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If only those who are most fit to reproduce are the ones reproducing the most, it makes sense that traits that prevent one from reproducing would eventually disappear from a population.

But what about traits that have no effect on reproduction, like the presence of wisdom teeth? Humans don't need them, but they don't inhibit us from reproducing either. So how do humans evolve past the point of having wisdom teeth or other similar traits with no negative effects?

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  • $\begingroup$ Maybe in some cases the expression of the trait with no negative effects is linked to one that is, so thereby if the organism is expressing the negative trait it would also express the non negative trait, and then that would be selected against. I am just a layman though, I would wait for a more clear response... $\endgroup$ – Ro Siv Jun 5 '16 at 23:18
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    $\begingroup$ They DON'T. Remi's answer down there is pretty comprehensive, but it really should point out with much more emphasis that non-negative traits can remain in the populations for millions of years, if not forever, unless a random mutation+drift removes it, which is a pretty low chance event. $\endgroup$ – Davor Jun 6 '16 at 7:40
  • $\begingroup$ Following up on the comment by @Davor vestigial organs are an example of useless traits sticking around. Having an appendix won't help your offspring to flourish, but it won't diminish their chances either $\endgroup$ – Malhar Khushu Jun 7 '16 at 11:46
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Oops I wrote a lot! This is almost a very brief introduction to some concepts of population genetics.

A little bit of terminology first

Locus

A locus (plur. loci) is a position on a chromosome.

Allele

At a specific locus, different individuals may have different variants. One individual might be ATTCTA while another might be ATTCAA for example. These variants are called "allele". The whole goal of evolutionary genetics is to understand the creation (mutation) and change in frequency of the different alleles in the population.

Heterozygosity

You probably know that in many species, we carry two copies of each chromosome. One individual can therefore carry twice the same allele at a given locus or can carry different alleles. Expected heterozygosity ($H$) at a given locus is the fraction of the population that carries two different alleles at this single locus.

Fixation

When an allele reaches a freuqency of 1 (100%), that is only this allele exists in the population (and therefore the other alleles have been wiped out), we say that it has reach fixation.

Natural Selection

The process you describe in your first paragraph is called natural selection.

Natural selection is a fitness (=a function of both reproductive success and survival) difference between different genotypes. As you said (but with other words), deleterious alleles will decrease in frequency through time and will eventually end up being weeded out from the population through natural selection.

Genetic Drift

There are other evolutionary processes than natural selection. Typically, the one we will be interested in to address your question is genetic drift.

What is genetic drift?

Genetic drift refers to the random sampling of a population in order to build up the next generation population. There are different ways to model genetic drift (Wright-Fisher model, Moran model, coalescence model) but without having to talk about the details of these mathematical models, it is important that you can conceive the role of random sampling in the change in allele frequency through time. For this, I invite you to first read Why is the strength of genetic drift inversely proportional to the population size? and when you have finished, come back to this post.

One the below figure, you can see the effect of drift through time. The below graph consider a bi-allelic locus (a locus where 2 and only 2 different alleles exist in the population) starting with an initial frequency of 0.5 (=50%). Each line is a separate simulation and shows the frequency of only one of the two alleles.

enter image description here

The above graph considered a population of size $N=100$ individuals. Below, the same types of simulation with a population size of $N=25$ individuals. As you can see that the genetic drift is stronger

enter image description here

Every single locus is subject to genetic drift whether it is under selection or not. However, if a locus is under strong selection, then genetic drift with have little power to explain what allele will get fixed in comparison to when the locus is neutral.

Genetic drift and loss of alleles

It is clear from the graph above that genetic drift also wipe variant out of the population. We just don't know for sure which variant will be wiped out. To be more accurate, the expected heterozygosity due to genetic drift is reduced by a factor $\frac{1}{2N}$ each generation. One can make plenty of calculations from basic models of genetic drift. For example the expected time to being wiped out is $\bar t(p_0)=-4N\left(\frac{p_0}{1-p_0}\right)\ln(p_0)$, where $p_0$ is the starting frequency of the allele of interest.

