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.
So, it is perhaps no wonder that this early generation of models concluded that sex would evolve only under very restrictive conditions. Subsequent models have relaxed these assumptions in a number of ways, attempting to better capture many of the complexities involved in real-world evolution. The results of these second-generation models are briefly summarized in the following sections. Current models indicate that sex evolves more readily when a species' environment changes rapidly.
When the genetic associations built up by past selection are no longer favorable, sex and recombination can improve the fitness of offspring, thereby turning the recombination load into an advantage. One important source of environmental change is a shift in the community of interacting species, especially host and parasite species. This is the so-called "Red Queen" hypothesis for the evolution of sex, which refers to the need for a species to evolve as fast as it can just to keep apace of coevolving species.
The name of this hypothesis comes from Lewis Carroll's Through the Looking Glass , in which Alice must run as fast as she can "just to stay in place.
Sex can also be favored when selection varies over space, as long as the genetic associations created by migration are locally disadvantageous. Whether this requirement is common in nature remains an open question.
Organisms that reproduce both sexually and asexually tend to switch to sex under stressful conditions. Mathematical models have revealed that it is much easier for sex to evolve if individuals that are adapted to their environment reproduce asexually and less fit individuals reproduce sexually.
In this way, well-adapted genotypes are not broken apart by recombination, but poorly adapted genotypes can be recombined to create new combinations in offspring. Models that account for the fact that population sizes are finite have found that sex and recombination evolve much more readily. With a limited number of individuals in a population, selection erodes easily accessible variation, leaving only hidden variation Figure 2.
Recombination can then reveal this hidden variation, improving the response to selection. By improving the response to selection, genes that increase the frequency of sex become associated with fitter genotypes, which rise in frequency alongside them. Interestingly, the requirement that fitness surfaces exhibit weak and negative curvature is relaxed in populations of finite size; here, fitness surfaces may be uncurved or positively curved and still favor sex.
Figure 2: Selection in finite populations leaves hidden variation. This diagram depicts a population consisting of 14 haploid individuals who carry plus or minus alleles at each of four sites in their genome left panel.
In a new environment favoring the plus alleles, selection will, over time, increase the frequency of the plus alleles throughout the genome right panel. For example, in a hotter climate, alleles conferring tolerance to higher temperatures would rise in frequency. Selection favors the good gene combinations here, the ones containing two plus alleles and eliminates the bad gene combinations. In the absence of sex, the only variation that remains after several rounds of selection is hidden in the sense that plus alleles at the first site are found with minus alleles at the second site or vice versa.
This problem is irrelevant in an infinitely large population, because mutation will immediately create beneficial combinations e. Two populations are represented as black circles with fourteen line segments, each composed of four black plusses or minuses. The population at left, representing the Initial population, contains two line segments with two plus signs, seven line segments with one plus sign, and five line segments with zero plus signs.
Arrows point to another population at right.. This resulting population also contains fourteen line segments, each containing two plus signs and two minus signs. Eight of the line segments contain a minus sign, two plus signs, then one minus sign, whereas six of the line segments contain alternating plus and minus signs.
This last result is particularly interesting, because it suggests that August Weismann might have been right all along in arguing that sex evolved to generate variation. Modeling Weismann's hypothesis with infinitely large populations failed because variation is too easily generated by mutation and too easily maintained by selection within these populations. Altering this size-related assumption by modeling selection among a finite number of individuals reveals just how important sex and recombination are as processes that allow genes residing in different individuals to be brought together, thereby producing new genotypic combinations upon which selection can act.
De Visser, J. The evolution of sex: Empirical insights into the roles of epistasis and drift. Nature Reviews Genetics 8 , — doi Felsenstein, J.
The cervix dilates. In the second stage, expulsion , powerful contractions push the head and the rest of the body through the dilated cervix, and out through the vagina and the vulva. The baby is born. Further contractions expel the placenta to complete the placental stage. Reproductive structures begin to form in the embryonic stage. By week 6, gonads and genitalia are present but undifferentiated. Whether they become male or female is determined by one chromosome delivered by the sperm.
This pair contains an X sex chromosome from the female egg and either an X or a Y sex chromosome from the male sperm. If the chromosome pair is XY, the gonads develop into testes starting in week 7. If the chromosome pair is XX, the gonads become ovaries starting in week 8. Testes secrete testosterone, forming male genitalia around week Without testosterone, female genitalia form. All reproductive structures are in place at birth or shortly after.
At puberty, an increase in sex hormones will grow them to their adult size and reproductive capability. Download Reproductive System Lab Manual. See more from our free eBook library. An article in Science Daily on a research study that involves creating a synthetic model of the placenta to better understand pregnancy. Female Reproductive Structures. Male Reproductive Structures. When you select "Subscribe" you will start receiving our email newsletter.
Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe. See our privacy policy for additional details. The sperm cells surround the egg cell if it is present. When a sperm enters an egg cell, they unite and their chromosomes mingle together.
This is called conception or fertilization. If the egg is fertilized by the sperm cell, the fertilized egg cell begins to divide into two cells, then four, then eight, and so on, while it travels the rest of the way through the fallopian tube to the uterus.
There it implants and grows into the unborn baby. Skip to main content.
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