Evolution, short term or long term adaptation?

[Suraj M]

I had a small question, which one contributes more to evolution:
1)short term adaptation and heritable
2)long term adaptation and heritable ?

 

[Pythagorean]

Short term what? I'm not sure I understand your question.

 

[Suraj M]

oh sorry, i meant short term adaptation or long term adaptation.

 

[Ryan_m_b]

Natural selection works on the traits currently present in the population, not on potential future adaptations.

 

[Pythagorean]

Could you expand on your question even more?

 

[Suraj M]

This was the original question 'Pythagorean'

 

[Pythagorean]

Thank you Suraj. Around here, we like to see some effort put into homework-style questions. What do you think about the options?

 

[Suraj M]

Surely it can't be inheritable, so deciding between short term and long term adaptation, I feel Short term adaptation should be more contributing because the environment is not going to stay as it is, it will change, so short term adaptation should help the organism to survive, more variations in the off spring, contributing to evolution? Is this right?

 

[Pythagorean]

I don't know what the correct answer is. I would think short term adaptations lead to long term adaptations and long term adaptations are what survive selection. So both seem important to me. Certainly a long term adaptation like multicellularity is what allowed further adaptation of things like limbs and brains. If evolution is to build on itself, there needs to be long term adaptation to stabilize traits like multicellularity.

 

[Mike Dacre]

Fundamentally from the perspective of natural selection there isn't any reason to distinguish between 'long-term' and 'short-term' in such a broad way, without clarifying what you mean by short and long term, and particularly without mentioning the survival benefit of the adaptation.

The reason for this is that is a trait has a very powerful survival benefit, even if that benefit only lasts for a few hundred years (the stronger the benefit, the less the time required for the adaptation to become fixed, a 100% survival benefit would become fixed effectively instantly) there is a high probability that it will become so abundant in a population that it will drift to fixation eventually. Conversely, if a trait has a benefit, but that benefit is very small, then even if the benefit lasts for millennia, if the population size is large, it probably will never become fixed.

The way to think about this issue is to realize that the actual process of evolution happens over very short terms, based on adaptations that are beneficial 'right now', and that that process is actually a relatively weak one, particularly for smaller benefits. Because the process of natural selection is quite weak on all but the most powerful adaptations however, the longer that an adaptation is beneficial, the more time evolution has to act. For the homework question, the difference between short term and long term are irrelevant, the question is actually asking about the heritability. However, in your question 'Which one contributes more to evolution', the answer is that if everything else is equal, adaptations that are beneficial for longer will be more important to the evolution of a given organism, because evolution has more time to work. However, I hope you can tell from my answer that the question is a gross oversimplification of the processes of evolution, and other factors are actually more important.

As an interesting aside, a good example of something like this question in action is present in the tropical regions of the world. The tropical rain forests of our planet are old. Very, very old. They also happen to contain the largest species diversity of any region on the planet, with the result that the vast majority of tropical species have very tiny niches - which is the reason they are become extinct so quickly in our rapidly changing climate. A very plausible theory for this diversity is that given the huge amounts of time the rain forests have existed, even small advantages in a given niche end up becoming fixed in a population, resulting in extensive speciation. In other parts of the world, where ice or desert occasionally kill almost everything, there just isn't the time for that kind of super-specialized adaptation to occur, and so there are fewer species.

 

[Almeisan]

If something isn't inheritable, it can't be part of evolution as it won't be passed on. Think adaptation in behavior.
Mutations that are inherited that are selected for on the short term proliferate in the gene pool
Evolution doesn't do any long term planing or design. Each step needs to be advantageous.
It doesn't really make sense to talk about short or long term, as stated before. Only the 'now' matters for a set of genes to survive and be proliferated or to be diminished. You can't hold on to a bad gene until it becomes a good one, as a long term strategy/adaptation.
So I don't even know what it would mean to have a long term adaptation, right now.
Really, the whole word 'adaptation' is a misnomer and can lead to confusion.

