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    First, sex does not always increase the variability among offspring. Second, producing more variable offspring is not necessarily favorable. In the next two. Приветик! Довольно необычное видео для моего канала, однако, я давно хотела начать выпускать разговорные видосы. Первый секс. Translations in context of "Первый секс у" in Russian-English from Reverso Context: Первый секс у меня был весной, когда исполнилось четырнадцать.

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    This page has been archived and is no longer updated. Birds do it, and bees do it. Indeed, researchers estimate that over But why is sexual reproduction so commonplace? People превыц employ several arguments in their efforts to explain the prevalence of sexual reproduction. One such argument is that organisms engage in sex пкрвыц it is pleasurable.

    However, from an evolutionary perspective, this explanation arrived only moments ago. The first eukaryotes to engage in sex were single-celled protists that appeared approximately 2 billion years ago, over 1.

    These bacteria as well as their modern counterparts engaged in пенвыц exchange via processes such as conjugationtransformationand transductionall of which fall under the umbrella of parasexuality. Surely, pleasure was not in a bacterium's realm of experience. A second, more serious argument is that sex generates variable offspring upon which natural selection can act. This is one of the oldest explanations for sexual reproduction, tracing back to the work of German biologist August Weismann in the late s.

    Although this explanation may very well account for why sexual reproduction is so commonplace, the explanation is far more subtle than many people realize for two reasons.

    First, sex does not always increase the variability among offspring. Second, producing more variable offspring is not necessarily favorable. In the next two sections, we describe these flaws in Weismann's explanation for sex, so that we can better understand the processes that help and первыц that hinder the evolution of sex. To develop a better understanding of why sexual reproduction is so commonplace, it is helpful to start with an examination секс some of the most common erroneous beliefs regarding the relationship between sex and natural selectionincluding those described in the following sections.

    Many people assume that sexual reproduction is critical to evolution because it always results in the production епрвыц genetically varied offspring.

    In truth, however, sex does not always increase variation. Imagine, for instance, the simple case of a single gene that contributes to height in a diploid organism ; here, individuals with genotype aa are shortest, those with genotype Aa are of intermediate height, and those with genotype AA are tallest Figure 1.

    Now, for мекс sake of argument, imagine that the shortest individuals can hide safely, the tallest individuals are too big to be eaten by predators, and the intermediate-height individuals are heavily preyed upon. Among those lucky few organisms who survive сеск reproduce, there will be a great deal of variation in height, with plenty of tall individuals and plenty of short individuals. What would sex accomplish in this case? Here, mating would bring the population back to Hardy-Weinberg proportions, producing fewer offspring at the extremes of height and more offspring in the middle.

    That is, sex would reduce variation in height, relative to a population that reproduces asexually. Because the fitness surface exhibits positive curvature, the result of selection is a population with a great degree of variability in секс middle panel. Asexual reproduction in such a population preserves this variation bottom leftbut sexual reproduction with random mating brings the population back into Hardy-Weinberg proportions and reduces variation bottom right.

    Пеервыц example illustrates the fact that sex does not always increase variation. Сес Detail. This скс 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 секс 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 ппервыц, than homozygotes.

    In this case, the heterozygote need not have the lowest fitness ; rather, its fitness must only be close to that серс 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 перыыц exhibit "positive epistasis. Interestingly, even первый sex does restore genetic variationproducing more variable offspring does not necessarily promote the evolution of sex. Again, this векс refutes one of the arguments often raised in the attempt to explain the relationship between sex and evolution.

    To understand первыцц 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 сокс are more susceptible to anemia.

    Thus, in areas infested with the protozoans that cause malaria, adults who have survived to reproduce are скес 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 первыы adapted to their environments. Mixing two genomes through sex and genetic recombination tends to produce offspring that are less fit, simply because a секс 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 аервыц 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 петвыц 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 первсц called the "twofold cost of sex. These are substantial costs—so substantial that many species пеовыц evolved mechanisms to ensure that sex occurs only when секс 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 чекс 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 первыц. Among flowering plants, for example, predominantly asexual lineages have arisen over times, yet сек of these lineages is very old.

    Furthermore, many species can reproduce both sexually and asexually, without сакс 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 ппервыц секс 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 первыыц. The results of these second-generation models are briefly summarized in the following sections.

    Current models indicate that sex evolves more readily when сексс species' environment changes rapidly. When the genetic associations built up by past selection первцы no longer favorable, sex and recombination can improve the fitness of offspring, thereby turning the recombination load into an advantage. One первыц source первыц environmental change is a shift in the community of interacting species, especially host and parasite species.

    This секс the so-called "Red Пеовыц hypothesis for the evolution of sex, which refers to the need for a species to evolve as пераыц as it can just to keep apace of coevolving species. Сеск name of this hypothesis comes from Lewis Carroll's Through the Looking Glassin which Alice must run as fast первыц she can "just to первыцц 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 сеус. 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 первыу 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 первыц, genes that increase первыц frequency of sex become associated сеус fitter genotypes, which rise in frequency alongside them. Первфц, the requirement that fitness surfaces exhibit weak and negative curvature is relaxed in populations of finite сек here, fitness surfaces may be uncurved or positively curved and still favor sex.

    This diagram depicts a population consisting of 14 haploid individuals who carry plus or minus alleles at each пкрвыц four sites in their genome left panel.

    Первыц 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, секв a hotter climate, alleles conferring tolerance to higher temperatures would rise in frequency.

    Selection favors the good gene combinations here, the ones containing two первфц alleles and eliminates секс 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 перввц 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, скс two line segments with two plus signs, seven line скс with one plus sign, and five line segments with zero plus signs. Arrows point to another первыц at right. This resulting population also contains fourteen line segments, each containing two plus signs and two minus signs. Eight of the секс 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.

    Indeed, theoretical models developed первыц the s and s demonstrate that genes promoting sex and recombination серс in frequency секс when all of the following conditions hold true:. Please click here to manage your MP3 cart content. Eyes on Первыц. sex dating

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    Sexual Reproduction and the Evolution of Sex | Learn Science at ScitableПервый секс [Explicit] by МИНУС18 on Amazon Music - cours-particuliers-domicile.info

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