Cycads are a group of plants that resemble tree of ferns or palm trees. But they’re not closely related to either: rather they constitute an ancient group of gymnosperms (naked-seeded plants) that originated around 300 million years ago. They reached their peak of abundance during the Jurassic, the age of the dinosurs, when they were abundant throughout the world. Now, though, they are restricted to tropical and subtropical areas, and most of them are rare and endangered, though some are grown as ornamentals.
Cycads are one group of organisms known as “living fossils,” because the living species highly resemble ancient ones (other “living fossils” include the coelocanth, horseshoe crabs, and the tuatara of New Zealand). In other words, living fossils show the “morphological stasis” so beloved of Steve Gould and other advocates of punctuated equilibrium.
The term “living fossil”, by the way,was coined by Darwin in Chapter 4 (“Natural selection”) of The Origin (my emphasis):
And it is in fresh water that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain extent orders now widely separated in the natural scale. These anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition.
Note that Darwin attributed their long-term morphological stasis to a lack of competition: Darwin always thought that most natural selection resulted from competition between individuals of a species or between members of different species.
Other explanations for the stasis, have, however, included a lack of genetic variation (you can’t evolve if there’s no variation in your genes), or the possibility that the group either lives in a constant environment or seeks one out, so there is no selection to produce change.
Both of these explanations have had problems. The first—lack of genetic variation—is almost certainly wrong, for surveys of genetic variation in “living fossils” like horseshoe crabs show that they’re just as variable as species that have changed more over time.
The “constant environment” explanation, which seems more plausible to me, suffers from the fact that some living fossils seem to live in the same environments as species that have evolved more rapidly. And some, like cycads, can’t behaviorally seek out the environments to which they’re adapted.
We should not, however, think that just because “living fossils” look like their ancient relatives, that they haven’t changed. When we compare extant with ancient species we see largely the external morphology: the hard parts that are on the outside and replaced by minerals. Perhaps some “ancient fossils” have changed extensively on the inside since their origin—either in the internal anatomy or physiology, or simply via wholesale DNA changes that were largely caused by random genetic drift. Still, the constancy of external morphology over hundreds of millions of years is an evolutionary puzzle.
In our book Speciation, Allen Orr and I note that “living fossils” such as gingko trees and horseshoe crabs not only haven’t changed much on the outside, but never were particularly speciose: that is, the unchanging lineages also didn’t produce a large number of species over their evolutionary history. (It is possible for a group to produce lots of species but not change very much in external appearance. That just doesn’t seem to have happened.)
Here’s a cycad showing its “cone,” which resembles that of a pine tree (another gymnosperm):
Cycas circinalis (from Wikipedia)
A new paper in Science by Nagalingum et al. sheds some light—or purports to shed some light—on the puzzle of living fossils.
There are now about 300 living species of cycads in three families, and the authors used DNA from both the nucleus and chloroplasts to a) make a family tree of 199 of the species, and b) estimate when the living species diverged from each other. Estimates of divergence time are made by using calibrated “molecular clocks,” which assume (and there is support for this), that divergence in DNA sequence is proportional to the absolute time in the past when species diverged. With calibration, then, we can get a decent handle on when living species of cycads diverged from each other.
The “family tree” of cycads yielded no surprises: the families, and relationships between species within families, were pretty much related in the same way that previous data based on DNA and morphology had indicated.
But the surprise was this: all living cycad species formed recently: within the last 5 to 12 million years. That means that although the group itself hasn’t changed much in appearance in 300 million years, the species we have today (i.e., the lineages that are reproductively isolated from one another) are relatively new. That is, reproductively isolated lineages have arisen much more recently than previously thought, and the species we see today are not identical (in the sense of being reproductively compatible) with their ancient ancestors. Nor are they the products of a single unchanging lineage that extends back to ancient times.
That is a surprise, but doesn’t, I think, have much bearing on the question of “living fossils,” that is, the question of why some lineages don’t change in their appearance over long periods of time.
And it’s clear that cycads don’t change much over millions of years, at least judging by the authors’ own statement highlighting the “morphological conservatism” of the group. So that mystery remains.
The authors do note that cycads show “low levels of genetic diversity” compared to other groups, so it’s formally possible that their morphological stasis is due simply to a lack of genetic variation. But that is contradicted by the fact that they’ve formed so many new species in the past few million years—for the formation of new species requires genetic variation. No, the explanation for the morphological conservatism of cyads—and of other living fossils—probably lies elsewhere.
Another mystery remains: why was there a relatively sudden radiation of cycad species in the last 12 million years? The authors think that climate may be involved:
The near-simultaneous initiation of diversification of six of the living cycad genera across the globe (in Australia, Africa, south-east Asia, and central America) indicates a single trigger may have been responsible. During the late Miocene, the global climate shifted as the world’s landmasses largely assumed their current positions (28). This closed the last of the equatorial seaways that had allowed warm tropical water to circulate the globe, leading to a shift from globally warm, equable climates to present day cooler, more seasonal climates (29). The majority of cycad species live in tropical or subtropical climates in regions of predominantly summer rainfall (2). Thus, it is possible that cycad diversification was largely driven by the global climate change that increased the geographic extent of those subtropical and tropical biomes that became marked by seasonality.
What is disturbing is that most cycads are endangered: the authors note that fully 62% of all cycad species are on the IUCN Red List of Threatened Plants, a higher proportion than in any other plant group.
So what the author have shown is that morphological conservatism of a group does not reflect the lack of ability of that group to produce new species. What they haven’t shown is why morphological conservatism exists in the first place. There are ways to rule out some possibilities. If genetic diversity for external characters were pervasive, you could rule out the lack-of-variation explanation. You could, for instance, perform artificial selection on living cycads to see if you could change them into plants that looked markedly different, as we’ve done with the ancestors of corn and broccoli. But ruling out alternative explanations doesn’t always tell us the real explanation unless only one remains.
Nagalingum, N. S., C. R. Marshall, T. B. Quental H. S. Rai, D. P. Little, and S. Mathews. 2011. Recent synchronous radiation of a living fossil. Science (published online, 20 Oct. 2011).