- Proof that crabs are the ultimate goal of evolution.
One of the many attempts of Nature to evolve a crab (Borradaile, 1916; McLaughlin & Lemaitre, 1997)
Generally speaking, convergent evolution means that two organisms independently evolve a feature or set of features that were not present in a common ancestor, or that these features occur in different epochs in time.
Of course, convergent evolution is not that rare. The other wiki has a handy list of other examples (Wikipedia contributors, 2021). Carcinization is just a particular example: crab-like features occurring independently, several times.
Why the meme? Why this particular example out of all examples of convergent evolution? As it’s often the case, it’s hard to pin down to a single cause, but I guess the extensive study of carcinization has to do with it. After all, it’s been studied and documented since the 19th century (Keiler et al., 2017), though not without its fair share of scrutiny and proposed alternate hypotheses (see McLaughlin et al., 2004, §Introduction).
Of course, the meme itself can be more or less pinpointed (Knowyourmeme contributors, 2020) but it’s building on a scientific fact that was definitely not new at all.
As with many other products of popular science, the meme itself contains scientific inaccuracies that nevertheless help disseminate itself. For instance, the relevant xkcd mentions that (Explainxkcd contributors, 2021):
Evolution just loves making crabs, I guess!
…which is definitely not true: «Evolution» or «Nature» do not «decide» anything and evolution is a process without guidance, preexisting designs or goals. However, the idea of an anthropomorphic force setting things up for crabs to exist at all costs is way funnier than the dry biological explanation.
And hey, it even serves as a good science topic to introduce people to evolutionary biology!
References and Bibliography
Borradaile, L. A. (1916). British Antarctic ("Terra Nova") Expedition, 1910: Natural history report. Zoology, III(3), 111–126. https://research.nhm.org/pdfs/31940/31940.pdf
Explainxkcd contributors. (2021). 2314: carcinization. https://www.explainxkcd.com/wiki/index.php?title=2314:_Carcinization&oldid=205268.
Keiler, J., Wirkner, C. S., & Richter, S. (2017). One hundred years of carcinization the evolution of the crab-like habitus in anomura (arthropoda: crustacea). Biological Journal of the Linnean Society, 121(1), 200–222. https://doi.org/10.1093/biolinnean/blw031
Knowyourmeme contributors. (2020, November 19). Carcinization. https://knowyourmeme.com/memes/carcinization
McLaughlin, P. A., & Lemaitre, R. (1997). Carcinization in the anomura - fact or fiction? I. Evidence from adult morphology. Contributions to Zoology, 67(2), 79–123. https://doi.org/10.1163/18759866-06702001
McLaughlin, P. A., Lemaitre, R., & Tudge, C. C. (2004). Carcinization in the anomura fact or fiction? II. Evidence from larval, megalopal and early juvenile morphology. Contributions to Zoology, 73(3), 165–205. https://doi.org/10.1163/18759866-07303001
Wikipedia contributors. (2021). List of examples of convergent evolution — Wikipedia, the free encyclopedia. https://en.wikipedia.org/w/index.php?title=List_of_examples_of_convergent_evolution&oldid=1033301974.