Originally posted by starblue:
[b]
when people say, "humans evolved from apes", what do you people
think about this?? if i were to tell you that this phrasing is
inherently flawed, what would you think about the evolution of
humans as a species?
b]
You haven't said why you think this statement is inherently
flawed.
indeed, for mutations to play a role in evolution, it is
imperative that "advantagous" mutations occur in genes that will be
passed down to the children, that is, the mutation must occur in an
individual's gametes (egg cell or sperm cell's genetic makeup).
considering the fact that the average male produces millions of
sperm cells a day (which means, a super duper high rate of
proliferation), do you think that mutations are still too rare to
occur in the gonads?
Oh no, there are plenty of mutations, but only a small number of
those may give a survival advantage. But it does not matter that
there are millions of spermatozoa in the testis, only one can
fertilize an egg. What are chances that a mutation that confers a
survival advantage is present in that particlar spermatozoa?
The human female releases only one egg each month, and once
pregnant, is rendered anovulatory until the cessation of lactation.
In a life time, how many offspring can she bear? Not many. If she
indeed gives birth to a child who carries a 'good' mutation, what
are the chances that she can give birth another child with this
survival advantage? Not likely....given that in her reproductive
life she is unlikely to bear more than 20 children.
The female is the limiting factor. The male can impregnante
numerous females almost throughout his life after puberty. The
number of offspring possible is limited by the finite number of
eggs his mate possesses.
Unlike spermatozoa which are produced continuously through life,
the eggs in a female are fixed at birth. If you get pregnant at 40,
your eggs are actually 40 years old and that's why pregnancy is
less likely with age. It is also true that these eggs acquire
mutations with increasing maternal age and that most of these
mutations are deleterious...accounting for a higher miscarriage
rate with increasing age of the mother.
If we add to that the cultural norms that limit a man to only one
mate and the effect of late marriages and a dropping fertility
rate, I think further evolution of man is going to be rather
slow.
There is another issue. For evolution to work, you need genetic
variation accorded by mutation etc, but you also need pressure for
natural selection. The white moths in England almost disappeared in
the Industrial Revolution because all the buildings were black with
soot, making it easy for the the white moths to be spotted and
eaten by the birds. That's a selection pressure. If a white moth
acquires a mutation that turns it darker and given the right
selection pressure, it is more likely to survive and passed on its
genes. Its white brothers diminish in number and become extinct if
the selection pressure is strong enough. This sort of incremental
changes build up over long periods of time and perhaps the changes
may become big enough to constitute a new species...even genus
maybe? That takes longer if the selection pressure is weak and if
the reproduction rate is low.
In human society, we artifically reduce the selection pressure. We
bring heating to the house, so those with thin fat layers survive.
We use arrows to shoot deer, so even the congenital dwarf is not
worse off than the fastest man. The 'weaker' genes get transmitted
as well. This is right and to be expected in a humane, civilized
society but without this selection pressure, why would there be
further evolution? There may be increased diversity, but without
natural selection, there's no evolution, is there?
Darwin thought that adaptation to a changing environment will lead
to natural selection and therefore persistence of successful
adaptive traits. I believe that happens. But not for man...cos'
instead of adapting to a changing environment, we have been
adapting our environment to suit us. I would have thought it will
be other species that are under tremendous selection pressure
imposed by us. Evolution, if it occurs, is going to happen in other
species much faster than it will in humans.
