Byte Size Biology

Blog for Darwin: on Mass Extinctions

By Iddo on February 12th, 2009

Happy Darwin Day! If you are reading this then you probably do not need an introduction to Charles Darwin, the importance of his work, how his theory of evolution by natural selection shaped modern Biology. Or if you do, I bet there are plenty of posts in today’s blogswarm that will address this.

There is one  evolutionary force that I would like to talk about on this special day, and that is the force of mass extinction. Over the history of the Earth, fossil records show us that there have been five mass extinctions, or as palaeontologists refer to them “The Big Five”.  The most famous and best understood  of mass

extinctions is also the most recent one: the Cretaceous-Tertiary or  KT extinction 65.5 million years ago (Ma),thought to be caused by an extraterrestrial object impacting in the area of what is now Chixclub in the Yucatan peninsula, Mexico. The impact (or a combination of the impact and previous volcanic activity) caused the mass extinction of some 60% of terrestrial and 70% of marine species; most famously, the dinosaurs. Fossil records tell us of four more mass extinctions, some larger in scope of species death than the KT.

The real killer was the Permian-Triassic or PT extinction, 250 Ma, where it is estimated that 95% of aquatic species and 70% of terrestrial species were annihilated. Also called  The Great Dying, it is the closest that life on Earth got to being completely extinguished; it is still unclear what caused the PT mass extinction. There were three other mass extinctions, and a collection of other multi-species extinctions to a lesser extent, global or local.

Obviously, mass extinctions are a strong evolutionary force to that we need to understand.  They clean a vast tract of the biospheric slate, eventually fostering a whole new and vastly different biosphere. At the same time,  the Big Five are estimated to have destroyed all together only some 5% of  animal and plant species that ever existed. The remaining 95% went extinct by lesser calamities, or by background extinction.  Background extinction refers to the normal, non-catastrophic and gradual extinction of species that occur for primarily Darwinian reasons: reduced fitness due to losing competition with other species,  lack of the ability to adapt to a slowly changing environment, competition from invading species and so on.

I like using the city analogy to explain this: in the normal course of urban history, houses are renovated or simply demolished and replaced. Roads are extended or their course changes. Business districts are re-zoned to residential and vice-versa, necessitating structural changes. All this creates a slow but perpetual change of the urban landscape and function in different areas of the city. The evolutionary parallel is  those species extinctions that occur over geological time-scales. Most houses over the course of a city’s history get replaced in that manner (if the city is old enough). Very rarely, a catastrophe happens: fire, flood, earthquake or a war. Many houses and other elements of the urban landscape and infrastructure are demolished over a very short period. These sweeping and catastrophic events disrupt the normal course of urban affairs in a grand manner. The rebuilt city (if it is rebuilt) may look quite different than the previous one.

How did Darwin view those catastrophic mass extinctions? How did he try to  fit them his theory of natural selection that stressed the gradual changes in species as the primary contribution to their fitness and survival? The short answer is: he did not. While Darwin did address background species extinction, he, and most of his contemporaries, simply did not accept the mass extinction phenomenon;

it just seemed too improbable,  even though they were well-aware of the sharp boundaries and the sudden drop in the fossil record, both at the PT and the KT events. Some 50 years before Darwin wrote  Origin,  Georges  Cuvier (1769-1832) proposed a catastrophic explanation to the gaps in the fossil record.  Cuvier was also the first to propose that fossils are in fact the remains of extinct animals, and not, as was thought at his time, that they were still living in Africa or some other remote and yet unexplored (by Europeans) corner of the Earth. The theory that species go extinct as a matter of course did  eventually  gain a general acceptance: by the time Darwin wrote Origins there was little doubt that the fossil record contains mostly extinct species.  At the same time, Cuvier proposed  that mass extinctions also occur, as a result of global catastrophes:

All of these facts, consistent among themselves, and not opposed by
any report, seem to me to prove the existence of a world previous to
ours, destroyed by some kind of catastrophe.

Darwin indirectly countered Cuvier in Origin:

So profound is our ignorance...that we marvel when the hear of
the extinction of an organic being;  and as we do not see the cause,
we invoke cataclysms to desolate the world... -- Origin. Chap 10

So how did Darwin explain the abrupt changes in the fossil record? the KT as well as the PT geological boundaries were well known at his time (albeit by different names), as well as the paucity of the fossil record on the latter side of each boundary. Darwin rationalized:

With respect to the apparently sudden extermination of
whole families of orders, as Trilobites [...] (PT extinction IF)
and the ammonites at the close of the secondary
period (KT event.. IF), we must remember what has already
been said on the probable wide intervals of time between
our consecutive formations". -- Origin, chap 10.

