Evolution - Chapter 2

In 1819, an unusual moth emerged from its cocoon in a woodland somewhere in northern England. It belonged to a species called the peppered moth but it didn't look quite like all the other peppered moths that were emerging around it. Normal peppered moths have speckled pale wings, but this moth didn't have wings like that. Instead, it had beautiful, sooty black ones. The moth was, in fact, a mutant.

There had been a change to some of the letters in its genome, so its cells were producing more black pigment than normal. Now, we've already seen that most mutations are harmful, so we might prophecy doom for this new arrival. But instead, naturalists would watch in awe as this moth and its descendants became wildly successful. How?

During the 19th century in northern England, the Industrial Revolution was well underway. Victorian engineers had worked out how to burn coal to heat water, to produce steam, to drive newly invented machines so they could produce things cheaper and faster than ever before. But coal is a dirty fuel. Burning coal produces clouds of black soot and these settle out onto houses, schools, lungs, and even trees.

Worse, coal contains impurities, like sulphur, so when you burn coal, you also burn sulphur, and that produces sulphur dioxide, which is an acid gas, and that's very toxic to the crusty grey-green lichens that normally plaster trees. So all this soot and this sulphur dioxide transformed the tree trunks of northern England. Where they'd formally been speckled grey-green, they were now columns of purest black.

If you were a pale, peppered moth sitting on a tree trunk, then this transformation was disastrous. For birds, moths make a good meal, and if they can see them, they will eat them. These pale speckled wings of the original form of the peppered moth used to provide very good camouflage against the tree trunk in unpolluted woodlands. But now, against the newly blackened tree trunks, these moths were sticking out like sore thumbs.

In fact, you could say that the peppered moth was on the verge of extinction. Salvation lay in the mutant moth. Its sooty black wings provided perfect camouflage against the newly blackened tree trunks, and so the birds ignored it. When it had offspring, they too inherited the black wings, and so over a few decades, the black moth became more and more common, while the original pale speckled form was almost impossible to find.

The story of the peppered moth reveals that species are not set in stone. They can evolve as the typical genome changes over time. This change begins with a mutation in a single lucky individual, but it spreads through a population because individuals that carry the mutation are more successful than those that don't. This almost ludicrously simple process forms the core of the Theory of Evolution by Natural Selection.

It was independently proposed by Charles Darwin and Alfred Russell Wallace in 1858. And although it has been attacked by various religious figures over time, its position at the heart of biology has never been in serious doubt.

So let's take a look at this theory. Now, I have to tell you, it's a pretty unusual kind of theory, unique in biology. The way it's laid out is we look at a set of assumptions, there are three crucial ones, and we have to decide whether or not we believe that those assumptions are true. Because if they are, then there is a logical conclusion, and that conclusion is that species will evolve to become better adapted to their environments. So let's take a look at the assumptions.

The first one is that there is heritable variation among individuals. Well, that sounds a bit of a mouthful, but what does it mean? Well, heritable variation is genetic variation, variation that can be passed on from a parent to offspring. And if we look at the peppered moth, we see that we do have that heritable variation because we have the two forms, the pale speckled form and the black form. And we now know that the black form was caused by a mutation in the genome, so it is heritable.

The second point, the second assumption, is that there is going to be competition among the individuals in a population so that not all of them are going to survive and thrive. And that happens because generally more individuals are born than the environment can support. The pressure in this environment is predation by birds. Some of them are going to be eaten by birds, but which ones? And that leads us to point three. And point three says that not all individuals are equal.

This heritable variation from point one actually matters. So some individuals are more likely to survive and have more offspring perhaps than others. In the case of the peppered moth, in the polluted woodlands, it's the black moths, they are going to survive better and therefore have more offspring than the pale ones. And so because those three assumptions are true, we are going to see evolution in action. The peppered moth is going to evolve to be better adapted to its environment.

The population, in fact, is going to change from one that looked like this into one that looked like this. And that in a nutshell is evolution in action.

So since Darwin's day, the theory of evolution has been tested extensively, and there's a mountain of evidence to support it. But in his day, Darwin himself was concerned with one or two issues, and the biggest thing he was concerned about was time. So he was absolutely convinced that all life on Earth was descended from a single common ancestor, something that we believe today very strongly.

But if that's true, then it must have taken a really long time for natural selection to have produced that extraordinary array of creatures. So one of the best pieces of evidence we have that there was other life on Earth in the past is of course fossils. Now, Mary Anning is one of the great figures of palaeontology. She lived in Dorset, around the same time as Darwin and Wallace, and she looked for fossils in the cliffs near her home.

She found the first ichthyosaurs and a wonderful skeleton of a long neck plesiosaur, and those were huge marine reptiles that lived at the same time as the dinosaurs. So what these fossil finds showed is that the Earth had looked very, very different in the past. And that was a brand new idea in the 19th century, the idea that the Earth had been populated by fabulous beasts.

So today we are confident that the Earth is about four and a half billion years old. Now, that is just a span of time that any puny human is going to find difficult to comprehend. And of course, we appear very, very late in that time. So there's various comparisons that are made. If we just squeeze the history of life into a single day, then we are going to appear on the last stroke of midnight. If we give it a year, then we are going to appear at 11:35 PM on the last day.

But any way you look at it, humans are not a major part of the history of life. Now, amazingly, we actually think that life itself evolved pretty quickly. The early Earth was molten, but by about 4 billion years ago, we think the first cells were getting up and running. But then until about half a billion years ago, all life on Earth was single celled.

So half a billion years is about 540 million years ago. The first animals appear, and the good news about them is they have hard parts. So some of them have shells and some of them have bones and some of them have teeth. And then we start to get a much better fossil record. So that period, from 540 million years ago to the present day, is known as the Eon of Visible Life. Now that aeon can be split into three great eras.

There is the Paleozoic, which means ancient life, the Mesozoic, which is middle life, and the Cenozoic, which is recent life. And those eras are dominated by different creatures. So the Paleozoic is the longest era and that oversaw therefore the biggest changes to the planet. For example, it was during the Paleozoic that the plants colonised the land and the continents became green and then some animals followed them.

The Mesozoic was dominated by dinosaurs on land, but also the ichthyosaurs and plesiosaurs in the ocean and winged pterosaurs in the skies. The Cenozoic, the recent era, has been dominated by mammals and birds. So that's the one that we are most familiar with. Now, the great eras are punctuated by two mass extinction events. The first one that lies between the Paleozoic and the Mesozoic is often called the Great Dying. And that's just to emphasise how bad it was.

It was caused by massive sustained volcanic eruptions that pumped out carbon dioxide and hiked up global temperatures very rapidly. And that led to the loss of about 90% of species on Earth, which is a pretty catastrophic event. The mass extinction between the Mesozoic and the Cenozoic was different. That had an extraterrestrial cause.

So a huge asteroid hit the Earth in what's now the Gulf of Mexico and that caused wildfires to spread around the globe and would've looked, I don't know, almost like Armageddon. So that gives you the span of time and the major time points. And of course, what you can see is that the diversity of life on Earth has really fluctuated, building up and then crashing back down again during a mass extinction.

But what's interesting is today there are probably more species on our planet than at any time in the past.

Well, I do hope you enjoyed that episode, and if you did, then please do share it with friends and colleagues. If you'd like to have your own copy of the book, then there is a link below. There's a lot more in chapter three about evolution. In particular, there's a lot of stuff about how multicellular beings evolved. That's actually quite a puzzle for evolutionary biologists. Otherwise, don't forget to join us next time for episode three, which is all about sexual reproduction.