live_wire at cix.compulink.co.uk (Andrew Clifton) writes:
>The Molecular Clock Hypothesis
> A few questions....
>II. INTRA-SPECIFIC SEQUENCE VARIATION IN "LIVING FOSSILS"
> If, as the molecular clock hypothesis asserts, sequence variations
> between two organisms are dependend only on the time which has
> elapsed since they separated from their common ancestor, one would
> expect that in species which have survived for a remarkably long
> period of time, such as the lungfish, coelocoanths and horse-shoe
> crabs, there would be much more intra-specific sequence variation
> than there is in more recently evolved species. Is there any data
> on the amount of intra-specific variation in these species, as
> compared to, say, homo sapiens?
One thing to remember is that variation created by mutation can be
erased by genetic drift or by founder effects --- if a
species went through a "population bottle-neck" (in which only one
small population survived out of a previously larger set of populations),
then the current diversity would reflect what had evolved since the
bottleneck and not since the formation of the species. For example,
the attempts to date "Eve" cannot tell us when the first human
existed, only (possibly) when the last common female ancestor did.
However, I believe there is data (isozymes?) which shows that horseshoe
crabs and sharks do show a high degree of variation.
Program in Biochemistry, Molecular, Cellular, and Developmental Biology
robison at ribo.harvard.edu