11.1: Adaptation's dark twin

Natural selection and related processes control the evolution of organic life. Elsewhere in this text, we have explored how random mutations in the genetic material inside organisms’ cells produces variations in their size, shape, and abilities, some of which will provide a selective advantage over other individuals. In those circumstances, the mutated individual is likely to enjoy enhanced survival, and thus enhanced reproduction, which passes the gene for the new trait on to the next generation. The new trait is an adaptation. Through repeated cycles of copying and culling, the adaptive gene spreads through the population and comes to dominate. If the changes are sufficiently pronounced, we call the organism by a different name; we infer that speciation (or “origination”) has occurred.

Figure \(\PageIndex{1}\): Theoretical comparison between evolution “tree of life” diagrams with and without the “pruning” effect of extinction. (Callan Bentley redrawing of an original diagram by David Raup.)

However, this narrative emphasizes the spread of a new trait (and the gene that codes for it). Equally critical in the history of life is the cancellation of some old traits (and the genes that code for them). Without extinction, there would likely be insufficient ecological “space” available for new species. The pattern we see in the fossil record is not one of continuous diversification with new species being added, but none ever removed. Instead, the average species lasts a few million years, and then vanishes forever from the face of the planet. It goes extinct. A handful of species have persisted with relatively minimal change (sometimes we call them “living fossils”), but most lineages show changes through time, gaining some species but losing others. As you look around the world today and (rightly) marvel at the diversity of organisms who share your planet with you, it is sobering to realize that Earth’s deep time hosted many, many more — species that got snuffed out somewhere along the way. Estimates are that 99.9% of all species that ever existed on Earth no longer exist — the vast majority have gone extinct. Most of these occur as part of the normal “background extinction” rate, a regular coming-and-going of species due to habitat loss, changing physical environments, and biologic pressures over time.

Periodically, entire lineages will go extinct. The trilobites, for instance, were exceptionally diverse and widespread, but they never made it out of Paleozoic Era. Similarly, sauropod dinosaurs are a Mesozoic-only group. When multiple lines of descent all terminate at the same moment in geologic time, we call it a mass extinction. Mass extinctions potentially imply a common cause for the synchronous extinctions of many unrelated groups. We study mass extinctions because they are fascinating and dramatic, but also because we worry about our own future. The study of mass extinctions demonstrate that Earth’s biosphere can periodically be wrecked, and then healed, but healed with a new suite of organisms. Put another way, every time there is a mass extinction episode, it is followed by a time when the survivors diversify and adapt, taking advantage of the lack of competition. This is an adaptive radiation. For instance, after the reptilian dinosaurs, mosasaurs, and pterosaurs went extinct at the end of the Cretaceous, mammals underwent an adaptive radiation, diversifying to fill all sorts of ecological niches on land, sea, and even in the air.

Figure \(\PageIndex{2}\): Geologic ranges for select index foraminifera from the Cretaceous and Paleogene. (For educational purposes from the Geologic Timescale Foundation. Modified by Shelley Jaye.)

We know about mass extinctions because of careful study of the appearance of fossil forms in the stratigraphic record. Each species has a geologic range defined by its first appearance (oldest stratum in which it is known) and its last appearance (youngest stratum in which it is known). Like any scientific conclusion, these ranges are always subject to change with new data (such as in the famous case of the coelacanth), but for the most part the ranges are well established, having stood repeated testing over the past two centuries of paleontological documentation.

Mass extinctions lead to declines in both diversity and abundance of organisms, are typically global scale events, and cut across taxonomic groups affecting organisms in a wide array of environmental habitats.