Red, theory; black, fact.
6-01-2018; Since about 1950, the world population has been increasing along a remarkably steady ramp function with no slackening in the rate of increase yet apparent, although one cycle of oscillation in the slope occurred during the Sixties. Malthusian reasoning predicts an exponential increase, which this is not. From several lines of evidence, I keep coming back to the idea that humans must have a subconscious population controller in their heads, and yet such a controller would have leveled out the increase by now. Until now, no theory has sufficed to explain the facts.
I here propose that the natural population curve for humans in good times is a saw-tooth waveform, with population ramps alternating with political convulsions that result in a large group being expelled permanently, resulting in the precipitous but limited drop in local population density that ends the saw-tooth cycle. This cycle accomplishes the ecological dispersal function to which I allude many times in these pages. The population must ramp up for a time to sustainably create the numbers needed for the expulsions. The WHO population curve shows only a ramp because it is a worldwide figure and therefore population losses in expelling regions are balanced by population increases in welcoming regions. This also implies that human population has been increasing in a way unrestrained by food or resource availability or any other external constraint since 1950, to now.
Clearly, human population is being controlled by instinctive factors, but not to a constant absolute density, but rather to a constant rate of increase. Population density would go up along the much faster, steeper, and more disastrous exponential curve of Malthus if there were actually no controller.
My formal training in engineering and neuroscience justifies a bit of speculation as to mechanisms at this point. Look first for such a controller in the hypothalamus, already known to control other variables, such as temperature, by feedback principles.
In school, I was taught that nature does not reinvent the wheel, which I understand to mean that once a brain structure evolves to serve a particular computational function, it will be tapped for all future needs for such a calculation. This process may make it grow larger or develop sub-nuclei, but additional, independent nuclei for the same computation will never evolve.
I will continue to assume that the controller is a conventional PID controller, as in previous posts. To make it control rate of increase rather than absolute population density, you put a differentiator in the feedback pathway. Look first in the amygdala for such a differentiator. If you are of the opinion that human population control is urgent, then you must knock out this differentiator and replace it with a simple feed-through connection. Fortunately, one common way for evolution to implement differentiation in mammals is to begin with such a feed-through connection and supplement it with an inhibitory, slow, parallel feed-forward connection. If this is the case here, then you just inhibit the feed-forward pathway pharmacologically as specifically as may be, and the job is done. Subjectively, the effect of such a drug would be to take away people's ability to get used to higher population density in deciding how many children to have. An increased propensity to riot should not occur.
I assumed in the last post that the political convulsions that produce dispersal are triggered by the value on the integrator of the PID controller rising above a threshold. However, in the above design solution, the convulsion would be triggered by the raw, undifferentiated population-density signal rising above some threshold. Look in the amygdala for this signal as well. Consistent with this, bilateral removal of the amygdalae and hippocampi in monkeys is known to have a profound taming effect accompanied by hypersexuality, known as the Kluver-Bucy syndrome.
6-17-2018: To be consistent, I would have to say that the differentiator for the population signal is more likely to be in the hippocampal formation by the argument of nature not reinventing the wheel, because in an earlier post, I interpreted the hippocampus as the site of four successive differentiations that carry out a Fourier transform by mapping sinusiodal waves back onto themselves at a particular best frequency, in the presence of a map of such best frequencies.
However, this setup would require the creation of two neuron-to-neuron connections for its evolution; a first connection to send the amygdalar raw population signal to the hippocampus, and a second to send the differentiated result back for further processing. At best, this would require two simultaneous mutations. Either change by itself would be at best useless and could never be selected. This appears to be another example of irreducible complexity requiring the bi-mutation mechanism described in the previous post.
The mechanisms usually offered to explain cases of apparent irreducible complexity, such as spandrelling, exaptation, and scaffolding, all appear to lack time efficiency and processiveness. I previously said that in evolution there are no (absolute) deadlines, but relative deadlines can easily be created by an interaction of processes. In the presence of relative deadlines, such as adaptive footraces to be the first clade to exploit a newly-habitable area or a new niche, time is of the essence and selection for speed and evolvability can be expected. Such selection will create mechanisms such as crossing over that enhance evolvability.
