emerge? Why, for example, isnât there just one gender? If there were, we could mate with everyone.
While hermaphrodites â those with both male and female reproductive organs â do exist in nature, they remain exotic.Evolution has clearly favoured two sexes. Schilthuizen argues that âmalesâ and âfemalesâ evolved to prevent a conflict between little units inside cells called organelles. These units make proteins and control cell activity. A cell needs only one type of each organelle. During intercourse, the sex cell of one animal fuses with another to make offspring, so if both cells had the organelles, they would be in competition â effectively warring with each other. Consequently, it was more advantageous to have a large sex cell packed with organelles â known as the femaleâs egg â and a small sex cell that needed only to carry DNA. Since eggs take more energy to produce, itâs a better bet for females to invest in quality males to fertilise the âexpensiveâ eggs, says Eric Haag at the University of Maryland. The âcheapâ sperm favours a slutty approach.
But itâs not just about sperm and eggs. âItâs about who invests more time and energy in the offspring,â says Schilthuizen. âIf the male invests more, youâll have a reversal of roles.â In April, Kazunori Yoshizawa of Hokkaido University in Sapporo uncovered the ultimate role reversal: a group of insects called Neotrogla , where females have penises and males have vaginas, at least of a sort. These insects are found only in extremely dry caves in Brazil, where food is scarce. Here, the males invest heavily to make nutrient-packed ânuptial giftsâ to give to females during sex. According to Yoshizawa, this has created strong sexual competition among females for the food, so they display typically âmaleâ traits, including penises â quite dominating ones, too, with spines that anchor the males into the sexual positions to force the males into sex for days (the longest act detected by the scientists was 71 hours).
In the few species where the investment in offspring is equal, this classic conflict of the sexes evaporates, says Schilthuizen. In humans, for example, making eggs has become almost a negligible burden for females, at least compared with the maininvestment of caring for offspring for some two decades after theyâre born, and thatâs a burden the sexes are expected to share.
But the more common sexual conflict exemplifies one of the greatest misconceptions about evolution: itâs not about species. Evolution is driven by individuals, whose only concern is to carry their genes to the next generation. That brings us to âone of the most fundamental puzzles in evolutionary biologyâ, as Darrell Kemp of Macquarie University in Sydney puts it: why do animals have sex at all? Natural selection should favour those who pass most of their genes into future generations, but sex with another automatically wipes out half their stock. Itâs also risky â in some cases, one can get eaten while in the act â and courting uses time and energy. But while there is a tiny minority of species that have done away with intercourse â such as self-cloning Timema stick insects in North America â the advantage of sex must be âprofoundâ, says Kemp.
Studies into critters suggest that mixing genes with a mate increases the chance their offspring will survive over time. Initially, an individual loses half their genes, but mingling their DNA with another may allow the kids to cope with future environments and new diseases that they themselves couldnât handle. Witness the asexual populations of the New Zealand snail Potamopyrgus antipodarum , which have more parasitic worm infections than sexual populations of the same species.
While there is much to learn from the intimate antics of snails, spiders and
While hermaphrodites â those with both male and female reproductive organs â do exist in nature, they remain exotic.Evolution has clearly favoured two sexes. Schilthuizen argues that âmalesâ and âfemalesâ evolved to prevent a conflict between little units inside cells called organelles. These units make proteins and control cell activity. A cell needs only one type of each organelle. During intercourse, the sex cell of one animal fuses with another to make offspring, so if both cells had the organelles, they would be in competition â effectively warring with each other. Consequently, it was more advantageous to have a large sex cell packed with organelles â known as the femaleâs egg â and a small sex cell that needed only to carry DNA. Since eggs take more energy to produce, itâs a better bet for females to invest in quality males to fertilise the âexpensiveâ eggs, says Eric Haag at the University of Maryland. The âcheapâ sperm favours a slutty approach.
But itâs not just about sperm and eggs. âItâs about who invests more time and energy in the offspring,â says Schilthuizen. âIf the male invests more, youâll have a reversal of roles.â In April, Kazunori Yoshizawa of Hokkaido University in Sapporo uncovered the ultimate role reversal: a group of insects called Neotrogla , where females have penises and males have vaginas, at least of a sort. These insects are found only in extremely dry caves in Brazil, where food is scarce. Here, the males invest heavily to make nutrient-packed ânuptial giftsâ to give to females during sex. According to Yoshizawa, this has created strong sexual competition among females for the food, so they display typically âmaleâ traits, including penises â quite dominating ones, too, with spines that anchor the males into the sexual positions to force the males into sex for days (the longest act detected by the scientists was 71 hours).
In the few species where the investment in offspring is equal, this classic conflict of the sexes evaporates, says Schilthuizen. In humans, for example, making eggs has become almost a negligible burden for females, at least compared with the maininvestment of caring for offspring for some two decades after theyâre born, and thatâs a burden the sexes are expected to share.
But the more common sexual conflict exemplifies one of the greatest misconceptions about evolution: itâs not about species. Evolution is driven by individuals, whose only concern is to carry their genes to the next generation. That brings us to âone of the most fundamental puzzles in evolutionary biologyâ, as Darrell Kemp of Macquarie University in Sydney puts it: why do animals have sex at all? Natural selection should favour those who pass most of their genes into future generations, but sex with another automatically wipes out half their stock. Itâs also risky â in some cases, one can get eaten while in the act â and courting uses time and energy. But while there is a tiny minority of species that have done away with intercourse â such as self-cloning Timema stick insects in North America â the advantage of sex must be âprofoundâ, says Kemp.
Studies into critters suggest that mixing genes with a mate increases the chance their offspring will survive over time. Initially, an individual loses half their genes, but mingling their DNA with another may allow the kids to cope with future environments and new diseases that they themselves couldnât handle. Witness the asexual populations of the New Zealand snail Potamopyrgus antipodarum , which have more parasitic worm infections than sexual populations of the same species.
While there is much to learn from the intimate antics of snails, spiders and
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