Effect of zero selection (pressure) on the population health

Question

Evolution naturally produces better features: stronger muscles, teeth and minds. Killing the weakest, evolution wipes defective genes out of populations.

The mutations are necessary for advance. However, they are random and, thus, mostly negative. Right? How does the nature eliminate them from the population?

I see that one mechanism is polygamy: male have higher mutation rates. They tend to reproduce as much as possible. Female have lower mutation rates, they couple with only with best men and contribute much more to their offsprings. So, men generate as much random solutions as possible, whereas the role of female is to conserve the best of them. This means that the most of the male (bad mutations) die unreproduced. This eliminates the bad mutations and favours the progress of good qualities. The higher animals started to form harems and tournaments over their ownership. This further fosters the elimination of weak men and profileration of strong genes. However, human monogamy has disabled this selection mechanism. Best men are dedicated to only one woman. Other women cannot mix their genes with other good genes from previous generations. They are forced to look at the low quality men, who carry degenerate mutations. Might be it is monogamy that makes us human but it implies that all genes reproduce.

Yet 100 years ago it was not a big deal since we still had another filter: our grandfathers were born in families that had normally 10 children (I am speaking about Russia) and only 2-3 survived until reproduction (the population increased really slowly). I bet that those who survived had really good health: thanks to the bad health and immense selection pressure, the negative mutants are eliminated immediately and population maintains a perfect genome (aka biologic health). Killing more weakest in the current generation improves the health of next generations.

However everything has changed during the 20th century. The advances in economy and medicine have almost eliminated the selection. The selection pressure has relaxed to essentially zero so that everybody survives and leaves the same amount of offspring (and some scientists are even concerned that degradatory individuals leave more offsprings than the prosperous ones). We have even eliminated the infant mortality. We use baby incubators so that the people with the weakest health could survive and reproduce as normal and we are proud of it. It may sound strange but the mutation rate has not changed at all. Does it mean that the human genome is in danger?

I see the situation like you have inherited a perfect mechanism and decided not recovering it from the inevitably adverse action of entropy, that degrades it constantly. There is a constant chance, d, that every good gene is affected by a mutation. It is a fraction of healthy genes that will fail passing to the next generation. This will leave only (1-d) genes healthy in the next generation. The fraction of healthy genes will melt like 1, (1-d), (1-d)², (1-d)³, an so on, with every generation.

You see the fraction that stays healthy over generations. It decays exponentially. Any utility turns into entropy (aka garbage) exponentially, if not protected. Is this model correct? How quickly does this dissipation go? What is d? Let's assume absolute survival and equality in the reproduction rate among all groups people. How many generations is needed to lose 50% of good qualities?

Answer 1

I need to point out one thing, natural selection does not bring species to perfection. The best mutant may not be selected for many reasons.

When you have no selection pressure then you have neutral evolution concurring and what takes over instead of natural selection is genetic drift. Genetic drift is just sample error. Say you have 1,000 individuals in a population and all of those individuals reproduce, in the next generation all the genotypes would be in the next population. Now imagine if you only had 10 individuals in the population and only 5 of those individuals were randomly selected to produce 10 individuals in the next generation (the parents die every year). There is no selection pressure but just by chance some individuals reproduce and some don't. If you do this over and over again what you will see is that some genes become fixed in the population, meaning that there is no other genotype in the population.

This fixation at small population sizes can occur with either beneficial or deleterious mutations. Obviously if a mutation is very deleterious the individual will die but slightly deleterious mutations can become fixed in the population as well.

There is an entire field of biology devoted to this study. It is called population genetics. The probability for fixation for an allele is its proportion in the population. This proportion can be calculated by 1/2N, where N is the population size.

Answer 2

Even if a species lives in abundance, there is still an evolutionary arms race: who reproduces the fastest? Even slight advantages in reproduction rate multiply over the generations.

There are, however, cases where selection pressures on specific traits have vanished. For example, for fish living in caves, there is not selection pressure to be able to see, and they lost their eyes. A more complete discussion can be found in this paper: "Relaxed selection in the wild" -- see also this summary with a funny cover image of a species living in abundance.