to you teenagers?
ACNE! Also known as pimples, zits, goober. However, many of us have erroneous notions of what acne is, what it is caused by, and how to make it go away. Below are the top
Myths of Acne - Dispelled!
adapted and retrieved 30 June 2011 from http://www.acne.org/myths.html
1: Washing your face more often will help clear up acne.
No! Facial blemishes are not caused by dirt. Contrary to what you may have seen in commercials, pores do not get blocked from the top down due to "impurities". Rather, the walls of a pore stick together deep within the skin, starting acne formation. Far from preventing acne, frequent washing may actually irritate pores and cause them to become clogged. A washcloth can add even more irritation. The best bet is to wash very gently with bare hands, and only wash twice a day. Steer clear of exfoliants or scrubs, which can irritate your pimples. Also avoid products that contain alcohol because they can dry out and irritate your skin.
2: Stress causes acne.
Wrong again! Stress may have an effect on hormones and theoretically can promote acne. However, an effective acne system is more powerful than a bout of stress any day. Some psychiatric medications may have acne as a side effect, but stress itself is no big deal. Your time is better spent determining the right course of acne treatment rather than feeling guilt about stress.
3: The sun will help get rid of acne.
Not true. The sun may work in the short-term to hasten the clearing of existing acne while reddening your skin, thus blending your skin tone with red acne marks. However, a sun burn is actually skin damage. It's important to prevent damage to your skin while trying to get rid of acne. Sun exposure causes irritation which can make acne worse. People will often notice their skin breaking out as it heals from sun damage. The sun is a short-term band-aid which will often bite back with more acne in the weeks following exposure. Having said that, I don't want to give the impression that the sun is evil. It is not. We get our vitamin D from the sun for instance. Limiting sun exposure on acne prone areas of your body is most likely prudent, but some exposure from time to time is not only unavoidable, but is perfectly okay.
4: Diet and acne are related. Chocolate and fried foods are the common miscreants.
Thankfully, no. Although it sounds logical, it is also not true, which is a good thing if you love eating chocolate and the occassional hamburger or fried chicken. No studies have proven this and it may be only a psychological belief that this is true. Of course, it sure will not help if you have greasy hands and wipe them on your face while eating - that will not help.
Bottom line is we need more research. We do know that people in some indigenous societies do not experience acne whatsoever across the entire population. This is in stark contrast to the widespread presence of acne throughout all modern society. It leaves us to ponder the question of whether the indigenous people's diet contributes to their acne-free skin. Discovering a dietary way of preventing acne may be a future reality, however, we may live so differently from our hunter/gatherer ancestors that it has become close to impossible to replicate our ancestral diet.
5: Popping your pimples is the best way to get rid of them.
Step away from that mirror! Some people might tell you that popping your zits will make them less noticeable and help them heal faster, but they're wrong. Picking or popping your pimples pushes germs further under your skin, which could cause more redness, pain, and maybe even a nasty infection. And popping zits can lead to scarring, which could last forever.
If pimples always seem to show up at the wrong time, like before a big event such as a dance, talk to your parent about seeing your doctor or a dermatologist (say: der-muh-tah-luh-jist), a doctor who specializes in treating acne. A doctor can help get your acne under control.
Thursday, 30 June 2011
1.3. Will a couple reproduce everytime they have sex?
No! Beause there are fertile and infertile phases during the female menstrual cycle, and fertilisation can only take place during the fertile phase.
Watch the menstrual cycle explanatory video here.
However, pre-menstrual symptoms (PMS), which are observed characteristics in females just before menstruation occurs, can pose severe problems to a female. In fact, PMS also affect males, when they cannot recognise them and hence are perplexed by the crazy mood swings and inexplicable irritability that their partners display. Read on to understand more about PMS (and your future wife's behaviours)!
In her study, Marván questioned 49 women -- all students at a private university in Puebla, Mexico, all white, and all from middle-to-upper class families. None of the women knew this was a study about PMS, she says. They were "asked if they wanted to be part of a study of lifestyle factors and health," Marván writes.
She questioned each woman numerous times, on the days both before and after menstruation. Among the questions: Did they have cramps, swelling, headache, muscle stiffness, painful breasts, nausea? Did they feel irritable, depressed, anxious, distracted, have trouble concentrating?
