How does evolution work?

Most people have some pretty vague ideas about what evolution is, but most of us don’t really know how evolution actually happens.

(Some people even claim evolution isn’t real, but that’s about as sane as saying the Earth is flat, or that the Phantom Menace is better than the Empire Strikes Back).

In this post I’ll try and explain how evolution happens as simply as possible, and tell you why in the future human’s won’t evolve six fingers, wings, or two heads.

Evolution means change. If you remember anything about evolution remember this. Evolution does not mean improvement, or progress, or anything like that, it just means change. If a species evolves, it changes, it doesn’t “improve” or become more “highly-evolved”, just different.  Never forget this.

I’m sure most people know that evolution is something to do with species changing to become better adapted to their environments over time, and eventually this change causes them to become a new species altogether. A common example of evolution is usually something like; “giraffes developed longer necks to help them reach leaves that other species couldn’t reach”.

This makes sense; you can see why it would be good for giraffes to have long necks. So the why may be obvious, but the how is less obvious. Just because there is a reason for giraffes to have long necks, this doesn’t explain how they grew longer.

This is a giraffe by the way. And no, I’ve no idea why it looks so furious.

In fact, no scientist understood this until Charles Darwin and Alfred Russel Wallace came along and explained it to them. Darwin and Wallace didn’t discover evolution, they discovered how it works (although Darwin is more famous, Wallace also made the discovery too).

Before them, many scientists recognised that animals and plants had changed over time, as palaeontologists could track the evolution of life in the fossil record. They could see that life began in the sea, that fish evolved, and that some fish developed into amphibians on land, which evolved into reptiles, and then mammals and so on. But no one understood how this actually happened.

People put forward a number of different explanations, usually something like; life has an inbuilt destiny, and that this destiny will cause life to rise from “lower species” to “higher species”, just as humans begin as babies and grow into adults. Many people saw humans as the ultimate destiny, the pinnacle of evolution, that life was inevitably going to spawn mankind.

But this is absolute nonsense. Life has no destiny or goal, and there is no hierarchy, there are no higher or lower species, just many different types of life. Darwin and Wallace destroyed this view when they showed that evolution could happen without any guidance, that it was a random process without design or direction.

And this is Darwin. I don’t know why he looks so furious either. Maybe he’s been talking to that giraffe?

What Darwin and Wallace discovered was called Natural Selection. And Natural Selection is the process that causes most evolution. It requires three things in order to happen, variability, hereditability and a struggle for survival.

1. Variability means that individuals within a species must be different from each other. Variability is easiest for us to see in our own species. Its obvious when you look around you that everyone is different. We come in different heights and shapes, we have different colour hair, skin and eyes, different personalities, different skills and so on. And this is true for all species. No two red-eyed leaf frogs are the same, or no two mountain gorillas.

2. Hereditability means that some of these differences must be passed-on from parents to their children. For instance, if you have two tall parents, then you are more likely to be tall too. Darwin and Wallace didn’t know how this happens, but today we know it’s due to DNA. Your DNA controls many of your characteristics, from your height and eye-colour, to aspects of your personality and your skills and weaknesses. Your DNA is a combination of your parent’s DNA, thus you inherit much of what makes you unique from your parents.

3. The struggle for existence means that not all individuals in a species will have children, or as many children as others. The natural world is a tough place, and lots of plants and animals don’t successfully mate before they die, as they may be eaten by predators, or killed by disease, or starve because they can’t find enough food. Therefore, the individuals who are best adapted to an environment, who can best find food, avoid predators and resist disease, are likely to survive for longer and have more children. Darwin called this the “survival of the fittest”. By fit he didn’t necessarily mean able to do lots of exercise, he meant best adapted to an environment. The fittest individuals might be the fastest cheetahs, or the whitest polar bears, or the dolphins that can stay under water for the longest. Fitness is whatever makes an individual more likely to have more children.

Here the tallest giraffe is the fittest, and least furious.

These three combine to make Natural Selection. Because individuals in a species are different, some will be more likely to have more children than others. And if the difference that makes them more successful is hereditable, then their children will be more likely to be more successful too. Over time, these more successful individuals will have more and more children and eventually will come to dominate the population.

The giraffe is a great example. Giraffes evolved from an animal that looked quite like an antelope, with a much shorter neck and legs than giraffes today. These animals ate leaves, but lots of other animals ate leaves too, thus there was competition for food, and only the most successful individual’s will have survived and had offspring. These antelope-like animals will have varied and perhaps some had DNA that gave them unusually long necks. Long necks may have allowed these individuals to eat leaves higher-up in trees than other animal could reach. Therefore these long-necked animals may have found it easier to survive and likely had more offspring than the shorter-necked members of their species. Thus over-time the long-necked individuals may have come to dominate the species.

This is probably what the antelope-like ancestor of giraffes looked like. More sad than furious.

As time progressed, variation in the long-necked animals may have caused individuals to be born with DNA for even longer necks, who then in turn came to dominate the population. If this continued, then over many generations the necks of these antelope-like animals will have become longer and longer until the giraffe’s neck was achieved.

Thus evolution did not cause the giraffe’s neck to magically grow, Natural Selection meant that variation in the population was rewarded by an increased chance of survival and increased chance of having offspring for longer-necked individuals. Natural Selection showed how a species can change over time, and that if Natural Selection happens for long enough, it can change a species so much that it becomes a new one.

So why won’t humans evolve six fingers, wings or two heads?

