The species:
Eckert and Barrett surveyed flower morph frequencies in 102 populations of L. salicaria in Ontaria and 35 populations in Europe, where the species is native. Results:
1. Populations
in which one or more morphs have been lost tend to be relatively small
populations, as is expected if morph loss is due to genetic drift.
The figures are frequency histograms of population size (i.e. the fraction
of populations falling in different size classes). Black portions
of bars represent populations exhibiting morph loss. Note that these
are concentrated in smaller populations.
2. Large European populations tend to have morph frequencies closer to the equal frequencies expected if natural selection is the primary influence on morph frequencies. By contrast, in the smaller Canadian populations the effects of genetic drift are more pronounced and are manifested in two ways:
a. morph frequencies do not cluster as tightly around the expected equal frequencies under selection
b. in quite a few populations, one or more morph has been lost (squares and circles).
In this figure,
gene frequencies are plotted using a De Finetti diagram. In such
a diagram, each side of the triangle represents particular morph
(or allele). For a given point, the frequency of that morph is proportional
to the distance from the point to the side corresponding to that morph.
Thus, a point in the interior represents a population with all three morphs
present, while a point on a side of the triangle represents a population
from which the morph corresponding to that side has been lost (distance
of point to that side is zero).
3. The S-morph is lost more frequently than the L or M morph, as can be seen in the following table.
Figures and
Table from C. G. Eckert and S. C. H. Barrett. 1992. Stochastic
loss of style morphs from populations of
tristylous Lythrum salicaria and Decodon verticillatus (Lythraceae).
Evolution 46: 1014-1029.
.
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