(picture naturally selected from here.)
(picture naturally selected from here.)
Reposted by OH special request, from a minor skirimish with the Salted Slug on my home patch.
Make of it what you will.
Evolution and natural selection are two different though admittedly related things.
Natural selection can make a species taller, shorter, fatter or thinner without actually making it a new species.
If you happen to live somewhere where all the food is fruit on tall trees, your taller siblings will get more fruit than you. They'll be better fed, fitter, and more likely to reproduce. They'd have more kids than the short ones and the kids would be taller and out-compete shorties. Eventually all the population would be tall, but crucially, this does not need to involve a change in a single gene. All it takes is a difference in gene expression. They are taller than a hundred years ago but they are the same species. Exactly the same. That's natural selection but there's no change in the species so it's not evolution.
Now, if the shorties all decide to eat roots instead while the tall ones eat fruit, then the species splits into two groups and you have the start of speciation.
Evolution without natural selection - well, if you want a current example of evolution happening before your eyes, look at chihuahuas and great danes (brace yourself, Maisie). They are the same species. They look different but they are biologically the same. They are biologically capable of interbreeding. Physically, it's not going to happen unless a particularly well-endowed chihuhua learns to use a stepladder. The other way round is too horrible to contemplate. Their environments are the same, there's no selective pressure here, they can live in the same place at the same time, eat the same food, but they cannot interbreed.
Dogs haven't been domesticated long enough to separate into different species. It will happen in time because some breeds simply cannot mate any more. So they will, over time (a lot of time), pick up enough variations in their genomes that they will no longer be compatible biologically. It takes a lot of time to do it but it doesn't need to take many changes. We are currently 98% the same as a chimp but that 2% is far, far more than enough to prevent interbreeding. That, and the smell.
Or take horses and donkeys. They are still close enough to interbreed but they are different species because the offspring (a mule) is always sterile. So the result of that interbreeding is no longer a viable species in its own right. That's an example of evolutionary process as it happens - eventually, horses and donkeys will no longer be able to produce mules. It doesn't happen overnight. It can't. If it did, you'd need two identical mutations of different sexes or the new species, no matter how wonderful, is gone when the single example dies. It's more likely that the moon will spontaneously reform into an Ikea advert than that two identical mutations, both biologically distinct from the parent species, would appear at once. Evolution of one species into another takes a long, long time.
Where natural selection and evolution link is in the example of tall and short above. Selective pressure on the shorties means they have to find another food source because the growing population of tall ones have eaten all the fruit. If they fail, they die, and the population becomes a taller version of the same species. Natural selection without evolution. The species has developed into a taller form but the crucial part is, it's still the same species.
If the shorties succeed, they will gradually separate from the tall tribe, perhaps move somewhere there is more ground-based food, and although they are both biologically and physically capable of interbreeding, it's not likely to happen because they are acting like a different species even though they aren't one yet. Eventually they will be. They will have evolved as an offshoot of the original species but that original species does not need to vanish for that to happen. If all the fruit trees died, the tall group would die out, otherwise the original species is still there and might still throw out an occasional shortie now and then.
Biology is a convoluted subject and we might never understand all of it. One thing does not necessarily lead to another, even if it seems absolutely certain that it must.
Example - There's a particular antibiotic that used to be fed to pigs all the time, not because they were always sick but because farmers initially noted that the pigs getting the antibiotic grew faster. It was because they were originally sick, treated, got better and grew faster but nobody wanted to hear that, least of all those selling the antibiotics. It ended up in pig feed as a matter of course, with no more testing until someone decided to have a look. That was when antibiotics in animal feed were under serious scrutiny to deflect attention from antibiotic resistance arising in hospitals.
This antibiotic killed Gram positive bacteria so in the lab, I tested it against Gram positive bacteria to make sure it was working. It was.
We fed it to healthy pigs and they not only didn't grow any faster, the Gram positive bacteria in their guts increased significantly. The work was commercial so I can't name the antibiotic and the company weren't interested in pursuing the anomaly because, having shown the stuff didn't work as a growth enhancer, they weren't investing any more in it. So it's still not clear why an antibiotic can kill a particular group of bacteria in the lab, yet enhance that same group in the gut. Most likely they've developed resistance over the years of exposure but I never had the chance to test it - nor to test whether the resistance would vanish without the antibiotic.
In biology, things don't always do what it says on the tin.