Significant numbers of women reported such symptoms during their pre-period days. Yet when asked later about their premenstrual days, many reported much worse symptoms.
Other researchers have noted the impact of popular women's magazines in "bombarding women" with headlines, anecdotes, and studies implying that extreme mood swings are an inevitable part of our menstrual cycle, she writes.
"Many women have a misperception about the meaning of PMS," Marván writes. "Consequently, they amplify their premenstrual changes, reflecting women's cultural stereotypes rather than their actual experience."
Marván's study "confirms what a lot of literature has been showing -- that PMS exists in the minds of women, that it's not clearly an entity," says Alice Domar, PhD, director of the Mind/Body Center for Women's Health in Boston and author of Self-Nurture: Learning to Care for Yourself as Effectively as You Care for Everyone Else.
"The enormous strength of this study is that the women didn't know it was about PMS," Domar tells WebMD. "It shows that this is more of a psychological issue than we previously thought."
"If you're having a really bad day, feeling yucky and bloated on day 15, you're likely to attribute it to your boss, fight with the husband, bad grade at school," she says. "If you feel that way on day 16, you think it's PMS."
In reality, many women have slightly different premenstrual symptoms, says Domar. But true PMS "is much more severe and much rarer than women are led to believe. Only a minority of women report disabling, debilitating symptoms during PMS. I've certainly had patients come in say they've lost relationships because of PMS. They feel so physically ill that can't go to work."
Antidepressant medicines and relaxation techniques can work to combat the depression and anxiety of PMS, she tells WebMD. Although researchers aren't certain why, calcium supplements have also been shown as effective in reducing symptoms.
"There's a consistent impression among women that they feel physically and psychologically different premenstrually," she says. "Mild symptoms are pretty normal. If you find you have symptoms, certainly try some relaxation techniques. It's not going to hurt to add some calcium to your diet. And if symptoms are really severe, see your doctor and see if you're eligible for an antidepressant."
Watch the menstrual cycle explanatory video here.
However, pre-menstrual symptoms (PMS), which are observed characteristics in females just before menstruation occurs, can pose severe problems to a female. In fact, PMS also affect males, when they cannot recognise them and hence are perplexed by the crazy mood swings and inexplicable irritability that their partners display. Read on to understand more about PMS (and your future wife's behaviours)!
PMS: Fact or Fiction
Reviewed by Charlotte E. Grayson Mathis, MD
Retrieved 24 June 2011 from http://women.webmd.com/pms/news/20010910/pms-fact-fiction
Sept. 10, 2001 -- You feel cranky, icky, just plain yucky -- so of course, it's PMS, right? Many women swear they suffer as their period approaches. Yet a new study shows that for many women, that's a self-fulfilling prophecy. They expect to feel bad, so they do.
"The more a woman believes in the phenomenon of menstrual distress, the more she exaggerates ... the negativity of her symptoms during her last period," writes María Luisa Marván, a psychology researcher at Universidad de las Americas-Puebla. Her study appears in the current issue of the journal Health Psychology.
"The more a woman believes in the phenomenon of menstrual distress, the more she exaggerates ... the negativity of her symptoms during her last period," writes María Luisa Marván, a psychology researcher at Universidad de las Americas-Puebla. Her study appears in the current issue of the journal Health Psychology.
In her study, Marván questioned 49 women -- all students at a private university in Puebla, Mexico, all white, and all from middle-to-upper class families. None of the women knew this was a study about PMS, she says. They were "asked if they wanted to be part of a study of lifestyle factors and health," Marván writes.
She questioned each woman numerous times, on the days both before and after menstruation. Among the questions: Did they have cramps, swelling, headache, muscle stiffness, painful breasts, nausea? Did they feel irritable, depressed, anxious, distracted, have trouble concentrating?
Significant numbers of women reported such symptoms during their pre-period days. Yet when asked later about their premenstrual days, many reported much worse symptoms.
Other researchers have noted the impact of popular women's magazines in "bombarding women" with headlines, anecdotes, and studies implying that extreme mood swings are an inevitable part of our menstrual cycle, she writes.
"Many women have a misperception about the meaning of PMS," Marván writes. "Consequently, they amplify their premenstrual changes, reflecting women's cultural stereotypes rather than their actual experience."
