Taxonomy - The Potato: Botany, Production and Uses - page 14

was one of the progenitors of the tetraploid
S. stoloniferum (Wang et al ., 2008). This would
indicate a different, more direct relationship be-
tween the Mexican diploids and polyploids.

overlap to draw boundaries easily between spe-
cies. Spooner and Hetterscheid (2005) pointed at
the lack of species-specific characters and advo-
cated a polythetic morphological species concept,
which defined species by the statistical covari-
ance of characters that were deemed important,
not all of which were necessarily displayed by all
individuals of that species (Mayden, 1997). Spe-
cies are thus characterized by (unique) combin-
ations of characters. Cladistic analyses of mo-
lecular data sets made it possible to circumscribe
cladistics species concepts based on monophyly,
and Olmstead (1995) proposed the terms “apo-
species” for the monophyletic daughter species
and “plesiospecies” for the remnant paraphyletic
progenitor species. This terminology was also
used in potato taxonomy; for example, by Castillo
and Spooner (1997), in a paper on series Conici-
baccata , and by Van den Berg et al . (2002), in a
paper about the reduction of the number of spe-
cies in series Longipedicellata .
Camadro et al . (2012), in a recent paper on
the species problem in wild potatoes, argue
that—since barriers against hybridization are
incomplete, introgression within and between
populations takes place, and populations are at
different stages of genetic divergence—the taxo-
nomic species concept cannot be applied. And
since potatoes can propagate both sexually and
asexually, the overlapping of uni- and biparental
generations hinders the application of the bio-
logical species concept. They make the point
that potato taxonomists often work with gene
bank accessions and that it is not certain at all
that these accessions are representative sam-
ples of the genetic diversity of the originally
sampled populations. In taxonomic studies in
potato, genetic, reproductive, and environmen-
tal aspects must be taken into account. The
authors indicate the need for a set of genetic
markers at various exploratory levels and for
sampling strategies to capture the dynamics of
wild populations.

Remaining problems
in potato taxonomy

A large number of scientists have devoted a con-
siderable amount of time and effort to arrive at a
classification of the group of wild relatives of the
potato. Still, controversies about species delimi-
tations and competing classifications have
arisen. What are the causes of these problems in
potato taxonomy? On the one hand, as men-
tioned above, some of the potato taxonomists
displayed an unfortunate splitter approach, cre-
ating a multitude of names and extensive syn-
onymy. But also, the plasticity of potato plants,
causing their morphological characteristics to
be influenced easily by environmental factors,
has contributed to the problem. Notwithstand-
ing this plasticity, there is not that much vari-
ation in the morphology of leaves and flowers,
leaving the taxonomist with only a few charac-
ters to work with. As most of these characters
display an almost continuous variation in char-
acter states, it is very hard to draw boundaries
between species. At least part of section Petota
consists of a large group of closely related en-
tities, many of which are able to hybridize, again
confounding species boundaries. Finally, since
these species are related to the cultivated potato,
human influence is likely to increase the taxo-
nomic problems. All this led Spooner et al . (2000)
to conclude “a practical taxonomic treatment of
the group using morphology likely will remain
an elusive goal”.
A number of classification problems are ap-
parent: how to define species; how many species
should be recognized; how to subdivide species;
how to cluster species in natural groups; and
how to deal with hybridization.

Overclassification

Species concepts

There has been a tendency to recognize too
many species, distinguishing minor variants at
the species level on the basis of morphology. Many
morphological characters, however, proved to
be influenced easily by environmental factors,
such as the effect of shade on leaf dissection

Paradoxically, the species—the basic building block
used by taxonomists to produce classifications—
has proven to be difficult to define in the potato.
Potato taxonomists relying on morphological
characters have found that there is too much