Roles of Natural Selection and Cognition in Evolution and Population Size?

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The current thread, Does neutral evolution explain the genetic differences between humans and chimpanzees, is limited to scientists, and I do not wish to go off on a tangent there. One of the concepts which has been expressed there, however, seems to me very naive.

Clearly, humans could and do live in the same environment that chimpanzees do and achieve far larger populations. In fact, one of the reason that chimpanzees might go extinct is human encroachment on the chimpanzee environment. That is classic Darwinian competition. So, how many beneficial mutations do you think it required for humans to get in this evolutionary position?

…and…

You’ve claimed that you can account for most of the genetic differences between humans and chimpanzees using neutral evolution but there are now over 7 billion humans and only 300,000 chimpanzees.

There have been a number of effective replies in that thread challenging and qualifying the idea that population size correlates to fitness, but I think the concept is worth exploring a bit further.

Natural selection and ecological limits on population dynamics in the wild may be analyzed in terms of the adaptation of an organism to its environment. I do not see how such constraints apply to humans, because of one glaringly obvious distinction between humans and the rest of the animal kingdom. Whatever the proportion of neutral and beneficial mutation led to the threshold of abstract thought and verbal communication, once these were in place, the limitations of natural selection had to be modified as applied to humans; this for the fundamental reason that we are able to live under circumstances and in places for which we are not physiologically equipped.

No other creature can claim this. Too cold, too hot, too dry, too wet, these we can approach as problems to be solved. We do not need to grow winter hair, we can wear the skin of other beasts. We can build shelters and change our environment like beavers, but unlike beavers we are not a one trick wonder; as required we can build shelters of snow and hide and eventually concrete and glass. Despite our physiology, we can go from the bottom of the Mariana Trench to the moon.

This disconnect between physiology and environment is something new. It is emergent, in that the phenomena is coherent with the fundamental laws which gave rise, but the subsequent behavior cannot be fully explained by reduction only to those fundamental laws. Whatever its initial state, it takes on a life of its own from there. The ability for human cognition to overcome physiological limitations and coordinate activity of city state populations represents a basic discontinuity in nature. If there was a correlation between fitness and population prior, that must be reexamined.

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Sure they can. Many creatures modify their environments to better fit their needs. Beavers are a famous example. Of course we don’t live in places for which we aren’t physiologically equipped; we change those environments, sometimes very locally, sometimes limited to immediately around our bodies, as when we put on coats in winter. Similarly, termites create mounds engineered to cool their immediate environments. It’s all a matter of degree, not kind.

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This is true, but I don’t think it invalidates the point Ron is making (carried over from another thread). Intelligence brings about a change in state, or a phase shift, that changes the rules for survival. There are other social animals, but humans have raised the game to where we are primarily competing with other humans. Many environments constraints no longer apply.

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I trimmed your post title down so more key words will be visible.

This is a tough question, and I’ve seen it asked in different ways. Where do we evolve from here now that we can change the rules of the game? Social dynamics are very hard to predict - selection still applies but it is not clear what constitutes a selective advantage. If we decide to modify our own genome, then all bets are off. This also becomes an ethical quagmire.

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Quantity has a quality all its own, you’re saying; still a difference in degree, not kind.

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You are not wrong either. With respect to humans quantity doesn’t mean what it used to. By sheer numbers we are outcompeting most other animal in any environment we occupy. Our main competition now is other humans and limited resources.

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Even in those places where we are physiologically equipped, our population sizes are fairly small without agriculture. And they remain small, regardless of how much artificial help to our physiology we provide, in those areas where we can’t sustain agriculture. So I would say that it is agriculture, not our artificial adaptions to harsh environments, that makes the difference.

And to @John_Harshman’s point: Agriculture is not unique to humans either. So still a matter of degree, not kind.

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Certainly, many other creatures modify their environments. As I mentioned in the OP, beavers are an example, in that they profoundly impact their territory. But as is typical of such animal behaviors, they are still confined to ranges limited by wider environmental constraints. I’ve helicoptered over hundreds of beaver ponds in northern Canada, and those boreal plains are their happy place. I have no idea what goes on between the ears of a beaver, but the human ability to innovate as required I take to be a difference in kind.

