5
$\begingroup$

Inspired by https://worldbuilding.stackexchange.com/questions/58533/is-intelligence-the-natural-product-of-evolution , I'm wondering if creatures have ever become less intelligent as a result of natural selection and evolution.

It seems plausible, because the brain can use a lot of energy, and I've heard of other aspects of an animal atrophying because they're not useful any more, such as vision in the Mexican blind cavefish.

One possible candidate I can think of is the koala: it's described as having a low brain to body mass ratio, doesn't have a great need for intelligence, and may have a need to minimise energy usage.

I'm mainly interested in cases of natural selection, as opposed to artificial breeding - I wouldn't be surprised if we selected animals to be less intelligent.

$\endgroup$
2
$\begingroup$

Domestication is often given as an example, though it seems to be debated.

I'm guessing almost all parasites have become less intelligent in their evolution to parasitism, you could probably find some slam-dunk examples there.

More specific but anecdotal and possibly opinion-based, I recall in the book "Adventure among the Ants" the author draws a line between the size of an ant society and how competent and intelligent its individual members are. He specifically talks about how ants evolved, how the species with the biggest colonies are comparatively recent, and how there is an inverse correlation between an individual ants' size, brain size, complexity of behavior etc and the size of the colony it's a part of.

In other words, the evolutionary history of ants involved the evolution of larger and larger colonies, and in the lineages where this happened, as the species evolved to exist in larger colonies, the individuals in that species evolved to become simpler and less intelligent.

You could plausibly find examples if you look for examples of organisms that evolved to be smaller - if only because that can involve smaller brain size.

$\endgroup$
2
$\begingroup$

Unfortunately, measuring intelligence (assuming we would have a strict definition) is not a thing that can be done from fossil record or genetic data. The only thing one can do is to compare brain mass (or at least size of the cranial cavity) from fossil record. In this regard, no need to go very far for an example, humans are an example. From wikipedia

The evolutionary history of the human brain shows primarily a gradually bigger brain relative to body size during the evolutionary path from early primates to hominids and finally to Homo sapiens. Human brain size has been trending upwards since 2 million years ago, with a 3 factor increase. Early australopithecine brains were little larger than chimpanzee brains. The increase has been seen as larger human brain volume as we progressed along the human timeline of evolution (see Homininae), starting from about 600 cm3 in Homo habilis up to 1500 cm3 in Homo sapiens neanderthalensis which is the hominid with the biggest brain size. The increase in brain size topped with neanderthals; since then the average brain size has been shrinking over the past 28,000 years. The male brain has decreased from 1,500 cm3 to 1,350 cm3 while the female brain has shrunk by the same relative proportion.

Note however, that wikipedia does not cite any reference and I failed to find one!

Note, also that one should not confound cranial cavity (the thing that is actually measured) with brain size and more importantly no confound brain size with intelligence. Wikipedia goes on saying

However it is argued that another essential element of brain evolution in humans is rearrangement (Hoffman et al. 2004). Larger brains require more wiring, but more wiring can become inefficient (Hofman 2001). The brain has therefore become reorganized for efficiency. Furthermore, the average body size of nethanderthals was larger which led to bigger brain size (see Brain-to-body mass ratio).

I could not find the Hofman papers but I probably just did not look hard enough (and am a bit tired right now)! One might want to read Healy and Rowe 2007 for a criticism of the inference that can be done from measures of cranial cavity.

$\endgroup$
1
$\begingroup$

Yes, there is one great example just off the top of my head,Tunicates which have evolved to lose their brain as they mature. The simple reason this happens is brains are expensive they cost calories and material that could be spent elsewhere so just like anything else under the right conditions (when they are not helpful (such as in sessile filter feeding sea squirts) a reduction and/or lose will be favored.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.