Geneticist Boasts of Her Just-So Story
She should be ashamed, but she gloats in her story about “how the pangolin got its scales.”
Ricki Lewis has a PhD in genetics. As an evolutionary geneticist, she should know better than to trade in just-so stories. Why, then, did she title her entry on PLoS Blogs in Kipling fashion? It reads, “How the Pangolin Got Its Scales – A Genetic Just-So Story.” She doesn’t report, mind you, a critique of that kind of explanation. She seems to celebrate it.
Darwin and Lamarck pondered the advantages of the giraffe’s long legs and neck, while a few decades later Rudyard Kipling explained how the leopard got its spots. Today genome sequencing is fleshing out what we thought we knew about some distinctive animal adaptations, from the giraffe to the leopard.
Adaptations are inherited traits that increase the likelihood of an individual surviving to reproduce. A zebra’s stripes rendering it invisible when it runs and the fennec fox’s giant ears that dissipate heat and hear distant predators are adaptations.
A report in this month’s Genome Research provides the basis for a “just-so story” about how the pangolin – aka the scaly anteater — got its scales. They protect, but in a way beyond the obvious. According to the genome, the armor of the pangolin replaced part of its immune response.
If a scientific explanation is just-so (i.e., ‘believe it because I said so’), then no amount of jargon can atone for that sin, nor can a recounting of known facts about an animal. The focus should be on the explanation for the trait. (Incidentally, zebra stripes do not render it invisible when it runs; see 2/09/12 and 12/22/13). Lewis spends inordinate time describing the life and habits of the pangolin (a.k.a. spiny anteater). That’s fine in a biology text or TV show, but those facts are distractions when her point is to explain how the pangolin got its scales.
We can commend Lewis at one point: she correctly identifies Darwinian evolution with the Stuff Happens Law, warning media reporters to stop attributing purpose to evolution:
Evolutionary geneticists probe genomes for signs of positive and negative natural selection. Genes that don’t vary much in DNA sequence from individual to individual indicate positive selection, because whatever the sequence is, the encoded protein is working: an if-it-ain’t-broke scenario. In contrast, a gene that’s no longer functional can be riddled with mutations, varying greatly among individuals – if it’s protein isn’t useful or even produced, it doesn’t much matter what the underlying DNA sequence is. (Note to The New York Times and other media outlets: avoid “to evolve”. Change driven by mutation and natural selection is not a desire or choice. It just happens. Giraffes didn’t yearn to reach the treetops and alter their DNA accordingly.)
Lewis calls certain genes pseudogenes because they have premature stop codons, compared to counterparts in other mammals. The moral in this part of her story is ‘use it or lose it’. Then she identifies more active genes involved in hair and scale formation. Here comes the completed just-so story:
Natural selection tells the tale. At some point in time, a few pangolins, thanks to chance mutations, had harder hair. Other mutations somehow guided those hairs to eventually overlap, providing shielding. Individuals whose hairiness began to become overlapping scaliness were less likely to succumb to bacterial infections, and thereby more likely to survive to pass on those traits. Perhaps the armor also made them more attractive and they had lots of pangolin sex, assuming their privates are reachable, passing on the desirable trait.
But if mutations were “guided… to” do something, she has violated her own sermon. The genes “suggest strongly that the armor has replaced part of the immune response,” she claims. “The tightly-knit, tough scales deter not only predators, but keep the animal free of infection.” But if this were a law of nature, it would happen to all the other mammals in the same ecosystem. If, on the other hand, she explains it because “stuff happens,” then yes indeed: she has told a just-so story, despite her boasting that her story is better:
Although it is intriguing to imagine reasons why animals are as they are – from the giraffe’s neck to the leopard’s spots to the pangolin’s armor – clues in DNA sequences can provide a broader and less biased view of adaptive traits, from those that have stood the test of evolutionary time to those relegated to the genomic junkyard.
Could not a skeptical reader think that Lewis has done just that: imagined reasons why pangolins are as they are?
Here’s Lewis’s just-so story in a nutshell:
- Some genes mutate, because stuff happens.
- Genes that don’t disappear end up in a mythical ‘genomic junkyard’.
- Male pangolins got more dates with female pangolins who thought scales were cool.
- The males with scales over their privates couldn’t copulate.
- Bacteria stayed away from the scales rather than evolving to live in them, because stuff happens.
- Evolutionary time is a lab technician that makes sure things stand the test.
- Natural selection works in strange ways. It gave the pangolin scales and the zebra stripes.
- I’m broadminded and not biased. I’m a scientist, after all.
- With the power of suggestion, you can believe my imaginary tale.
Add some artwork and you would have a very nice book for the kiddies to put them to sleep.