6.17 - Darwin & Wallace

Darwin and Wallace

• Charles Darwin proposed the theory of evolution by natural selection in the nineteenth century.
• He developed it after around five years on a round-the-world expedition on HMS Beagle, studying plants, animals, and fossils in many different places.
• The idea was supported by years of further experiment and discussion, and by the developing science of geology and fossils.

Darwin's key observations

1. Members of a species show a wide range of variation.
2. Organisms must compete for limited resources.
3. The individuals with characteristics best suited to their environment are the ones more likely to survive, breed, and pass those characteristics on to their offspring.

He reasoned that, over many generations, the helpful characteristics become more common and the species gradually changes - it evolves. He set this out in On the Origin of Species (1859), bringing together a large body of evidence.

Why Darwin's theory was incomplete

• The relevant science did not yet exist, so Darwin could not explain why new characteristics appeared, nor exactly how an organism passed beneficial characteristics on to its offspring.
• Genes and mutations weren't discovered until about fifty years after the theory was published.
• The inheritance mechanism we now know underpins natural selection was therefore a gap in Darwin's account.

Why the theory was only gradually accepted

• It was controversial - it challenged the widely held religious belief that God had made all the animals and plants on Earth, offering instead the first convincing explanation of how life could develop without a creator.
• There wasn't yet enough evidence to convince many scientists, as little other research had been done into how organisms change over time.
• The unknown mechanism of inheritance left an obvious hole in the argument.

Acceptance came later, once genetics showed how characteristics are passed on. The theory is now widely accepted and underpins much of modern biology, including the idea that all living things share common ancestors and the way organisms are classified by how closely related they are.

Alfred Russel Wallace

• Alfred Russel Wallace was a scientist working at the same time as Darwin who independently arrived at the same idea of natural selection.
• He travelled the world making observations that provided evidence for the theory.
• He is especially known for his work on warning colouration - the bright colours some species, such as certain butterflies, use to deter predators, itself a characteristic that evolved by natural selection.
• He also did early work on speciation - the way new species form.
• Wallace and Darwin published their work on evolution jointly in 1858, acknowledging each other's contributions. This prompted Darwin to bring out On the Origin of Species the next year.
• Because that book set out the case so fully, Darwin is usually the better remembered of the two.

Lamarck's rejected hypothesis

• Darwin wasn't the only person to suggest how evolution might happen. Earlier, Jean-Baptiste Lamarck put forward a rival hypothesis.
• Lamarck's idea: characteristics an organism acquires during its lifetime are passed on to its offspring.
  1. If a feature was used a lot, it would become more developed during the organism's life.
  2. That more-developed feature would then be inherited by the offspring.
• Example: Lamarck reasoned that if a rabbit used its legs a lot to run from predators, its legs would grow longer, and its offspring would be born with longer legs.

Why Lamarck's hypothesis was rejected

• Experiments didn't support it - a characteristic gained during life is not passed on.
• For example, if you dyed an animal's fur a different colour, its offspring would still be born with the normal colour, because the new feature isn't inherited.

Why Darwin's hypothesis was accepted

• The later discovery of genetics explained how organisms born with beneficial characteristics can pass them on through their genes.
• Further evidence supported it:
  • The fossil record showed how organisms changed slowly over time.
  • Bacteria evolving resistance to antibiotics gave an observable example of natural selection at work.

What this tells us about how science develops

• Scientists can form different hypotheses from similar observations, partly because they bring different beliefs and influences to the same evidence.
• The way to decide between competing hypotheses is to look for evidence that supports or disproves each one.
• Over time, the weight of evidence settled the question in Darwin's favour and against Lamarck's.