Alfred Russel Wallace independently came up with the theory of evolution by natural selection. He worked around the world gathering evidence, and he is best known for his work on warning colouration in animals and for his theory of speciation. In 1858 he and Darwin published joint writings, which prompted Darwin to publish On the Origin of Species the following year.
The ideas of Darwin and Wallace had a huge impact on biology. They gave scientists a single explanation for how species change over time and for the huge variety of living things. Wallace did pioneering work on speciation, and more evidence gathered since has led to our current understanding of it.
Speciation is the formation of a new species. It happens in steps. First, populations of the same species become isolated, for example by a physical barrier such as a mountain range or a stretch of sea. This is called geographical isolation. The populations can no longer breed with each other, so genes are not shared between them.
Next, the environment of each population is different, so natural selection favours different characteristics in each one. Over many generations the populations become more and more different. Eventually the two populations are so different that they can no longer interbreed to produce fertile offspring. They are now separate species.
