EVOLUTION OF SEX

[caonacl]

I posted this because I read about a theory that descibes the very close relationship between sex and death. Think about it. Sexual reproduction would be of no benefit to a species if not closely tied to the species death rate. Long livng animals don't reproduce nearly as fast as short living animals.

Sex, in effect, was natures way of correcting genetic errors in the species. But the corrective mechanism would not have been nearly so effective if the adult organism would not die and remove the bad/corrupted genes form the gene pool.




EVOLUTION OF SEX
Why is there sex? We assume, anthropocentrically, that reproduction requires two individuals. But in many organisms that is not true. Life originated without sex (as best we can tell) so sexual reproduction is something that had to evolve.


First what is sexual reproduction? 1) production of haploid gametes by meiosis, a reduction division, 2) fusion of these gametes produces a zygote and restores the full diploid complement of chromosomes.

There are thus two parts to the evolution of sex: 1) the origin of sexual reproduction (cellular evolution) and 2) the evolution and maintenance of sexual reproduction and recombination (recombination is like sex in that it reassorts genetic material) Recombination probably evolved ~ 3 billion years ago as a mechanism of DNA repair; sex evolved ~ 1-2 billion years ago in the early eukaryotes; the reason is unclear but it its likely that it is maintained in the current day by selection.


One "story" about the origin of sex is as follows (from J. Maynard-Smith, The Evolution of Sex, Cambridge University Press, 1978):
1. Binary cell fusion (advantage ~ hybrid vigor; masking deleterious mutations)
2. Evolve the use of one spindle apparatus (advantage = maintaining both sets of chromosomes and hence any hybrid vigor effects
3. Homologous pairing and chiasma (next step but advantage unclear; generates variation but also creates regions of genetic homozygosity)
4. Reduction division + syngamy (favored to restore heterozygosity)

This may be one plausible scenario but why higher eukaryotes spend so much of the haploid-diplod life cycle in the diploid stage is unclear

Subsequent evolution and maintenance of sex and recombination. Observation: one can select (both up and down) for rates of recombination between two loci without affecting other rates of recombination (see figure 8.6, pg.213). This means that there is genetic variation affecting recombination, which in turn means that selection can alter the rates of recombination.


The central problems with sex: 1) sex and recombination mix up any "coadaptation" that a genotype might have to a particular environment; why disturb this if its is adaptive; 2) there is a cost of meiosis associated with putting genes into males that cannot produce eggs. Consider the following model: k = number of eggs, S = probability of survival, Parthenogenetic = unfertilized eggs develop into females.
# adults# eggsadults next generationParthenogenetic femalesnknSknsexual femalesN1/2kN1/2SkNsexual malesN1/2kN1/2SkNRatio Partheno/totaln/(2N + n)(Skn) / (SkN + Skn) = n / (N + n)



If n is small, the parthenogenetic strain has effectively doubled in the next generation. This assumes that the egg production by a sexual and a parthenogenetic female are Å same and that the eggs of sexual females are equally divided between males and females. Why be sexual?


Advantage of sex in terms of genetic variation and rate of evolution. Consider two loci, A and B and new mutations to alleles a and b which can interact to produce a genotype of high fitness in a novel environment.. Since mutations are rare originally the only new chromosomes in the population will be Ab (from a B -> b mutation ) and aB (from an A -> a mutation). An asexually reproducing stain would have to wait for the second mutation because it has no way of reassembling the existing alleles into new combinations. The sexual strain could produce the high fitness genotype much faster by recombination (see figure 11.1 page 286).

This advantageous effect of recombination would be reduced in small populations because the number of mutations occurring is the product of mutation rate and population size, so the advantage of sex and recombination would be reduced in small populations.


These two observations suggest that sex might evolve by group selection: we have 1) genetically isolated "groups" (sexual and parthenogenetic strains), 2) disadvantage to the individual and 3) advantageous to the long-term survival of the group.

But G. C. Williams notes that the cases of facultative parthenogens (species that can reproduce either parthenogenetically or sexually) strongly suggest that there must be some short-term advantage to sex (otherwise it would be selected out of population as a strategy; note that we are dealing with the same group here so the advantage of sex at the group level does not apply). Some possible advantages:


Unpredictable environment theory: sex and recombination produces more genetically varied offspring, individual using this strategy will have higher fitness since they have a high chance of leaving an offspring that will survive in an unpredictable and changing environment.

