Genetic Drift: How Long Does It Take?

Question

From that first cell until this point, there was countless genetic mutations, and the ones that were beneficial carried on due to natural selection. What I'm asking is, how long is "long"? Say a change in a bird's beak: How long would it take before it mutated and was naturally distributed throughout the population?

And humans: How many genes code the formation of the human brain? How long before a mutation changed the correct genes to enlarge and benefit the human brain capacity?

Answer 1

I think you are confusing genetic drift and selection. Drift is when a trait is neither selected for or against in a population. Because the trait is not under either positive or negative selection it is free to "drift" in frequency in the population due to random chance in mating. Sometimes a trait will drift to fixation in a population, it depends upon the frequency of the trait, the size and demographics of the population and the mating style of the species. Under selection, a beneficial trait can become fixed in a population in a very short period of time but time becomes a relative factor.  A new trait in bacteria can become fixed in a matter of days due to the short generation time. This happens in humans who carry the HIV virus and is what makes HIV so hard to vaccinate against. it is not uncommon for a person with HIV to carry several strains of HIV that are unique to that individual as the virus has mutated within the individual. In species with longer generations fixation will take more time. It also depends upon the factors mentioned above than influence drift and how beneficial the trait is. Changes in allele frequency have been observed within one generation due to selective pressures. See the Grants work on Galapagos finches for a a good example of this.

A large number of genes influence growth and development of the human brain and a good idea of the time that it has taken for the human brain to grow can be found by looking at the gradual increase in brain size during human evolution. The last common ancestor that humans shared with chimpanzees is estimated to have lived 4 - 6 million years ago and it has taken that long for our brains to evolve to their present size.

Remember, mutation is a random process and there is not "purpose" to it. If, by chance, a mutation arises that is beneficial it will be selected for. I beneficial mutation may arise at any time or it may never arise. Most selection actually takes place on mutations that have previously arisen and have been neutral so that have remained in the population through drift. A change in the environment (e.g. a new pesticide or heavy metals entering the soil) can suddenly make the previously neutral mutation beneficial and it will then be selected for. Both of these situations have been observed in the wild. This can allow a change in the allelic frequencies to happen very rapidly as the now beneficial mutation is already available.

New mutations do, however, play a real role in the evolution of new traits and good evidence for this is the fact that bacteria have genes that increase their mutation rate when faced with a hostile environment. This increases the chance that a beneficial mutation will arise and is one of the reasons that bacteria can change so rapidly when faced with new antibiotics.

New mutations can also occur in clusters in the germ cells. Instead of just one new mutation having to survive in the population you can now have the mutation in a group of individuals all in the same generation, this increasing the chance that the allele will not be lost due to random chance.

Sorry this is rather long but the effect of environmental changes producing new mutations etc. is what I study.

Answer 2

You are asking valid questions.  But you are using several terms incorrectly, and they are getting in the way of your understanding.

First, genetic drift is not the same as natural selection.   These are the two main sources of evolution.  

Genetic drift is when there is *no* selective pressure for a gene, and it just spreads randomly.  For example, the presence of blue eyes in the population of humans with European descent is pretty much due to random genetic drift, not any *advantage* conferred by blue eyes.  

Natural selection is when one gene has some advantage or disadvantage over the others in the gene pool, and therefore will either propagate (spread into the population) or disappear over time.  Natural selection is a much stronger mechanism than genetic drift for producing evolution (change in the gene pool over time), and is ultimately what causes the big changes that we associate with long-term evolution.

>"Say a change in a bird's beak: How long would it take before it mutated and was naturally distributed throughout the population?"

Be careful.  Something like beak length is not the result of the beak "mutating" and this mutation spreading throughout the population.   It is important to understand basic *VARIATION*.   In a population there is a *range* of beak lengths, caused by all sorts of small differences in genetics (some caused by mutations, some caused just by different recombinations from mother and father).   The availability of food sources will select for beak lengths ... short beaks better for cracking the hard berries in the tree branches, long beaks better for picking the softer berries off the ground ... until the population can start to split as the short-beaked birds start mating more often with other short-beaks (because they are all hanging out together in the higher branches) and the long-beak birds start mating with other long-beaks (because they're hanging out on the ground together).   So in this case we've got *two* kinds of genes that are spreading at the same time in the same population.

>"And humans: How many genes code the formation of the human brain?"

Several thousand involved in coding for the brain itself.  If you mean the genes that separate the human brain from, say, the chimp brain, it could be as little as just a few dozen.  The human brain is not that different *in structure* from the chimp brain ... just different in the *size* of key areas, and in the sequence of *development* of new neurons..  These differences can be contained in a surprisingly small number of genetic differences.

>"How long before a mutation changed the correct genes to enlarge and benefit the human brain capacity?"

Again, it's misleading to think of it as "a mutation".   It was always about a *range* of brain sizes and capacities, coupled with a *constant* selection pressure towards bigger and better brains.  There were probably several dozen mutations over this time that produced some small jump in brain capacity (not just the size of the brain, but the timing of brain development, the ability to rewire itself during childhood, etc.) that were incorporated into the human gene pool.   It was not "a mutation."

And second, I should re-emphasize that it's not just a difference in *size*, but rather the way that the brain develops (now it adds neurons and rewires itself during critical stages of embyonic development and childhood development).

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So after all that correction, I can address your "how long" question now.

For beak sizes, modern scientists have documented a change in beak sizes of Darwin's finches (because of competition with another species) within the course of only 20 years.

http://en.wikipedia.org/wiki/Darwin's_fi...

And as far as brain capacity.  The fossil evidence (size of brain case in skulls) shows that the size of the hominid brain has roughly tripled in the course of about 3 million years.

http://www.pubmedcentral.nih.gov/article...

Answer 3

Life, presumably from a common ancestor, is about 3.5 billion years old, so that;'s how long it took to get to this point.  But a new phenotype that results from a single gene can spread through the population in just a few generations. It depends on the demographics.