Mass extinction and “morphospace”

Theory tells us that after a mass extinction, an event where the diversity of species is drastically reduced, nature should rebound with a flurry of creativity. Species should quickly proliferate to refill desolate ecosystems, something called adaptive radiation.

Yet, the paleontological record suggests that this doesn’t happen at anywhere near the expected pace. Now, research published in the journal Nature Ecology and Evolution argues that understanding something called “morphospace” might help us find the cause.

Extinction events happen with alarming regularity: there’s the “big five”, but a host of slightly smaller, yet still devastating extinctions have peppered the planet’s history. 

Scientists now worry that we might be in the middle of one of our own making, so this makes it all the more important to understand how the natural world bounces back from such catastrophes. 

Perhaps the most well-known of the earth’s mass extinctions is the Cretaceous–Paleogene (K–Pg) extinction event. This took place 66 million years ago when an asteroid smacked into the earth next to what is now the Yucatán Peninsula, creating the nearly 200-kilometre-wide depression known as the Chicxulub crater. This impact drove the extinction of all the non-avian dinosaurs, and much else besides.

In the new research, Christopher Lowery of the University of Texas Institute for Geophysics, and Andrew Fraass from the Sam Houston State University, also in Texas, US, argue that the recovery of various forms of plankton – or, more formally, planktic formanifera – was unusually slow.

What should have been “a classic example of explosive adaptive radiation” happened in waves over time, “the latter of which were delayed for millions of years”, they write.

The diversity of species did not return to mid-Cretaceous levels for 10 million years, and only reached late Cretaceous levels after 20.

The hypotheses put forward to explain the slow recovery marine plankton have mainly centred on the environment, such as toxic metals left by the asteroid or impact-linked volcanism. Lowery and Fraass note, however, that evidence is sparse for these explanations.

Instead they argue “that ecology, rather than environment, controls diversification after a mass extinction”, and an important component of this is “the time needed to reconstruct morphospace within ecosystems”. This they call the morphospace reconstruction hypothesis.

Morphospace, short for “morphological space”, is a way for scientists to visualise the possible shape and structure of organisms, the physical phenotype. A morphospace can show all possible forms, and how many of those have been taken on by organisms in the real world.

Ecological niches are formed by the pairing specific environmental conditions with the phenotype of species and “this can be more properly conceived of as morphospace (that is, the range of morphologies occupied by a clade)”, the authors write. This represents “the range of successful strategies that a clade has evolved to adapt to its environment and pressure from other organisms.”

As organisms undergo adaptive radiation after an extinction, they begin to colonise different parts of the morphospace. These new morphologies, or phenotypes “can serve as a jumping-off point for further evolutionary innovation”.

This, in turn, provides a new point of evolutionary departure and further opportunity for morphological innovation and complexity.

What Lowery and Fraass discovered is that the fossil record for marine plankton shows that increasingly complex morphologies are linked to episodes of taxonomic diversification, with the latter being dependent on the former. Newly occupied morphospaces opened up room for increasing radiation, just as the morphospace reconstruction hypothesis predicted.

Importantly, their work indicates that the constraints of morphospace colonisation might impose a “speed limit” on post-extinction taxonomic diversification. This is a sobering reminder that if we are in the midst of an anthropogenic extinction event, the biosphere will take millions of years to get over it.

And even when it does, it will be a very different world from the one we inhabit now.