How can One thing become Two?

One of the ‘arguments’ against evolution is some peoples reluctance to accept that one species can give rise to two separate species. How can, they say, one thing become two things? They’re usually happy with the idea that creatures can change over time, become a different colour and so on but how can something change into something else?

To answer that question we need to start with an understanding of what a species actually is:

A species is a group of organisms that can interbreed in nature to produce a fertile offspring. Stated in another way, species are reproductively isolated groups of populations. Organisms classified in the same species have very similar gene pools.

So, for example, mice & rats are separate species because they can’t interbreed. Horses and donkeys can produce offspring but the resultant mule is sterile and so the breeding isn’t viable. Oak trees can’t interbreed with Elm trees, dogs can’t interbreed with cats and so on.

The best way to show that one species can become two is by example.

Imagine a group of 100 River Fish happily living in a fresh water river. They are a single species and can easily interbreed with each other. They are adapted for their particular environment and are prospering. But during a particularly heavy rainy season a flood sweeps down the river washing 50 of the fish into the much saltier estury. Almost immediately half of the fish die from the shock of suddenly being exposed to salt water. The remainder have the ability provided by their genes to cope with higher salt levels than their dead relatives but all do not cope equally well. Of the 25 only 5 can cope adequately with their new environment. They are far from perfectly adapted but can still manage to function. The other 20 though are suffering. They are sluggish and prone to disease, both of which make them easy prey. Few successfully breed. Of the 5 fish better suited to their new environment most successfully breed but many of the resulting eggs die quickly due to the levels of salt in the water. Of the eggs that hatch many produce sluggish ill fish that are quickly taken by predators but a significant percentage are like their parents and are able to survive and grow to breeding age. A smaller percentage are actually better adapted to the new conditions than their parents and prosper by both laying more viable eggs and producing more viable offspring.. and so it goes on.

Each new generation of Estury Fish takes a year to grow to maturity. After 100 generations most of the eggs are hatching and most of the hatchings are fairly well adapted to the salty water. After 500 generations – or 500 years – few if any non-salt adapted offspring are being produced and there is a growing number of fish now fully adapted to the salt water. Some are becoming adapted in other ways too by changing fin shape and colour and by growing in size.

Even at this early stage it would be difficult for descendents of the original River Fish to meet and sucessfully breed with the descendents of the original Estury Fish though probably not impossible, at least not yet. However, if we move forward another 500 generations you can see that the two types of fish are moving further and further apart. Due to adaptation to their environment and genetic drift, whereby differences accumulate over time, the Estury Fish are now quite different from the River Fish though both share a common ancestor. When another flood surge washes a group of River Fish into the Estury the survivors meet a type of fish they have had no experience of, know nothing of their mating rituals, and would fail to fertilise their eggs even if they tried.

Over 1000 generations – only 1000 years – and original species of River Fish has given rise to a new species of Estury Fish as well as descendents still living in the river. In this way one species becomes two.