What would you do if you were Robinson Crusoe’d up on some desert island? I expect there would be an inner conversation involving threat assessment, inventory of potential resources, prioritizing of objectives, and execution of a plan to provide for as much security and comfort as possible. It is that conscious deliberation which animals do not possess in any degree that involves language and abstraction, but is universal in humans.

When I said that “we are able to live under circumstances and in places for which we are not physiologically equipped”, perhaps it would have been clearer if I appended “absent the problem solving skills distinct to humans.” That is really the point. It is our ability to consciously address environmental challenges which permits people to migrate globally. We don’t just run around naked and await cold resistant proteins. Adam and Eve were provided with a kick start of animal skins.

Therefore, a population model which is parameterized by environmental constraint in a narrowly defined ecological niche must be reexamined as applied to humans. The genetic developments which might have had an immediate effect of simply being more competitive within the initial range also allowed us to escape. So contrary to @kleinman’s contention, whatever the validity or relation there may have been initially between population and number of mutations, that would distort the correlation in respect to the anthropogene. More broadly, it raises consideration as to how human evolution may be distinct due to our consciousness, and how to incorporate that into our models.

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Both are important. Agriculture vastly increases the population density we can achieve, while our adeptness at modifying our environment is critical to our (quite large) range expansion.

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While I agree, consider what the population sizes of these expanded ranges would be without agriculture. Obviously a larger range can support more individuals, so this range expansion would allow for a larger population size, but how large? I think that range expansion is a multiplier, but agriculture is exponential.

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Human colonization of the planet preceded appreciable agriculture, but I would agree it is indisputable that agriculture made possible order of magnitude population growth from early on. In modern times yields dramatically increased and starvation averted by mechanization and plant breeding practices as pioneered by Norman Borlaug.

I would note that physiological independence from environment, and agriculture, have the same wellspring in that human intellect makes possible purposeful control over environment. As environmentalists point out, a tree farm is not a forest. For better or worse, we are now far from passively rooting about hunting and gathering whatever bounty nature serves up. Our farming practices have transformed the face of the earth to facilitate the feeding of our families. Agriculture has raised the carrying capacity of the earth to allow for dramatic increase in population. So this is really an extension of the premise that human intellect introduces a qualitative factor not present in models of animal variation and fitness vs. population. This is because fitness involves the success of an organism in its environment, but we are no longer passive before our environment, we act upon it far more extensively than any other creature.

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The historic and potential output of the North American prairie and Brazillian pampas farmlands are quite impressive. The estimated pre-contact population of the Americas was around 100 million which is dwarfed by current populations on those same continents and the agricultural output that feeds many people outside of the Americas. To be fair, indigenous populations did experiment with farming, but nothing compared to modern techniques. A notable addition to mechanization and breeding practices is the Haber process that made large scale fertilizer production possible.

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They did more than ‘experiment’.

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“Experiment” seems a very pale term for 12,000 years of intensive agriculture.

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That’s a fair criticism.

I’ve always pictured agriculture in the pre-contact Americas to be sporadic or episodic, which could be completely wrong. The same could probably be said for the early Western civilizations, with only the Nile River communities possibly being the only exceptions. It’s been 25 years since my Western Civ classes, and I never took any courses on American indigenous civilizations, so it may be time to educate and reeducate myself.

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‘Completely wrong’ isn’t the half of it. Farming practices in the Pre-Columbian Americas were at least as efficient as those in contemporary Europe, and likely more so (accounts are somewhat sketchy, as you might imagine). By all evidence, the entire center of the North American continent was subject to widespread and sustained agriculture.

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With regard to “neutral” mutations, I’m not even sure what that means when a game-changing adaption like tools use comes along. The relative fitness of many genes may change because the environment suddenly offers opportunity that simply did not exist before.

Edit: I’m going to answer my own question and say that neutral changes are neutral at the time they arise, but subsequent variation may change the relative fitness.

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