Problems/qualifications: 1) if selection were sufficiently strong in the unpredictable environment then there is a chance that the genetic variation produced by sex/recombination might not be sufficient to "cover" the range of genotype needed. Considers the hard selection/soft selection continuum

2) if there were continuous selection such that genetic variation was continually removed from the population, there might be little variation remaining for sex to have an advantage.

3) theoretical models show that one needs to have the unpredictability be in the form of a switch from hot-dry vs. cool-wet environments in one generation to hot-wet and cool-dry in the next generation for recombination to really have a significant advantage.

Sib competition theory states that offspring in the next generation will be competing for the same resources and thee genetically identical offspring will experience more severe competition that sexual siblings because the latter will be genetically different and will not utilize resources in identical manners. Supporting evidence from plants where offspring were shown to have higher fitness when grown in competition with different genotype than in competition with its own genotype.


Muller's ratchet and recombination. In a strain of asexual species the number of deleterious mutations accumulate with time. The only way to get rid of them is by death. When the genotype with the smallest number of mutations nx dies , the genotype with the smallest number of mutations in now nx+1 and thee "ratchet" has clicked up one notch. With recombination one can mate two genotypes and their offspring might have fewer mutation (and more as well), so variation is generated that can be a higher fitness than any of the existing genotypes.

http://biomed.brown.edu/Courses/BIO48/Image18.gif http://biomed.brown.edu/Courses/BIO48/Image19.gif
The lower chromosome now has fewer deleterious mutations than either parental chromosome.


Hitchhiking effect Deleterious alleles can become associated with advantageous alleles by virtue of finite population size which will impose disequilibrium on loci (or association could be due to lack of recombination and a deleterious mutation at an adjacent locus). Any other locus which increases the rate of recombination will be favored because it will tend to break up the association between the advantageous allele and the linked disadvantageous allele.


Converse of this is that any increase in fitness that occurs as a result of the interaction between two loci, selection will act to reduce recombination because recombination would break up favorable associations of alleles.

Parasites may also have played a role in the evolution of sex. Various data suggest that more genetically diverse (heterozygous) organisms can mount a more effective defense against parasites than homozygous hosts. Sexual reproduction might again have a short term advantage in terms of sexually produced organisms having a greater defense system against parasites and other infectious agents.

http://biomed.brown.edu/Courses/BIO48/19.Evol.of.Sex.HTML

[caonacl]

SEX AND DEATH, CONNECTION OF

Sex and death have a number of connections other than having been taboo topics in polite company and controversial subjects in school curriculums. As is the case with many taboos, both can lead to fetishes and eroticisms, and their mere mention holds shock value for young adults.
Few question that life's greatest drives are to reproduce and to avoid death. The Austrian psychoanalyst Sigmund Freud and the French social theorist Michel Foucault argued that the two are fused, that the death instinct pervades sexual activity—a connection easily seen by a Frenchman whose language frames orgasms as petit mort, or "mini-deaths." With the AIDS epidemic their view has become particularly poignant. A 1992 study from Amsterdam, for instance, found that about one in six U.S. soldiers surveyed said that sex without condoms was worth the risk of getting the AIDS virus. A year later a story released by Planned Parenthood counselor offices in San Antonio, Texas, explained how teenage girls were demonstrating their toughness by having unprotected sex with an HIV-infected gang member. It seems that, for some, sexual desire is intensified in the presence of taboos and boundaries, even deadly ones.

The Scientific Perspective: Death As the Cost of Reproduction

Early lessons about the connection between reproduction and death often come from exceptional stories from the animal kingdom. Pacific salmon, for instance, return after three or four years in the ocean to battle hundreds of miles upstream— against gill nets, predators, and dams—to the tributaries where their lives began, to spawn and to die. Their remains fertilize the streams, providing food for the tiny microorganisms on which their offspring will feed. In addition, one cannot forget the story of how the female praying mantis bites off the head of her partner while mating. Or how in several marsupial mice species, the immune systems of the mice collapse after their first mating, leading to death shortly thereafter.
It has been observed that death is the price multicellular creatures must pay in order to reproduce. The biologist William Clark observed, "Obligatory death—as a result of senescence (natural aging)—may not have come into existence for more than a billion years after life first appeared. This form of programmed cell death seems to have arisen at about the same time cells began experimenting with sex in connection with reproduction" (Clark 1996, p. xi). Perhaps one legacy of this original immortality is the telomerase, the so-called immortality enzyme, found within the cells of testes and ovaries. Absent from normal cells that age and die, telomerase is what allows cancerous cells to reproduce without limits.