Marván's study "confirms what a lot of literature has been showing -- that PMS exists in the minds of women, that it's not clearly an entity," says Alice Domar, PhD, director of the Mind/Body Center for Women's Health in Boston and author of Self-Nurture: Learning to Care for Yourself as Effectively as You Care for Everyone Else.
"The enormous strength of this study is that the women didn't know it was about PMS," Domar tells WebMD. "It shows that this is more of a psychological issue than we previously thought."
"If you're having a really bad day, feeling yucky and bloated on day 15, you're likely to attribute it to your boss, fight with the husband, bad grade at school," she says. "If you feel that way on day 16, you think it's PMS."
In reality, many women have slightly different premenstrual symptoms, says Domar. But true PMS "is much more severe and much rarer than women are led to believe. Only a minority of women report disabling, debilitating symptoms during PMS. I've certainly had patients come in say they've lost relationships because of PMS. They feel so physically ill that can't go to work."
Antidepressant medicines and relaxation techniques can work to combat the depression and anxiety of PMS, she tells WebMD. Although researchers aren't certain why, calcium supplements have also been shown as effective in reducing symptoms.
"There's a consistent impression among women that they feel physically and psychologically different premenstrually," she says. "Mild symptoms are pretty normal. If you find you have symptoms, certainly try some relaxation techniques. It's not going to hurt to add some calcium to your diet. And if symptoms are really severe, see your doctor and see if you're eligible for an antidepressant."
Tuesday, 28 June 2011
1.2. Why is Puberty an important stage in Human Reproduction?
Without puberty, our reproductive systems will not be capable of producing gametes (i.e. sperms and ovums)!
But do you know how long a female's ova last? And why do males release 300 million sperms in an ejaculation when all it needs is just one for fertilisation?
The following content was taken from Columbia University's Health Q&A Internet Service at http://www.goaskalice.columbia.edu/1639.html. 19 Jan 2007.
Unlike men, who produce new sperm daily throughout most of their lifetime, women are born with all their eggs in one — okay, two baskets (ovaries). To be more precise, a woman is born with about one to two million immature eggs, or follicles, in her ovaries.
Throughout her life, the vast majority of follicles will die through a process known as atresia. Atresia begins at birth and continues throughout the course of the woman's reproductive life. When a woman reaches puberty and starts to menstruate, only about 400,000 follicles remain. With each menstrual cycle, a thousand follicles are lost and only one lucky little follicle will actually mature into an ovum (egg), which is released into the fallopian tube, kicking off ovulation. That means that of the one to two million follicles, only about 400 will ever mature.
Relatively little or no follicles remain at menopause, which usually begins when a woman is between 48-55 years of age. The remaining follicles are unlikely to mature and become viable eggs because of the hormonal changes that come along with menopause.
The following content was taken from Scienceline at http://scienceline.org/2008/06/ask-olson-sperm/. By Eric R. Olsen, 2 June 2008.
“Every sperm is sacred. Every sperm is great. If a sperm is wasted, God gets quite irate,” goes the song from Monty Python’s movie The Meaning of Life. If the lyrics strike you as funny, it’s most likely because calling a sperm cell “sacred” sounds ridiculous when men can produce so many of them.
In fact, the average male will produce roughly 525 billion sperm cells over a lifetime and shed at least one billion of them per month. A healthy adult male can release between 40 million and 1.2 billion sperm cells in a single ejaculation.
In contrast, women are born with an average two million egg follicles, the reproductive structures that give rise to eggs. By puberty, a majority of those follicles close up and only about 450 will ever release mature eggs for fertilization.
But if it only takes one sperm and one egg to meet and create a baby, then why do men produce such a whopping number of sperm? Wouldn’t it be less wasteful for a man to release a single sperm, or at least fewer, to meet one egg?
But if it was just a matter of ‘more is better,’ then animals of all species would have evolved ridiculously large testicles in a bid to overwhelm the competition. But it’s not quite that simple—numbers are important, but so is proximity. Fertilizing an egg is not just about how much sperm you can produce. It is also about how close you get your sperm to it.
In the early 1980s, researchers in the United Kingdom and the United States realized that both proximity and number were important factors in the physiology of primates, including humans. In primate societies with rigid social structures and one dominant male who mates with all the females, testes trend towards the small. In gorillas, for example, they are very small relative to body weight. (Don’t tell them that.) In gorilla society, one male defends a harem of females to ensure only his sperm gets anywhere near their eggs. In this case, making a lot of sperm doesn’t really help the male gorilla get the job done.
