I was reading an article about so-called “man-flu,” which postulated that mens’ weaker immune systems might have been the result of some evolutionary benefit conferred by saving that energy (that a more aggressive immune system would use) for the rigors of mate competition. This seems like quite a stretch. And suddenly it seemed to me that this whole thing about natural selection has become a little too convenient of a go-to explanation for everything.
Back in caveperson days, there were surely many fewer people than now. So, say somebody has some kind of random genetic variation and they happen to hook up with a mate that is especially fertile, or long-lived, or both. Maybe they happen to live in a relatively secluded valley with few dangers, and maybe they are experiencing a decades-long cycle of clement weather that results in bountiful food available. So they have numerous fortunate offspring, who then get wanderlust, and find secluded valleys far afield where they are safe to produce lots of offspring of their own, only they’re a month’s travel away.
The fewer your potential mates are (in a still sparsely-populated world), the more likely you are to get two copies of a random mutation that doesn’t confer any benefit in personal survival or competing for mates, but is simply common.
What I’m saying is, isn’t it possible for some things to become rampant, or even universal, through sheer happenstance?
And, wouldn’t one random variant that does result in evolutionary advantage potentially take a lot of survival-neutral etraits along with it?
Is it necessary for every trait to have had an evolutionary benefit for it to have become the norm?
I think there is a tendency to reach for some sort of evolutionary adaptation explanation for things that may well just be random.
Blue eyes, for example, are the result of a single mutation several thousand years ago. The gene for blue eyes has spread widely, but I doubt it’s due to any advantage that blue eyes confer. It just happened that some of the first blue eyed people had a lot of kids who survived, as so here we are.
Pretty close to any science REPORTING that doesn’t actually come from a peer-reviewed journal (i.e., most mainstream science reporting) should be taken with several grains of salt, because science journalists are horrible at drawing reasonable conclusions and science editors are ridiculous with their headlines.
Also, not everything happens for some purpose in evolution. Mutations are generally random and accidental and they’re not so much selected for as selected AGAINST. IOW, mutations that aren’t fatal prior to reproductive age or that don’t interfere with reproduction can ride along for a very long time without serving a “purpose” or aiding in the survival of the species.
There’s tons of randomness. Evolutionary survival comes after surviving all the random events that could wipe out a species. Random mutations don’t have to provide a definite advantage to survive, they only need to not provide too great a disadvantage. And it is randomness that drives evolution in the first place.
This is called genetic drift, and is completely uncontroversial and accepted among actual biologists. It just doesn’t make as good headlines, so you won’t read much about it in the popular press.
This is called genetic drift, and is completely uncontroversial and accepted among actual biologists. It just doesn’t make as good headlines, so you won’t read much about it in the popular press.
Absolutely. As others have said, it’s called genetic drift. And it’s an important part of evolution. “Happenstance” does not necessarily require a complex series of events like you describe. Most genetic drift is simply a result of sampling error in the genes that happen to make it into the next generation - so genetic drift is much higher for small populations. For the same reason, a small population will tend to lose genetic diversity rapidly, because some gene variants are lost in each generation by chance.
Having said that, at the phenotypic level, it’s still true that natural selection for fitness is usually the dominant effect. But not at the molecular level. Molecular evolution is the evolution of genes or proteins, i.e. DNA or amino acid sequence change in a population.
For DNA it is true that
(a) Most mutations have no effect;
(b) Of those mutations that have any effect, most are deleterious.
In other words, only a tiny proportion of DNA mutations (and a small proportion of sequence evolution) is attributable to “positive” selection, i.e. natural selection in favor of a new advantageous mutation. To a first approximation, positive natural selection can be ignored at the molecular level!
The Neutral Theory, the fact that most sequence change is neutral drift, gives us a molecular clock that allows quantitative inference about the evolutionary history of species.
Yes, this is called hitchhiking. It’s due to the fact that genes are strung together end-to-end on chromosomes. The process of meiotic recombination still shuffles genes on the same chromosome over time, but in the short run genes that are “linked” (located physically close to one another on the same chromosome) tend to be inherited together.
One would agree with the OP that reaching for natural selection every time for every little trait is overreaching. Perhaps an overlooked point is that selection needs some reason to select. In times of easy life - abundant food, few predators etc - there is little pressure and drift is going to bring about all sorts of unimportant oddities that just don’t matter. Maybe in some later time of difficulty pressure might come to play and select for or against some of them, but if the trait has become widespread in the populace there is no alternative for selection to work with, everyone already has it.
The usual example for a deleterious mutation is Vitamin C.
IANA Biologist, and maybe I’m misunderstanding this, but surely selection is, in the big picture, far more important than anything else. Having blue eyes, or green eyes, or brown eyes, may be a result of drift; but having eyes in the first place is surely the result of natural selection, and a hell of a lot of it. I can’t see how organisms could have simply “drifted” into the massive and overwhelming amount of exquisitely complex anatomy, metabolism, and behavior–all promoting survival and reproduction–which all living things display.
The source for all the man-flu articles is the British Medical Journal http://www.bmj.com/ which posts a whole bunch of jokey articles in every year’s Christmas edition.
With that article removed from its original context, and rewritten by content farms of dubious reliability, the joke is lost.
No, but we humans like to have explanations for things, because [insert evolutionary explanation here], and natural selection is one handy place to look for such explanations.
No. People do often try to find adaptive reasons for traits, but they can certainly become established by chance (that is, if not directly detrimental). Some of these factors have already been mentioned.
Some other factors include:
-Genetic drift. Traits can become fixed in small populations by chance and persist even when the population becomes larger.
-Developmental constraints. Some traits may be due to developmental pathways that can’t easily be changed without disrupting other processes. (Nipples on male mammals could be a trait like this.)
-Traits that are linked to or a side effect of a beneficial trait that is strongly selected for that are dragged along for the ride.
S.J. Gould discusses some of the limitations to adaptationist explanations in his article The Spandrels of San Marcos.
This said, just because natural selection might not explain something is not a reason to say there is “no reason” for it. It’s reasonable to look for explanations for traits, but not to assume that natural selection must be the only one possible.
To oversimplify one argument for illustration, consider this: every positive selection event that occurs to one gene carries along with it a whole slew of nearby genes despite their having nothing at all to do with the selection. As far as those genes are concerned, they lucked out - in other words, drift. Thus, the total number of selection events at the gene level is far smaller than the total number of drift events.
This is called “neutral theory” and yes, it’s absolutely and unquestionably a real force in natural history. Less important in general than natural selection, but a real and important force nonetheless. Neutral theory of molecular evolution - Wikipedia
To clarify terminology, what you’re describing is not genetic drift, which by definition is due to random events rather than selection. The phenomenon you describe is termed a selective sweep or selective hitchhiking, in which genes in the vicinity of a strongly selected gene are dragged along with it even though they are not under direct selection themselves.
Just to build on this: imagine a mutation that makes a guy’s heart dissolve on his 100th birthday. Granted, that’s silly, but I figure it’s memorable and gets the point across: if my great-great-grandfather had it, (a) nobody would’ve known, since he died of something else first; and (b) it in no way interfered with him having kids who had kids who had kids and so on: each of them sired just as many kids as they would have without that mutation, and they all did just as good a job of looking after said kids likewise, because none of them made it to 100 anyway.
So long as it’s been irrelevant, it’s had no trouble sticking around.
It’s important to note that the Neutral Theory is a theory of molecular evolution. It is not applicable to phenotypic evolution. The point is that most DNA mutations have no effect on the phenotype, and most of those that do have an effect are deleterious. It’s a much more open question to what degree phenotypic evolution is attributable to selection vs drift.