Re: The role of radioactivity in evolution

"Tim Tyler" <seemysig@xxxxxxxxxxxxxx> wrote in message
news:ekpo6o$2evn$1@xxxxxxxxxxxxxxxxxxxxxx
an.rasmussen@xxxxxxxxx wrote:
Tim Tyler wrote:

Radiation is the main reason people whose ancestors lived in Africa
look so different from northerners.

Tim is referring to solar radiation - sunlight.

I think Tim's comment is clearly right, for exactly the reasons
he states in his explanation. Furthermore, it answers the
question the original poster raised about radiation affecting
evolution - even though the O.P. may not have realized he raised
the question in the form that Tim answered.

We should note however that what Tim is talking about is totally
different from what others are discussing in this thread. The
other posters have focussed on the effect of radiation as a
mutating agent. Tim is talking about the effect of radiation on
survival and reproduction of organisms, producing an evolutionary
pressure towards adaptive mechanisms - in this case, dark skin.

On the question of whether radiation affects evolution by
directly mutating DNA, the answer is clearly yes. Studies on all
kinds of plants and animals have shown that "ionizing" radiation
(i.e. radiation with energy levels sufficient to knock electrons
off atoms) does cause mutations in DNA. Ionizing radiation does
occur in nature, mostly from the sun and from natural radioactive
substances like uranium and radon gas.

It is no doubt true that the vast majority of all mutations are
either neutral or negative in the life of the organism and so
play no role in the evolution of that organism (except in Tim's
sense of giving survival value to those individuals who are more
resistant to it.) However when we talk about gazillions (a
technical term for very large numbers :)) of reproductions over
billions of years, it's hard to imagine that there haven't been
at least some positive mutations caused by naturally occurring
radiation.

I'm not really qualified to speculate on how the magnitude of
these mutations compares to the magnitude of mutations caused by
other sources. But since this newsgroup has an ancient tradition
of speculating freely without solid research behind it, I'll go
ahead and speculate anyway.

I speculate that radiation has played a very minor role in
comparison to other causes of mutations. The reason for this
speculation is that the process of DNA replication is inherently
error prone. Modern eukaryotic cells, such as in humans, are
said to have replication error rates after all proof reading and
correction of about 10^-9. That amounts to an average of 6
errors in a full double set of human chromosomes on each
replication. Prokaryotes have much higher error rates, and early
organisms must have much higher error rates still.

I therefore speculate that natural replication errors greatly
outnumber radiation induced errors in the history of mutations to
DNA.

I could, of course be very wrong. There have probably been
periods in the history of the earth where radiation was much
greater than today.

Alan



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Relevant Pages

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