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  Figure 2. Biological evolution is not like designing the innovative, first full-flying monohull sailing boat (A). It often uses unrelated components and thus more resembles a boat construction using re- cycled materials (B). (A) is courtesy of and with permission from ©SEAir and (B) is courtesy of and with permission from Rame Pen- insula Beach Care.
For mechanoreceptors such as touch, pressure, and acoustic receptors, by contrast, the physical properties of numerous structures play major roles in capturing, transmitting, and receiving the stimulus. An examination of the complex func- tion of the ears shows that it is their physical properties that dominate signal processing, from sound capture to sensory transduction. The specific structure of different ears is, how- ever, by no means determined by physical laws; as we show here, it is mainly powerful biological processes that govern the evolution of the vertebrate ear. A brief description of the general anatomy and physiology of the ear is given by Bruce (2017; see Figure 1).
In view of the variety of the structures in ears, it might be expected that the particular constellation in various animals and the way they have evolved might be strongly, and logi-
cally, traceable to the laws of physics that govern how struc- tures capture, transmit, and respond to sounds of different frequencies and amplitudes. Indeed, this expectation has led some authors (e.g., Lorimer et al., 2015) to propose that the physics governing the responses to sound have dominated the evolution of the structure of inner ears of mammals. In this article, we intend to show that this is a misleading concept. Although it is a given that the responses of mecha- nosensory organs will depend on the physical properties of their components, the mechanisms of evolution are different from the deterministic nature of physics. There is one ex- tremely important aspect of the laws of biological evolution that can lead to evolutionary trends that a physicist might find counterintuitive; this aspect is known as historical con- tingency.
The Concept of Historical
Contingency Illustrated
The idea of historical contingency can be illustrated using a hypothetical situation. Imagine that a physicist in a modern laboratory is asked to build a small, seaworthy boat. Numer- ous erudite works and plans might be consulted, and a boat that is both functional and appealing might be designed “from scratch,” using new materials and working with the best machines (Figure 2A). On the other hand, there is an engineer stranded on a remote island who needs to put to- gether an adequate flotation device using only components left scattered over the island by previous inhabitants (Figure 2B).
The way biological evolution works is very much akin not to the first situation but to the second (Futuyma, 2008). The evolution of any “new” structure in the biological world al- most always proceeds from preexisting structures that may, indeed, originally have had a completely different function. They may only poorly correspond to what anyone may in- tuitively conceive of, but the changes that happen in small evolutionary steps, driven by mutations and random mix- ture of genomes, are adequate to provide a small advantage over previous constellations and thus provide the owner of the new structure with a reproductive advantage.
Thus, evolution is a creative and yet highly conservative phenomenon whose trajectories are greatly influenced by evolutionary coincidences rather than by any deterministic dominance of physics. As an example, chance rather than the principles of physical design determined that the mam- malian middle ear has three ossicles rather than one (Figure
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