For chimpanzees, on the other hand, sperm competition is a serious issue. In chimpanzee society, many males and females live together in large troops, and females have sex with many males in a short span of time.
This is why male chimpanzees possess the largest testes of all the great apes, weighing in roughly 15 times larger than gorillas, relative to their body weight. This gives them a better shot at swamping out the competition.
But do you know how long a female's ova last? And why do males release 300 million sperms in an ejaculation when all it needs is just one for fertilisation?
The following content was taken from Columbia University's Health Q&A Internet Service at http://www.goaskalice.columbia.edu/1639.html. 19 Jan 2007.
Unlike men, who produce new sperm daily throughout most of their lifetime, women are born with all their eggs in one — okay, two baskets (ovaries). To be more precise, a woman is born with about one to two million immature eggs, or follicles, in her ovaries.
Throughout her life, the vast majority of follicles will die through a process known as atresia. Atresia begins at birth and continues throughout the course of the woman's reproductive life. When a woman reaches puberty and starts to menstruate, only about 400,000 follicles remain. With each menstrual cycle, a thousand follicles are lost and only one lucky little follicle will actually mature into an ovum (egg), which is released into the fallopian tube, kicking off ovulation. That means that of the one to two million follicles, only about 400 will ever mature.
Relatively little or no follicles remain at menopause, which usually begins when a woman is between 48-55 years of age. The remaining follicles are unlikely to mature and become viable eggs because of the hormonal changes that come along with menopause.
The following content was taken from Scienceline at http://scienceline.org/2008/06/ask-olson-sperm/. By Eric R. Olsen, 2 June 2008.
“Every sperm is sacred. Every sperm is great. If a sperm is wasted, God gets quite irate,” goes the song from Monty Python’s movie The Meaning of Life. If the lyrics strike you as funny, it’s most likely because calling a sperm cell “sacred” sounds ridiculous when men can produce so many of them.
In fact, the average male will produce roughly 525 billion sperm cells over a lifetime and shed at least one billion of them per month. A healthy adult male can release between 40 million and 1.2 billion sperm cells in a single ejaculation.
In contrast, women are born with an average two million egg follicles, the reproductive structures that give rise to eggs. By puberty, a majority of those follicles close up and only about 450 will ever release mature eggs for fertilization.
But if it only takes one sperm and one egg to meet and create a baby, then why do men produce such a whopping number of sperm? Wouldn’t it be less wasteful for a man to release a single sperm, or at least fewer, to meet one egg?
The reason for this predicament boils down to two words: sperm competition. Since the dawn of the sexes, males have vied with each other to get as many of their own sperm near a fertile egg as possible. Getting more of your sperm closer to an egg means there is a greater probability that it will be you and not your neighbor fertilizing it.
This kind of competition is an evolutionary imperative for males of any species. If a rival’s sperm fertilizes an egg, then an opportunity to pass on your genes is lost. Through many generations, as the reproductive spoils continually go to the highest sperm producers, their genes are passed on. The genes of the smaller sperm producers are eventually weeded out of the population and become a footnote to evolutionary history.But if it was just a matter of ‘more is better,’ then animals of all species would have evolved ridiculously large testicles in a bid to overwhelm the competition. But it’s not quite that simple—numbers are important, but so is proximity. Fertilizing an egg is not just about how much sperm you can produce. It is also about how close you get your sperm to it.
In the early 1980s, researchers in the United Kingdom and the United States realized that both proximity and number were important factors in the physiology of primates, including humans. In primate societies with rigid social structures and one dominant male who mates with all the females, testes trend towards the small. In gorillas, for example, they are very small relative to body weight. (Don’t tell them that.) In gorilla society, one male defends a harem of females to ensure only his sperm gets anywhere near their eggs. In this case, making a lot of sperm doesn’t really help the male gorilla get the job done.
For chimpanzees, on the other hand, sperm competition is a serious issue. In chimpanzee society, many males and females live together in large troops, and females have sex with many males in a short span of time.
This is why male chimpanzees possess the largest testes of all the great apes, weighing in roughly 15 times larger than gorillas, relative to their body weight. This gives them a better shot at swamping out the competition.
