Why do humans reproduce

Asexual reproduction in such a population preserves this variation bottom left , but sexual reproduction with random mating brings the population back into Hardy-Weinberg proportions and reduces variation bottom right. This example illustrates the fact that sex does not always increase variation.

Figure Detail. This example is overly simplified, but it serves to illustrate a general point: Selection can build more variation than one would expect in a population in which genes are well mixed. In such cases, sex reduces variation by mixing together genes from different parents.

This problem arises in the case of a single gene whenever heterozygotes are less fit, on average, than homozygotes. In this case, the heterozygote need not have the lowest fitness ; rather, its fitness must only be close to that of the least-fit homozygote. In general, mathematical models have confirmed that selection builds more variation than expected from randomly combined genes whenever fitness surfaces are positively curved, with intermediate genotypes having lower-than-expected fitness.

In such cases, sexual reproduction and recombination destroy the genetic associations that selection has built and therefore result in decreased rather than increased variation among offspring. The term " epistasis " is used to describe such gene interactions, and cases in which the intermediate genotypes are less fit than expected based on the fitness of the more extreme genotypes are said to exhibit "positive epistasis.

Interestingly, even when sex does restore genetic variation , producing more variable offspring does not necessarily promote the evolution of sex. Again, this reality refutes one of the arguments often raised in the attempt to explain the relationship between sex and evolution. To understand how this operates, consider another simple case involving a single gene, but this time, assume that heterozygotes rather than homozygotes are fittest.

The gene responsible for sickle-cell anemia provides a great real-life example. Here, people who are heterozygous for the sickle-cell allele genotype Ss are less susceptible to malarial infection yet have a sufficient number of healthy red blood cells; on the other hand, SS homozygotes are more susceptible to malaria, while ss homozygotes are more susceptible to anemia.

Thus, in areas infested with the protozoans that cause malaria, adults who have survived to reproduce are more likely to have the Ss genotype than would be expected based on Hardy-Weinberg proportions. In such populations in which heterozygotes are in excess, sexual reproduction regenerates homozygotes from crosses among heterozygotes. Although this indeed results in greater genetic variation among offspring, the variation consists largely of homozygotes with low fitness. Yet again, this simple example illustrates a more general point: Parents that have survived to reproduce tend to have genomes that are fairly well adapted to their environments.

Mixing two genomes through sex and genetic recombination tends to produce offspring that are less fit, simply because a mixture of genes from both parents has no guarantee of functioning as well as the parents' original gene sets. In fact, mathematical models have confirmed that when selection builds associations among genes, destroying these associations through sex and recombination tends to reduce offspring fitness.

This reduction in fitness caused by sex and recombination is referred to as the "recombination load" or the " segregation load" when referring specifically to segregation at a single diploid gene. The reason that the recombination load is a problem for the evolution of sex is better appreciated by looking at evolution at the level of the gene. Imagine a gene that promotes sexual reproduction, such as by making it more likely that a plant will reproduce via sexually produced seeds as opposed to some asexual process e.

Carriers of this gene will tend to produce less fit offspring because sexual reproduction and recombination break apart the genetic associations that have been built by past selection.

The gene promoting sex will fail to spread if the offspring die at too high a high rate, even if the offspring are more variable. Indeed, theoretical models developed in the s and s demonstrate that genes promoting sex and recombination increase in frequency only when all of the following conditions hold true:.

Unfortunately, empirical data have not indicated that fitness surfaces curve in just the right way for these models to work in real-life situations. To make matters worse, sexual reproduction often entails costs beyond the recombination load described earlier. To reproduce sexually, an individual must take the time and energy to switch from mitosis to meiosis this step is especially relevant in single-celled organisms ; it must find a willing mate; and it must risk contracting sexually transmitted diseases.

This last cost is often called the "twofold cost of sex. These are substantial costs—so substantial that many species have evolved mechanisms to ensure that sex occurs only when it is least costly.

