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. platyglossa
L. fremontii
L. munzii
Limitation Morphological Species Concept- variation
• Layia glandulosa
Flower color
• Layia gaillardioides
Leaf shape
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. platyglossa
L. fremontii
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. platyglossa
L. munzii
L. chrysanthemoides
L. fremontii
L. glandulosa
L. discoidea
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