Human males fall somewhere in between gorillas and chimps. The average man’s testes are roughly two and a half times as big as a gorilla’s but six times smaller than a chimp’s, relative to body weight. This has led some researchers to question whether sperm competition was ever at work in human societies, or whether our relatively large testes are just a hold over from an earlier period in our evolutionary history.
Monday, 27 June 2011
Myths About Homosexuality - debunked!
Disclaimer: The following post does not represent any of my views on the issue of homosexuality and its morality. Its purpose is to provide you with knowledge of some of the scientific findings of homosexuality, in this case, with regards to zoology and genetics.
The following content was retrieved 29 June 2011, from http://www.livescience.com/13409-myths-gay-people-debunked-sexual-orientation.html
Adapted from 5 Myths about Gay People Debunked
Despite a popular perception that male-female pairings are the only "natural" way, the animal kingdom is actually full of examples of same-sex couples. Penguins, dolphins, bison, swans, giraffes and chimpanzees are just a few of the many species that sometimes pair up with same-sex partners.
Researchers are still mulling over the evolutionary reason, if any, for gay animal sex, since it doesn't produce offspring. Some ideas are that it helps strengthen social bonds or encourages some individuals to focus their resources on nurturing their nieces and nephews, thus boosting their own genes indirectly.
Or, it may simply be fun. "Not every sexual act has a reproductive function," said Janet Mann, a biologist at Georgetown University.
Being gay is a choice
While some claim that being gay is a choice, or that homosexuality can be cured, evidence is mounting that same-sex attraction is at least partly genetic and biologically based.
To test whether genes play a role, researchers have compared identical twins (in which all genes are shared) to fraternal twins (in which about 50 percent of genes are shared). A 2001 review of such twin studies reported that almost all found identical twins were significantly more likely to share a sexual orientation – that is, to be either both gay, or both straight – than fraternal twins, who are less genetically close. Such findings indicate that genes do factor into a person's orientation.
Other studies have found that biological effects, such as hormone exposure in the womb, can also play a role
in shaping sexual orientation. And findings of physiological differences, such as different inner ear shapes between homosexual and heterosexual women, contribute to this idea.
"The results support the theory that differences in the central nervous system exist between homosexual and heterosexual individuals and that the differences are possibly related to early factors in brain development," said Sandra Witelson of McMaster University in Ontario, an author on the 1998 inner ear finding published in the journal Proceedings of the National Academy of Sciences.
The following content was retrieved 29 June 2011, from http://www.livescience.com/13409-myths-gay-people-debunked-sexual-orientation.html
Adapted from 5 Myths about Gay People Debunked
by Clara Moskowitz, LiveScience Senior Writer. 25 March 2011
Animals are all straight.Despite a popular perception that male-female pairings are the only "natural" way, the animal kingdom is actually full of examples of same-sex couples. Penguins, dolphins, bison, swans, giraffes and chimpanzees are just a few of the many species that sometimes pair up with same-sex partners.
Researchers are still mulling over the evolutionary reason, if any, for gay animal sex, since it doesn't produce offspring. Some ideas are that it helps strengthen social bonds or encourages some individuals to focus their resources on nurturing their nieces and nephews, thus boosting their own genes indirectly.
Or, it may simply be fun. "Not every sexual act has a reproductive function," said Janet Mann, a biologist at Georgetown University.
Being gay is a choice
While some claim that being gay is a choice, or that homosexuality can be cured, evidence is mounting that same-sex attraction is at least partly genetic and biologically based.
To test whether genes play a role, researchers have compared identical twins (in which all genes are shared) to fraternal twins (in which about 50 percent of genes are shared). A 2001 review of such twin studies reported that almost all found identical twins were significantly more likely to share a sexual orientation – that is, to be either both gay, or both straight – than fraternal twins, who are less genetically close. Such findings indicate that genes do factor into a person's orientation.
Other studies have found that biological effects, such as hormone exposure in the womb, can also play a role
in shaping sexual orientation. And findings of physiological differences, such as different inner ear shapes between homosexual and heterosexual women, contribute to this idea.
"The results support the theory that differences in the central nervous system exist between homosexual and heterosexual individuals and that the differences are possibly related to early factors in brain development," said Sandra Witelson of McMaster University in Ontario, an author on the 1998 inner ear finding published in the journal Proceedings of the National Academy of Sciences.
69 children?!
