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Chromosome Numbers and Evolution
Name: Walter R.
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: August 2002
Question:
Something has always puzzled me about the theory of evolution. When
two individuals of a species reproduce the male and female each supply
50% of the genetic material which matches up in the egg. For years I've
also heard that earthworms, for example, have many more chromosomes
than humans, so presumably numbers of chromosomes doesn't relate to
genetic complexity.
So, how does a creature with a certain number of chromosomes create an
offspring with a different number of chromosomes (which evidently must
happen occasionally) which will still be viable with the parent
species? Don't ALL of the chromosomes have to pair up in the egg with
its counterpart from the other parent? Since the order and alignment
of the chromosomes matters wouldn't adding a new one anyplace in the
chain cause the whole chain to be misaligned and thus useless?
So unless mutations always occur in identical pairs how does evolution
produce a new species?
Replies:
Chromosome numbers are usually even, and you are right, if one organism
spontaneously mutates to having more chromosomes than its species, how can it
mate and pass that on? There are conditions in which an organism has an
extra chromosome or perhaps one too few. Most of the time these arrangements
are fatal and the organism fails to develop. But occasionally the extra or
deletions allow life to go on. Down Syndrome is one of these, the person has
3 #21 chromosomes. When that person makes eggs or sperm, half will have the
normal number and half will have the odd number. Remember that in
evolutionary theory, mutations can be harmful, and make the organism less
fit, or neutral, it does not have an effect at the time, or beneficial, it
allows the organism to do something it did not do before. Perhaps along the
way, having the extra chromosome provided some advantage or was not harmful
and was carried through. If it happened in more than one individual in the
population, and the two happened to mate, perhaps an offspring with a new
number of chromosomes could appear.
It appears also that organisms that have many chromosomes have small ones and
organisms that have few have larger ones, which implies that all the info is
there it is just divided up differently. If you compare chimp, gorilla and
human chromosomes, chimps and gorillas have 48 chromosomes and humans 46. It
appears that two chromosomes joined together in the human to become one, but
if you line up the chromosomes side by side, the banding patterns match up
quite well, with a some changes over time. So perhaps if the info was still
there in the offspring it was still able to function. The jury is still out
on this one and it is a perplexing and excellent question.
vanhoeck
The short answer is...NO......In plants and cancer cells, all sort of
chromosome doubling and mismatching (respectively) can occur without the
cells dying. There are some quite interesting theories of the divergence of
chromosomes in primates. They follow generally along the lines of a piece
of one chromosome breaking off and attaching to another, or not attaching
and forming a new chromosome. If you consult articles or web-sites that
deals with the Evolutionary Cytogenetics of primates you can get many of the
fascinating details. Much of the early work began with fruit flies see
http://www.iisc.ernet.in/~academy/resonance/Oct2000/Oct2000p61-65.html
Warning...this requires you understand the mechanisms of DNA replication in
eukaryotic cells.
Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy
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Update: June 2012
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