Score one for punk eek

Speaking of macroevolution…

I think it’s fair to say that the concept of punctuated equilibria is one of the most famous and most misunderstood ideas in 20th century evolutionary biology. PE, or “punk eek” was proposed by palaeontologists Niles Eldredge and Stephen Jay Gould  (Eldredge and Gould, 1972) as a reconciliation of the Modern Evolutionary Synthesis and the fossil record. Its core idea is that most (visible) evolutionary change happens during the formation of new species, and that this process is usually quick compared to the lifetime of a species. (An excellent layperson-friendly explanation of punk eek is available here.)

Of course, punk eek is not a law of nature – it’s only one way evolution might proceed, and it’s a decent explanation of the dearth of low-level (species to species) transitions in the fossil record. But there’s nothing to say that this is how evolution always proceeds, and consequently, exactly how often it does so is a valid (and still actively debated) question in evolutionary biology.

A related question is how often new species arise by the wholesale transformation of the ancestral species (anagenesis) or by the splitting of the ancestor into two or more descendants (cladogenesis). Since punk eek posits that most new species come from small isolated populations of the ancestor, under punk eek scenarios you’d expect most speciation to occur by cladogenesis.

However, assessing the exact contribution of each requires an exceptionally good fossil record where ancestor-descendant relationships and precise times of appearance and disappearance can be determined. This makes the investigation difficult to impossible in most groups. In the latest issue of PNAS, Strotz and Allen (2013) went to one of the few groups with a good enough record to answer such questions and analysed the living shit out of them.

Foraminiferans of forams for short are single-celled creatures that build hard shells to live in. They are very abundant, widely distributed in the world’s oceans, and because of their shells they make excellent (if tiny) fossils. Their relationships have also been studied with molecular methods, so we have a pretty good understanding of who’s related to whom and how well morphology meshes with genetics.

Therefore, as Strotz and Allen point out, we can say with a fair amount of confidence that what we’ve identified as species in the fossil record are likely to actually be species, not just varieties. (It doesn’t always work the opposite way – some “species” that look exactly the same on the outside are known hide several genetically distinct lineages.) The genetic data also help sort out who begat whom.

Armed with this knowledge of genetics and the detailed fossil record of planktonic forams in the last 65 million years, the pair formulated criteria for identifying cladogenetic events:

  • If morphologically distinct ancestor and descendant(s) overlap in time (factoring in dating and classification error), the descendant must have arisen by cladogenesis.
  • Likewise, cladogenesis must have occurred if the two species occur together in the same sample even if their morphologies overlap at that point.
  • Third, if an ancestor gave rise to a series of descendants, all but the last of those must have formed by cladogenesis – the ancestral form has to continue existing for it to sprout more descendants!

Thus, the possibility of anagenesis only remains for ancestor-descendant pairs that didn’t get caught on any of the above filters. And the number of those turns out to be very low.  Depending on how you estimate the errors associated with identifying fossils, only around 43-64 out of 337 speciation events (less than a fifth of the total) in the last 65 million years shows no evidence against anagenesis. The numbers are even lower, dipping below one-tenth of all events, if you only consider the last 23 million years, for which more precise dating information is available. In conclusion, for planktonic forams since the death of the dinosaurs, splitting an old species has been by far the more common way of forming new species.

It’s important to talk about the things this paper doesn’t say. It doesn’t, for example, say that its findings apply to all organisms. Speciation need not work the same way for all groups, and a subset of forams need not be representative of anything. It also doesn’t say – and the authors are quite explicit about this – that morphological evolution only occurs when species split. Instead, they argue, their findings support a modified view of punk eek in which species do change throughout their lifetimes – but the changes are fluctuations due to short-term influences, and they only persist if populations get isolated.

(Myself, I just think the simple fact that we have a fossil record where such ideas can be tested is pretty amazing. You can complain about the patchiness of the record all you like, but in the meantime it’s worth stopping and appreciating what we do have!)

***

References:

Eldredge N & Gould SJ (1972) Punctuated equilibria: an alternative to phyletic gradualism. In Schopf TJM (ed) Models in Paleobiology. Freeman, Cooper & Co., pp. 82-115

Strotz LC & Allen AP (2013) Assessing the role of cladogenesis in macroevolution by integrating fossil and molecular evidence. PNAS 110:2904-2909

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