Evolution's Rainbow. Joan Roughgarden

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СКАЧАТЬ are we to decide whether rainbows are good or bad? Who is correct, the diversity affirmers or the diversity repressers? To answer this most fundamental question of evolutionary biology, let’s compare species with full rainbows to species with very limited rainbows. Species who manage to reproduce without sex have limited rainbows. By sex, I mean two parents mixing genes to produce offspring. Lots of species propagate without sex. In such species, everyone is female and offspring are produced without fertilization. In addition, in many species offspring may be produced either with or without fertilization, depending on the season.

      If you go to Hawaii, look at the cute geckoes on the walls. You’re seeing an asexual species—all these geckoes are female.⁶ Females in all-female species produce eggs that have all the needed genetic material to begin with. In sexual species, like humans, an egg has only half the genetic material needed to produce a baby; a sperm has the other half, so combining these yields the required material. In addition, eggs from an all-female species don’t need fertilization by a sperm to trigger the cell divisions that generate an embryo. Females in all-female species clone themselves when they reproduce.

      The Hawaiian all-female geckoes are locally abundant and widespread throughout the South Pacific, from the lovely Society Islands of French Polynesia to the Marianas Islands near New Guinea. More all-female species live in Mexico, New Mexico, and Texas—all varieties of whiptail lizards These small, sleek tan and brown-striped animals dart quickly along the ground looking for food. The all-female species of whiptail lizards live along streambeds, while sexually reproducing relatives typically live up-slope from the streams in adjacent woods or other vegetation. Every major river drainage basin in southwestern North America is a site where an all-female whiptail lizard species has evolved. More than eight all-female species are found in this area. Still more all-female species of lizards are found in the Caucasus Mountains of Armenia and along the Amazon River in Brazil. All-female fish occur too. Indeed, all-female animal species are found among most major groups of vertebrates.⁷

      Also, some species have two kinds of females: those who don’t mate when reproducing and those who do mate. Examples include grasshoppers, locusts, moths, mosquitoes, roaches, fruit flies, and bees among insects, as well as turkeys and chickens.⁸ Fruit flies grow easily in the laboratory and are especially well studied. Over 80 percent of fruit fly species have at least some females that reproduce entirely asexually. Although the majority of females in these species reproduce through mating, selection in the laboratory increased sixtyfold the proportion of females not needing to mate, yielding a vigorous all-female strain.⁹

      Thus all-female species are well known among animals. So why don’t even more all-female species exist? Indeed, why aren’t all species all-female? To answer this question, let’s look at the costs and benefits of reproducing with and without sex.

      Sexual reproduction cuts the population’s growth rate in half—this is the cost of sex. Only females produce offspring, not males. If half the population is male, then the speed of population growth is half that of an all-female population. An all-female species can quickly outproduce a male/female species, allowing an all-female species to survive in high-mortality habitats where a male/female species can’t succeed. (This result is also true in hermaphrodite species, in which the fifty-fifty allocation of reproductive effort to male and female function reduces the female allocation used to make eggs by half.)

      The potential for doubling production in an all-female species hasn’t escaped the attention of agricultural scientists. In the 1960s, turkeys and chickens were bred to make all-female strains.¹⁰ Indeed, the cloning of a sheep in Scotland reflected a fifty-year-old aspiration to increase agricultural production by taking the sex out of reproduction. However, despite the big advantage in population growth rate that all-female species enjoy and the many examples of all-female species that do occur, clonally reproducing species remain a tiny minority. Far and away most species are sexual. Nature has experimented many times and keeps experimenting with clonal species, but with little success. Sex does work. Why?

      The benefit of sex is survival over evolutionary time. Lacking sex, clonal species are evolutionary dead ends. On an evolutionary time scale, almost all clonal species are recently derived from sexual ancestors. On the family tree of species, asexual species are only short twigs, not the long branches.¹¹ The advantages of sex are also demonstrated by species who can use sex or not, depending on the time of year. Aphids (tiny insects that live on garden plants) reproduce clonally at the beginning of the growing season, switching to sexual reproduction at the end of the season. Aphids benefit from fast reproduction when colonizing an empty rose bush, but the anticipated change of conditions at the end of the season makes sexual reproduction more attractive.¹²

      

      Clonally reproducing species are “weeds”—species specialized for quick growth and fast dispersal, like plants that locate and colonize new patches of ground. The common dandelion of North America is a clonal reproducer whose sexual ancestors live in Europe.¹³ Weeds eventually give up their territory to species who are poorer colonizers but more effective over the long term.¹⁴ The geckoes who colonized the South Pacific and the whiptail lizards of New Mexico streambeds make sense in these contexts, where dispersal is at a premium or the habitat is continually disturbed.

      Clonal reproduction is a specialized mode of life, not recommended for any species that fancies itself a permanent resident of this planet. But we haven’t answered why sexual reproduction is good over the long term. Two theories have been offered for why sex benefits a species, one diversity-affirming, the other diversity-repressing. Both theories agree that asexual species are short-lived in evolutionary time relative to sexual species and that sex guarantees the longer species survival. Both theories therefore agree that sex is beneficial to a species. Both theories also agree that the purpose of sex isn’t reproduction as such, because asexual species are perfectly capable of reproducing. But the theories have different perceptions of why sex is good. The diversity-affirming theory views diversity itself as good and sex as maintaining that diversity. The diversity-repressing theory views diversity as bad and sex as keeping the diversity pruned back.¹⁵ Let’s start with the diversity-affirming theory.

      THE DIVERSITY-AFFIRMING THEORY

      According to the diversity-affirming theory for the benefit of sex, sex continually rebalances the genetic portfolio of a species. Think of a savings account and jewelry—a rainbow with two colors. How much can both colors earn together? When demand for jewelry is low, one can’t sell jewelry, even to a pawnshop, and earning 2 percent from a bank account looks great. When jewelry is hot, interest on a bank account looks cheap and selling jewelry turns a good profit. The overall earnings are the total from both investments.

      A species earns offspring instead of money from its investments. The long-term survival of a species depends on being sufficiently diversified to always have some offspring-earning colors. Although biologists may talk about the rainbow as a source of genes for new environments, it is in fact more important for surviving the regular fluctuations between hot and cold, wet and dry, and the arrival and departure of new predators, competitors, and pathogens like the bubonic plague or AIDS.¹⁶

      The social environment within a species is always changing too. Concepts of the “ideal” mate change through time. Among humans, men have sometimes preferred the amply proportioned Mama Casses among us, at other times the skinny Twiggys, as recorded in the portraits of women from art museums. Other aspects of our social environment have also changed over the centuries, like the fraction of time spent with others of the same sex or the opposite sex, or the number of sex partners a person has. Changes in the social setting within a species, as well as changes in the ecological and physical environment, all affect which colors of the rainbow shine the brightest at any one time.

      A clonal species can accumulate diversity through mutation, or it may have multiple origins, thereby starting out with a limited rainbow. In fact, several genetically СКАЧАТЬ