After natural selection is finished, it merely appears as though an organism has adapted.

Answer has to be short term heritable. But,.. this question is why MC is bad and why evolution is poorly understood.

 

[Pythagorean]

I'd argue that now isn't the only thing that matters. Our evolutionary history has a lot do with the long term adaptations. Obviously, it's only in hindsight that we can call them long-term, but multicellularity (as one example) still plays a huge role in who we are today; so does the spinal chord and segmentation.

 

[Mike Dacre]

Pythagorean,

How can a trait that has no value in a population provided a fitness benefit? I think that what you are arguing is essentially a semantic point. 'long term adaptation' really just means that a trait has continued to be useful for a long period of time. However, the driving forces behind evolution are natural selection and drift; if a trait has no value at any point in time, natural selection ceases to have any role on that trait at all, and then the sole driving force behind the trait is drift. If a trait has been valuable for long enough that it has accumulated to close to 100% frequency in the population then if it ceases to be valuable for a period of time, it is likely to stay present in the population anyway, as long as that time is not too long.

Whether something has long term value is essentially a judgement call that the forces of selection are blind to. Animals do not survive and choose mates based on whether their offspring will be successful in two million years, they survive and choose mates based on how well they and their mates are adapted to the environment they find themselves in 'right now'.

The reason evolution appears to only have profound influence over huge time scales is that most adaptations are not stunning life-savers, they are little changes. Combine this with the fact that beneficial mutations occur very rarely, and thus accumulate slowly, and it is obvious that large changes to a population must happen over long time periods. This does not mean that the forces of selection are acting directly over those time periods, that is impossible. Selection happens based on the fitness of an organism in the environment it is in 'right now'. What you are referring to is essentially the accumulation of millions of 'right now's.

 

[Pythagorean]

How can a trait that has no value in a population provided a fitness benefit? I think that what you are arguing is essentially a semantic point. 'long term adaptation' really just means that a trait has continued to be useful for a long period of time.
[...]
What you are referring to is essentially the accumulation of millions of 'right now's.

Hrm... perhaps it's just semantics. Let me tell you my thinking: what about the adaptations that led to multicellularity but aren't around anymore? Doesn't that make multicellularity distinct (in that's in unlikely that we will ever "evolve" back to unicellularity because that pathway is gone now). And multicellularity has provided a platform for which many other adaptations have built off of. Whereas a more recent short-term adaptation could more easily disappear before the organism becomes so dependent on it?

No that they necessarily follow my reasoning, but here's a couple papers that finds the distinction useful:
http://gbe.oxfordjournals.org/content/5/3/572.full*
http://www.sciencemag.org/content/303/5659/793.short

 

[Ygggdrasil]

I think an important concept that Pythagorean is alluding to is the concept of path dependence and historical contingency. Because the fitness of certain traits often depends on the presence or absence of other traits (a concept known as epistasis), certain evolutionary innovations can become "locked in" by subsequent mutations that prevent reversion of the trait (essentially, this means getting trapped in a local maxima of the fitness landscape rather than finding the global maximum).

The classical non-biological example of path dependence is the QWERTY keyboard. It was originally designed to slow down typing (so typewriters were less likely to get jammed by fast typing), but it has persisted to the computer age because it is too costly (in terms of time and effort for retraining) to switch to a better keyboard layout. There are also biological examples of this phenomenon (for example, see http://www.nature.com/nature/journal/v461/n7263/full/nature08249.html for a biochemical explanation of how genetic drift can close off evolutionary paths and prevent reversion of gain of function mutations).

These and other studies demonstrate that the evolutionary history of an organism influences the evolutionary pathways available for that organism, so clearly the past and present influence evolution. However, I'd agree with Mike that evolution is not anticipating future conditions (except by creating and maintaining variability within populations in order to allow faster adaptation to new conditions).

