It’s hard to start writing an overview to evolution without feeling that everything I’m saying is either already established to the point of tedium, or blindingly obvious – but, given how little some people seem to know about it, I suppose I shouldn’t presume too much about what’s obvious or what “everybody” already knows.
This is a summary of my understanding of the subject, but it’s an understanding gleaned somewhat incidentally, by casually reading various books and blogs, some more academic than others. I’m not really an academic myself, and certainly not in the field of biology. In this piece and its future sub-articles I’m trying to recount things as best I can in my own terms, but you may find plenty of useful corrections and clarifications elsewhere: Wikipedia’s entry on evolution and the links cited, The TalkOrigins Archive, and the writings of any number of people smarter than me, might be a good place to start. I’m very much a layman at this, which will be obvious from my general avoidance of any kind of terminology (and of factual accuracy too, most likely).
Evolution, of the contentious biological variety, basically has three fundamental aspects you need to understand.
1. A population of replicators
I don’t know whether Stargate SG-1 fans are ever likely to form a large part of my reader-base, but don’t worry about the replicators. Sci-fi villains aside, this just refers to some entity which can produce copies of itself, with no particular desire to deconstruct every particle of matter in the universe to do so.
A bacterium will split apart to produce two identical bacteria, which will in turn both split and reproduce, rather slower than you’ve probably seen in those rather eerie microscope videos. Plants produce seeds, which produce more plants. A chicken lays an egg, out of which hatches another chicken. And people do all manner of freaky things, some of which result in creating more people. There are replicators all over the place.
2. A source of variation among the replicators
Nothing’s going to evolve if every new generation is exactly like the last. There needs to be something to cause slightly different copies to be made, at least some of the time. In creatures that sexually reproduce, a random selection of genes is inherited from each parent, which contributes to things getting mixed up – so, I have my dad’s eyebrows and occasionally abrasive laugh, but not his snoring; I have my mum’s long arms, but not her tolerance of high temperatures; I have myopia from both of them, but my militantly atheistic streak came out of nowhere. (Or something. Genetically speaking, this might be a really bad example, but hopefully you see what I mean.)
Also, stuff can come along and randomly throw a spanner in the works, mutating an offspring’s DNA so that it turns out different from that of its parent or parents – stuff like radiation, or viruses, or just errors in the DNA copying itself, when a carbon atom doesn’t quite behave itself or a protein doesn’t bother to proofread its work.
My technical ignorance is dazzling even myself here. Suffice it to say that some organisms of any species will be harder, better, faster, stronger, or whateverer, than their fellows, simply by virtue of not being entirely identical in every way. This happens, and is necessary for the process of evolution by Darwinian natural selection to take place. There’s one more thing we need, though.
3. Competition among the varying replicators
You’re also not going to see much evolving going on if every creature, and their children, and their children’s children, has all the space to run around and play and frolic that they could want, and all the food they can eat, and all the mating opportunities they have the energy for. It’s a jungle out there, in the scary place we call nature, and not everyone’s going to make it. Only a few of any population of organisms are likely to make it over all of life’s hurdles, keeping sufficiently warm and sheltered and nourished and healthy to get as far as passing on any of their genetic material; many more will become an evolutionary dead-end, also known as “lunch”.
But it’s not a total crapshoot which organisms are lucky enough to get lucky, so that their genetic information gets another shot at doing it all over again. There are some variations that provide a definite advantage over other creatures. My shag-carpet eyebrows might not be of any particular advantage to me, but a cheetah (or a gazelle) could certainly benefit hugely from happening to have just slightly stronger leg muscles than its equally hungry (or edible) friends.
Or some birds might happen to have slightly differently shaped beaks, which turn out to be more suitable for acquiring whatever food is available to them; or some butterflies might randomly have a pattern of colour on their wings which makes them look a bit scary and possibly dangerous, to the kind of thing that might normally want to eat a less dangerous looking butterfly; or some line of apes might find that their DNA has provided them with slightly bigger skulls than before, and room for slightly bigger brains, with neurons connected slightly differently, which gives them abilities that make them more effective predators and survivors than the other apes. Whatever the scenario, some of the variations that arise will make particular creatures better suited to their environment; those creatures will have a better chance of passing on exactly those traits to their offspring, which means more and more instances of those traits will be seen.
That’s the bare bones of it. Organisms that are better equipped to survive long enough to reproduce and pass on their genes are, tautologically, more likely to pass on to their offspring the very genes that made them so well equipped. All other things being equal, certain randomly varying traits will turn out to be more advantageous than others in any particular population, and those traits will become more prevalent over an increasing number of generations. The Theory of Evolution is a model that describes how these processes account for the entire variety of life observed on the planet, including all human life, down from those very first amino acids, or whatever exactly kicked it all off.
This merits a huge number of subcategories and more specific discussions, some of which I hope to get around to writing sometime before the eventual heat death of the universe. This is just a useful starting point.