This is the latest post in our regular OUPblog column SciWhys. Every month OUP editor and author Jonathan Crowe will be answering your science questions. Got a burning question about science that you’d like answered? Just email it to us, and Jonathan will answer what he can. Today: how do organisms evolve?
By Jonathan Crowe
The world around us has been in a state of constant change for millions of years: mountains have been thrust skywards as the plates that make up the Earth’s surface crash against each other; huge glaciers have sculpted valleys into the landscape; arid deserts have replaced fertile grasslands as rain patterns have changed. But the living organisms that populate this world are just as dynamic: as environments have changed, so too has the plethora of creatures inhabiting them. But how do creatures change to keep step with the world in which they live? The answer lies in the process of evolution.
Many organisms are uniquely suited to their environment: polar bears have layers of fur and fat to insulate them from the bitter Arctic cold; camels have hooves with broad leathery pads to enable them to walk on desert sand. These so-called adaptations – characteristics that tailor a creature to its environment – do not develop overnight: a giraffe that is moved to a savannah with unusually tall trees won’t suddenly grow a longer neck to be able to reach the far-away leaves. Instead, adaptations develop over many generations. This process of gradual change to make you better suited to your environment is called what’s called evolution.
So how does this change actually happen? In previous posts I’ve explored how the information in our genomes acts as the recipe for the cells, tissues and organs from which we’re constructed. If we are somehow changing to suit our environment, then our genes must be changing too. But there isn’t some mysterious process through which our genes ‘know’ how to change: if an organism finds its environment turning cold, its genome won’t magically change so that it now includes a new recipe for the growth of extra fur to keep it warm. Instead, the raw ‘fuel’ for genetic change is an entirely random process: the process of gene mutation.
In my last post, I considered how gene mutation alters the DNA sequence of a gene, and so alters the information stored by that gene. If you change a recipe when cooking, the end product will be different. And so it is with our genome: if the information stored in our genome – the recipe for our existence – changes, then we must change in some way too.
I mentioned above how the process of mutation is random. A mutation may be introduced when an incorrect DNA ‘letter’ is inserted into a growing chain as a chromosome is being copied: instead of manufacturing a stretch of DNA with the sequence ATTGCCT, an error may occur at the second position, to give AATGCCT. But it’s just as likely that an error could have been introduced at the sixth position instead of the second, with ATTGCCT becoming ATTGCGT. Such mutations are entirely down to chance.
And this is where we encounter something of a paradox. Though the mutations that occur in our genes to fuel the process of evolution do so at random, evolution itself is anything but random. So how can we reconcile this seeming conflict?
To answer this question, let’s imagine a population of sheep, all of whom have a woolly coat of similar thickness. Quite by chance, a gene in one of the sheep in the population picks up a mutation so that offspring of that sheep develop a slightly thicker coat. However, the thick-coated sheep is in a minority: most of the population carry the normal, non-mutated gene, and so have coats of normal thickness. Now, the sheep population live in a fairly tempera