Just thousands, to be precise the last two thousand.
We know for a cast iron fact that this is false. Human populations show the signs of natural selection for resistance to various diseases.
The population increase that humans went through in the last 2-3 thousand years has few parallels in nature, and there is plenty of evidence showing that selection does not operate in expanding populations, I can dig up refs if you want. But to put it simply, natural selection operates under the premise that some of your progeny will die (and with it presumably your "weak" genes), so in order for natural selection to operate, a certain percentage of the population has to perish. But in today's society this is not happening anymore, or in other words, both the fittest and the weakest are surviving. If both are surviving there is no selection.
I would be interested in your refs because the description you have given makes no sense. All selection needs to act is inheritable differences in relative fitness - a gene that produces more living offspring can be selected for in an expanding population as easily as one that produces a drop in living offspring in a reducing population. It would only make no difference if both the fittest and the weakest survive unless they both also go on to have exactly the same number of surviving offspring themselves - at which point they have the same fitness anyway.
It's also simply not true that everyone was surviving for last two thousand years, throughout most of that period the majority of people born never lived long enough to have children of their own.
quote:The population increase that humans went through in the last 2-3 thousand years has few parallels in nature, and there is plenty of evidence showing that selection does not operate in expanding populations, I can dig up refs if you want. But to put it simply, natural selection operates under the premise that some of your progeny will die (and with it presumably your "weak" genes), so in order for natural selection to operate, a certain percentage of the population has to perish. But in today's society this is not happening anymore, or in other words, both the fittest and the weakest are surviving. If both are surviving there is no selection.
As stated, this is incorrect. Natural selection works just fine in expanding populations, and does not require any deaths at all. Selection requires differential reproduction, not death, and differential reproduction can occur in an expanding population just as well as in any other population.
What is true is that fixation of beneficial alleles is less likely in a rapidly expanding population.(*) So if alleles for high intelligence were being selected for in the expanding human population, most new mutations conferring high IQ would not reach 100% in the population -- but the population would still get smarter, on average.
(*) Fixation is less likely, but certainly not impossible. Loss of the less fit allele does not require death; it just requires failure to reproduce enough copies to maintain the allele at its existing frequency. In the real world, many humans fail to reproduce at all, and some of them have even been known to die, despite our increase in population size.
I think Mr. Jack and sfs have raised the main counter points to your argument that I would have made, principally the fact that there is considerable evidence of selective pressures on human populations shaping their evolution in the last few centuries and that a large expanding population does not prevent evolution according to any current model of population genetics, although it will reduce the likelihood of fixation as sfs suggests.
I'd also like to point out that apart from the theoretical issues the more factual claims you make are also highly dubious. While we may be seeing a reduction in certain selective pressures in the west in 'today's society' these have not been the situation pertaining for the preceding 2000 years, perhaps a case could be made at the outside for a couple of centuries. Further these conditions do not pertain in a very large proportion of the modern world either where survival is still very much a competitive struggle.
Certainly the idea the 'the weakest are surviving' seems highly suspicious in evolutionary terms unless you are claiming that there is no longer any child mortality to go along with the equally specious necessary corollary that everyone can now reproduce.
Also your argument about Einstein and Hawking seems to be based on a very naive concept of what constitutes fitness. Your own argument in fact seems to directly admit that natural selection is still in operation but that what an evolutionary perspective on natural selection considers the most fit (the less intelligent person who gives rise to many more descendents) does not comport with your own idea of who should be considered fittest (Einstein and Hawking).
You seem to be saying in one sentence that there is no selection and then in the very next to be admitting that in fact natural selection is still in operation. To be honest your point is unclear, you seem to be saying that Einstein and Hawking are the result of a relaxation of selection and presumably therefore you consider them less fit and you go on to assure us that there are more fecund/reproductively successful people out there, the very core of the mechanism by which selection operates.
So what is it? Do you consider reproductive success a key element of fitness? Do you think there is no differential reproductive success in humans today or that if it exists it is independent of genetics?
Perhaps a more cogent example if you were putting forward Einstein and Hawking as less fit would have been to point out that both of them had several children. Of course I'm not sure what your rationale for considering them to be examples of those that selection would naturally weed out before reproduction is. Even in the case of Hawking his motor neurone disease didn't really strike until he was in his 20's, certainly old enough to have reproduced.
..... looked a lot of ape and human brains, compared them and found that there's actually not much difference in size between great ape and human brain frontal cortices
Which destroys a lot of previous wisdom and leaves me scratching my head.
The idea that humans have disproportionately larger prefrontal lobes (which is where our cognitive functioning occurs) seems to have been founded on a dodgy sample size ie it came from possibly one half-lobe sample.
It does go on to speculate about human brain specialisation:
There is already some evidence .........[to] suggest that the internal organization and size of individual cortical areas are specialized among the hominoids. In a previous study, we found that the relative volume of white matter underlying prefrontal association cortices is larger in humans than in great apes ... This is compatible with the idea that neural connectivity has increased in the human brain. More recently we have shown that orangutans have a smaller orbitofrontal sector than other apes or humans ... which suggests that some differences can be found in small subsectors of the frontal lobe at a gross level. Thus, it seems possible, and even likely, that either subsectors of the frontal lobes or individual frontal cortical areas have become specialized during hominoid and hominid evolution. Cognitive specialization in each hominoid species would be related to mosaic evolution and reorganization of specific areas in this and other parts of the brain.
We really do know next to nothing about our brains and what we do think we know seems tentative..
But thats basically what the question being asked is. Why do humans have the mental faculties we have. Scientifically we can only talk about the mind naturally. No spirit or anything outside of chemistry is allowed.
And the conspiracy was strong, for the people increased continually - 2 Samuel 15:12
Francis Crick says, "The Astonishing Hypothesis is that "You", your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules... The hypothesis is so alien to the ideas of most people alive today that it can truly be called astonishing."
Other scientists such as Richard Dawkins have said similar things. The question is, do these scientific facts apply to Crick and Dawkins aswell or just the members of the public?
And the conspiracy was strong, for the people increased continually - 2 Samuel 15:12