A few weeks ago, the science world briefly noted an Imperial College London study concerning rotifers. The remarkable news about the rotifers in question was that though they had not had sex for 40 million years — which fact prompted a few predictable jokes about marriage — the animals had nonetheless managed to diversify into a number of distinct species in that time.
Bdelloid rotifers don’t reproduce sexually, as mentioned above, but rather through parthenogenetic oögenesis, a phrase I like enough that I’ll use it instead of the equivalent term “parthenogenesis,” because really, how often do I get a chance to? A bdelloid rotifer produces eggs that are, in essence, clones of herself, almost more like Agave offsets than eggs as we usually imagine them. Those eggs develop into new rotifers which lay new eggs. It’s the anti-feminist’s nightmare made flesh: every rotifer a mother and no males needed, not even to move little rotifer pianos or open little rotifer jars.
The finding that bdelloids have managed to give rise to different species despite their reproductive strategy is interesting, but not especially surprising. Making endless copies of the same genome rather than swapping genes with every generation is an evolutionary handicap, but we’re talking at least 40 million years here. Some scientists put the bdelloid loss of sex back around 100 million years ago. Though bdelloids are remarkable for surviving periods of dessication almost a decade long, a month is a more typical bdelloid lifespan, so making the supremely conservative assumption that each rotifer reproduces only once, that’s at least 480 million generations to play with, plenty of time to accumulate diversity even just through copying error.
The reason the study was at all notable outside the Rotiferologist Community was that it offered an Exception that Proves the Rule, the rule being that evolution is an epiphenomenon of sex.
Evolution is an epiphenomenon of sex.
Reductionists define evolution as change in a population’s allele frequency over time. This is a supremely unsatisfying definition, about like describing mammalian sex as stimulation of epidermal nerve endings through friction. You can’t really argue with the definition, but you’ve lost a lot of nuance in translation. Still, that’s the arena in which sexual reproduction turns the evolutionary speed dial up to eleven. A bdelloid rotifer is surpassingly unlikely to possess a genome different from that of her great great grandmother: a sexually reproducing organism is unlikely to have a genome identical to its mother’s (though such things do happen). Asexual organisms play genome solitaire with the same deck of cards in the same order, while sexually reproducing organisms shuffle the deck and play, I dunno. Pinochle. Given forty million years, sexually reproducing creatures can evolve into about any niche available. A mass extinction in the Cambrian wiped out the planet’s reef-building multicelled animals, and forty million years later in the Ordovician metazoans were building reefs again, and paleontologists consider that intervening stretch of time puzzlingly long.
But sex does more than increase the shuffling of genes as the generations progress.
The simple definition of a species we learned in high school has been eroded significantly. You can start bar fights among biologists by ingenuously asking what a “species” is, especially if the crowd includes a bacteriologist, a botanist, and an expert on salmonids. The word has an utterly different meaning when applied to kangaroo rats than it does when used in the context of blue-green algae. But for the world on our scale, the world of living things which we can apprehend directly with no more magnification than a hand lens or a good pair of binoculars, the world of bats and bugs and trees and moss, you can mainly go with the canon. The word “species”as used in one’s daily life can still be defined in the old familiar language of the Biological Species Concept as related by Ernst Mayr:
“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”
Dogs and wolves and coyotes bear fertile offspring, and few biologists advocate putting coyotes in the same species as wolves and dogs. The male calf of a female cow and male bison is often fertile; his sister usually won’t be. Don’t get me started on the orchids, whch can produce fertile progeny from parents of two different genera. Often enough, closely related species whose populations are separated geographically will produce abundant fertile offspring when brought into proximity. There are abundant fuzzy edges to the Mayr version of the species concept, and it’s lost a lot of utility for those doing serious biology.
But those fuzzy edges are edges, exceptions to the rule. Maybe the Biological Species Concept definition falls apart when you’re at Tehachapi Pass discussing the Ladderbacked and Nuttall’s woodpeckers, whose ranges overlap there. But if you’re discussing the differences between the Nuttall’s woodpecker in my backyard and the raven chasing it, the cat watching them both from the garden wall, the rabbit hiding from the cat and the oak tree the woodpecker retreats to, when you’re discussing the vast majority of the biological diversity that makes up our everyday world, the rough generalization holds. The difference between ravens and oak trees and the yeast in your beer boils down to this: ravens have sex with ravens, and not with oaks or brewer’s yeast.
Sexual reproduction facilitates evolution into greater genetic diversity, and then the outward expression of that diversity is preserved when sexual exchange of genes is halted between two divergent populations. If sexual reproduction had never been invented, life on Earth would likely consist of a layer of mucilage in various shades of green, red and brown, an entire planet coughed up by a cosmic cigarette smoker. The very existence of bees and birds is due to the subject matter euphemized by 1950s parents as “The Birds And The Bees.”
You can even give sex credit for geology, a lot of the time. The rock in the hill I’m sitting on is the fossilized shells of diatoms, which evolved through sexual reproduction. Much of the limestone in the world can likewise be credited to sex, though the coccolithophores that made the Chalk Cliffs in Dover haven’t yet been caught in flagrante delicto. Coal? Came from big sexy ferns and seed plants. And as icing on the cake, consider the flashiness many species evolved for sexual purposes: brilliant feathers, blaze-orange and red petals, majestic antlers, birdsong.
In sum: Sex created the world as we know it.
This, I think, offers an answer to those wondering what has motivated biblical literalists to choose opposition to teaching evolution as their last-ditch stand against rationality and empiricism. They had a bit of trouble with Galileo, but at one point or another all but a few crackpots accepted that the earth revolved around the sun. There are other parts of the Bible whose literal truth they have happily abandoned, severe proscriptions against prosciutto-wrapped chicken breast in cream sauce or coveting one’s neighbors’ Rolex, prohibitions against admitting people with skin problems to the priesthood and the like.
But if they have been willing to grant that Genesis 1:14-18 might not be the literal truth, with its description of the sun and moon as lights stuck on a firmament with much-smaller stars as an afterthought, why the steadfast adherence to Genesis 1:11-12 and 1:20-27?
Simply this: heliocentrism can be swallowed, as can prosciutto, without denying primacy to the literalists’ God.
But Verses 11-27 of Genesis Chapter One explicitly credit Yahweh with conjuring up every last bit of the biodiversity on the planet. The fundamentalists might be willing to accept challenges to the literal truth of those verses, but look at what reality offers them as the alternative! All that diversity that was credited to God, the great sea monsters and every winged bird, the cattle according to their kinds, and everything that creeps upon the ground? Reality would have them accept that those were created by sex instead.
Evolution replaces God with sex, in other words, and that’s too much for them to bear.
And the compromise path offered by scientists of Christian faiths, that God created Deep Time and set evolution in motion? That’s saying God created the world with sex, and nothing bothers the fundies more than saying God used sex to create something.