Question..

in article [email hidden], Dohpaz at
[email hidden] wrote on 9/12/03 8:35 AM:

Quoted message said:

mate with? At some point, a nearly-human ape must have given birth to a nearly-ape human. This
child would be the only one of its speices, so how could that species survive?

This question has an important false premise. Speciation does not generally

Speciation is generally a population level process that takes time to unfold. Subpopulations develop
that are initially recognizably distinct, but clearly represent alternative forms of a single
species. Individuals in these subpopulations can interbreed, and many cases do exhibit interbreeding
at this stage. The process of speciation then proceeds to the point where these subpopulations
become more different and interbreeding is diminished. Conventional wisdom is that geographic
separation of the subpopulations is most often responsible for diminishing or eliminating contact
between individuals from the two subpopulations. At some point, usually a somewhat arbitrary point,
biologists accept that the degree of difference and the lack of interbreeding marks the
subpopulations as different species. This is not quite as arbitrary as it might sound, because the
process often results in two populations that cannot produce viable hybrid offspring should
individuals of different types try to mate.

Guy

"Dohpaz" <[email hidden]> wrote in message

Quoted message said:

Quoted message said:

One mechanism is "allopatric speciation", which is a fancy way of saying that two populations become
divided by a physical barrier, maybe a stretch of water or a desert, and then evolve differently.

Quoted message said:

Who would a mutant mate with? At some point, a nearly-human ape must have given birth to a nearly-
ape human. This child would be the only one of its speices, so how could that species survive?

Ignoring the special case of humans (Chomsky, a famous linguist, believes that language must have
arisen "at a stroke", religious people believe in a soul which distinguishes man from beast,
consciousness and free will may be restricted to humans, and may be all or nothing phenomena), there
is no one point at which a species becomes another.

For instance, big dogs and small dogs cannot mate with each other. If we were to eliminate all dogs
except for St Bernards and miniature Yorkshire terriers, we would have effectively two species.
However as it is there is a

compatible with the ones nearest in the chain.

Quoted message said:

At some point, a nearly-human ape must have given birth to a nearly-ape human.

Your basic premise is based on ignorance of evolution not to mention biology. The dividing line
between more human apes and less human apes would not have occured in one generation but would have
been a seemless transition over (at least) tens of thousands of years.

Thanks, that helps a lot. However, if it is true that
1) Animals with different numbers of chromosomes cannot interbreed and
2) All animals are decended from a common anscestor and
3) Not all animals have the same number of chromosomes.

Then there must have been at least one incident in which an animal (or animals) were born with a
different number of chromosomes than the rest of their species. Would these animals be forced to
mate with their equivalently mutated brothers and sisters? Or was one of my premises wrong?

"Guy Hoelzer" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

in article [email hidden], Dohpaz at [email hidden] wrote on 9/12/03
8:35 AM:

mutant

Quoted message said:

Quoted message said:

mate with? At some point, a nearly-human ape must have given birth to a nearly-ape human. This
child would be the only one of its speices, so

how

Quoted message said:

Quoted message said:

could that species survive?

This question has an important false premise. Speciation does not

generally

Quoted message said:

Speciation is generally a population level process that takes time to unfold. Subpopulations
develop that are initially recognizably distinct, but clearly represent alternative forms of a
single species. Individuals

in

Quoted message said:

these subpopulations can interbreed, and many cases do exhibit

interbreeding

Quoted message said:

at this stage. The process of speciation then proceeds to the point where these subpopulations
become more different and interbreeding is

diminished.

Quoted message said:

Conventional wisdom is that geographic separation of the subpopulations is most often responsible
for diminishing or eliminating contact between individuals from the two subpopulations. At some
point, usually a

somewhat

Quoted message said:

arbitrary point, biologists accept that the degree of difference and the lack of interbreeding
marks the subpopulations as different species. This is not quite as arbitrary as it might sound,
because the process often results in two populations that cannot produce viable hybrid offspring
should individuals of different types try to mate.

Guy

"TWINBLUE" <[email hidden]> wrote in message

Quoted message said:

Quoted message said:

At some point, a nearly-human ape must have given birth to a nearly-ape human.

Your basic premise is based on ignorance of evolution not to mention biology. The dividing line
between more human apes and less human apes would not have occured in one generation but would
have been a seemless transition over (at least) tens of thousands of years.