This should be explained in light of the times: Darwin was a colleague and admirer of Charles Lyell (1797-1875), another founder of modern geology. Lyell was strictly opposed to the idea of catastrophes shaping Earth, as was suggested by Cuvier. Darwin, and eventually most of his contemporaries accepted that view. Actually, it was not until the early 1980s that neo-catastrophism became a well accepted theory, with the discover of the record of the meteorite impact that caused the KT mass extinction. Darwin attributed the sudden disappearance of many species from the fossil record chiefly to  imperfections in the record. He held that mass extinctions were not really massive, certainly not on a global scale. At worse a synchronous extinction of several species was due to sudden invasion of species that are better adapted to the invaded environments; the invaders may trigger a chain reaction that causes a local, cross-species extinction.  As far as Darwin was concerned,  the chief role extinctions place in evolution is the one of gradual transitions to better adapted forms:

No one I think can have marvelled more at the
extinction of species, than I have done[...] The theory of natural
selection is grounded on the belief that each
new variety, and ultimately each new species, is produced and
maintained by having some advantage over those with which it comes
into competition; and the consequent extinction of less-favoured forms
almost inevitably follows. [...] The competition will generally be most severe, as formerly
explained and illustrated by examples, between the forms which are
most like each other in all respects. Hence the improved and modified
descendants of a species will generally cause the extermination of the
parent-species; and if many new forms have been developed from any one species, the nearest allies of that species, i.e.
the species of the same genus, will be the most liable
to extermination. -- Origin chap. 10

So Darwin held that extinction was gradual, and was caused chiefly by species of the same genus that are simply better adapted to their environment than their competitors. Thus Darwin has explained that large-scale evolutionary events (extinctions) are still caused by small scale forces (gradual adaptation, or rather the lack of it).

But today we know that catastrophes and mass extinctions do happen, so gradualism is not the only explanation for species adaptation and survival, as many aspects of evolutionary success are irrelevant during times of mass extinctions. It has therefore been suggested that the subsequent repopulation of the biosphere is subject to contingency as well as to gradual natural selection. Contingency — synonymous with “cosmic dumb luck” —  plays a role in which species would become dominant and survive as well as natural selection, but also, that it may be that just being in the right place in the right time (as remotely as possible from the crashing asteroid, outside the burning forest or the suffocating ocean) will play a role. The chief land vertebrate that survived the PT mass extinction was the Lystrosaurus, a rather unimpressive pig-like lizard. Very little about it suggests why it survived the Great Dying while its contemporaries did not. It is estimated that some 95% of land vertebrates during the early Triassic, just after the Great Dying,  were Lystrosaurs.

So what is the role mass extinctions play in evolution? Obviously a dramatic and all-encompassing one, but also a highly random and arbitrary one. This offends our human sensibilities: first Darwin came along, teaching us that we are not the crowning achievement of Creation, but the  mere terminus of a shortish and rather recent limb on a vast tree of life, created by a culmination of  selected random events.  Hey, but at least they were selected for survival, and we like to think of us humans as a species that made good use of our evolutionary endowments: using our opposable thumbs and our big brain enabled us to create human civilization, complex technologies, art, literature, music, architecture and science; we reach for the stars and we work at the core problems of the universe. Our intelligence that lets us greatly modify the environment lends perceive ourselves to be masters of our fate, more than any of our cohabitants on this planet. Then we learn about the total catastrophes that are the Big Five extinctions come along and suddenly another element of randomness is inserted in the development of life: the whole intricacy beauty and diversity of Permian life is swept away from the biosphere, only to have this “shovel lizard” (literal meaning of Lystosaurus) survive, with no apparent adaptive merit. As humans, we would at least like to believe that survival is dependent upon some adaptive traits, rather than plain luck (even if those adaptive traits are acquired by dumb luck in the first place).  Unfortunately, nature does not cater to our sensibilities; all the trait richness acquired piecemeal over eons of gradual selection does not do much good when faced with an impacting asteroid, or a mega-volcanic eruption.

Nevertheless, we can take some comfort that classic Darwinian natural selection still works most of the time, at least until the next catastrophe:

Natural selection remains the only viable, naturalistic explanation
we have for sophisticated adaptations like wings and eyes. We would
not be here without natural selection. Extinction by bad luck merely
adds another element to the evolutionary process, operating at the
level [of] species, families, and classes, rather than the level
of local breeding populations of single species. 

-- David Raup Extinction: Bad Genes or Bad Luck? (Norton, 1991)

---

Sources, and further reading.

Books:

C. Darwin, The Origin Of Species

M. Benton, When Life Nearly Died: The Greatest Mass Extinction of All Time (Thames & Hudson, 2005), first edn.

T. Hallam, Catastrophes and Lesser Calamities: The Causes of Mass Extinctions (Oxford University Press, USA, 2005).

D. M. Raup, Extinction: Bad Genes or Bad Luck? (W. W. Norton & Company), first edn.

Papers:

Raup DM (1994). The role of extinction in evolution. PNAS, 91 (15)

D. Jablonski (2001). Lessons from the past: Evolutionary impacts of mass extinctions Proceedings of the National Academy of Sciences, 98 (10), 5393-5398 DOI: 10.1073/pnas.101092598

S. Nee (1997). Extinction and the Loss of Evolutionary History Science, 278 (5338), 692-694 DOI: 10.1126/science.278.5338.692

G. Keller (2004). Chicxulub impact predates the K-T boundary mass extinction Proceedings of the National Academy of Sciences, 101 (11), 3753-3758 DOI: 10.1073/pnas.0400396101

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