Sex is a pretty cool phenomenon, and it's not just because it's kinda fun.
How exactly sexual reproduction first evolved isn't well understood, but its advantages are clear. Asexually-reproducing species, like most bacteria, a good many protists, and a handful of plants and animals, result in genetic copies -- clones, really -- of the parent organism.
The problem with this is twofold. First, clones (being identical) are susceptible to the same pathogens, so a communicable disease that is deadly to one of them will wipe them all out. In a genetically-diverse population, chances are there'd be some that were resistant or entirely immune; in a monoculture, one epidemic and it's game over. (That's basically what caused the Irish Potato Famine; a one-two punch of cold, rainy weather and an outbreak of late blight killed nearly all of the island's potato crop, resulting in massive starvation.)
The second problem, though, is subtler, and causes problems even if there's no external environmental risk involved. It's called Muller's Ratchet, named after American geneticist Hermann Muller, who first described the phenomenon. Asexual species still undergo variation because of random mutations; at each generation, the DNA picks up what amount to typos. The whole thing is like a genetic game of Telephone. Each time the genes pass on, there are minor replication errors that accrue and ultimately will turn the whole genome into unintelligible garbage.
Various asexual species have evolved mechanisms for coping with Muller's Ratchet. Some bacteria have multiple copies of critical genes, so if one copy gets knocked out by a mutation, they have other copies that still work. Some evolved conjugation, which is a primitive form of sexual reproduction in which cells pair up and exchange bits of DNA, with the goal being the sharing of undamaged copies of important genes (as well as copies of any novel beneficial mutations that may have occurred).
So asexual reproduction is fast, efficient, and doesn't require finding a partner, but ultimately makes the species susceptible to the double whammy of disease proneness and Muller's Ratchet; sexual reproduction requires finding a partner, but increases overall fitness by improving genetic diversity.
Is there any way to gain both advantages without picking up the disadvantages at the same time?
This is one of the main drivers of evolution in flowering plants. Some flowering plants can reproduce both sexually (through flowers) and asexually (through rhizomes, bulbs, and so on). Grasses, for example, are pretty good at both. A very few -- the commercial variety of bananas is one of the only ones that comes to mind -- only reproduce asexually. (Which is why bananas have no seeds, and also why growers are in a panic over the spread of fusarium wilt.) A lot of plant species only reproduce sexually, and this brings up the problem of finding a partner of the opposite sex -- which is difficult when you are stuck in place.
This is where pollinators come in. Some flowering plants are wind-pollinated, and rely on the air to carry the pollen (containing the male gametes) to the ovules (containing the female gametes). Others use nectar or color lures to bring in insects, birds, and even a few mammals to act as couriers. But this risks having the pollinator simply double back and fertilize a flower on the same plant, meaning that the offspring is (more or less) identical to the parent -- obviating the advantage of sexual reproduction.
So a great many species have evolved mechanisms for facilitating cross-pollination and avoiding self-pollination. Some of the brightly-colored flowers of plants in the genus Salvia have evolved a mechanism where there's a spring-loaded trigger -- a visiting bee trips the trigger and gets smacked by the pollen-bearing stamen, with the intention of startling it enough that it decides to move along and visit a different individual of the same species. Many orchids have wildly byzantine mechanisms for maximizing the likelihood of cross-pollination. Other species, such as some of the fruiting trees of the rose family (including cherries, apricots, and peaches) have bisexual flowers, but the stamens of one tree mature at a different time than the ovules do -- making self-pollination impossible. Apples have a genetic barrier to self-pollination -- if pollen from an apple flower is brought to another flower on the same tree, it recognizes the ovule as genetically identical and simply doesn't fuse.
The reason this comes up is a study that appeared last week in the journal Science, looking at the genetics of gender and pollination in walnuts. Walnuts, and most of the other members of the family Juglandaceae (which also includes hickories and pecans), are pollinated by the wind.
Wind-pollinated plants are most at risk for accidental self-pollination; the wind, after all, isn't going to be attracted or deterred by any kind of mechanical contrivance, and wind-pollinated plants often produce tons of pollen (to maximize the likelihood of at least some of it hitting the target, since inevitably a lot of it is simply blown away and wasted). This is, incidentally, why most allergy-inducing pollen comes from wind-pollinated plants like grasses, willows, birch, oak, cedar, and (especially) ragweed.
Walnuts, it turns out, solve this problem by switching sex every few weeks -- a particular tree only produces male flowers during one interval, then only female ones the next. The following year, they do it again -- but changing the order of who is male when. This renders self-pollination not just unlikely, but impossible. And the paper, which came out of research at the University of California - Davis, describes the genetic mechanism for how this is controlled.
Oh, but you bigots, do go on and explain to me how in the natural world sex and gender are simple and binary, they're both fixed at conception, male-and-female-He-made-them, and so on and so forth.
Even after years of studying biology, and evolutionary biology in particular, I'm still astonished by the diversity of life, and how many solutions species have evolved to solve the problems of survival, nutrition, and reproduction. It seems fitting to end this with the final paragraph of Charles Darwin's Origin of Species, which echoes that same sense of wonder:
It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.
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