6.3.2 Speciation

The work of Wallace

1. Alfred Russel Wallace travelled the world collecting evidence that supported the developing theory of evolution.
2. He independently reached the idea of natural selection, and his joint work with Darwin in $1858$ prompted Darwin to publish.
3. Two ideas in particular made his name: warning colouration in animals, and an early theory of speciation.
4. His pioneering ideas on speciation have since been refined as more evidence has been gathered.

What is a species

1. A species is a group of similar organisms that can breed together to produce fertile offspring.
2. Two different species may sometimes breed, but their offspring are infertile.

How a new species forms

1. Speciation begins when two populations of one species become isolated, often geographically by a river, a mountain range or the sea.
2. Once isolated, the two populations can no longer interbreed, so their genes are kept separate.
3. The two environments usually differ, so different alleles are advantageous in each.
4. Natural selection acts separately on each population, and new mutations add different alleles, so the populations become increasingly different.
5. Eventually they are so different that, even if brought back together, they can no longer interbreed to produce fertile offspring.
6. At this point they are two separate species, and speciation has occurred.