Taxonomy and Species concepts

• Idea (Greek) species (Latin) synonymous.• Eidos (Greek) “the visible look” or “outward

form”• Plato – eidos -> immaterial unchanging

forms, the eternal ideas (exist only in the mind).- the objects we sense are ephemeral manifestations of universal archetypes (the ideal oak tree).

• Aristotle – used “idea” as group sharing common form

• Aristotle’s “idea” translated to “species” in Latin

The concept of the species

• Linnaeus – taxonomy (species were created, distinct forms, unchanging).

• Evolution – species evolve BUT what is the “species”

• Selection – individual levelgroups of individuals (populations) show a change in gene frequency

Taxonomy

• Taxonomy is naming and classifying objects• Classification can be based on similarities or

differences.

Linnaean Taxonomy

• 1735• Binomial nomenclature• Genus species• Based on physical appearance (phenotype)• Did not assume any form of evolutionary

relationships

Species Definitions

– groups of actually or potentially interbreeding individuals within an area, which are reproductively isolated from other such groups (Mayr 1942).  

Biological Species Concept

Phylogenetic Species Concept

(Cladistic Species Concept) – the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent (Cracraft 1983).

Evolutionary Species Concept

- a lineage which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all other lineages outside of its range (Van Valen 1976).

Recognition Species Concept

– defines species from the basis of gene recombination (Paterson 1985).

Cohesion Species

– emphasizes the mechanisms (gene flow, natural selection) that results in species cohesion (Tempelton 1989).

Evolutionary Significant Unit (ESU)

• a population of organisms that is considered distinct for purposes of conservation.

- Definitions of an ESU generally include at least one of the following criteria:

1. Current geographic separation 2. Genetic differentiation at neutral markers among

ESUs caused by past restriction of gene flow 3. Locally adapted phenotypic traits caused by

differences in selection.

Why do we have so many definitions for a species?

Problem with species concept

• We are trying to impose a static concept on a dynamic process.

Evolutionary classification

• Phylogeny – evolutionary treeprovide inference as to which organisms evolved from which other ones

• Systematics is the reconstruction and study of phylogenies

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Two basic schools of classification

• Phylogenetics: classification based on ancestry and shared derived traits.

• Phenetics: classification based on overall similarity of organisms based on morphology or molecular traits.

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Cladistics• A simple and objective way of constructing

phylogenies (phylogenetic trees)• Focus on shared derived characters• Clade – group of organisms related by descent• Synapomorphy – shared derived character of

a clade• Cladogram – the branching diagram

representing the phylogeny

Example

• Why does a cladogram require shared derived characters (homologies vs analogies)?