“It has only been in the last twelve million years that life has existed on Earth, that sexual reproduction has flourished — a mere fraction of geological time. Now it is the predominant form of reproduction for species on Earth.”
Dr. Anthony F. Bogaert,Understanding asexuality, chapter 3.
Is it, though? Does he mean certain species, or all Earth’s species in general? ‘Cause I think there a lot more predominantly asexually reproducing species than predominantly sexually reproducing ones. Isn’t the most common form of reproduction on Earth binary fission or something?
That’s actually a very hard question to answer, since we don’t have good estimates for the numbers of species out there. Wikipedia gives this estimate which implies that sexually reproducing species are probably more common.
Ah. The more you know! Though there are probably more individuals whose reproduction is predominantly asexual.
Almost certainly, yes.
(Fun fact: while bacteria can reproduce asexually, they also do this thing called conjugation that is like sex in that it mixes up genetic material.)
Also, there are a lot of species that reproduce asexually at some times and sexually during other points in their life cycle. Many of these reproduce asexually during times when resources are relatively abundant and sexually when it’s relatively more difficult to survive, so their form of reproduction is condition-dependent. Would those be classified as “predominantly asexual” or “predominantly sexual?”
I also know of an experimental evolution study that produced different C. elegans strains with varying ratios of sexual to asexual reproduction. Wild-type C. elegans have two sexes, hermaphrodites and males. The hermaphrodites are capable of either self-fertilizing their own eggs (asexual reproduction) or mating with males (outcrossing). The study produced lines ranging from obligately selfing with zero males produced all the way up to obligately outcrossing, with half-and-half sex ratios and the spermatogenic gonadal tissue effectively disabled in the hermaphrodites so that they had to mate with males to reproduce. Wild populations of C. elegans are also known to vary in the proportion of males produced, although selfing is usually more common. So how would you rank that species in terms of “mostly asexual” or “mostly sexual?”
In any case, there are very few species that only reproduce asexually* over a long period of evolutionary time—the only example I can think of off the top of my head is bdelloid rotifers, and I think there might be one other that I’m missing. And that information is currently under question. (Other species that reproduce only asexually do exist, but they tend to be lineages very recently split off of sexually-reproducing species like parthenogenetic lizards.) On the other hand, there are lots of species that are obligately sexual. So I’m pretty comfortable saying that sexual reproduction is the predominant form of reproduction on Earth, since the species that do engage in some form of sex vastly outnumber the ones that don’t.
*Counting conjugation as a form of sex in that it creates daughter cells with genetic mixtures from the “parent” cells, effectively mixing clonal lineages.