Evolution is an important topic on the AQA and OCR specs. It can be difficult to understand because the evidence for this theory lies very much in the distant past. Often the best way to understand a conceptual theory is to see real examples.

We teach about the general idea of divergent evolution by natural selection – the mutation led change to an individual, that gives it a selective advantage, so it survives to reproduce and pass the advantageous mutation onto the next generation. After many generations, the advantageous feature that the mutation was responsible for, becomes established in the population. So, this random process gives us a new species that has evolved from a pre-existing species. If we look back over millions of years we can see how many of today’s species have common ancestors. The idea of branching out from a common ancestor gives us divergent evolution.

But there is another format of evolution that creeps into exam questions – that of convergent evolution. As the environmental conditions change, different features become advantageous – so natural selection is driven by selection pressures. A situation can arise where two completely unrelated species evolve the same advantageous features simply because they are both exposed to the same selection pressure. There are some lovely examples of this in Biology that help to understand the way evolution works by natural selection. My favourite is the extra ‘thumb’ that a red panda and giant panda share – perfect for eating bamboo.

This article discusses another very interesting example – there have been five separate evolution events that have ended up with a crab! It’s called carcinization – evolution into a crab-like form. This might have happened because this body shape is very advantageous because it provides stability, protection, defence and an efficient movement along the seabed. This is convergent evolution because different species are evolving features that are more like one type of species, even though they are not closely related. The selection pressures that have driven this convergent evolution are: the threat from predators, water currents, accessing prey. Being a crab seems to solve a set of physical problems very nicely indeed!

Download the article from the popular mechanics page here