... but I've never completely bought the explanations of why the evolutionary process has seemed so erratic. From a strictly common sense point of view it would seem to me that evolution would occur gradually and evenly throughout time if it was stictly a natural process with no external intervention.
Why?
It's a matter of random mutations that occur unpredictably, and
It's a matter of natural selection of the mutations that happen to be beneficial over those that are not, what those selection systems involve are not constant either, as it can swing from one extreme environment to another for the subject species in the subject areas
With all the makings of a fractal\chaos system, with no steady state and no mean state. Is there any mechanism to drive it towards a steady state?
And then there is sexual selection - see R.A.Fisher and Feedback Sexual Selection for a system that can turbo-charge genetic change in a species.
The indications are that sexual selection played a role in the selection of increased creativity in humans, thus selecting for better brains -- from wikipedia
Sexual selection - Wikipedia
... Some of these traits also represent energetically costly investments for the animals that bear them. Because traits held to be due to sexual selection often conflict with the survival fitness of the individual, the question then arises as to why, in nature, in which survival of the fittest is considered the rule of thumb, such apparent liabilities are allowed to persist.
An often-cited theory, published by R.A. Fisher in 1930, that attempts to resolve the paradox, posits that such traits are the results of explosive positive feedback loops that have as their starting points particular sexual preferences for features that confer a survival advantage and thus "become established in the species." Fisher argued that such features advance in the direction of the preference even beyond the optimal level for survival, until the selection pressure of female choice is precisely counterbalanced by the resultant disadvantage for survival.
Certainly the brain size is at the limit for survival of the mother and child during the birth process, and thus an overly large brain qualifies as being "beyond the optimal level for survival"
That is what I think this data shows as well -- areas subject to intense selection rather than normal survival selection levels.
Enjoy.
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