medicines that kill bacteria, and they have saved countless lives. But bacteria are now becoming resistant to them, which is a serious problem for medicine. Here is how resistance arises and spreads, step by step — exactly the natural-selection sequence applied to bacteria:

flowchart TD
  A["A random mutation occurs in one bacterium,<br/>making it resistant to the antibiotic"] --> B["The antibiotic is used;<br/>it kills the non-resistant bacteria"]
  B --> C["The resistant bacterium survives<br/>(it has a survival advantage)"]
  C --> D["The survivor reproduces rapidly,<br/>passing on the resistance allele"]
  D --> E["Over time the whole population<br/>becomes resistant to the antibiotic"]

1. By chance, a random mutation in a bacterium produces a new allele that makes that bacterium resistant to a particular antibiotic.
2. When the antibiotic is used, it kills the bacteria that are not resistant, but the resistant bacterium survives — it has a survival advantage.
3. The surviving resistant bacterium reproduces (very rapidly), passing the resistance allele to its offspring.
4. Over time, more and more of the bacteria carry the resistance allele, until the whole population is resistant and the antibiotic no longer works.

This is exactly why doctors are careful about prescribing antibiotics. To slow down the development of resistance, antibiotics should not be over-used or used for non-bacterial (e.g. viral) infections, and patients should always complete the full course so that all the bacteria are killed and none of the more resistant ones are left to survive and reproduce. New strains like MRSA are difficult to treat because they have become resistant to several antibiotics.

Exam Tip: Antibiotic resistance is the classic exam example, and the marks come from the natural-selection sequence: a random mutation makes one bacterium resistant; the antibiotic kills the non-resistant ones; the resistant one survives and reproduces; resistance spreads through the population. Do not invent figures for how fast this happens — describe the mechanism.

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How New Species Form

If natural selection can change a population over time, what happens if a single species becomes split into two separate populations that no longer mix — for example, if a river changes course, or some individuals reach an island? Each population now lives in its own environment and experiences its own selection pressures. Different characteristics are advantageous in each place, so over many generations natural selection changes the two populations in different ways, and their gene pools drift apart. Eventually the two populations can become so different that, even if they meet again, they can no longer interbreed to produce fertile offspring — at which point they are classed as two different species. This formation of a new species is called speciation.

This idea ties the whole topic together. The variation within each population comes ultimately from mutation; natural selection acts on that variation differently in the two environments; and over a very long time the accumulated differences produce new species — which is exactly how the huge diversity of life on Earth has arisen from common ancestors. You do not need the detailed mechanisms for GCSE, but you should understand that populations becoming separated, followed by natural selection acting differently on each, can lead to new species over many generations.