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My ideas on hybridisation are rather unusual, I have to
admit, but I think they’re logical and make more sense than anything else
I’ve read so far.
Anyway, I'd like to invite comment from anyone who is interested in why
humans have 46 chromosomes whereas the other great apes have 48.
The ideas here are, I think, quite complex so this is a rather lengthy
piece...
The most popular scenario to explain chromosome number change in Hominoidae,
as I understand it, is based on a mutation in either sex (I've heard it
suggested that the female might be the more likely of the two, but I’m not
sure why that should be) that results in an n=23 haplotype gamete which then
fuses with a ‘normal’ hominoid (n=24) gamete of the other sex to make an F1
generation with 47 chromosomes. The idea is that this viable, mature, fit
individual then mates with other individuals with 2n = 48 chromosomes to
produce F2 with 50% chance of 2n = 48 and 50% with 2n = 47 and from this,
subsequent interbreeding would eventually produce a small population with 2n
= 46 chromosomes and from there... The rest is history.
My problem with this scenario is one, I suppose, of huge personal
incredulity. These odd (2n= 47) chromosome numbered individuals are somehow
considered to not only be viable at birth, but manage to survive to
adulthood and actually not only able to produce viable offspring but,
presumably, have sufficient sexual fitness that attracts ‘normal’ peers
(actually probably either a brother or sister) in the group. I know that
there are examples in nature where such odd chromosome numbers are indeed
viable but then again there are many examples in humans where even a single
deletion is fatal. Even when extra chromosomes are present (ie in trisomy)
it more often than not results in a foetus that is not even viable.
To make it work, it seems to require a very small population bottleneck to
justify that such rare individuals (although there must be at least four)
might actually interbreed – probably with siblings. This implies that both
the 2n=47 chromosome and, especially, the resulting (and much rarer, to
start with) 2n=46 chromosome individuals really must have some kind of
inherent ‘super-fitness’ from day one, otherwise how could they not only
survive but out-compete the parental (2n=48) population? However, if there
was a bottleneck then, presumably, the local population had recently already
gone through difficult times, and yet this novel, and seemingly death
defying, karyotype, manifest in just a handful of individuals, is proposed
to rescue the species from the edge of extinction.
I just don’t buy it.
My version of a hybridisation model is, I think, much more plausible.
Firstly, before going into it in detail, we need to remember that hybrid
zones exist everywhere and probably always have. All you need is two
populations separated by some relatively (in evolutionary terms) short-lived
geographical barrier, which is then removed again (as happens all the time
in coastal niches with sea levels rising and falling like clockwork) and you
have two groups of closely related species with a semi-permanent interface
between the two where repeated interbreeding events could happen again and
again over extended time periods. I think they provide a much healthier
environment for speciation events than does founder populations. Look at the
gibbons. There are many sympatric species, almost all of them with different
numbers of chromosomes. Clearly, very little geographic isolation has
occurred there. Hybrid zones provide a much better model |
Secondly, we should note that botanists have, for many
decades, used the fact of hybridisation in their models of speciation (see,
e.g. Grant (1971)) and that in fish species, hybrid speciation has
manifestly happenned many times. (see e.g. Hubbs (1955). Clearly, it is
rarer in mammals but it should be obvious from the above that it is
theoretically possible from a genetic point of view. All that is needed for
it to work in mammals is some way of overcoming mating barriers. In that
area primates are probably the strongest candidates, due to their sociality
and rather complex socio-sexual systems which tend to leave frustrated males
on the sidelines. It therefore seems perfectly plausible to postulate one
group of hominids on the edge of the range of another, where interbreeding
would actually become a fairly common occurrence.
What might result of such interbreeding events? Clearly it depends on the
length of time of the separation. If population a and b are separated for a
very brief amount of time (say 150ky) then, clearly, insufficient genetic
drift would have happenned to provide any barrier to viable offspring being
produced. Hence all human populations today can
successfully interbreed. If the length of time of separation is very long –
let’s say 13 My or so, then so much genetic drift will have occurred so as
to render any such F1 offspring unviable, even if no gross karyotypic change
had occurred. So, orangutans and gorillas (presumably) cannot produce viable
offspring even though they both have 48 chromosomes. My argument is that
there must be a point in time, intermediate between these extremes, where
interbreeding works, but only just. Sufficient genetic drift will causes the
mechanics of meiosis to be interfered with, but not fatally so. It is in
exactly this scenario that I suspect a hybridisation could produce an
instantaneous speciation event – where the mutation doesn’t happen in one
sex or another separately before the sex act, but actually during the fusion
of the gametes after copulation. So, to use my hypothetical ever-so-slightly
more aquatic hominids... if Homo maritimus (seaside man) male met Homo
fluvensis female (riverside woman), both with their ancestral hominoid 48
chromosomes and, after a brief sexual encounter, something went slightly
wrong with the meiotic fusion of the gametes, resulting in a telomeric
fusion of two chromosomes to form a new chromosome number 2 (as seems to
have happenned with H. sapiens) – this could result in a viable infant being
born with 46 chromosomes in one simple, if rather fortuitous, step.
