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