In the case of the life span of mammals, the period in which they have the greatest resistance to harmful environmental factors is when they have the greatest reproductive capacity. Evolution has little interest in the survival of those who have produced viable offspring and are in the post-reproductive period of life, hence the extreme rarity of senescent (old) animals in the natural order.

Humanity is not immune from this law of death as the cost of sex. This toll for reproduction has particularly been borne by women. Unlike at the start of the twenty-first century, when women held a seven-year life-expectancy advantage over males in developed nations, historically, because of their high maternal death rates, women were the shorter-lived sex. Maternal death rates remain high in poor nations of the world, where women are up to 200 times more likely than women in the richest countries to die as a result of complications of pregnancy, abortion, or childbirth—the causes of one-quarter of all deaths of those of childbearing age. In the United States, the Centers for Disease Control and Prevention reported in 2001 that black women were four times more likely than non-Hispanic white women to die of pregnancyrelated problems.
Even the sex act itself can prove lethal. Cardiovascular specialists have long warned how extramarital sex was dangerous for heart patients, as it increased their blood pressure and pulse rate more than when having sex with a familiar partner. Such activity killed a former American vice president, Nelson Rockefeller, who died of a heart attack during an extramarital tryst in 1979. In 1998, having sex shortly after having given birth proved fatal for two British women, who died of air embolisms.
Attempts to enhance one's sexual experiences can be deadly as well. In 1998 the Food and Drug Administration reported the deaths of several men taking the highly popular Viagra impotence pill. Each year, attempts at sexual self-gratification accidentally kill between 500 and 1,000 individuals, predominantly men, because of autoerotic asphyxia. To heighten their sexual orgasm during masturbation, these individuals cut off the supply of oxygen and blood to their head, often by tying a belt or rope around their neck. Consciousness may be lost, and the individual dies by strangulation.

Relationships between Sex and Longevity

The need for species to change over time underlies evolution's mechanisms for triggering death with sex. In addition, there are genetic clocks determining the time frame for species to produce and raise the next generation to the point where it can successfully pass on its genetic code. Thus the later in life a species reproduces, the longer its life expectancy. Fruit flies with special "longevity genes" have been created, allowing them to live twice as long as their normal counterparts. By breeding them at increasingly advanced ages, Carla Sgrò and Linda Partridge also found that fruit flies that produced eggs at a young age died earlier than those that reproduced when they were older. When the younger-reproducing flies were sterilized with X rays, they began living as long as their older counterparts.

How this phenomenon might apply to humans raises interesting questions. Will the trend toward postponing parenting ultimately lead to the delaying of senescence and death? And, given the trend of affluent older males beginning second families with their young "trophy wives," will an even greater longevity advantage develop in the upper classes?
Nevertheless, postponement of parenting indefinitely can also lead to premature death. In 1986 Evelyn Talbott found that women over the age of fifty who had been married but had never had children might face an increased risk of dying suddenly of heart disease. Several studies in the early 1990s found that men who had vasectomies increased their risk of testicular cancer and prostate cancer, the latter by 56 to 66 percent at all ages.

Another 1994 study of 1,800 Seattle women by Janet R. Daling and her colleagues for the National Cancer Institute found that abortion increased women's risk of breast cancer by 50 percent. In the same year, a study directed by Mats Lambe found that having a child increased a woman's risk of breast cancer in her younger years but protected her against cancer in later life. For example, a thirty-five-year-old woman who gave birth at age twenty-five had an 8 percent higher risk of breast cancer than did a childless woman the same age; at age fifty-nine, however, the former's risk was 29 percent lower than the latter's.
On the other hand, eliminating one's ability to reproduce has also been found to reduce the likelihood of death. A 1999 study by physician Edward M. Messing and his associates showed that castration increased the survival chances of men with spreading prostate cancer. And Canadian researchers in 2001 reported that women with a high probability of developing ovarian cancer could reduce their cancer risk by up to 72 percent with tubal ligations.