So while natural selection is linked to evolution, and can result in an evolutionary change, it doesn't always do that. Likewise, evolution can happen without natural selection. Evolution is playing with fair dice, no loading, and you get what you get and learn to deal with it or die. Biological theories won't end up as any form of grand unified theory because if we ever think we have biology all sorted out, it'll bung in a random change somewhere and we have to start again.
I use a specific definition of evolution because it's a specific process. It's not a catch-all subject, it's a very specific description of a particular event. That event involves a change within a species such that the resulting offspring won't interbreed and therefore have to be classified as separate species. The original might die out, leaving only the new species, but not always.
Mutation is also part of the mechanism of evolution - in fact, without genetic mutation, evolution won't happen. All you'd get would be variants of a single species based on gene expression, but all individuals would have the same genome. Mutation changes genes but not according to any plan or logical agenda. It's purely random and most mutations cause damage rather than advantage. With a big enough population and enough reproductive cycles, random chance throws up a good one now and then. You can then pick out a few with an advantage and select for them.
In a culture of a single species of bacterium, I'd expect growth to at least 1000,000 cells per ml and often much higher. Some species are fast, too, with reproduction every half an hour under ideal conditions. Bacteria rarely exchange DNA and when they do, it's not the main genome, it's some extra bit they've picked up, called a plasmid. So you should, from a small population growing overnight, get a mass of clones in the morning. You don't because they make mistakes when reproducing and throw up a lot of variants. It's not a separate species, it's a function of the species that they produce this variation.
So if I need an antibiotic-resistant variant (used to find the thing again after dropping it into a pot of crap), all I do is take a big population of bacteria and add the antibiotic. It'll kill most of them but any variant with resistance will grow. Out of a 100 ml pot with a million per ml, I only need to find one and I have my traceable variant. But it hasn't evolved. It's the same species. In the absence of antibiotic, the population can lose its resistance again and revert to the sensitive form. It's just a variant. But given millions of years, something reproducing at bacteria rates will easily throw up enough variants to produce a eukaryotic cell, then a simple cell group, then co-operative groups, then multicellular organisms. It doesn't matter how many asteroids you throw at the early planet because they are working at such high rates that they can throw out another set of oddities even if you wipe out the first. They might even be at it now, in a slimy pool somewhere, working on our replacements.
As for flu, its ablity to change surface protein is not evolution because the changing surface proteins are a feature of the flu virus, not a change in the virus. It's the same virus with a different hat on. If it moves from one infection site to another, it can be said to have evolved. So if HIV really did come from SIV, as seems likely, that's viral evolution. If bird flu becomes a human flu (and no longer infects birds), it can be considered a different species. It's not just the protein coat. The nucleic acid inside has to be sufficiently compatible with the host DNA to take over replication. Otherwise, even if it gets in, it can't do anything.
I think I'm straying from the point somewhat so I'll finish with this -
A virus might have half a dozen genes. A change in one gene might be enough to make it a different species, or it might not. It depends on what the new variant does. You have millions of genes. A change in one gene might give you brown eyes or red hair or colour blindness or enhanced hearing or a big nose or cancer. It doesn't make you a different species. You'd need to change to the extent that you can no longer interbreed with humans in order to be a different species and that will take a change in a specific set of genes. Or an accumulated set of differences over a very, very long time. To do that, you'd need to be part of an isolated population. A couple of generations won't make any difference at all. You'd need to be isolated for many thousands of years. You might only need 0.01% of a difference but it'll take millenia to do it. Even if the place you're isolated in has exactly the same environment as the one everyone else is in, so there are no selective pressures at all, your isolated population will become, eventually, a new species. Remember, the ones you're isolated from are changing too. If you're not mixing genomes with them any more, you're not evening out variations across the whole species.
Humans don't appear to be evolving because we haven't been human long enough for any new changes to show. Worse, we now travel the world in a matter of hours and mate with anyone so we're becoming less varied, not more. Evolution in humans is stalled because we've stalled it. There are no isolated populations. We don't let genetically-defectives die out. The tall people pass the fruit down to the short people in trade for roots and the species remains a mixture of forms but always the same species. We are mutating and producing new forms but those new forms have no real advantage now. Someone who can run really fast would be impressive if we didn't have cars. Someone who's born with wings would have an advantage if we didn't already have planes. Evolution is trying its best to introduce random variation but we just absorb the changes. Extra finger? Surgery takes care of that. Genetic disease? We're working on ways to fix them. Missing limb? Have a prosthesis. There you go, now you're just like everyone else.
It's not God that keeps us all the same. It's us.