Even the greatest of success stories often owe something to a bit of luck. As mammals, it might seem that we owe a little of our success to luck. Flash back to 65 million years ago. Our mammalian ancestors had their place among the organisms on earth, but these rodent-size, insect-eating, nocturnal creatures didn’t remotely possess the dominant position we hold today. It was the dinosaurs’ time, and the giant reptiles dominated the earth.

All that changed in an instant when the earth was struck by an asteroid, about 6 miles (10 km) in diameter, near what is now the eastern part of Mexico. This caused the environmental conditions on earth to change quickly and drastically. We’ll explore the details of this catastrophic event in the next section, but one outcome was extreme: almost all of the dinosaur species were wiped out. Our mammalian ancestors, though, survived and found themselves living on a planet where most of their competitors had suddenly disappeared. We were in the right place at the right time.

What followed was an explosive expansion of mammalian species. In a brief period of time, a small number of species diversified into a much larger number of species, able to live in a wide diversity of habitats. Called an adaptive radiation, such a large and rapid diversification has occurred many times throughout history (FIGURE 10-24).

Figure 10-24A rapid diversification of species.

Three different phenomena tend to trigger adaptive radiations. After one of these events, surviving species find themselves in locations where they suddenly have access to plentiful new resources.

1. Mass extinction events. With the near-total disappearance of the dinosaurs, a world of “opportunities” opened up for the mammals. Where previously the dinosaurs had prevented mammals from utilizing resources, mammals suddenly had few competitors. Not surprisingly, the number of mammalian species increased from perhaps just a few hundred to more than 4,000 species in about 130 genera. This happened over about 10 million years, barely the blink of an eye by geological standards. Following other large-scale extinctions, numerous other groups that suddenly lost most of their competitors experienced similar adaptive radiations.

It is interesting to note that although, from time to time, sudden and extreme events lead to mass extinctions that wipe out a large proportion of the species on earth, in every case these mass extinctions are followed by a time of explosive speciation in the groups that survive

2. Colonization events. In a rare event, one or a few birds or small insects will fly off from a mainland and end up on a distant island group, such as Hawaii or the Galápagos Islands. Once there, they tend to find a large number of opportunities for adaptation and diversification. In the Galápagos, as we learned in Section 10-7, 14 finch species evolved from a single species found on the nearest mainland, 600 miles away. In Hawaii, there are several hundred species of fruit flies, all believed to have evolved from one species that colonized the islands—perhaps blown there by a storm, or carried there stuck in the feathers of a bird—and experienced an adaptive radiation.

3. Evolutionary innovations. In the world of computers, software developers are always looking for the “killer app”—the new application so useful that it immediately leads to huge success, opening up a large new niche in the software market or greatly expanding an already-existing niche. The first spreadsheet, email program, and web browser were all killer apps. In nature, evolution sometimes produces killer apps, too. These are innovations such as the wings and rigid outer skeleton that appeared in insects and helped them diversify into the most successful group of animals, with more than 800,000 species today (more than a hundred times the number of mammalian species). The flower is another innovation that propelled an explosion of diversity and ensured the evolutionary success of flowering plants relative to the non-flowering plants, such as ferns and pine trees. Today, about 9 out of 10 plant species are flowering plants.