Read the following article, paying close attention to the words in italics. Then send in your comments about the article and/or the stories it is reporting about.
The following article was retrieved 30 June 2011, from http://www.allvoices.com/contributed-news/3978800-who-gave-birth-naturally-to-the-most-children-in-the-world-she-had-69-children-and-gave-birth-27-times-all-multiples
The following article was retrieved 30 June 2011, from http://www.allvoices.com/contributed-news/3978800-who-gave-birth-naturally-to-the-most-children-in-the-world-she-had-69-children-and-gave-birth-27-times-all-multiples
Who gave birth naturally to the most children in the world? She had 69 children and gave birth 27 times, all multiples
By AnneHart. Aug 22, 2009
Sacramento, California, USA
The woman who gave birth to 69 children in 17th century Russia had given birth 27 times in her marriage. Each time she gave birth at home without the help of anyone but neighbors when the midwife was not available, she produced twins, triplets, or quadruplets, all without modern medical help, without C-sections, and without anesthesia. She birthed a total of 69 children in her long life span.
THE greatest recorded number of children born to one mother in the world, according to the Guinness 2004 world records is 69. In 27 pregnancies, the first wife of Feodor Vassilyev of Russia gave birth to 16 pairs of twins, 7 sets of triplets and four sets of quadruplets. She also holds the world record for giving birth to the most sets of twins and sets of quadruplets.
Those were the days of no in-vitro fertilization. So you can imagine how rare it is to have multiple births every few years. In 17th century, you gave birth at home in bed or on the floor under the bed with a rope tied around your waist that would be pulled down to move the baby out using gravity.
Having twins, triplets, or quads every two or three years for 27 years meant you probably married at fifteen and gave birth until you were no longer fertile or had eventually reached menopause. It's rare, that's why the Russian woman probably is listed in a publication three hundred years later.
The source of this information is, "Twins in Russia," Eugenical News, Vol. IV, (1919), p. 52. The notation is cited on page 350 in chapter 17, and on page 493, in Notes, of the book, War Against the Weak, a national bestseller by Edwin Black, Avalon, 2003.
The Octo-mom has given birth to the largest number of surviving multiples--eight infants--born at the same time. There have been two other pregnancies recorded at this time in other countries of a woman carrying 11 fetuses and one carrying 12 fetuses, but in one case only two children survived when the woman chose to have the fetus number reduced early in the pregnancy so that two could reach normal size and be born healthy.
There's no news yet on what happened to the woman carrying the 12 fetuses. But in those cases, doctors usually ask the mother whether they can reduce the number of fetuses, usually to two or three so give a smaller number of babies the chance to be born healthy.
The official record for the most babies in a pregnancy (although not the most babies in one birth) was set in 1971, when an Italian woman on fertility treatment conceived 15 babies. None of the 10 girls or 5 boys survived to birth.
Before the Octo-mom's eight babies were born on Jan 26, 2009, the first known surviving set of healthy septuplets was born in the U.S. to the McCaugheys in November of 1997.
The smallest woman to give birth contacted the Guiness Book of Records. Watch an online news interview video of "The Smallest Woman to Give Birth," at: http://www.ebaumsworld.com/video/watch/231061/ . The baby is normal sized and healthy.
THE greatest recorded number of children born to one mother in the world, according to the Guinness 2004 world records is 69. In 27 pregnancies, the first wife of Feodor Vassilyev of Russia gave birth to 16 pairs of twins, 7 sets of triplets and four sets of quadruplets. She also holds the world record for giving birth to the most sets of twins and sets of quadruplets.
Those were the days of no in-vitro fertilization. So you can imagine how rare it is to have multiple births every few years. In 17th century, you gave birth at home in bed or on the floor under the bed with a rope tied around your waist that would be pulled down to move the baby out using gravity.
Having twins, triplets, or quads every two or three years for 27 years meant you probably married at fifteen and gave birth until you were no longer fertile or had eventually reached menopause. It's rare, that's why the Russian woman probably is listed in a publication three hundred years later.
The source of this information is, "Twins in Russia," Eugenical News, Vol. IV, (1919), p. 52. The notation is cited on page 350 in chapter 17, and on page 493, in Notes, of the book, War Against the Weak, a national bestseller by Edwin Black, Avalon, 2003.
