I am somewhat new to evolutionary biology, having studied it on my free time as a computer science student. There is one particular thing that has always bothered me for which I have not seen a good treatment, relating to adaptations to the environment with respect to genetic diversity. If it is possible for a population to adapt to rapid environmental changes, and they don't have an adaptation for dealing with change directly (such as a complex brain), it seems to me that every generation must have present within them almost every possible environmental adaptation that the population is capable of expressing (including many irrelevant ones and a few relevant to the particular environmental challenge). Otherwise, it may take too many generations to deal with a change, which may be disastrous for the population.
So my question would be: how does an evolutionary biologist explain the mechanics behind the ability for a population to adapt quickly? Are most environmental changes slow or gradual enough that the population has a few generations to happen upon the mutations that will allow it to survive, and have generally been successful in this regard for 3.5 billion years? Or, are a large majority of possible adaptations present in almost every generation, and just serve no purpose or advantage for most of the population if the provided "benefit" is unneeded (i.e., are effectively neutral)? Or something in between?
Answer
It is a good question. The question is hard to answer though because
- The answer is not completely resolved
- There are many influential parameters hidden behind this question.
Your question, as I understand it, can be formulated as
Do natural populations have enough genetic variance to directly respond to an environmental change or do they have to wait for this variance to be created through mutations?
To address this question, I will have to assume you have some intermediate level of knowledge in evolutionary biology.
How do we call these two alternatives?
Adaptation can occur through selection on:
- Standing genetic variance
- De novo mutations
How can we tell them apart?
This section is mainly inspired from Barrett and Schluter (2008).
Adaptation from standing genetic variance and from de novo mutations tend to yield different genetic signature.
In comparison to de novo mutations, adaptation from standing genetic variation is likely to lead to
- Faster evolution
- Because there the respond to the new environmental is immediate, there is no need to wait for more mutations.
- Because the fitness variance associated with the trait under selection is very low even when the first mutation occurs.
- Fixation of more alleles of small effects.
- Because phenotypic variance is alleles of large effects are likely to be deleterious in the previous environment and would therefore be purged out of the population quite quickly
- Because, if adaptation occurs from de novo mutation, it is likely that only few mutation would have occurred that would allow adaptation
- Spread of more recessive alleles
- Because recessive alleles can reach a relatively high frequency in the previous environment even if deleterious in the homozygote mutant.
- Because recessive alleles represent little to no fitness variance at low frequency and are therefore likely and it would therefore take much time for adaptation to occur from recessive alleles if they just appeared through de novo mutations.
You will note that my explanations are non-exclusive and are overlapping. They all boil down to
- Loci causing high fitness variance in the previous environment are unlikely to be source of adaptation from standing genetic variance
- Loci causing high fitness variance in the new environment are likely to cause adaptation.
If you have a hard time to understand these concepts, then you might want to have a look at Fisher's fundamental theorem, Mutation-selection balance and eventually follow some intermediate level course on the mechanism of natural selection.
Which mechanism is more common?
Do most adaptation occurs through selection on de novo mutations or through selection on standing genetic variance? This question is very hard to answer. It depends on
- Type of adaptation
- Population of interest
1. The type of adaptation
The answer is likely to differ depending on whether we are talking about
- adaptation to a changing environment over the whole species range
- How fast is this environment changing is important too as you noted in your post.
- A single population that detach from the rest of the metapopulation to slowly colonize a new environment
- Adaptation to a universally (independent of the environment) beneficial trait
- Coevolutionary process
- etc...
2. The population of interest
Different populations retain different level of genetic diversity. This level of genetic diversity depends on
- Demographic parameters
- such as the population size and its variation through time
- Evolutionary history
- Such as the number of recent selective sweep
- Environment
- Such as the diversity of environments present over the range of the species
- Such as temporal variations
- Genomic architecture
- Such as the presence of strong epistasis that could cause large amount of hidden genetic variation (see for example Hermisson and Wagner, 2004) and the eventual "revelation" of this previously hidden genetic variation through new mutation or a new environment (see for example La Rouzic 2008).
Example
Three-spined sticklebacks are present in salt water and fresh water. They occupy several geographically isolated bodies of fresh water (connected by salt water). All populations present in fresh water environments show similar phenotypic traits. We have first thought that sticklebacks have adapted repeatedly to this freshwater habitats (repeated evolution) through de novo mutations. However, some papers suggest that, as the marine population of sticklebacks is so big, that it is possible that there is standing genetic variation for these traits that are selected for in freshwater habitats. Below are some papers of interest
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