Genes on the
line
I have long been
fascinated by biological examples of clines. These are linear distributions of
species in which almost imperceptible variation between neighbouring
populations can lead to marked differences between those at the two ends of the
range. I first became of aware of genetic clines in certain bird species, but
in any organism with a long-range distribution there is the possibility of
gradually changing genetic make-up, especially if the distribution lies along
an environmental gradient. Just such a situation has been investigated by
Still
et al., Pomona, California, USA
(pp. 467–477). In North America,
Echinacea angustifolia occurs from
the plains of
Canada in the
north to
Texas
in the south. From within that range, the authors sampled ten populations from
North Dakota to
Oklahoma,
a 1500 km north–south range embodying a very marked temperature gradient.
Genetic variation within and between populations was assessed by amplified
fragment length polymorphism (AFLP) in which different pairs of primers were
used in polymerase chain reactions to amplify a range of DNA fragments within
the total genome. A total of 1290 fragments were scored for presence or absence
in a number of individuals from each population. The data clearly showed that
the smallest genetic distance between any pair of populations was nearly twice
the average genetic distance between individuals within a population. When the
data were used to construct a phenogram (effectively an intra-specific
phylogenetic tree) there was clear evidence of four major groupings of
populations that fell along the north–south gradient, correlating with
temperature. One key feature of AFLP analysis is that it does not focus on DNA
sequences under selective pressure in respect of, in this instance,
temperature. It thus gives a good overall picture of microevolution.
Accordingly, this paper provides good evidence for restricted gene flow between
separated populations along a climatic gradient—a beautiful example of a cline.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