The Octo-mom has given birth to the largest number of surviving multiples--eight infants--born at the same time. There have been two other pregnancies recorded at this time in other countries of a woman carrying 11 fetuses and one carrying 12 fetuses, but in one case only two children survived when the woman chose to have the fetus number reduced early in the pregnancy so that two could reach normal size and be born healthy.
There's no news yet on what happened to the woman carrying the 12 fetuses. But in those cases, doctors usually ask the mother whether they can reduce the number of fetuses, usually to two or three so give a smaller number of babies the chance to be born healthy.
The official record for the most babies in a pregnancy (although not the most babies in one birth) was set in 1971, when an Italian woman on fertility treatment conceived 15 babies. None of the 10 girls or 5 boys survived to birth.
Before the Octo-mom's eight babies were born on Jan 26, 2009, the first known surviving set of healthy septuplets was born in the U.S. to the McCaugheys in November of 1997.
The smallest woman to give birth contacted the Guiness Book of Records. Watch an online news interview video of "The Smallest Woman to Give Birth," at: http://www.ebaumsworld.com/video/watch/2
1.1 Why do Humans undergo Sexual Reproduction?
Adapted from Otto, S. (2008) Sexual reproduction and the evolution of sex. Nature Education 1(1), retrieved July 25, 2010 from http://www.nature.com/scitable/topicpage/sexual-reproduction-and-the-evolution-of-sex-824.
Sexual Reproduction and the Evolution of Sex
Birds do it, and bees do it. Indeed, researchers estimate that over 99.99% of eukaryotes do it, meaning that these organisms reproduce sexually, at least on occasion. But why is sexual reproduction so commonplace?
People typically employ several arguments in their efforts to explain the prevalence of sexual reproduction. One such argument is that organisms engage in sex because 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 1.3 billion years before development of the first animals with neurons capable of assessing pleasure. These bacteria (as well as their modern counterparts) engaged in genetic exchange via processes such as conjugation, transformation, and transduction, all 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 1800s. 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 those that hinder the evolution of sex.
The Importance of Sexual Reproduction
To develop a better understanding of why sexual reproduction is so commonplace, it is helpful to start with an examination of some of the most common erroneous beliefs regarding the relationship between sex and natural selection, including those described in the following sections.
Sex Does Not Always Generate More Variable Offspring
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Many people assume that sexual reproduction is critical to evolution because it always results in the production of 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. Now, for the 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 to 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.
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.) The problem also arises in more complicated cases involving multiple genes whenever those genes interact in such a way that intermediate genotypes have lower fitness than the average of the extreme genotypes
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."
Producing Variable Offspring Can Hinder the Evolution of Sex
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.
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).
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.g., budding, asexual seeds, etc.). 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 1980s and 1990s demonstrate that genes promoting sex and recombination increase in frequency only when all of the following conditions hold true:
Indeed, theoretical models developed in the 1980s and 1990s demonstrate that genes promoting sex and recombination increase in frequency only when all of the following conditions hold true:
- The population is under directional selection. (This means that increased variation can improve the response to selection.)
- Fitness surfaces are negatively curved. (This means that sex and recombination can restore variation eliminated by past selection.)
- The surface curvature is not too strong. (If too strong, the recombination load is severe).
Unfortunately, empirical data have not indicated that fitness surfaces curve in just the right way for these models to work in real-life situations.
Sex Can Be Too Costly to Evolve
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.
Moreover, an individual that reproduces sexually passes only half of its genes to its offspring, whereas it would have transmitted 100% of its genes to progeny that were produced asexually. (This last cost is often called the "twofold cost of sex.") Thus, unless the individual's sexual partner contributes enough resources to double the number of offspring, an organism that reproduces sexually passes on fewer copies of its genes than an organism that reproduces asexually.
Moreover, an individual that reproduces sexually passes only half of its genes to its offspring, whereas it would have transmitted 100% of its genes to progeny that were produced asexually. (This last cost is often called the "twofold cost of sex.") Thus, unless the individual's sexual partner contributes enough resources to double the number of offspring, an organism that reproduces sexually passes on fewer copies of its genes than an organism that reproduces asexually.
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.
Why, Then, Is Sexual Reproduction So Common?
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 300 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.
Sex Evolves When Selection Changes Over Time
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.") Increased allocation to sexual reproduction can evolve because of "Red Queen" interactions, but only if selection is strong enough to cause rapid switches in which gene combinations are favorable.
