Convergent Evolution

I've been working on a project recently which involves convergent evolution, so I thought I'd talk about that.

When we see the same trait in different species it is often because these species have an ancestor in common that developed the trait, passing it on to the species that later develop from it. This is similar to a parent passing on a trait to their children, such as eye colour.

A nice trait to think of in this way is the ability to incubate young in the womb versus laying eggs.

We assume that egg laying was the original condition and that an ancestor species of mammals (another term for mammals which incubate their young in-utero is Eutherians) developed the ability to incubate young in its womb instead - a feature which has a lot of benefits for the safety of the unborn young, such as protection from the elements or from predators. From phylogenetic studies we know that all mammals have a common ancestor which they don't share with reptiles, birds or fish - all of which lay eggs.

There are other species which help to show the development of this trait - Marsupials, for example, which don't lay eggs but incubate their young in external pouches, such as the kangaroo. There is also a further group of mammals which lay eggs (Monotremes), the platypus is an example of this. They are grouped with other mammals because of other mammalian traits they have, such as warm bloodedness, but they show us an intermediate stage of the development of the ability to incubate their young in the womb.


(image from http://palaeo.gly.bris.ac.uk/palaeofiles/marsupials/Index.htm)

Anyway, that was a bit of a sidetrack, because that trait is not an example of convergent evolution. Convergent evolution is when, rather than multiple species inheriting a trait from an ancestor species, the trait evolves independently in different species.

An example of this is the ability to fly. Both birds and bats possess the ability to fly. However, the bat is a mammal and much more closely related to other, non-flying mammals than it is to bats. If you only know this information it would be possible to assume that flight was lost in other mammals and it was an ancestral trait retained in bats. However, if you look at the structure of a bird and a bat wing, you can see that they are very different.


(image from http://evolution.berkeley.edu/evosite/evo101/IIC1Homologies.shtml)

With some traits, like flight, it is fairly simple to determine if covergent evolution has occurred. However with other traits it can be much more difficult, at which point a comparison at the DNA level is needed to determine the similarity between species.