Date post: | 03-Jan-2016 |
Category: |
Documents |
Upload: | elisabeth-sims |
View: | 226 times |
Download: | 5 times |
Chapter 11 (Plant Taxonomy, pp. 137-152)
Species Concepts
Morphological Species Concept
• A species is a community, or a number of related communities, whose distinctive morphological characters are, in the opinion of a competent systematist, sufficiently definite to entitle it, or them, to a specific name (Regan 1926)
Layia (Asteraceae)
Layia discoidea Layia glandulosa
Layia spp.
L. chrysanthemoides L. fremontii
L. platyglossa L. munzii
Limitation Morphological Species Concept- variation
• Layia gaillardioides
Leaf shape
• Layia glandulosa
Flower color
Limitation Morphological Species Concept- subjectivity
• How many species are represented?
Biological Species Concept• Species are groups of interbreeding natural
populations that are reproductively isolated from other such groups (Mayr 1969)
Layia discoidea Layia glandulosa
F1= 8/8, 100% fertile!
Layia spp.
L. chrysanthemoides L. fremontii
L. platyglossa L. munzii
Limitations Biological Species Concept
• Difficult to measure the true extent of gene flow among natural populations
• Inapplicability of this concept to asexual species
• Interspecific hybridization between species of flowering plants is common
• What qualifies as reproductively isolated? 25%, 50%, 75%, or 100%?
Phylogenetic Species Concept
• Species are the terminals in a phylogenetic tree (Davis and Goldman 1993)
• The smallest diagnosable monophyletic unit with a parenteral pattern of ancestry and descent (Rosselló-Mora and Amann 2001)
Layia Phylogeny
L. chrysanthemoidesL. fremontii
L. munzii
L. platyglossa
L. discoideaL. glandulosa
Limitations Phylogenetic Species Concept
• Species are mosaics of genetic variation and often of reticulate origin
• Some species are monophyletic and others paraphyletic, and species are therefore not amenable to cladistic analysis
• It is unrealistic to assume that all species are bifurcating, completely evolved entities that form at the same evolutionary rates
Barcode Species Concept
• Identification of species is based on comparing DNA sequences of specimens to sequences in databases (Hebert et al. 2003)
Limitations Barcode Species Concept
• Many sequences in databases are based on misidentifications
• Interspecific and intraspecific variation exhibited by rbcL and matK varies between plant taxa
• Genetic variation considered intraspecific by some workers is treated as interspecific by others
• Species discrimination using the two locus combination rbcL + matK is successful in only 72% of cases
Ecological Species Concept
• A species is a lineage which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range (Van Valen 1976)
• Limitations- not all species fit this model, and it is hard to confirm without extensive genetic study