So, how does evolution work?
What I’m wondering about is the actual biological process. Take any animal in a particular environment. Say, they encounter a new migrant predator. How does the evolution process over hundreds of years work that enables these prey to adapt to their new predator and develop some defense or instinctive flight mechanism, assuming the prey last that long?
Random mutations and selection.
When DNA is transcribed, errors creep in. This is unavoidable, the transcription mechanism isn’t perfect. Each generation of animals born will carry some random mutations. The vast majority of those errors are completely neutral - they have no effect whatsoever on survival. Some of the mutations will cause the animal to be less successful in passing on its genes to the next generation. These will be eliminated by natural selection - they won’t get passed on to the next generation. A very few mutations will be beneficial, and help the animals which inherit them to survive and pass on their genes. Over time these advantageous genetic mutations will become widespread in the gene pool.
It’s pretty simple; random changed and variation culled by environment-driven selection. It’s a slow process, but it has been observed in nature and in the lab to occur.
It usually takes a lot more than hundreds of years for complex organisms like animals.
The specifics of evolution have filled many books and been studied and debated for a long time, but the very basic idea is as follows:
Every time a lifeform reproduces, there is a slight chance for mutation of the genetic structure. With organisms that produce sexually, like most animals, there is also the mixing of genes from the parents.
Both of these processes cause the new organism to have certain traits. The passage of two recessive genes from the parents may cause the child to exhibit a certain trait the parent does not have, and a mutation may cause the child to have a completely new trait.
If a certain trait is beneficial with regards to the new predator, then there is a good chance that children with that trait will survive and reproduce, while the other ones get eaten. Evenetually, all of the species will have that trait.
If a certain trait is negative with regards to the new predator, then there is a good chance the child will be eaten before reproducing.
Most mutations are bad – it’s only a very few that cause interesting new things to happen. Further, a trait that is bad in one instance may not be bad in another. In humans, the recessive gene that causes sickle-cell anemia also causes an immunity to malaria. So you’ll find a lot of people carrying the genes in Africa, where malaria kills lots of people without it, but not many in Europe or North America, where anemia is a bigger danger.
Every time a lifeform reproduces, there is a slight chance for mutation of the genetic structure. With organisms that produce sexually, like most animals, there is also the mixing of genes from the parents.
So, if I understand this correctly, organisms do not adapt to their environment. If by good fortune, should a random mutation turn out to be beneficial to the threat at hand, then the organism has “adapted” (or gone a small way towards having adapted).
Ah, see that’s the thing: the key lies in those who do last that long! Those who survive the introduction of a new predator likely do so because of their specific traits, rather than random happenstance. Thus, those who survive will pass on those beneficial traits to their offspring, who will in turn be able to outrun / outsmart / evade / whatever the predator themselves, and will pass on those genes, etc. Over time, then, the population will adapt such that most healthy adults will be able to avoid becomng a meal (key concept: natural selection is not a guarantee of survival for any individual - it’s a statistical thing). Of course, at the same time, the predator is evolving to be able to catch its evolving prey…
That’s right. According to current theories of evolution, there is no directed or intentional adaption - merely the fact that those creatures with genes that allow them better survival tend to pass those genes onto the next generation, and new variations are constantly being introduced to the gene pool by mutation.
Individuals do not adapt – populations do. Once a beneficial trait, as determined by the prevailing circumstances, has become the norm in population, the population is considered to be “adapted” to its environment. Note also that a population adapts to the sum of environmental effects, not to specific effects.
Of course, at the same time, the predator is evolving to be able to catch its evolving prey…
That’s the thing. This above sentence makes it sound like evolution is a sentient process. Which seems amiss to me. How does the predator know how to evolve? Isn’t it possible that out of all random mutations in the predators, none are germane to their hunting skills with regards to the prey.
Individuals do not adapt – populations do.
But surely, these mutations occur at the individual level. If three particular preys was born with a beneficial mutation, then all future adapted offsprings will trace one of their parents to those three animals. Especially given the fact that if mutations are random and numerous, a very small initial sample might get the beneficial mutation.
The predator doesn’t know how to evolve. But in the population of predators, some may be better hunters than ever. If food is scarce, the better hunters are going to get the food and the inferior ones will die. If food is abundant, then it may not matter at all.
Precisely. But remember, just because a specific individual has an advantage doesn’t mean everyone else is going to die immediately. Just that he and his descendents are more likely to survive and reproduce then the others. The two classes will be living side-by-side, and breeding with one another, for quite some time.
That should read “some may be better hunters than others.”
:smack:
All this means that it’s possible to devolve as well. In other words, as time passes, the survivors aren’t necessarily better overall, they’re just the progeny of those who had a combination of being luckier/better-arsenaled against immediate threats.
