Darwin and Wallace

Alfred Russel Wallace was a scientist who proposed the theory of evolution by natural selection independently of Charles Darwin. In 1858, Darwin and Wallace worked together to publish a scientific paper. The following year, Darwin published his book, On the Origin of Species. This introduced the theory of natural selection to a wider audience.

Wallace is best known for:

  • His research and evidence provided on warning colouration in species (such as butterflies). These colours serve as a warning to predators, letting them know that they are toxic or dangerous. This is an example of a characteristic that has evolved by natural selection.
  • Developing the theory of speciation. Although over time, more evidence has led to our current understanding of speciation.


Speciation is an evolutionary process that leads to the formation of new and distinct species. When populations within a species can no longer successfully interbreed to produce fertile offspring, they are considered different species.

A new species can emerge if two populations of the same species are isolated, which can be:

  • Reproductive isolation (sympatric speciation)
  • Geographical isolation (allopatric speciation)

Reproductive isolation is when populations are unable to breed and produce fertile offspring due to differences in behaviour or physiological processes. For example, if two populations of the same species develop different mating rituals (e.g. mating schedules), it can permanently isolate them over time. This typically happens when both populations are in the same geographical area

An environmental event, such as a flood or an earthquake, can cause members of a species to be physically separated. So, they will have to live apart from each other. This is called geographic isolation.

When Geographic isolation happens:

1. The population divides into smaller groups, which are exposed to different environments.

  • These environments may differ in several ways. For example, environment A may have a warmer climate and more food availability than Environment B.

2. If the conditions in either environment are more favourable for reproduction, the population in that environment is likely to grow faster.

3. Over time, the alleles that produce the best advantages are selected. Whereas, the alleles that do not produce an advantage are selected against. Gradually, each population becomes adapted to their specific environment.

  • So, the phenotypes of each population will change due to natural selection

4. At this point, the two groups will have become so distinct that producing fertile offspring together is impossible.

  • Therefore, the two populations are considered to be separate species

Darwin’s Finches

Charles Darwin studied birds on the Galapagos Islands, collecting evidence on the speciation of finches. Although the finches were isolated from one another by the ocean, Darwin noticed variation among them.

Each species of finch has a different beak that is adapted to the type of food that it eats. For example, some finches have long, thin beaks, allowing them to eat cactus flowers without being poked. Whereas, some finches have blunt beaks to crack the shells of nuts and seeds.

Based on these observations, Darwin concluded that the finches became separate species due to geographical isolation.

The finches arrived on different islands and the phenotypes of each population changed due to natural selection. They adapted to their new environments and eventually became so distinct that they could no longer interbreed.

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