Earlier this month, there was a flurry of headlines in the pop-sci press that exasperated me. “Have scientists discovered reversible evolution?” was one; “Evidence of Reverse Evolution Seen in Dust Mites” was another. They failed because they always tried to express a subtle idea in a fluffy way that screwed up a more fundamental concept in evolution — it was one step forward in trying to explain a legitimate science paper, and ten steps back in undermining understanding of evolution. This was just awful:
Researchers who deny the idea that evolutionary traffic can only move forward saw their arguments bolstered this week with the publication of a study suggesting that house dust mites may have evolved from free-living creatures into full-time parasites, only to abandon that evolutionary track and go back the way they came, reverting to the free-living creatures that live invisibly in your carpet, bed, and other places in your home that it’s probably best not to think about them living.
“Evolutionary traffic can only move forward”? Please, define “forward” in this context for me. Evolution doesn’t have a direction. You can talk about a temporal sequence of historical changes in a gene, for instance, but from the point of view of the process, there’s no “forward” or “backwards”, only change over time. Is a genetic deletion a backwards step? Is a duplication a forward step? If a mutation changes a cytosine to an adenine, is that going forward, and if there is a revertant, a mutation that changes that adenine back to a cytosine, is that going backwards? I keep hearing this talk about directions, and it doesn’t even fit into my understanding of the process of evolution. Direction is always something people infer retrospectively.
The paper all this comes from, Is Permanent Parasitism Reversible?–Critical Evidence from Early Evolution of House Dust Mites, by Klimov and O’Connor, isn’t that bad, but still it has some bits that annoy me.
Long-term specialization may limit the ability of a species to respond to new environmental conditions and lead to a higher likelihood of extinction. For permanent parasites and other symbionts, the most intriguing question is whether these organisms can return to a free-living lifestyle and, thus, escape an evolutionary “dead end.” This question is directly related to Dollo’s law, which stipulates that a complex trait (such as being free living vs. parasitic) cannot re-evolve again in the same form. Here, we present conclusive evidence that house dust mites, a group of medically important free-living organisms, evolved from permanent parasites of warm-blooded vertebrates. A robust, multigene topology (315 taxa, 8942 nt), ancestral character state reconstruction, and a test for irreversible evolution (Dollo’s law) demonstrate that house dust mites have abandoned a parasitic lifestyle, secondarily becoming free living, and then speciated in several habitats. Hence, as exemplified by this model system, highly specialized permanent parasites may drastically de-specialize to the extent of becoming free living and, thus escape from dead-end evolution. Our phylogenetic and historical ecological framework explains the limited cross-reactivity between allergens from the house dust mites and “storage” mites and the ability of the dust mites to inhibit host immune responses. It also provides insights into how ancestral features related to parasitism (frequent ancestral shifts to unrelated hosts, tolerance to lower humidity, and pre-existing enzymes targeting skin and keratinous materials) played a major role in reversal to the free-living state. We propose that parasitic ancestors of pyroglyphids shifted to nests of vertebrates. Later the nest-inhabiting pyroglyphids expanded into human dwellings to become a major source of allergens.
It’s actually rather interesting that these mites have a phylogenetic history that shows some dramatic changes in lifestyle. Parasitism is a specialized pattern that typically involves a loss of shedding of generalized abilities that allow for autonomous living; they can get rid of functions that won’t be needed in the conditions they’ll be living in. A mammalian parasite is swimming in a sea of nutrients provided by the host; it can lose genes for the synthesis of many amino acids, for instance, and still survive because it’s immersed in those amino acids, provided by the mammalian bloodstream. But that makes it difficult to leave the parasitic life — if it moves out to the more limited diet available in the external world, it may find itself starving to death, unable to synthesize essential building blocks. Yet here they have evidence that mites shifted from parasitism to free-living.
But I have two complaints. One is this framing as a refutation of Dollo’s Law — I really don’t give a damn about Dollo’s “Law” at all. The second is that they haven’t really shown any evidence of molecular/genetic reversibility.
I just roll my eyes at papers that talk about Dollo’s Law anymore. Do people realize that it was a macroevolutionary hypothesis formulated in the 1890s, before anyone had a clue about how genetics worked, much less how genetics and evolution worked together? It was a reasonable prediction about how traits would distribute over time. A horse, for instance, runs on a single robust toe on each leg, the other digits reduced to vestigial splints; Dollo’s law says that those splints won’t re-expand to reform toes identical to those found in horse ancestors. Why, he didn’t know.
A modern understanding of the principle, informed by the underlying genetics, would instead say that a complex character involving multiple genetic changes and relying on a particular background for its expression is statistically unlikely to be reconstituted by stochastic changes in a different genetic background, in exactly the same way. It’s not a ‘law’, it’s a consequence of probability.
The authors have only found reversion to an ancestral pattern on a very coarse scale: there are a great many ways to be a free-living organism, and there are a great many ways to be a parasite. They can say on a very gross level that mites have changed their niches in their evolutionary history, but they can’t claim there has been an evolutionary reversal: if we compared the ancestral free-living form (pre-parasite phase) to the modern free-living form (post-parasite phase), I have no doubt, and there’s nothing in the paper to contradict me, that there would be significant differences in form, physiology, biochemistry and genome, and further, that the parasitic phase would have left evolutionary scars in that genome.
Dollo’s Law is archaic and superficial, and I have no problem agreeing that Klimov and O’Connor have refuted it. But the more interesting principle, founded in a modern understanding of microevolutionary and genetic events, has not been refuted at all — it’s just confusing that we’re still calling that Dollo’s Law, and that we mislead further by talking about a direction for evolution and ‘reversibility’ and all that nonsense. The only source of direction in this process is time’s arrow, and that doesn’t go backwards.