AQA GCSE Biology: The Theory of Evolution and Speciation

This whole guide is Triple award only. The theory of evolution took a long time to be accepted, and several scientists shaped it. This guide covers Darwin's theory and why it was resisted, Lamarck's rejected alternative, Mendel's overlooked work on inheritance, and how new species form. The theme is how scientific ideas develop and are accepted over time.

Darwin's theory

Charles Darwin proposed the theory of evolution by natural selection, published in On the Origin of Species in 1859. It was based on his observations during a worldwide voyage, on experiments, and on growing knowledge of geology and fossils.

His theory was only gradually accepted, for three reasons:

• it challenged religious beliefs;
• there was insufficient evidence at the time to convince other scientists;
• the mechanism of inheritance was not known until about 50 years later.

The third reason is the scientific one, and it is the one worth understanding. Darwin could see that offspring resemble their parents and that some individuals survive better than others, but he had no idea how a characteristic was passed on. Without that, a critic could reasonably ask why an advantage would not simply be diluted away in each generation. The answer was already sitting in Mendel's work, unread.

Alfred Russel Wallace independently arrived at a similar theory, and he and Darwin published joint writings in 1858. Wallace also did important work on warning colouration in animals and on speciation.

Lamarck's theory

Lamarck proposed an earlier theory, now disproved, that changes acquired during an organism's lifetime could be inherited, for example that giraffes stretched their necks and passed the longer necks on. We now know that acquired characteristics cannot be inherited.

The reason is simple once you know about gametes. A characteristic acquired during life is a change to body cells, and only gametes pass DNA to the next generation. Stretching a neck, building muscle or losing a limb does not alter the DNA in the sex cells, so none of it is handed on.

Mendel's work

Gregor Mendel, in the mid-19th century, showed through experiments on pea plants that inheritance involves separate units (what we now call genes) passed unchanged to offspring. His work went unrecognised for a long time because chromosomes and DNA had not yet been discovered, it was published in obscure journals, competing theories existed, and he was not an established scientist.

The word unchanged is the important one. Mendel's units do not blend, which is exactly what Darwin's theory needed: an advantageous characteristic is kept intact in the offspring rather than being watered down. Chromosomes were seen behaving the right way in the early 1900s, and by the 1950s DNA had been identified as the molecule involved, at which point the two ideas became one.

Speciation

Speciation is the formation of a new species, and it happens when populations become separated.

• Populations become geographically isolated, so they can no longer interbreed.
• The environments on each side differ, so natural selection acts differently on each.
• Over time, and with different mutations, the two populations become so genetically different that they can no longer interbreed to produce fertile offspring, and they are now separate species.

The barrier can be a river, a mountain range, a stretch of sea, or a new island. What matters is only that genes stop flowing between the two groups.

After that, three things push them apart. Different mutations arise in each population, because mutation is random. Natural selection favours different characteristics, because the conditions are not the same. And nothing is mixing the two sets of changes back together. The moment the two populations could no longer produce fertile offspring even if they met again is the moment they became separate species.

How this fits together

The theory needed evidence to be accepted, and that evidence, from fossils and from living examples like antibiotic resistance, is the subject of the fossils and evidence for evolution guide.