See this link for more
detail on how this is proposed to have happenned.
Consider the improvements this model offers over the 47 chromosome
intermediate idea:
1) The infant is viable from day one because it hasn’t got an odd number of
chromosomes and because, it actually has very little difference in its
karyotype from its parents other than the fact that two chromosomes got
stuck together at the end of meiotic fusion. Odd numbers of chromosomes, in
contrast, would in all probability be fatal very early in utero.
2) Because this infant was born on the edge of two, probably large and
healthy populations, this kind of event would probably be very common. As
long as the populations had been separated for sufficient time for the
meiotic mechanism to be half-damaged, the scenario seems rather likely. Of
course it is a rather massive assumption in my model that such a separation
would lead to this kind of meiotic clanger happening on fusion, but I argue
that it is still rather more feasible than the alternative. |
Contrast this with the 47-er scenario: It assumes that a
small bottleneck population becomes isolated; That for some unspecified
reason a rare mutation occurs to fuse chromosomes in the gametes of one sex
or another; That the offspring is viable, lives to maturity and is fertile;
That at least four (assuming it is the F2 generation that finally have the
2n=46 condition) such individuals are born at the same time and place; That
the breeding of the two 2n=47 individuals produces a 2n=46-er that is not
only also viable and fertile but actually has relatively high selective
fitness compared to its parental 2n=48-er ancestors (as it is destined to
replace them) and that this happens quickly enough to rescue the bottleneck
population from the brink of extinction.
3) The nascent 2n=46-ers would inevitably accumulate, as such events would
be fairly common. It is likely that 2n=46-ers would be immediately
genetically isolated from their 2n=48-er parental groups allowing the new
karyotype to become quickly fixed in the population. 2n=46-ers could
therefore start interbreeding almost immediately, soon producing a high
degree of genetic variability from which selection would quickly select out
the best trait combinations. This is an ideal scenario for punctuated
evolution. It is well known that introgression is a force of increased
diversity in hybrids. (see, e.g. Rieseberg et al (1999) and so, placed in
such a context, a great deal of novel genetic material can be postulated to
have been formed from which rapid selection would take place and from which
saltatory changes in the human phenotype (e.g. encephalisation and language)
could feasibly emerge. The 2n=47 chromosome idea, proposes almost the
opposite scenario – very little genetic variability, very low fitness and no
great genetic isolation between the new 2n=46 chromosome group and its
parental populations.
I imagine that one objection to this hypothesis could be the same as my own:
personal incredulity. But is it really more likely that a mutation to fuse
two chromosomes should happen in one sex or another in isolation in the
early phase of meiosis rather than in a later stage during fusion, when the
two individuals have been separated for, say, 1.5 million years? I cannot
see how one could argue so. In addition, at least the hybridisation model
can argue that if sufficient interbreedings occur between two relatively
large and healthy populations living adjacent to each other then the chances
of this sort of scenario actually happening is multiplied greatly. By
positing it in a low population bottleneck seems to make its likelihood, in
comparison, vanishingly small.
So, I hope that clarifies my position.
The hybridisation idea is neither help nor hindrance to any form of AAH,
other than it might explain why humans have some anomalous ‘aquatic’ traits
which are rather odd. For example our tears are salty but nowhere near
sufficient to act as any kind of excretory function. We appear to have lost
our salt hunger but infants are highly sensitive to high levels of salt. It
seems to me that these anomalies might be explained better if one of our
ancestors was a more marine-coastal living hominid and another was more
fresh-water living. And, of course, estuaries make perfect hybrid zones.
References
Grant, Verne (1971). Plant Speciation. Columbia University Press (New
York)
Hubbs, C L (1955). Hybridization between fish species in nature.
Systematic Zoology 4 1-20
Rieseberg, Loren H; Wayne, Robert K; Archer, Margaret A (1999).
Transgressive segregation, adaptation and speciation. Heredity 83
:363-372 |