You will have to imagine that the icecaps have melted and the earth now has 2 island continents separated by oceans. On the coast of each continent are domestic cats with no human intervention and no contact with the cats on the other island. The cats hunt normal prey and because they live on the coast they also do a bit of swimming to catch fish.

An Adaptation

On one island, there is a random mutation in one cat that gives it webbed feet. This makes it a stronger swimmer and being a stronger swimmer means it is more successful at catching fish. This is an adaptation because a body part has changed slightly. The cat with the webbed feet catches fish in 2 out of every 3 fishing trips. The cats without webbed feet only catch fish in 1 out of every 3 fishing trips. This means the web-footed cat is better nourished than the non-web-footed cats. Being stronger makes it more successful at breeding. If the web-foot cat was a male it can drive away other male cats and mates with several females. If it was female it has stronger kittens and can feed them better so that more survive. Either way, the web footed cat has an adaptation which gives it an advantage when competing with the other cats for food and mates. After a number of years, the non-web-footed cats have become extinct and there are only web-footed cats. They breed so successfully they spread along the coast into new territories and spread inland, especially near rivers.

Meanwhile, on the other island, there isn’t a web-foot mutation. The cats continue to hunt and fish just as they always did. Because they are not competing against web-footed cats they are not at any disadvantage.

Evolution

Over the next 50,000 years random mutations occur among the cats on each island (say one mutation every 500 years on average). Perhaps some cats have a mutation that makes them better able to digest fish. It’s equally likely that there is a mutation that means they can’t digest fish very well. Some mutations given them the competitive edge over non mutant cats. Some mutations put them at a disadvantage. It all depends on whether the cats are in an environment where the mutation is helpful or not helpful. The majority of mutations are neutral – not an advantage or a disadvantage – but there is a change to the DNA even if it doesn’t seem to cause any physical change. Natural selection means that advantageous or neutral mutations will be passed on to the next generations, but disadvantageous mutations will die out.

During that time the earth’s crust is moving and the islands eventually join together. The cats look exactly the same except that cats on one island have webbed feet and cats on the other island don’t have webbed feet. But something odd has happened – the cats can’t breed with each other! Because the cats on each island have had different mutations during 50,000 years their DNA has become different from each other and is now incompatible. While they have been separated they have evolved into two different species. They can co-exist alongside each other because the web-footed species have become specialist fishing cats and rarely hunt land animals or birds. However the non-web-footed species does not depend so much on fish and they can thrive by hunting mice, birds, reptiles etc. The two species now occupy different ecological niches where they don’t compete for food (a fishing niche and a non-fishing niche) and can occupy different territories (a waterside territory or an inland territory) and they will continue to evolve separately. Because they can’t interbreed their difference will become greater and greater due to random mutations over the next several thousand years.