Special Case of AIDS

During the late twentieth century it was the AIDS epidemic that most universally symbolized the lethal aspects of sexuality, particularly acts outside of monogamous relationships. While the popular conception in the United States initially saw the sex-death connection largely confined to specific high-risk groups, particularly homosexual populations, throughout most of the world the epidemic spread through heterosexual unions.
At the start of the twenty-first century, the highest rates of HIV infection were in sub-Saharan African countries, occurring within the most sexually active segment of the population, those fifteen to forty-nine years of age. Here the cultural sex order made for an epidemiological nightmare, where individuals were more likely than their European counterparts to have numerous sex partners. Once again women were disproportionately the victims, more likely being the one infected than infecting—owing to greater male promiscuity and female subservience—and being the sex to most quickly develop full-blown AIDS infection and dying of its effects. Projections, made in 2000, were that men would outnumber women by eleven to nine. Without AIDS, life expectancy in 2010 was projected to be 70 years in Zimbabwe, 68 in South Africa, and 60 in Zambia. With AIDS, life expectancy was expected to fall below 35 years in Zimbabwe, to 48 in South Africa, and to 30 in Zambia.

The HIV deaths resulting from sexual relations extend from individuals to entire social orders. The epidemic has killed family structures—producing a huge generation of orphans—and severely diminished chances for economic development. In 1999 at Eskom, South Africa's electric utility, 11 percent of the workers were infected with HIV, as were an estimated 40 percent of the Ugandan military and one-third of that country's teachers. In South Africa, Zimbabwe, and Kenya, some of Africa's most industrialized countries, gross domestic product was predicted to be 20 percent lower by 2005 than it otherwise would have been without the epidemic.

When the Sex Drive becomes Deadly

Evolution has endowed human males with a high sex drive coupled with considerable aggressiveness—especially regarding matters of breeding rights. In Pakistan, husbands are often acquitted for the "honor killing" of their spouses, whose "crime" may range from a simple flirtation to an affair.

Violent sexual assaults on the weak and unempowered occur throughout the world. In Juárez, Mexico, during the late 1990s, at least seventy women had been raped and murdered by sexual predators, their bodies dumped in the Chihuahua Desert. In the United States, the rape, maiming, and murder of children are frequent news items, leading to most states passing sexual predator legislation and demands that the public be informed of where these individuals reside when released.

Violence between sexual intimates is unfortunately common. Domestic violence is the leading cause of injury to women in the United States, resulting in more injuries than muggings, rape by strangers, and car accidents combined. In about one-third of killings of women, the killer is either her spouse or boyfriend. And of females murdered by strangers, prostitutes comprise a disproportionate number of victims.

Rough sex has produced its share of fatalities as well. One highly publicized death occurred in the mid-1980s in New York City's Central Park, where an eighteen-year-old woman was found strangled. Robert Chambers, a nineteen-year-old from an affluent family, confessed to having accidentally killed her while engaging in "kinky sex." This turned out not to be the case, but the defense has been used several times since for deaths resulting from sadomasochistic sexual activities.

Psychiatrists have long seen the underlying sexual motivations behind serial killers, typically featuring elements of sadism and necrophilia. Tim Cahill's psychobiography of John Wayne Gacy— who between 1972 and 1978 raped, tortured, and then murdered thirty-three young men in the Chicago area—detailed Gacy's feelings of inferiority and unworthiness in his father's eyes, guilt about his homosexual tendencies, and feelings of inadequacy in male-female relationships.

Connections between Sexual and Death Moralities in the American Mind

Moral codes often contain messages of restraint regarding matters of sex and harm of others—the antithesis of social chaos with its orgies of sex and violence. It is worth noting the internal consistencies of these issues in individuals' minds, for how they coalesce affects outlooks toward a host of political and religious matters.