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.") Increased allocation to sexual reproduction can evolve because of "Red Queen" interactions, but only if selection is strong enough to cause rapid switches in which gene combinations are favorable.
Sex Evolves When Selection Changes Over Space
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.
Sex Evolves When Organisms Are Less Adapted to Their Environment
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.
Sex Evolves When Populations Are Finite
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. 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.
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.
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References and Recommended Reading
De Visser, J. A. G. M., & Elena, S. F. The evolution of sex: Empirical insights into the roles of epistasis and drift. Nature Reviews Genetics 8, 139–149 (2007) doi:10.1038/nrg1985 (link to article)
Felsenstein, J. The evolutionary advantage of recombination. Genetics 78, 737–756 (1974)
Otto, S. P., & Lenormand, T. Resolving the paradox of sex and recombination. Nature Reviews Genetics 3, 252–261 (2002) (link to article)
Genome Evolution (1)
- Origins of New Genes and Pseudogenes
The formation of new genes is a primary driving force of evolution in all organisms. How exactly do these new genes crop up in an organism’s genome and what must occur in order for them to be passed on?
Macroevolution (1)
- The Molecular Clock and Estimating Species Divergence
What is the molecular clock? Levels of molecular variation could be used, in principle, to estimate divergence times, serving as evolutionary “clocks” that “tick” at different rates.
Microevolution (7)
- Sexual Reproduction and the Evolution of Sex
Birds do it, and bees do it. Indeed, researchers estimate that over 99.9% of eukaryotes reproduce sexually. What, then, are the true costs and benefits of sex? - Neutral Theory: The Null Hypothesis of Molecular Evolution
In the decades since its introduction, the neutral theory of evolution has become central to the study of evolution at the molecular level, in part because it provides a way to make strong predictions that can be tested against actual data. The neutral theory holds that most variation at the molecular level does not affect fitness and, therefore, the evolutionary fate of genetic variation is best explained by stochastic processes. This theory also presents a framework for ongoing exploration of two areas of research: biased gene conversion, and the impact of effective population size on the effective neutrality of genetic variants. - Negative Selection
How are humans contributing to negative selection? It’s a part of evolution that can also drive some species to extinction; models of negative selection help us understand biodiversity. - Evolutionary Adaptation in the Human Lineage
Are you lactose intolerant? Many people are. In fact, the ability to digest lactose may be an example of adaptive evolution in the human lineage. - Natural Selection: Uncovering Mechanisms of Evolutionary Adaptation to Infectious Disease
The evolutionary link between sickle-cell trait and malaria resistance showed that humans can and do adapt. But are the “bugs” that make us sick evolving as well? - Negative Selection
How are humans contributing to negative selection? It’s a part of evolution that can also drive some species to extinction; models of negative selection help us understand biodiversity. - Genetic Mutation
Is it possible to have “too many” mutations? What about “too few”? While mutations are necessary for evolution, they can damage existing adaptations as well.
Phylogeny (2)
- Trait Evolution on a Phylogenetic Tree: Relatedness, Similarity, and the Myth of Evolutionary Advancement
What do phylogenetic trees illustrate? These diagrams don’t just organize knowledge of biodiversity--they also show us that living species are the summation of their evolutionary history. - Reading a Phylogenetic Tree: The Meaning of Monophyletic Groups
Phylogenies are a fundamental tool for organizing our knowledge of the biological diversity we observe on our planet. But how exactly do we understand and use these devices?
Speciation (4)
- Hybrid Incompatibility and Speciation
Often, hybrids between closely related species are often inviable or sterile. How does this sterility and inviability happen? Genetics helps provide insight into answering this question. - Haldane's Rule: the Heterogametic Sex
Why are there deformities in male hybrid flour beetles while female hybrids are spared? Haldane’s rule: the male beetles have the heteromorphic sex chromosomes. - Hybridization and Gene Flow
What are ways species exchange genes with each other? Hybridization and gene flow are shortcuts to biodiversity that don’t always involve differentiation. - Why Should We Care about Species?
The questions "What are species?” and “How do we identify species?” are difficult to answer, and have led to debate and disagreement among biologists. See how consensus on answers to these questions can steer global, political, and financial pressures that affect conservation efforts.
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