Sensory Drive and the Predictability of Natural Selection

I have been getting into a number of non-sense argumentative spats with people who otherwise will never change their mind about the process of natural selection, and I thought I might try and clear things up in long-form for my followers who might be confused.

One of the main criticisms of natural selection is that despite numerous observations that certain traits benefiting the fitness of individuals move into fixation in populations, it is impossible to predict which traits will do so. This is a main criticism by strict neutralists who usually respond to biologists’ real world examples of natural selection beautifully predicting functional traits by moving the goalpost deep into epistemological grounds. For this blogpost, I’m just going to run through three examples of natural selection being both predictable and real, and then after that I will permanently stop engaging with these actors online.

Case 1: Wallace’s Moth

Xanthopan morgan
Xanthopan morganii from Africa

In 1862, Charles Darwin described a species of orchid named Angraecum sesquipedale originating from Madagascar which contained a nectary so deep that Darwin could not imagine an insect which would be able to probe it. He predicted that a moth with a proboscis long enough must exist to take advantage of the orchid’s nectar. Alfred Russel Wallace, reading Darwin’s remarks, noted that such a species of moth, Xanthopan morganii, existed in mainland Africa, and stated, “That such a moth exists in Madagascar may be safely predicted; and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune,–and they will be equally successful!” It wasn’t less than forty years later that such a moth, now named Xanthopan morganii praedicta (roughly in Latin: Morgan’s above mentioned all yellow moth) was found. This is one of the earliest and most extreme examples of natural selection’s predictability, but is the most illustrative and textbook of them all. Other predictions for natural selection often come in more subtle forms.

Case 2: Sensory Drive

In 1992, professor John Endler of the University of California Santa Barbara published a paper titled, “Signals, Signal Conditions, and the Direction of Evolution,”1 outlining a new theory which has governed the field of sensory ecology ever since. His theory, now titled sensory drive, argued that due to environmental constraints pertaining to visual cues, acoustic impedance, and other natural components of the environment, animal signals should develop in a predictable manner giving biologists the ability to predict which signals might arise in a given microhabitat.

Huia-cavitympanum.jpg

Thus far, sensory drive has been met with great success, with about 104 or more studies in the literature correctly predicting sensory drive in at least the visual domain of sensory ecology2. This may be due to selections of the studies sampled in the meta-analysis I am referring to, but sensory drive has also been found in auditory domains. One such example comes from the concave-eared torrent frog (Odorrana tormota) from eastern China.3 This species of frogs, which lives in the Huangshan Mountain landscape around rapidly moving brooks, is marked by its high-frequency calls and hearing which can reach up to 128kHz (the highest frequency of human hearing is around 20kHz, and speech is generally below 5kHz). Why would such an extreme form of hearing have evolved in this species? Due to the high-incline of the brooks in its microhabitat, their acoustic landscape is constantly inundated with low-frequency background noise, and the high frequency calls are thought to be an acoustic channel outside of the range of these environmental sounds. Likewise, another species of frog from Borneo, the hole-in-the-head frog (Huia cavitympanum), exhibits similar adaptations for communicating around waterfalls.4

Case 3: Ongoing Work in Science Labs

Finally, we know natural selection happens and is predictable because scientists around the world are testing selection versus drift hypotheses in their labs every day. Labs are often used to test antibacterial resistance to a number of different antibiotics, sodium levels, and all other sorts of arbitrary environmental factors scientists throw at them. These experiments are so prevalent, you can even watch videos of them in labs.5

Conclusion

People are always going to be moving the goalposts for this sort of debate, and that’s fine. Like creationists, there’s no way we are going to convince them. But to say that natural selection isn’t predictable is not only wrong, it’s outright lying given the evidence we’ve accumulated all over the world and the frameworks we use for testing our hypotheses. Whether natural selection is a more important component than say, genetic drift, is a different debate, but I think such predictability exists for either of them. Arguing otherwise is, in any form, intellectual dishonesty.

References

1Endler, J.A., 1992. Signals, signal conditions, and the direction of evolution. The American Naturalist, 139, pp.S125-S153.

2Cummings, M.E. and Endler, J.A., 2018. 25 Years of Sensory Drive: the evidence and its watery bias. Current Zoology.

3Liu, J., Yang, H., Hu, G.L., Li, S., Xu, Z.M., Qi, Z. and Shen, J.X., 2015. Little effect of natural noise on high-frequency hearing in frogs, Odorrana tormota. Journal of Comparative Physiology A, 201(10), pp.1029-1034.

4Arch, V.S., Grafe, T.U. and Narins, P.M., 2008. Ultrasonic signalling by a Bornean frog. Biology Letters, 4(1), pp.19-22.

5Baym, M., Lieberman, T.D., Kelsic, E.D., Chait, R., Gross, R., Yelin, I. and Kishony, R., 2016. Spatiotemporal microbial evolution on antibiotic landscapes. Science, 353(6304), pp.1147-1151.

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