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

In the opening story of this chapter, we discussed the evolutionary arms race between echolocating bats and the moths that evolved the ability to detect bat ultrasounds, and then avoid the predators. Moths are not the only potential prey of bats that have evolved the ability to hear ultrasounds. Many other groups of insects can hear the sounds produced by bats, and act to avoid these predators. In many cases, the insects can already hear, as they have evolved complex communication systems to attract mates. This produces the natural variation in the trait (hearing) that is needed for natural selection to act.

As we learned in the experiment discussed in Investigating Life: Do Long Wing Tails Help Moths Escape Bat Predation?, many non-hearing insects have evolved other strategies for confusing or avoiding echolocating bats. Several species of moths have evolved long extensions of their wings, called wing tails. The wing tails flutter as the moths fly, and the fluttering wing tails serve to distract the echolocating bats. A bat that attacks the long wing tail of a moth may get nothing but a tiny piece of moth wing, while the moth survives the attack.

Some bats locate their prey using the sounds produced by the prey themselves. There are species of bats that locate prey by the sound of calling frogs, insects, or even the ripples made by fish on the surface of the water. These prey species often must limit or modify the sounds they produce to balance the benefits of producing a sound with the costs associated with bat predation.

Species rarely evolve to a state of perfection and then stop evolving. Prey species are constantly evolving new ways to avoid predation, so predator species must evolve improved means of capturing prey. Although there are often historical, physiological, or mechanical limits to what can evolve, species are constantly changing in response to changes in their physical environment and species interactions. This constant selection for change, over millions or even billions of years, has produced the enormous diversity of life we see on Earth.

Future directions

Bats are not the only animals that have evolved the ability to echolocate. Toothed whales and dolphins, as well as some burrowing shrews and cave-dwelling birds, also use echolocation to “see” their environment with sound. So echolocation has evolved repeatedly in species that live in environments in which sight is limited. But how does a trait like echolocation first begin to evolve in a species? Some rudimentary form of the trait has to be present before selection can act to refine this sensory mode. Often, species co-opt a trait that evolved for other purposes, and then selection refines it over time for a new purpose.

Most humans do not use echolocation. We rely heavily on sight, and most of us feel awkward moving around unfamiliar spaces in the dark. But what would happen if humans were forced to live in an environment where we could not see? We already hear, of course, so we have a system that could be co-opted for echolocation. We are also capable of making sounds. Echolocation simply requires the production of sounds, the ability to detect the reflected sounds, and the ability to process the information appropriately. Indeed, some blind humans have trained themselves to use echolocation to sense their environment (see Media Clip 20.4). An individual learning a behavior is not the same as evolution of a behavior within a species, however. Before natural selection can act, there has to be genetic variation in the population for the trait in question. The fact that a few humans are able to use simple forms of echolocation shows that variation for this trait is already present in our species. The same is true for many species, which is why it is not surprising that echolocation has evolved multiple times in species that live or feed in the dark.