20

key concepts

20.1

Evolution Is Both Factual and the Basis of Broader Theory

20.2

Mutation, Selection, Gene Flow, Genetic Drift, and Nonrandom Mating Result in Evolution

20.3

Evolution Can Be Measured by Changes in Allele Frequencies

20.4

Selection Can Be Stabilizing, Directional, or Disruptive

20.5

Multiple Factors Account for the Maintenance of Variation in Populations

20.6

Evolution Is Constrained by History and Trade-Offs

An Evolutionary Arms Race Between Bats and Moths

Many species of bats roost by day and fly out as night approaches. The mass exodus of thousands or even millions of bats from a roost is an impressive sight, often attracting crowds of human bat-watchers. In urban areas, the cavities under bridges can attract enormous colonies of bats. People once feared these urban bat colonies, but we now know that many species of bats prey on night-flying insects, including mosquitoes and agricultural pests. Each bat can consume a third of its weight in insects per night, so just 1,000 bats consume about 4 tons of insects every year.

Most bats catch insects in flight at night. About 50 million years ago, bats evolved the ability to echolocate, which allows them to detect flying insects (such as moths) in darkness. The bats produce ultrasonic sounds, and compare these outgoing sound pulses with the returning echoes of the pulses that bounce off the flying moths. Because bats are among the greatest threats to night-flying insects, any insect that can escape detection by bats is much more likely to survive and reproduce. Inherited traits that lead to greater survival increase in the population over time as surviving moths pass these traits on to their offspring.

Many groups of night-flying insects, including several groups of moths, have evolved the ability to detect the ultrasonic sounds of bats. When such a moth hears a bat, it flies away from the source of the sound. If the bat is too close, the moth may dive rapidly to avoid capture. The moths in each generation that are better able to detect and avoid bats leave more offspring, so moths gradually evolve more effective escape mechanisms.

As moths get better at avoiding bats, why don’t the bats starve? Because populations of bats are evolving too. In each generation, the individual bats best able to detect and capture flying insects get the most food, which allows them to put the most effort into reproduction. Their offspring inherit their prey-detection capabilities, so populations of bats evolve more efficient insect-catching abilities, even as the moth populations evolve better escape strategies. This gradual change in both moth and bat populations is known as evolution by natural selection—a process that is constantly shaping and changing all species of life on our planet.

How do complex traits like echolocation, or the ability to avoid detection by echolocation, evolve in the first place?