An example of convergent evolution in whales and bats

The sun bathes the world around us in light, and we use the reflected light to construct an interpretation of that world — that is, to see. Some other animals rely on sound rather than sight to build their picture of the world. Because there's no universal source keeping the world uniformly awash in sound, these creatures have to emit sound in order to hear it reflect off objects around them. They emit high-pitched sounds and listen to the echoes, using changes in pitch and volume to create an image of the world around them. This strategy, called echolocation, is mainly used by toothed whales (like dolphins and sperm whales) and by certain groups of bats, both of which hunt under poor light conditions, although some shrews and bats are also known to echolocate.

Bats and whales echolocate in very different ways. Bats use their larynx to emit a pulse of ultrasound from their mouth or nose. Each species uses a different set of frequencies depending on their environment and the target. By listening for changes in the echoes, bats can distinguish targets less than half a millimeter apart and even tell whether their prey is flying and in which direction. Bats accomplish these amazing feats using specialized structures in their inner ear and auditory cortex. Unlike bats, echolocating whales don't produce sound in their larynx or emit with from their mouth. Instead, they make sound by pushing air through their nasal cavity, creating vibrations in a special fatty organ called the 'melon'. The melon focuses and modulates the sound, which then passes out through the head into the water around the whale. Whales hear the reflections of these sounds through fatty structures in their lower jaw and around the ear, which pick up the echoes and pass them along to the middle ear. (Researching whales is a tough undertaking; Alexis Rudd has written an excellent explanation of some of the techniques scientists use.)

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Although the anatomical differences in echolocation in bats and whales are strong evidence of independent evolution, recent research has found that several genes involved in hearing are nearly identical in these two groups. In 2010, Ying Li, Zhen Liu, Peng Shi, and Jianzhi Zhang looked at the DNA sequence of the gene Prestin in 25 different animal species, including a dolphin species, echolocating and non-echolocating bats, and animals like pigs, cows and humans. Prestin is involved in the motility of the outer hair cells in the ear, so changes in this gene are likely to be important in hearing the ultrasonic frequencies used in echolocation. To their surprise, the team found that the gene from dolphins clustered together with the version from echolocating bats rather than with cows and pigs, which are more closely related to dolphins. Later that year, Yang Liu, Stephen Rossiter, Xiuqun Han, James Cotton and Shuyi Zhang confirmed that the same was true in other dolphin species, as well as in beaked whales and sperm whales. Last year, another team extended these results to include other genes involved in hearing. Yong-Yi Shen, Lu Lian, Gui-Shen Li, Robert Murphy and Ya-Ping Zhang studied two genes involved in hair bundle motility and another gene thought to play a role in transmitting signals from the ear to the brain. Although the DNA sequence of these genes differed between bats and toothed whales, when the team looked at the protein made by each gene they again saw bats and dolphins grouping together. DNA encodes the information to create proteins which are the molecular machinery that actually do work in a cell; because the DNA code has in-built redundancy, it's possible for groups to evolve different DNA sequences to create the same protein and accomplish the same task.



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The presence of similar characteristics in different groups is sometimes evidence of an evolutionary relationship. Some examples are similar limb structures used as evidence for relationships between various tetrapods or the presence of mammary glands uniting mammals into a single group. However, these hearing-related genes aren't similar because they share an evolutionary origin; there's lots of good evidence that bats and toothed whales are only distantly related. Instead, these genes have grown to resemble each other due to convergent evolution, a process where distantly related creatures evolve very similar solutions to a problem they face. In this case, it's remarkable that the similarities are solely at the level of genes and proteins while the anatomical mechanism of echolocation is quite different. To me, it's a wonderful example of the ubiquitous interactions of the many levels of biology, from molecular genetics to ecology, and of evolution's role in binding and shaping them.

Further reading
Li, Y, Liu, Z, Shi, P, and Zhang, J (2010) The hearing gene Prestin unites echolocating bats and whales. Current Biology 20(2):R55-R56. doi:10.1016/j.cub.2009.11.042
Liu, Y, Rossiter, SJ, Han, X, Cotton, JA, and Zhang, S. (2010) Cetaceans on a molecular fast track to ultrasonic hearing. Current Biology 20(20):1834-9. doi: 10.1016/j.cub.2010.09.008
Shen, Y-Y, Liang, L, Li, G-S, Murphy, RW, and Zhang, Y-P (2012) Parallel Evolution of Auditory Genes for Echolocation in Bats and Toothed Whales. PLoS Genetics 8(6): e1002788. doi:10.1371/journal.pgen.1002788

Image credits
The sperm whale diagram is by user Kurzon on Wikimedia Commons. The phylogenies are from Shen et al (2012).