Lets take the example of the six fingers. Firstly you need variation to produce six fingers, and humans are born with six fingers. So condition 1 is satisfied. We would also need this to be hereditable. I’m no expert on this, but lets pretend that having six fingers is inherited from parents. Thus condition 2 is satisfied. Now there would also need to be a reason why people with six fingers are more likely to have more children, and thus six-fingered humans would could to dominate humanity. I’m pretty sure that this isn’t the case, that people with six fingers don’t have more kids than people with only five fingers. Therefore humans won’t evolve a sixth finger. And the same applies to wings, two heads, female beards, three eyes etc. There must be a reason why a hereditable variation allows individuals to have more offspring, and come to dominate a species.

Lots of snake species produce individuals with two heads, but there’s no reason why they would have more offspring (the opposite is usually the case) thus two-headed snakes have no come to dominate. (I’m guessing you’re bored of the whole furious thing now, right?)

So that’s it. Evolution means change. Evolution happens by Natural Selection. And Natural Selection happens because individuals have hereditable variations that cause them to have more offspring, which come to dominate a population over time.

Now, you may have spotted that earlier I said Natural Selection causes most evolution. There are a couple of other ways that evolution can happen too, but Natural Selection is the most important and the one that everyone should understand. I’ll talk about the other mechanisms in a later post.

Evolution by Natural Selection also has a number of important implications. But this post is already long enough, so I’ll save the implications for a later post.

See you there.


9 Responses to “How does evolution work?”

  1. 1 Alexander September 2, 2011 at 5:21 pm

    Absolutely wonderful!
    Just like your other posts!

    (I submitted your “What is the the Universe?” post to that book of science blog posts – I really liked it!)

  2. 3 Louis October 16, 2011 at 10:43 pm

    Are you familiar with epigenetic evolution? This doesn’t involve natural selection. It is a way that gene information can be passed from one generation to another directly in response to environmental influences e.g. the hunger response. Epigenetics is really neo-Lamarckism not neo-Darwinism.

    Another phenomenon that isn’t really natural selection, is where the whole genomes of one species are captured by another – amazing but true!
    See this reference:

    I have also read some theorists argue that diet has a direct role in evolution – which I think must potentially make sense, given DNA is made of chemicals. and food is chemicals.

    I think there are some philosophical issues with natural selection theory. We know for instance that in the 20th century Americans on average gained about 8 inches in height thanks to a wondrously improved diet. Let’s assume for the sake, that this nutritious diet is maintained for a million years. Does it make sense to say a million years later that the increase in height was the result of “natural selection” or does it make more sense to say it is the result of direct influence of diet?

    My own view is that we are still learning a lot about evolution. I think it is much more directly interactive with the environment than we yet understand.

    • 4 Dan G Swindles October 20, 2011 at 11:07 am

      Yep, I’ve studied epigenetics a little, but didn’t mention it here as I wanted to keep this post reasonably short and simple. Epigenetics has certainly had an immense impact on evolutionary theory though, so I’m sure I’ll write a post about it at some point

      I’d never really considered it as Neo-Lamarckism either, but you’re right, it does sounds more like “soft inheritance” than Darwinian inheritance

      I don’t think there are any issues with Natural Selection though, although there are a number of other mechanisms by which evolution can occur, such as genetic drift and gene & genome transfer (as you mentioned), they compliment rather than challenge Natural Selection, for instance, epigenetic systems themselves can be inherited and thus epigenetics itself can be subject to Natural Selection

      On the height question, evolutionary biology says that “phenotype is both the expression of genotype and the environment” i.e. your genes interacting with the environment defines what you become. With height, your genes will determine what your maximum possible height will be, but then your environment will determine how close you get to the maximum height. So I’d argue that the 8″ growth in height of Americans isn’t evolution, as their genes for height haven’t changed. If you placed an American baby from today in an environment from 200 years ago, then the baby would not grow the extra 8″ in height. Unless I’m reading your question wrong though, is there evidence that taller men are having more babies and thus sexual selection in human populations is leading to a height increase?

      I certainly agree though that we’re still learning a huge amount about the details of evolution

  3. 5 daniel maris October 21, 2011 at 8:33 pm

    I think we are moving from an era of dogmatism towards a more open approach to evolution.

    I recall seeing Richard Dawkins lecture a few years back. I was never impressed by his assertions that genes explained virtually all animal behaviour. I think the tide is turning. I read recently that recent studies show that Bower bird nest building is “learnt” behaviour rather than determined directly by genes. But of course, that begs another question – how birds with such tiny brains learn such complex behaviour.

    Another related issue is that the human genome has so few genes – far fewer than some quite simple vegetables. That was certainly not what was expected a few years back. What it suggests to me is that some form of interactive evolution, that we don’t yet properly understand, came to dominate as animals became much more complex.

    I wasn’t suggesting that the diet-related increase in height = evolution. But I think it does call into question what constitutes evolution exactly. I guess if one restricts it to something like “changes in the genome of a species” you can exclude everything that doesn’t fit in with neo-Darwinian theory.

  4. 6 andrew beechtree May 19, 2012 at 4:08 am

    Teaching new course this fall, Anthropology. Looking forward to it.
    Good stuff here. Interesting. Easy to understand. Fits in with the way I teach. Thanks.

  5. 8 Ashlee February 18, 2013 at 4:24 pm

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  1. 1 » How does evolution work ? Trackback on February 23, 2012 at 11:41 pm

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