How does evolution work?

Cooksonia. Image by Smith609, licensed under Creative Commons 3.0

Cooksonia. Image by Smith609, licensed under Creative Commons 3.0

Evolution is one of those concepts that’s simultaneously well-known and misunderstood: everyone’s heard the word, but most people are a bit hazy on what it means, and how it works in practice. So if you’re interested in getting a bit of clarification, look no further.

At it’s base, evolution means how species change over time. It’s not, as it is portrayed sometimes, about survival of the biggest or meanest, since they may not be useful attributes for a given species. And it’s certainly not ‘social Darwinism’, which is a pernicious myth that claims to be all about perfecting the human race (and yes, you do hear unpleasant overtones there). It’s just a species changing over time in response to outside pressures.

So how does it work then? I’m glad you asked. A slightly longer definition of evolution that’s often used is “natural selection through random mutation”. Mutations happen in any population: nothing reproduces perfectly forever.

But I thought all mutations were bad? Hey, we’re not talking about Godzilla here. Many of these mutations are tiny, and make no difference to the individual; some are deleterious; and some are beneficial. Consider a cheetah: a change in claw colour would make no real difference to its survival; an ability to run faster and use less energy would be beneficial; while being rotund would make it less likely to survive (although if its prey were also rotund, it would be an even contest, as well as funny).

So what makes a mutation good? Basically, anything that makes an individual more likely to survive and reproduce is a beneficial mutation. This is the ‘natural selection’ part of evolution – the genes for the beneficial mutation gradually spread through the population until the whole species carries those genes.

Why hasn’t everything evolved to be exactly the same? Because what’s good on land is not great for what’s 10,000 metres under the sea, and what’s good in a hot dry climate is not useful in a tropical climate, and so on – each type of environment imposes constraints on what lives there, so the evolution tends towards different results. Take for example the evolution of the eye: each type of eye is useful for a different type of creature.

Why doesn’t something evolve to be the best they can be then stay there, then? There’s no ‘ultimate best’ for species: they’re constantly under pressure from outside influences: avoiding predators and pests, finding food, dealing with environmental conditions, and competing against others of their own species. A fairly stable environment favours species that are already adapted, but drastic environmental changes (which coincidentally cause massive extinctions) cause correspondingly rapid bursts of evolution.

Why is that? If the environment changes rapidly and drastically, some species are unable to adapt and survive, so they die out. Other species, which are able to adapt, do so, and survive. We might be tempted to say that faster or bigger is always better, but that’s not necessarily true: many of the larger animals became extinct during drastic changes in climate due to their longer generational times, while smaller creatures, with shorter generational times, survived.

Why do some die out and some evolve? The short answer is ‘flexibility’: if an organism can change under pressure, it survives. If not, it dies out. Organisms can also die out if they’re out-evolved by other species: an example of this is Cooksonia, picture above. This primitive plant was out-evolved by plants that were better able to survive and reproduce. You can find plenty of detail about the stages of plant evolution at the Wikipedia page.

So it sounds like species are running a constant race to survive? Very true, and that’s why some evolutionary biologists describe aspects of evolution using the Red Queen hypothesis. The name comes from Lewis Carroll’s Through The Looking Glass, in which the Red Queen says to Alice, “Now, here, you see, it takes all the running you can do, to keep in the same place”. Individuals within a species are constantly under pressure to avoid dying, to find food, and to reproduce, while predators (herbivores in the case of plants), prey, and others of their species strive to do the same thing. Any of the individuals in any of those species might spawn a beneficial mutation that gives them an edge.

Why don’t species continue to evolve? For example, why don’t apes continue to evolve into humans? Mainly because they’ve been in a fairly stable environment, which favours the status quo. That’s changing now, of course: the human impact on habitat is huge and rapid, which is why so many species are becoming extinct.

How do species know when or how to evolve? They don’t. Evolution is not a voluntary process, it’s entirely random – mutations happen, and some of those mutations give the individuals an advantage, which over time spreads through the population.

Evolution is always good, then? Well, depends on your definition of ‘good’: some species followed the ‘bigger is always better’ path until circumstances changed, at which time they were unable to evolve sufficiently quickly and died out. Other species, which evolve quite rapidly, are a serious problem for humans: bacteria have very short generational times, and respond to the human-imposed evolutionary pressures by evolving forms that are resistant to current antibiotics.

But we’ll survive because we’re clever, right? Not so fast, pardner. Intelligence is not necessarily a long term survival trait: the same constraints apply to us as to all the other species, and our intelligence also gives us the ability to make a catastrophe happen.

Hagfish, for example, have been around for roughly 300 million years, while we’ve only been in our current form for about 250 thousand. And while our intelligence allows us to do more than hagfish (hard to imagine one playing the bassoon or inventing computers, for example), we’re also impacting the biosphere at an unprecedented rate. This is leading to the 6th greatest extinction of life on earth, known as the Anthropocene.

So evolution happens, given enough time and the right circumstances, and some mutations turn out to be beneficial while others do not. It remains to be seen whether humanity will ultimately be a successful mutation.

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