Consider, for instance, attitudes toward the moralities of abortion, euthanasia, and the right of the terminally ill to commit suicide, and how they correlate with the perceived moralities of premarital sex and homosexual relations. According to the National Opinion Research Center's "General Social Surveys," (Davis and Smith 1998), between 1977 and 1998 the proportion of American adults supporting all three death matters increased from 26 to 38 percent, while the percent disapproving of all three declined from 33 to 26 percent. During the mid-1990s, 70 percent of those who endorsed all three death issues believed it was "not wrong at all if a man and woman have sex relations before marriage," compared to only 15 percent of those who opposed these three death issues. Similarly, 64 percent of those who endorsed all three death issues believed that "sexual relations between two adults of the same sex" was "not wrong at all," compared to 12 percent of those opposed to all three death issues.

Death-Sex Connections in the Arts

In a 1992 book, Camille Paglia claimed that it was in the West that sex, violence, and aggression are major motivations for artistic creativity and human relationships. There is little doubt that these are qualities of audience appeal. Hollywood has long known of the attractions to the erotic and the violent, which is why 60 percent of R-rated movies and nearly half of X-rated movies contain violence. The long-term success of the James Bond movie series derives from its fusion of sex and death.
According to Geoffrey Gorer, such seductions derive from cultural pruderies to matters of sex and death. William May observed that as sex becomes pornographic when divorced from its natural human emotions of love and affection, so death becomes pornographic when divorced from its natural emotion, which is grief. Perhaps the pornographic connotation is why designer Christian Dior chose in the 1990s to label one of its perfumes "Poison."

http://www.deathreference.com/Py-Se/Sex-and-Death-Connection-of.html

[Arianwen]

tl;dr

[caonacl]

tl;dr

The short version is that sex evolved to give the species the opportunity to remove damaged genes from the gene pool.

Both genetic repair and genetic disposal are done through the female.

[wetDirt]

It's not that simple. Viruses replicate asexually, but if two genetic variants infect the same cell, the bits and pieces can recombine, reassort, and deal the bad bits out, which amounts to exactly the same thing, or better. Most bacteria can exchange genetic material too. It turns out that there are several different ways for genetic material to efficiently and effectively eliminate deleterious mutations, select improvements, and store and disseminate genetic variants to use later when circumstances might favor them. As a matter of fact, random mutation is the lamest, slowest, and most inefficient method of trying to clean up a gene pool imaginable.

[Kassy]

It's a war out there:
http://www.oeb.harvard.edu/faculty/hartl/lab/publications/pdfs/Singh-05-BioEssays.pdf

[caonacl]

It's a war out there:
http://www.oeb.harvard.edu/faculty/hartl/lab/publications/pdfs/Singh-05-BioEssays.pdf
from link above


Summary

Charles Darwin remarked that ‘‘males, with their superior
strength, pugnacity, armaments, unwieldly passion
and love songs, are almost always the more active

and most often, the initiators of sexual interactions’’.
(1)

Here, we propose that such male sex drive directly
impacts the genome by leading to its progressive
masculinization—genes that possess sex-specific effects
on male fitness accumulate to a much greater extent
and are generally more diverged.

(2,3) The larger proportion
of male versus female fitness modifiers in combination
with stronger sexual selection may generate
evolutionary signatures such as a greater sensitivity to
male sterility

(4) and a paucity of X-linked male-specific
genes.

(5–8) Male sex-drive theory complements the
female-choice theory of sexual selection and allows
for the genetic variation of costly sexual traits to be
continuously replenished.

BioEssays 27:518–525,
2005. 2005 Wiley Periodicals

[Kassy]

I was looking for stuff on sexual antagonism but i couldn't find any better links...or the real tasty examples.

In 2000, Dr. Rice published his findings showing that when flies in normally promiscuous habitats were experimentally forced to be monogamous, the evolution of the seminal fluids proteins ultimately promoted the extension of mate survival time, rather than the reduction of it, relative to the promiscuous controls. In addition to this observation, Dr. Rice noted that monogamous males who were placed back into a promiscuous environment displayed an overall reduction of fitness. These data are of particular interest because they show a correlation between the rate of mutation of sexually antagonistic traits, and the degree of commitment between an individual and its mate. This rate is important because of the elevated evolutionary rate of reproduction-associated proteins. A 2002 article in Science indicates that the divergence of these proteins is especially noticeable in the primate lineage leading to humans, mice and rats, marine invertebrates, and D. melanogaster.

http://www.jyi.org/volumes/volume7/issue3/features/campbell.html