For instance, organisms including aphids and daphnia reproduce asexually when resources are abundant and switch to sex only at the end of the season, when the potential for asexual reproduction is limited and when potential mates are more available. Similarly, many single-celled organisms have sex only when starved, which minimizes the time cost of switching to meiosis because mitotic growth has already ceased. Although various mechanisms might reduce the costs of sex, it is still commonly assumed that sex is more costly than asexual reproduction, raising yet another obstacle for the evolution of sex.

The aforementioned points might lead one to conclude that sex is a losing enterprise. However, sex is incredibly common. Furthermore, even though asexual lineages do arise, they rarely persist for long periods of evolutionary time. Among flowering plants, for example, predominantly asexual lineages have arisen over times, yet none of these lineages is very old.

Furthermore, many species can reproduce both sexually and asexually, without the frequency of asexuality increasing and eliminating sexual reproduction altogether. What, then, prevents the spread of asexual reproduction?

The first generation of mathematical models examining the evolution of sex made several simplifying assumptions—namely, that selection is constant over time and space, that all individuals engage in sex at the same rate, and that populations are infinitely large. With such simplifying assumptions, selection remains the main evolutionary force at work, and sex and recombination serve mainly to break down the genetic associations built up by selection.

May 10, Male orb-weaving spiders cannibalized by females may be choosy about mating Jun 01, Mar 20, Sep 19, Recommended for you. More evidence of an evolutionary 'arms race' between genes and selfish genetic elements 14 hours ago.

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Your friend's email. Your email. In a few centuries, all traces of our existence as human individuals -- memories of us, all our accomplishments --will likely be gone and forgotten, except for genes that survive from those of us who successfully reproduced through the generations. But, of course, we don't experience the world from a gene's eye evolutionary perspective. One experiences the world as an individual person, not as a gene dispenser fun as that may be. The joy we get from parenting comes not from some abstract generic idea of gene propagation, but from specific love and interaction with our own children -- making your own baby son giggle uncontrollably when you make ridiculous animal noises, the bittersweet emotional rush you feel as you watch your daughter walk down the aisle.

We care about ourselves and others as persons, not as a gene menagerie. Humans create our own meanings. But -- reproduction as the answer to life's meaning cannot be dismissed quite so easily.

Genetic evolution is the meaning of biologic life, in that it is the why and how of it, as well as the stock of future biological existence. The genes that survive -- and in turn the organisms they make -- are the winners in the existence game. Can we just dismiss this when considering the meaning of our own individual human lives? Sure, evolution itself does not have a specific direction or teleology, and genes themselves are not conscious, so there is not meaning in that sense.

But evolution cannot just be shrugged off as something apart from us, take it or leave it. It is the biological explanation of who we are, how we got here, and the diversity of life. Over billions of years, life left the oceans, stretched limbs to cover the earth, raised wings to fly. Underlying it all are the replicating molecules that continue to copy themselves even now. We owe our existence to this process, and our future depends on it.

Perhaps the meaning of your life as a biological creature is to make babies and help ensure the survival of life. In discussing the children she had with Carl Sagan, Ann Druyan put it like this: "When we come closest to each other we can create new life forms that carry on that continuity that stretches back all those billions of years, and in them are the generations of human beings who have struggled. That is magnificent. The Field Museum fuels a journey of discovery across time to enable solutions for a brighter future rich in nature and culture.

Why are a quarter of a billion sperm cells needed to fertilize one egg? Are women really fertile for only a few days each month? How long should babies be breast-fed? In How We Do It , I draw on forty years of research to locate the roots of everything from our sex cells to the way we care for newborns. For instance, I explain why choosing a midwife rather than an obstetrician may have a greater impact than we think on our birthing experience, examine the advantages of breast-feeding for both mothers and babies, and suggest why babies may be ready for toilet training far earlier than is commonly practiced.

In connection with the book, I recently started a regular monthly blog with Psychology Today.