Evolution by natural selection, speciation and selective breeding

Evolution is the change in inherited features of a population over time, leading over many generations to new species. The mechanism, first proposed by Charles Darwin in On the Origin of Species (1859), is natural selection.

Natural selection — Darwin's logic

There are five steps you must be able to write out fluently:

1. Variation. Individuals in a population show genetic variation (different alleles).
2. Competition / overproduction. More offspring are produced than can survive (food, mates, space are limited).
3. Selection. Individuals with characteristics best suited to their environment are more likely to survive ("survival of the fittest") — fittest meaning best-adapted, not strongest.
4. Reproduction. Survivors pass on their advantageous alleles to their offspring.
5. Change over time. Over many generations, the favourable alleles become common in the population — the species evolves.

A classic GCSE example is the peppered moth during the Industrial Revolution: dark moths became common in polluted areas because they were camouflaged on soot-covered trees, while light moths were eaten by birds.

Antibiotic-resistant bacteria — natural selection happening now

Bacteria evolve quickly because they reproduce rapidly. The story (a near-certain exam question):

1. Random mutations sometimes give a bacterium antibiotic resistance.
2. When the antibiotic is used, susceptible bacteria die; resistant bacteria survive.
3. The survivors reproduce, passing on the resistance allele.
4. Over time, the whole population becomes resistant.

To slow this:

• Don't prescribe antibiotics for viral infections.
• Always finish the course — incomplete courses leave the more resistant bacteria alive.
• Restrict use in farm animals.
• Develop new antibiotics.

MRSA is a famous example, resistant to most antibiotics.

Speciation — how natural selection creates new species

A species is a group of organisms that can interbreed to produce fertile offspring.

Speciation usually requires isolation of two populations:

1. A barrier (mountain, river, distance) splits the population.
2. The two populations face different environments → different selection pressures.
3. Mutation and natural selection act independently on each.
4. After many generations, the two populations are so different that they can no longer interbreed to produce fertile offspring.

Famous example: Darwin's finches on the Galápagos islands — different beak shapes evolved on different islands depending on food sources.

Selective breeding (artificial selection)

Humans have been doing the same thing as natural selection — but on purpose — for ~10,000 years.

The process:

1. Choose individuals with the desired characteristic.
2. Breed them together.
3. Select the best offspring.
4. Repeat over many generations.

Examples:

• Disease-resistant wheat.
• Cows that produce more milk or meat.
• Domestic dogs from grey wolves (in only ~15,000 years).
• Larger or sweeter fruit.

Risks of selective breeding (and inbreeding)

Inbreeding — breeding closely related individuals — reduces the gene pool. Risks:

• More inherited diseases (recessive alleles meet up).
• Less variation in the population.
• Population is vulnerable to a new disease (no resistant individuals).

A classic example: pedigree dogs with hip dysplasia or breathing problems.

Links

Builds on B6.5 (variation, mutation). Leads to B6.7 (genetic engineering), B6.8 (cloning), B6.9 (Darwin/Wallace, evidence for evolution) and B6.10 (classification).