The Life Cycle and Natural Selection

We observe that all organisms tend to come into existence, mature, reproduce, and go out of existence. In humans this cycle is represented by birth, attainment of adulthood, giving birth, and dying.

We observe that human populations only survive because the old are replaced by the young. A population that did not reproduce would eventually die out and disappear. There thus appears to be a causal relationship between birth and death, and it appears that the purpose of birth is to compensate for death, which is widely considered to be a flaw in the scheme of things. But it sometimes happens that we confuse cause with effect, so for the sake of argument we might hold in reserve the possibility that the purpose of death is to compensate for birth, or even that the cycle of birth and death serves some unknown purpose.

We also observe that there are two kinds of reproduction, sexual and asexual. Asexual reproduction is a kind of self-cloning, in which new individuals are created using genes from a single parent. This is a much simpler and more reliable process than sexual reproduction, so we might wonder why humans and other complex organisms don't reproduce asexually, as many less complex organisms do.

The reason for sexual reproduction, I believe, has to do with the length of time it takes an organism to mature to the point where it can reproduce. A single-celled organism can reproduce asexually soon after it is created. Significantly, there are bacteria that begin reproducing after they are only 20 minutes old. Furthermore, there are strains of bacteria that are highly susceptible to mutations, i.e., errors in copying genes when they reproduce. Because of this short life-cycle, favorable mutations can propagate into the species rapidly.

The ability of some viruses to quickly make themselves immune to our latest antibiotic medications is an example of natural selection at work. A strain of viruses survives because some mutations produce individuals that can survive better in an altered, hostile environment.

But in the case of complex multicellular organisms, the length of time needed to reach reproductive maturity becomes too great for asexual reproduction to be effective. New mutations would enter the gene pool very slowly. Sexual reproduction then becomes more effective, because mutant genes from many different individuals are shared when those individuals combine their genes during sexual reproduction. Plants tend to bear this idea out. Almost all plants can reproduce both sexually or asexually. Sexual reproduction is the preferred method, but since plants can't roam around looking for partners, they can resort to asexual reproduction when partners are not available. The species stagnates, but survives, during periods of asexual reproduction, and it thrives later by introducing more mutations into the gene pool during periods of sexual reproduction.

It should be noted that single-celled organisms do not experience natural death. They clone themselves into two new organisms when they reproduce. There doesn't seem to be anything about their cells that wears out after they have reproduced themselves a given number of times.

Since multicellular organisms are simply organized collections of single cells, it seems reasonable to assume that single-celled organisms existed first, and that multicellular organisms evolved from them later. But for some reason, the cells of multicellular organisms can only reproduce a given number of times before they, and the organisms, die.

When single-celled organisms reproduce asexually, the parts of the parent organisms are reused, and the species propagates itself most effectively. But when multicellular organisms reproduce sexually, the bodies of the parents cannot be reused during the process. In time, the parents will have contributed all of their potentially useful mutations to the gene pool, and they will have served their genetic purpose. It may be for this reason that they are designed to eventually self-destruct. Which is to suggest that the cycle of birth and death exists to serve the purpose of natural selection.

Eugene Paul