Though we have one fewer pair of chromosomes than a chimpanzee, and that change must have been a
saltation (sudden jump).

in article [email hidden], Dohpaz at
[email hidden] wrote on 9/15/03 7:11 AM:

Quoted message said:

Thanks, that helps a lot. However, if it is true that
1) Animals with different numbers of chromosomes cannot interbreed

This is not always the case. It depends on the specific ways in which the karyotypes differ.
Chromosomal structure differences that interfere with the mechanisms of mitosis and/or meiosis won't
allow for a functional or fertile hybrid. However, there are many ways that chromosomal structures
can differ that have little if any deleterious effects in hybrid offspring.

Quoted message said:

2) All animals are decended from a common anscestor and
3) Not all animals have the same number of chromosomes.

Then there must have been at least one incident in which an animal (or animals) were born with a
different number of chromosomes than the rest of their species. Would these animals be forced to
mate with their equivalently mutated brothers and sisters? Or was one of my premises wrong?

Your first premise was wrong. There are two important things to consider. First, there is frequently
polymorphism within populations in terms of chromosomal structures. Offspring that are heterozygous
for these structures may suffer a reduction in function or fertility, but not necessarily a total
loss. Therefore, such polymorphisms do not necessarily result in speciation. They may however be
important in setting the stage for speciation, which leads me to my second point. I would ask you
again to think at the population level, rather than the individual mutation level. As populations
evolve polymorphism churns through the constant input of new forms and loss of old ones. Therefore,
chromosomal structure evolution within a population can change in a cumulative way. Individuals from
two populations may have chromosomal differences that remain largely compatible at one stage of the
speciation process, but after further changes begin to pile up independently in the two populations
successful hybridization can become far less likely.

It is also possible that a chromosomal level mutation might be highly disadvantageous in a large
population, and selection would effectively purge the population of this change. On the other hand,
if such a mutation happened in a small population, such as a potentially isolated subpopulation at
the edge of a species range, then it could drift to fixation in that local group. Such mutations are
said to be positively frequency dependent, because they are only disadvantages when they are rare.
If they achieve a high frequency, then the ancestral condition is the disadvantageous one. The
peripheral population would have to make it through a period of producing offspring with low
viability and/or fertility, but if it is lucky enough to avoid extinction the outcome would be a
significant divergence from the parental population. Such a scenario could potentially lead to rapid
speciation in a peripheral isolate, because the chromosomal structure common in that population can
quickly become somewhat incompatible with the parental population. Now we would expect reinforcing
selection to favor mating only with compatible types (those from one's own source population), and
speciation can follow.

Cheers,

Guy

"Dohpaz" <[email hidden]> wrote in message

Quoted message said:

Thanks, that helps a lot. However, if it is true that
1) Animals with different numbers of chromosomes cannot interbreed and
2) All animals are decended from a common anscestor and
3) Not all animals have the same number of chromosomes.

Then there must have been at least one incident in which an animal (or animals) were born with a
different number of chromosomes than the rest of their species. Would these animals be forced to
mate with their equivalently mutated brothers and sisters? Or was one of my premises wrong?

Let's look at Down's patients. Down's syndrome is caused by an extra copy of chromosome 21.
Generally this is because the mother is old, and her egg isn't dividing properly. However there are
some instances of Down's running in families. Downs patients have many phenotypic differences from
normal humans. In some situations these might be an advantage - for instance an evil empire might
prefer Downs patients as slaves, and kill off the normals.

Down's women are generally fertile and have a 50% chance of having a Down's baby. Down's men are
generally infertile, but there has been one reported case of a pregnancy (Sheridan et al, 1989).

This gives us a clue. Imagine that the evil empire creates a situation in which it is adaptively
advantageous to have Down's. For a long time you would have a large number of Down's babies born to
Down's women by normal men. Then a fertile Down's male arises, breeding true for Downs, and the
genotype spreads rapidly through the population.

Eventually the slaves would be all Downs, whilst people not living under the evil empire would
be normals.

Finally, what would happen would be that the extra copy of chromosome 21 would specialise. There's
not much point having three copies of one gene, so the extra copy would be free to mutate and
perform different functions.

Thus you would get a change in chromosome number, without the need for a male and a female to
simulataneously mutate, find each other, and have offspring.

Sheridan, R., Lierena, J., Natkins,S. & Debenham, P. (1989). Fertility in a male with trisomy
21.Journal of Medical Genetics, 26, 294-298.