The question of why people have sex plagues nearly everyone, evolutionary biologists in particular.
Important questions like why stomachs or eyes have evolved were answered long ago, but a species’ choice of sexual reproduction over asexual reproduction continues to irritate evolutionary biologists.
UCSB ecology, evolution and marine biology professor Bill Rice and post-doctoral fellow Adam Chippindale published a paper in the Oct. 19 edition of Science magazine showing the advantages of sex, based on a study involving 34 experiments using the fruit fly Drosophila melanogaster.
A biologist’s definition of sex is different from the rest of the population’s definition. To them, sex means the recombination of two individuals’ genetic material, or genomes.
“Students were probably told in high school that the advantage of sex is that it makes diversity, so you don’t look exactly like your parents and that it’s good for the population to have diversity,” Rice said. “That turns out to be too simplistic an explanation.”
The problem with sexual reproduction is mathematical. If there are four asexual organisms – asexual creatures are all female — and they produce two babies per female, eight offspring result. If there are four sexual organisms — two male and two female with each female producing two kids – only four offspring result. Asexual populations can therefore grow twice as fast as sexual ones.
“The males are basically parasites on the females,” Rice said. “In an asexual population, there aren’t any parasitic males. It’s all females so they expand in population with a two-fold advantage each generation.”
Sexual reproduction is also disadvantaged based on what Rice calls a dosage cost.
“My son only has half of my genes; the other half are from his mother. Only half of my genome is getting into the population,” he said. “However, if I were an asexual female, my offspring would carry all of my genome. With asexual reproduction, you get two times as many offspring and two times as many genes into the population.”
To balance out these heavy costs, sexual recombination needs a larger advantage.
“Most organisms evolve with their enemies, their predators, their competitors, their parasites and their pathogens, and if you stop evolving, they’ll catch up to you, and you’ll be in trouble,” Rice said. “What sex does is it makes species more competitive. The faster you can evolve, the better off you are in this evolutionary arms-race.”
Rice and Chippindale found that the rates of adaptation for sexual and asexual populations were different. They would start out similarly but over time those that reproduced sexually would adapt to a change in environment and asexual populations could not compete.
“If you have an asexual rotifer living in a pond, it may be the best clone of the season, perfectly adapted to its surroundings,” Chippindale said. “But take it out and put it in another pond, it may completely suck.”
In their experiments, the researchers introduced beneficial mutations into random individuals in the population, and, overall, the better the individual is at reproducing species with an adaptive mutation the more likely it is to spread to the rest of the population.
The only way an asexual individual can get a beneficial mutation is by inheriting it or being the origin of the mutation, while sexual individuals can combine their genomes and propagate this newly introduced beneficial mutation in a new genetic background. This advantage led to sexual organisms’ current prominence in the natural world, while on the scale of evolutionary time, Rice said, “asexual lineages are formed and they blink in and blink out.”
Anthropology professor John Tooby, who has worked on the arms-race theory of sexual reproduction, said the study is unique.
“This is one of very few works of experimental tests of competing theories on the evolution of sex … and just technically, it’s a stunning piece of experimental genetics in terms of technical difficulty and care with which it was achieved,” he said. “It’s very impressive and researchers will pay a lot of attention to this result.”
The study is an important piece of research because it is the first experiment of a grand enough scale to produce conclusive results. Chippindale said the reason this kind of experiment has not been done before is because it involves engineering several populations of sexual and asexual organisms at the same time.
“The large number of experiments allowed us to look at the average effect of sex in this case, just as a large number of car accidents would let you look at the average effect of a seatbelt,” Rice said. “Under these conditions if we didn’t find an effect, we could throw the theory away. It just wasn’t working.”
Rice and Chippindale also used complex cytological tricks, and took advantage of the idiosyncrasies of the fruit fly’s system.
“These are some of the most fun experiments I’ve ever done in my life because from generation to generation, every other week of fly-handling-hell had its real fun side because you could see who the winners and losers were in evolution,” Chippindale said. “People talk about ‘playing God’ in biology. I like to ‘play Darwin.'”
“Drosophila work is a great introduction to evolutionary biology,” Chippindale added. “Though I don’t want to encourage students to have random sexual encounters, or to equate biological ideas of sex with human social ideas of sex. We’re not living in the ’80s anymore.”