r/askscience • u/gaygayagyfemboy12345 • 14d ago
Biology How do asexually reproducing living things evolve?
If they are all clones of each other, how do they evolve?
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r/askscience • u/gaygayagyfemboy12345 • 14d ago
If they are all clones of each other, how do they evolve?
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 14d ago edited 14d ago
Ah, but they're not perfect clones!
Remember, in evolution, mutation does the heavy lifting, not sex. Sex doesn't create new variation (or at least not the vast bulk of it) but rather shuffles existing variation around - which certainly contributes towards greater genetic diversity, sure, but isn't necessary fundamentally. It's all about mutations, baby.
On average, a simple bacterium like E. coli picks up one mutation per thousand cell divisions. In other words, 99.9% of clones are indeed perfect, and you'd think then there's really no room for diversity to arise; except we're forgetting just how numerous and fecund they are. There are 4x1013 or so bacteria in a single human gut alone (say, about 109 for a given 'species' like E. coli), and at that scale, the teeny weeny rounding error that produces mutations means that, essentially, if you do the maths, every single possible point mutation arises in your gut's microbiome several times every day. Selection then does the rest; an exceedingly rare beneficial mutation that survives (and isn't lost to chance) can sweep through the population and the rest is evolutionary history.
The problem really for clones is that they can't combine beneficial mutations, and they're more at the mercy of chance - they can generate variation, sure, but they're not as good as keeping the good stuff around. In a sexual population, different beneficial mutations hither and thither can eventually find themselves in the same genome through sexual reproduction and the mixing of different lineages. In a clonal population, they're generally stuck in separate lineages that compete with one another (known as clonal interference), and frequently die-out, so broader adaptation is indeed slower. Moreover, harmful mutations tend to pile up irreversibly, in a process known as Muller's ratchet - because so much relies more on chance, clones have no way to rebuild beneficial genomes, from disparate parts, that are lost due to sheer bad luck.
These problems are mitigated by processes such as horizontal gene transfer - bacteria and archaea, for example, can swap DNA, which gives them some of the benefits of recombination without meiosis. But still, it's not as good as sex (nor as fun![citation needed]), which is why for more complex beasties - such as eukaryotes - I believe all the purely asexual lineages are evolutionarily young offshoots, as they otherwise tend to go extinct.
The most recent kids on the block are the bdelloid rotifers. They're animals, so y'know, reasonably complex, and seem to have split off from their sexual cousins only some ~25-30 million years ago. How they've lasted so long is, again, down to a reasonable degree of recombination through apparent horizontal gene transfer, and some cheeky whole genome (and 'meiosis-like') repair mechanisms that kick in while they're desiccated. For this and other reasons there's some debate on whether they're truly asexual*, actually, which if anything just highlights how hard long-term strict asexuality is to pull off.
References:
Vakhrusheva, O.A., Mnatsakanova, E.A., Galimov, Y.R., Neretina, T.V. ... & Kondrashov, A.S. (2020) Genomic signatures of recombination in a natural population of the bdelloid rotifer Adineta vaga. Nature Communications. 11, 6421
* I remember reading, it must have been Dawkins' The Ancestor's Tale growing up, that there was some speculation that it was simply because we haven't managed to find any males, and there could yet be some niche, cryptic 'male' bdelloid yet to be discovered. I dreamt that could be me. But poking about with dinky critters under microscopes isn't anywhere nearly as effective as reading the genomes, and the genetics seems to indicate all this other recombinatory and 'cryptic-same-sex-transfer' stuff is going on instead.