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almost exclusively on hearing to detect their quarry, is supe- rior to the sensitivity of most mammalian species (Manley, 1990). The extended frequency range of hearing in echolo- cating mammals is almost matched by frog species that use ultrasonic calls for communicating (Feng et al., 2006).
Figure 4. Tree of life of amniotes that includes reptiles, birds, and mammals. All these classes of vertebrates evolved from the same an- cestral form of the stem reptiles at the bottom of the tree.
Did Physics Direct the Evolution
of the Mammalian Cochlea?
The hearing organs of all animals perform very much the same task: that of detecting acoustic signals. However, when examined through the eye of a physicist, the adaptations that permit this may be seen as being suboptimal. For example, a physicist examining the mammalian cochlea as a fluid- filled tube in which mechanics are dominated by viscous damping might find this counterintuitive. Equally irrational is the concept of using a gel-like, energy-dissipating tecto- rial membrane that covers all the sensory cells (Figure 1) to transmit energy in the cochlea. From a physics perspec- tive, the mammalian cochlea may be misconstrued as being poorly “designed” for its purpose.
Despite this, the mammalian cochlea fulfills its role superbly and enables species survival. Observations of seemingly un- suitable design are therefore difficult to ascribe to an organ that has evolved over millions of years by natural selection and, obviously, like the middle ear, has been continuously
modified for its purpose. The hearing part of the ear evolved from a vestibular organ whose evolutionary origin (and es- pecially the sensory cells themselves) resembles that of the lateral line, a body surface organ used by fishes to detect fluid motion in water. The lateral line has sensory hair cells simi- lar to those of the cochlea whose responses are strongly in- fluenced by viscous damping and are connected by a gel-like structure; both features survived 500 million years of evolu- tion and are common to vertebrate inner ears. The problem of damping was solved very early by the evolution of active mechanisms of the sensory cells (Hudspeth, 2008; Brownell, 2017), and linking sensory cells via a gel-like structure in fact increases both their sensitivity and their frequency se- lectivity (Authier and Manley, 1995).
The problem is that “design” is often seen quite differently by a physicist and a biologist. The physicist may see the ear as a device whose construction has been based solely on physi- cal principles (Lorimer et al., 2015). A biologist would be acutely aware of how such devices evolved naturally through historical contingences (Manley, 2017) and avoid the notion of design. It is therefore of interest to highlight where these views of cochlear evolution oppose each other and consider some of the resulting questions to clarify the current un- derstanding of how hearing organs evolve. Does the con- straint of working within the same physical modality, such as acoustic pressure, inevitably lead to hearing organs that all have the same design? Does physics direct the evolution of hearing organ structure or are the principles of evolution different?
Genes, Evolutionary Compromise,
and the Idea of Perfection
Anyone who has watched a television series on animal life will have often heard that animals are “perfectly” adapted to something. The idea is that in some way, evolutionary pro- cesses will inevitably reach an ideal situation in which no improvement is necessary or even possible. This, however, is erroneous. In any species that is adapted to something, individual animals will always vary in their genetic makeup and abilities (one basis of evolution). In addition, the object of the adaptation, say a prey organism, is also itself continu- ously adapting, gaining some small advantage to keep one step ahead of the predator. Thus, adaptation is always an on-going process; perfection can never be reached. Bodily processes do not evolve in isolation from each other and are subject to sometimes competing selection pressures. Hence, evolution inevitably involves compromises.
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