 

[Mike Dacre]

I think an important concept that Pythagorean is alluding to is the concept of path dependence and historical contingency. Because the fitness of certain traits often depends on the presence or absence of other traits (a concept known as epistasis), certain evolutionary innovations can become "locked in" by subsequent mutations that prevent reversion of the trait (essentially, this means getting trapped in a local maxima of the fitness landscape rather than finding the global maximum).

I agree, this is a very important concept and many essential biochemical processes fall into this category. The original question is whether long term or short term adaptation contribute more to evolution. I think that question is nebulous enough that we are having a discussion largely about definitions. Long term adaptations are also short term adaptations, in that they have value in the short term - a non-sense mutation in an essential kinase is instantly selected out. To me the only distinction between long term adaptation and short term adaptation is how long they have been around. You could say that an ancient adaptation - for example the endosymbiosis of mitochondria - has 'more effect on evolution' in that it is likely to be more essential than a more recently developed adaptation. This whole distinction makes me uneasy though as it implies that evolution is some all-knowing force, when it isn't, it is just a combination of natural selection and drift that acts on all new mutations. If you asked whether ancient or recent adaptations were more likely to be essential to life, that would be better.

I honestly just don't see how you can make any distinction between 'long term adaptations' and 'short term adaptations' that amount to anything more than that one has been around longer, and thus is more likely to be essential.

 

[Ygggdrasil]

I agree. Heritable adaptations result from mutations to the genome. It's not clear how biology could make some mutations "short-term" and some mutations "long-term."

 

[Pythagorean]

However, I'd agree with Mike that evolution is not anticipating future conditions (except by creating and maintaining variability within populations in order to allow faster adaptation to new conditions).

I agree with this too. If they're meaningful, long-term adaptations would be coincidence between genetics and environment. My only consideration was that there may be functional differences between long term and short term evolution. For instance, the threshold between species for them being able to mate.

 

[Pythagorean]

This whole distinction makes me uneasy though as it implies that evolution is some all-knowing force, when it isn't, it is just a combination of natural selection and drift that acts on all new mutations. If you asked whether ancient or recent adaptations were more likely to be essential to life, that would be better.

But we can accept that evolution is not an all-knowing force and still talk about the difference between long-term and short-term adaptations and how they contribute to evolution, can't we. For example, with that uncomfortable religious overtone out of the way, we can generalize this implied question:

[QUOTE="Yggg']how biology could make some mutations "short-term" and some mutations "long-term."[/QUOTE]

to:

"How environment and biology arrive at long-term adaptations"

For instance, the cycle of the sun and gravity are not something we expect to change significantly over the course of meaningful evolution, so traits that relate to them are expected across species. We might turn to materials science to ask about the consistency of flesh and muscle and see why particular structures are favored.

Short-term adaptations, on the other hand, may only lend a small statistical advantage that varies among current populations and more quickly changing conditions (food availability, predator presence, etc). If that were the case, we could say that long-term adaptations contribute more (of course, we are defining long-term

I'm not claiming authority on the subject by any means, I'm just curious; I'm an interdisciplinary student that has had some lab-related exposure to evolutionary biology.

 

[Guco]

I agree. Heritable adaptations result from mutations to the genome. It's not clear how biology could make some mutations "short-term" and some mutations "long-term."

Epigenetics? These are heritable adaptations that might be considered short-term adaptations.

If you are strictly talking about the genome, I can think of some examples that influence the chance a gene is maintained in the future:
- genes can acquired multiple, distinct functions
- genes may have inactive gene copies that can change/revert into functioning genes (copies provide redundancy)
- movement of genes towards more stable location in chromosomes (unstable: end of tail or recombination zone)

You could argue that such factors are more likely to occur when a gene has existed for longer. Although the contribution would probably not be very significant and definitely not separate genes into short- and long-term genes.

There may be some examples of very conserved genes being 'revived' after temporarily losing their function in a species. I can't think of any examples right now... but what comes to mind is that Axolotls can still be induced to metamorph into an adult state despite not doing so in the wild (I think...).