This blog has been created partly as a companion to Chemistry for the Biosciences, the textbook that I co-author with Tony Bradshaw, and to act as an archive of posts I write for other sites (particularly the OUPblog). Like the book itself, it explores how life on the scale of atoms and molecules has an impact on biology - at the scale of cells, tissues, and organisms - and seeks to demystify a range of biological and chemical concepts.

The blog's name takes as its inspiration the cover of the first edition of Chemistry for the Biosciences, which depicts a gecko seemingly clinging to its surface. To find out what links geckos to chemistry, read this.

Sunday, 17 July 2011

How do organisms evolve?

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 temperate environment in which a slightly thicker or slightly thinner coat makes little difference to an individual’s chance of survival. So, it’s just as likely that our thick-coated sheep will reproduce – and pass on its mutated gene to the next generation – as it is that the normal-coated sheep will reproduce, and pass on their non-mutated gene. When we look at the population as a whole, we don’t see any real change, as the balance between normal-coated and thick-coated sheep is staying firmly tipped towards coats of a normal thickness. 

However, over a period of years, the climate grows chilly. As this change occurs, the thick-coated sheep – a minority in the population as a whole – find themselves at a distinct advantage. Their thicker coats keep them warmer than their thinner-coated cousins, who find themselves increasingly susceptible to cold-induced disease; in other words, in the population as a whole, the thick-coated sheep have a greater chance of survival. Vitally, this increased chance of survival makes it more likely that they, rather than the thinner-coated sheep, will pass their genes on to the next generation. And so we see the prevalence of the mutated ‘thick coat’ gene increasing as time goes on, and see a greater proportion of the sheep population sporting thicker coats. 

Over time, what started out as a little quirk – a thicker woolly coat – goes from being an anomaly to being the norm, a defining characteristic of a species. When this happens – when a change becomes pervasive – the species is said to have evolved.

Charles Darwin is famed for introducing the world to the process of evolution in his book On the Origin of Species by Natural Selection; what we have seen here is the process of natural selection in action: in a population with different coat thicknesses, environmental conditions are favouring those sheep with the thicker coat. As a consequence, it is more likely that their genes are passed on to the next generation. We’re seeing a random event – the mutation that generated the ‘thick coat’ version of the gene – having a non-random consequence, all thanks to environmental pressures.

This exploration of evolution shows how life is driven by an elegant interplay between the recipe for our existence – our genomes – and the environment in which we live. In biological terms, nothing much happens in a single lifetime: it seems almost as unchanging as a single frame in a film. It is only by playing the film of life through many frames, over many generations, that the real beauty of this interplay is revealed.