David embarked on his analysis without many preconceptions of where it would lead, but after looking at the data, it was clear that they supported one hypothesis over others: mass extinctions change the rules of extinction when compared to normal background times. During normal times, clades (specifically genera — a narrowly defined group of species all descended from a common ancestor) with certain traits resist extinction. Having offspring that can easily disperse to new environments, having a lifestyle that involves burrowing into the seafloor, and having many species included in your genus — all of these traits and a few others protect a genus from extinction during normal times. But during mass extinctions, none of these factors matter. “Mass extinctions,” David suggests, “represent a change in the rules of extinction, rather than the obliteration of all rules or just an intensification of business as usual.” To date, David has uncovered only one rule of life and death that comes into play during mass extinctions: genera with broad geographic ranges are more likely to survive these catastrophic events.
To see how this works, put yourself in the position of a clam shortly before the end of the Cretaceous period. Things are going along swimmingly. You are highly adapted to your local environment, with lots of refined characteristics that allow you to burrow into the safety of the seafloor. You’re a filter feeder with plenty to eat, and each year, you release several million eggs into the ocean where they are fertilized and metamorphose into larvae that drift in the currents before settling down to make their livings. You and the species closely related to you have remarkably low rates of extinction; life is sweet. The only catch? You and all of your relatives happen to live in a narrow zone from what is today Alabama to western Tennessee. Then, the asteroid strikes, triggering tidal waves and wildfires and ejecting many cubic miles of debris into the atmosphere. The resulting dust cloud blots out the sun, and as photosynthesis grinds to a near halt, food webs the world over are disrupted. Suddenly, all the traits that were previously so advantageous no longer matter. The rules have changed: even species protected in the sea floor are affected by the food web collapse, and having offspring that are able to disperse to new environments is no longer helpful. The only thing that matters now is having a broad, cosmopolitan distribution — and in this, you and your relatives are out of luck. Despite your handy adaptations, your entire genus goes extinct, while other genera who happen to live not only in Alabama, but in France, Tunisia, India and Japan manage to survive.
David's research deals with differences in species-level and genus-level geographic ranges. What's the difference and why does it matter? Take an advanced side trip to What's in a range.
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