The thistle Cirsium palustre exists over most of lowland
Britain as a predominantly purple flowered species. However the
populations of seacliffs and mountains exhibit a flower colour
polymorphism, occurring as homozygous white morphs, homozygous
and heterozygous intermediate morphs, and homozygous and heterozygous purple morphs.
The degree of polymorphism of the seacliff populations of
the Gower Peninsula is correlated with population size, the
larger populations being less polymorphic. This might be interpreted as indicating chance fixation of the white alleles. Such
an explanation would gain support from the very small size of
the more polymorphic populations and from the drastic fluctuations
in population size which have been demonstrated as having occurred
in these populations over a period of four consecutive seasons.
However other explanations based on selective effects are possible.
On seacliffs the frequency of the white morphs is inversely
related to exposure, and decreases in population size are accompanied
by differential survival of the purple morphs.
On mountains the distribution of the polymorphism is
markedly correlated with altitude. For the mountains of southern
Mid Wales, populations below 1000 ft. are strongly monomorphic
purple. Above 1000 ft. the degree of polymorphism increases
abruptly, with white frequencies reaching over 80%. Morph
frequencies among North Wales populations bear a similar relation
to altitude but the increase in polymorphism occurs at about
1200 ft. In each case the general trend is that purple frequency
declines with increase in white frequency, and that the frequency
of intermediates shows a unimodal distribution with a quite
precise peak. For both sets of populations this peak occurs at
an altitude about 250 ft. higher than that at which the increase
in white frequency occurs.
It is possible that the occurrence of the polymorphism on
seacliffs and mountains may be related to a limitation of cross
pollination consequent upon the exposure of seacliffs and the
combined climatic characteristics of mountains, which include
increase in exposure, mist and rain and decrease in temperature.
Evidence on the levels of outbreeding in these populations was
inconclusive but evidence in other species suggested that pollination might indeed be limited in these conditions.
An increase in homozygosity consequent upon inbreeding would
promote the frequency of the white morphs. Moreover the white
morphs were subject to preferential pollination and both this and
certain forms of heterogeneity in morph distribution were likely
to promote the frequency of inbreeding among white morphs.
However in conditions of limited pollination the degree of
general outbreeding of the white morphs will be increased by
preferential pollination and this may be assumed to be a fitness
advantage which may be of particular importance in the maintenance
of the polymorphism. In addition it is likely that the presence
of white morphs within a population may result in the attraction
of higher numbers of pollinators or encourage foraging for longer
periods in which case the polymorphism may be said to be adaptive
in the sense of Fisher (1930).
Other selective effects are also apparent. The occasional
presence of highly polymorphic populations in valley bases and
the regular occurrence of predominantly purple populations in
mountain forests may both to some extent provide evidence for an
effect of temperature other than upon pollination. Some evidence
suggests that both exposure and moisture may also be of individual
importance. Selection was apparent even by the arrangement of
morph types within a population subject to no obvious environmental heterogeneity. It is possible that the polymorphism is
maintained by a physiological heterozygous advantage and that
this may be responsible for the maintenance of white and intermediate morphs in low frequency in the predominantly purple
populations of inland lowland regions.
The maintenance of the polymorphism imposed a significant
selective mortality upon the species. This was indicated by the
above instance of selection within a uniform community and also
in several instances in which intrapopulational selection occurred
between segments of population subject to differing exposure.
However the growth in cultivation of seeds set in natural
populations revealed that the complexity of the genetic system
was sufficient to allow widely different morph frequencies to
be maintained in different populations without the necessity of
high selection in each generation.</p