Christoph Oberprieler The Systematics of Anthemis L. - Herbmedit.org

18 Oberprieler: Anthemis in N Africa
ciation are genotype segregation in the hybrid progeny as well as backcrossing, introgression,
and consequent resorption of the hybrid genotype. Grant (1981) lists seven strategies
that may overcome segregation of a hybrid genome and enhance its stability. Only two
such strategies, however, are connected with sexual propagation and the homoploid nature
of hybrids: speciation by recombination and isolation of the hybrids from the parent species
by external factors. Speciation by recombination may occur in the progeny of a sterile
or semi sterile hybrid formed by hybridisation of two parent species which differ by two or
more chromosomal rearrangements, when recombination types evolve that are homozygous
for these rearrangements; the resulting hybrid descendants are fertile and diploid, but
genetically isolated from the parent species due to a chromosomal sterility barrier. Isolation
may occur when new combinations of characters lead to a mechanical, ethological, or
ecological crossing barrier between descendants and their parent species. In the case of
A. ubensis, both possibilities exist, but evidence, which would require thorough molecular
and cytological studies, is lacking.
8. Delimitation of taxa - Concepts of species, subspecies, and varieties
In the present revision of Anthemis in W and C North Africa the taxonomic categories
of species, subspc;:cies, and variety are used to classify the encountered patterns of morphological
variation at and below the species level.
As noted by Jansen (1985), "much debate and little consensus exists among biologists
concerning the definition of species." A number of different species concepts exist (see
Rieseberg & Brouillet 1994: 24 for a comprehensive list), based on biological (e.g. the
biological, better termed reproductive, species concept of Mayr 1969, 1970), phenetic
(Cronquist 1988), ecological (Van Valen 1976), and phyIogenetic criteria (the phylogenetic
species concept of Cracraft 1989 and the autapomorphic species concept of Mishler
& Brandon 1987).
A primarily morphoIogical or phenetic species concept has been used in the present
study. As discussed in the previous chapter, morphological discontinuities are assumed to
be due to genetic differences and, together with cytoiogical and molecular information in
some taxa groups, to a certain degree are taken to reflect speciation processes and phylogenetic
relationships. Since patterns of morphological variation show marked differences
between different subgroups of the genus, the resulting species may not be equivalent. As
noted by Lewis (1955, cited in Stuessy 1990:179), "the pattern ofmorphological differentiation
may differ from one group of pIants to another and is a reflection of the diversity of
evolutionary processes. Consequently, species and subspecies are not necessariIy equivalent
in different genera or different sections of the same genus." As discussed above, the
prevailing modes of speciation in perenniaIs of Anthemis sect. Hiorthia are hybridisation
and polypIoidy, while in annuals of A. sect. Anthemis allopatric speciation and Iocal
(founder effect) speciation (Levin 1993) is usually encountered. Therefore, species acknowledged
in the former section tend to be wide-spread, poIymorphic, and often exhibit
clinal variation of morphoIogical characters; while those of the latter section tend to be
more local and Iess poIymorphic.