A interesting result is that the probability of an allele to reach fixation is equal to its frequency. If there are $N$ individuals in the populations, there are $2N$ alleles (but not $2N$ different alleles). If there are, say 235 alleles A in the population and $2n - 235$ alleles B in the population, then over the long run (so that one will necessarily fix) the probability of fixation of the allele A is $\frac{235}{2N}$ and the probability of fixation of the allele B is $\frac{2N-235}{2N}=1-\frac{235}{2N}$.

Wisdom teeth

In your question, you are interested about a supposedly purely neutral locus. You talked about wisdom teeth I suspect more as an example rather than because you are interested to this trait.

Let's assume that the presence of wisdom teeth is completely neutral. Note by the way that as a rule of thumb if $2 N s << 1$, (where $s$ is the selection coefficient on a given allele) then the allele will behave essentially as if it was a neutral allele. So, if wisdom teeth is neutral, then the probability of this trait to disappear over the long run depends on the frequency of the allele causing this trait. So far, I have assumed that we were interested in a single locus only. In reality, variation in many traits are caused by variation at several loci. In such case calculations are a little less straight forward (but still feasible). I do not know the genetic basis of wisdom teeth, so I cannot make good predictions for the evolution of this particular trait.

Other evolutionary processes

Here are a few example of interest

Gene flow

Note of course that there are other processes to consider when trying to predict the fate of a given allele or trait. For example, in a given population you might have a flux of migrants that come from a population where one allele is fixed therefore affecting the allele freuqency in the focal population. This is called Gene flow.

Hill-Robertson effects

Never forget that a locus is not alone on its chromosome. It is physically linked to other loci and selective pressures at other loci will affect the allele frequency at the focal locus. This is called Hill-Robertson effects.

Mutations

Of course, when a locus is not polymorphic (has only one allele), mutations is the ultime source of variation. When a locus is polymorphic, then it is likely the mutations will have a minor effect on the allele frequency (although it depends on the size and type of locus and depends on whether mutations can make up new functional alleles).

Pleiotropy and covariance between phenotypic traits

Phenotypic trait do not evolve independently. If wisdom teeth is a consequence of the activation of the molecular pathway for resistance to HIV (which it very probably is not), then wisdom teeth will not evolve as a neutral trait. A locus that affects several traits is said to have pleiotropic effects.

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You are correct in thinking that traits that improve (impede) ones reproductive rate should be spread through (removed from) the population by selection. However, the process of evolution rests upon more than just selection.

There are four mechanisms by which evolution operates; mutation, migration, drift, and selection. Mutation brings new variation in to populations, with changes occurring in the DNA (e.g. an A becomes a T in the genetic sequence). Migration can both add and remove variation from populations, but is a within species mechanism of evolution (by definition there should be no genetic migration between species). Drift is the change in genetic variation that comes from random variation in reproductive success. With drift there is no link between the genes and the rate of reproduction, i.e. carriers of one allele are equally likely to reproduce equally well as carriers of alternative alleles.

Selection operates when there is covariance between reproduction (fitness) and the genes one carries, i.e. there is a tendency for the carriers of one allele to produce more offspring than the carriers of an alternative allele. This might arise because, for example, the gene improves fertility.

When you are considering the evolution of neutral (or nearly neutral) traits you are considering evolution via the first three mechanisms (mutation, migration, drift). Small populations are prone to losing genetic variation by drift more so than large ones, here is a simulation of 10 populations over 250 generations, where the population size is 200 on the left, and 2000 on the right. You can see that the smaller populations have a higher tendency for fixation of one allele, this is with no selection occurring.

Many neutral traits persist for many generations because it is just a lottery, eventually they should be lost from the population, but we can't tell when.

enter image description here

Furthermore, the genes that cause neutral variation may be under no direct selection, but could be in linkage with loci under selection, thus be affected by indirect selection, or they could have multiple effects (pleiotropy; e.g. the gene causing the expression of a neutral trait also affects the expression of a selected trait).

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