Thanks very much for writing to us and asking this question. I’m writing from Scotland, and every biologist here has heard about, and condemns the decision of your education board. I hope I answer this question well enough, since you might not learn very much about this at school.

Basically, macroevolution is about big evolutionary changes, over long time periods, while microevolution is about smaller, more rapid changes. Macroevolutionary questions are generally considered to start with things like ‘how are new species formed’ and go on to questions like ‘what can we learn about conditions in the past from the history of species diversification’ and ‘how do ecosystems recover from mass extinctions’ and ‘how constant (or not) is the rate at which new species are formed and other species go extinct’, as well as questions about the evolution of certain groups, such as describing how humans have evolved from other apes, and apes from other monkeys. Microevolution focuses on evolution within species, such as adaptive changes in particular enzymes in particular conditions, so things like the evolution of resistance to drugs in disease-causing bacteria, of resistance to high concentrations of poisonous metals in plants growing on polluted sites, and the identification of individual populations of deer would all have micro-evolutionary bases.

So, as a rule of thumb, macroevolution looks at changes above the level of species, such as the generation of new species and bigger groups, such as the mammals, while microevolution is about changes within species. Some evolutionary biologists (such as Stephen Jay Gould, who has written several excellent, popular books on evolution and biology in general) think that some other processes are at work in macroevolution as well as natural selection, and many people think that more or less change at the molecular level (changes in the sequence of nucleotides in DNA) are basically neutral – have no effect on the success of the organism but, and I want to make this very clear: ALL RESPECTABLE BIOLOGISTS BELIEVE THAT EVOLUTION BY NATURAL SELECTION IS RESPONSIBLE FOR THE MOST IMPORTANT CHANGES IN ORGANISMS. There are so-called ‘creation scientists’ who attempt to reconcile what they see in the natural world with the biblical account of creation, but they do very bad science, and aren’t really scientists at all. The fact that evolution has happened, and mostly by natural selection (which is what Darwin suggested, and what all the fuss is about), doesn’t mean that god doesn’t exist. If you are religious, then you have to see the Genesis account as a mere metaphor, and I think most large Christian denominations accept that God didn’t play any active role in influencing the course of evolution. Personally, I’m an atheist, but that’s another story.

There is a massive wealth of evidence proving that evolution has occurred, and you should realize that, although biologists argue about the role of various processes in evolution, they all accept that evolution has occurred. Evolution is, to my mind, the most important unifying theme making sense of all the little observations that biologists have collected over the generations. Microevolution is easy to demonstrate, as we can pretty much see it in action – the most famous example is of a moth called Biston betullaria, which was originally a speckled colour, but more and more commonly became much more solid black when pollution began killing lichen on tree stumps. Lichens are white and live in patterns on the darker trunks of beech trees which Biston lives on. As the lichens died, the trunks became darker, and experiments show that predators eat more of the dark moths on lichen-covered trunks, and eat more of the speckled moths on dark, non-lichened trunks. As the lichens died, the darker moths survived longer, gave birth to more dark moths, and so the entire populations of high-pollution areas (the original study was near Birmingham, an industrial city in the West midlands of England) became dark. Interestingly, as anti-pollution laws have come into effect, the dark form has become much less common, and this correlates well with the spread of healthy new lichens!

This is pretty much as close to a direct observation of evolution happening as we can hope for, and other examples include the evolution of bacterial resistance to some drugs (particularly antibiotics). Observing macroevolution is impossible, as most of it happened a long time ago, and it happens enormously slowly, so perhaps the best evidence for macroevolution is that we can see similarities between different organisms, and that these similarities are hierarchical – i.e. many organisms have only a few things in common, and some of these have many more things in common, and some of these few have loads of things in common, and a couple of species will be almost identical. This hierarchical similarity is what enables us to put organisms together into groups and to work out how they evolved from their ancestors. There are many famous examples of macroevolutionary studies, such as Darwin’s finches on the Galapagos islands and certain species of bivavles, for which an amazingly large numbers of fossils are known, making it easy to see how they have changed over long periods of time.

Yours,

James Cotton