Possible? Sure. In which case, the predator species may very well find itself extinct. But the predator species doesn’t “know” to evolve, it either does (by which it is meant, “traits arise which yield an advantage in prey capture for some individuals relative to others of its population, or a competing species; those traits are then statistically likely to persist into the next generation”), or it does not.
Simply put, adaptation is a population-level concept, referring to the changes in a population to “match” its environment. An individual does not do this; it simply possesses whatever traits it inherited from its parent(s), plus any new mutations, but those traits do not change over the lifetime of the individual. Even if the genome of the individual is altered during its life, that organism will not gain any real benefit from such. After development is finished, genome changes are at best going to affect one’s offspring.
The process of adaptation may well begin with a single individual, but that individual is still not adapted; the “end result” (if such a thing can be said to exist) in a population may well be quite different from those beginnings.
Evolution does not strive (to anthropomorphize) for “betterness”, it strives for survivability. If what’s left survives well enough to maintain a steady population size or better, then it should be pretty obvious that they’re doing something right, given the current environment. If the population reaches a state of steady decline, then they are likely on their way out.
'Devolve" is a meaningless term. Evolution is not directional. If a species changes over time in such a way as to lose capabilities or become less resistant to a certain threat, that’s evolution too.
I meant “devolve” in the colloquial sense.
If a species changes over time in such a way as to lose capabilities or become less resistant to a certain threat, that’s evolution too.
Just as people use evolution colloquially : a process of continuous change from a lower, simpler, or worse to a higher, more complex, or better state
That’s pretty darn good. The term “adaptation” was a very bad choice. It implies some sort of “intent” in reaction to the environment. “Getting a lucky card” would have been far more scientifically accurate".
Mind you, evolution also has a purely stochastic (random–sort of) element to it, too. Sometimes, the “selection” force is “being the third guy in the line”. No genes make that condition exist, but the poor third guy’s genes don’t get passed on because he gets eaten, anyway.
In that case, then you don’t want to know the real biological theory of evolution, since this “colloquial” sense is not at all what evolutionary theory posits.
There is no guarantee in this game. For example, it seems that there are a few people who cannot get AIDS. We don’t know why, but the fact seems clear. Prostitutes in Africa who have been exposed repeatedly, who have AIDS antibodies in their blood but no active virus. In the long run, if we don’t find a cure, whatever genetic feature that protects them may spread throughout the human race.
Mammals have fast twitch and slow twitch muscles. Sprinters and marathoners. There is a trade-off; more of one implies less of the other. If there are speedy predators, then individuals who have fast-twitch muscles will be favored and the population will, over time, move in that direction. If, on the other hand, the main predators (e.g. H. sap) have specialized in endurance, then the individuals who have more slow-twitch muscle will be favored and the population will move in that direction. Notice the contrast between individuals and populations. Individuals are favored or disfavored, while it is populations that evolve. That is why, IMHO, the argument over whether the “true” agent of evolution is the population or the individual (or the gene, or even the protein) is so sterile. You can analyze it at all those levels and they are all valid. What they aren’t is exclusive.
Another point. Evolution is only local. It can work only with the variation that is present in the population. Yes, it may happen that a mutation comes after the environment changes, but I think the normal situation is that the mutation is already present and then becomes advantageous when the environment changes. It was doubtless the case that for whatever reason some mosquitos were naturally at least a little bit more tolerant of DDT than others. This was neutral (possibly slightly, but not seriously deleterious) before there was DDT in the environment. Along comes DDT and the individual mosquitos who have at least some tolerance survive and reproduce better. Then within that population, certain individuals are even better and so it goes until you have bred a population of resistant individuals, possibly some that actually revel in it. Agan the same interplay of individual fitness and population evolution.
One aspect of evolution I read about in a recent issue of Scientific American had previously escaped me: namely how can a species rapidly evolve when a new predator or disease is introduced.
It turns out they don’t, really. In the huge complexity of DNA, much of the strands are seemingly random or insignificant. But when some new population pressure is put on a species, random strand A, previously useless might actually become useful, if not essential. This could be something as minor as an ability to produce a particular protein, or offer increased disease resistance, or slightly faster healing, or some other previously insignificant trait. All the non-A animals are rapidly killed off, and all the A animals no longer have to compete as hard for food or territory. Their offspring (carrying A) rapidly multiply and refill the environment. Thus the species has changed in some minor fashion. Repeat this process for umpteen generations and the animal may physically change in dramatic ways.
The point is that anti-evolutionists frequently claim that it’s impossible for a species to adapt instantly, but the species doesn’t have to. All that has to happen is that 1 percent survives, and with a reasonably prodigious breeding rate, that 1 percent can multiply back up to the previous numbers rather quickly.
A species that has no DNA strands or traits that can help it goes extinct. Similarly, a species that has DNA strands to cover every possible emergency can carry on unchanged for millions of years. I’ll bet cockroaches has covered all the bases, which is why a pre-Jurassic roach is pretty much identical to a modern one.
Magic. Just like God intended.
