evolution and speciation

Stewart Schultz schultz at unixg.ubc.ca
Fri Oct 25 00:14:59 EST 1991

In article <stormo.688250251 at beagle> stormo at boulder.Colorado.EDU (Gary Stormo) writes:
>Given the large number of chromosome translocations and inversions
>that exist in Drosphilia, one ought to be able to construct some flies
>whose offspring are competent to mate with each other (or at least a 
>subset of their sibs) but would have very low fertility in matings to
>any other strains, even the parental.  If so, these would represent a
>new species, by the usual definition, in which there has been no change
>in allelic frequencies (at the gene level, I'm not counting a rearranged
>chromosome as a new "allele").  

About 40% of flowering plant species (both monocots and dicots) originated
through polyploidy (mostly allotetraploids) since the origin of their
genus, and about 80% arose through polyploidy since the origin of the
angiosperms.  About 95% of homosporous fern species are polyploid (although
many have been diploidized).  This mode of speciation occurs in
just a few generations, and is usually sympatric with one or both
of the parents.  Hybridization and doubling often continue repeatedly
between lineages, yielding complex reticulate evolution and regular
polyploid series from 4x to over 10x in many cases.  Species of the fern
genus Ophioglossum have the largest chromosome counts known, 
2n = 1260 = 84x.

Aside from polyploidy, speciation by "chromosome repatterning" is
common in flowering plants; see Verne Grant's _Plant Speciation_
for many detailed examples. 

I think the jury's still out on the question of the relative rarity of
polyploid or chromosomal speciation in animals (and other groups,
e.g. gymnosperms, some bryophytes, fungi).  Disruption of sex
determination or of dosage compensation of the X chromosome may
be factors in many animal groups (Orr, 1990), but I think that
problems of autosomal dosage imbalance are even more serious,
mainly because there are very few examples of animal polyploids
greater than 4x.

In any case, flowering plants provide an example where the standard
mechanism of speciation is by no means a gradual allelic divergence.

-S. Schultz

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