species concepts species concept: an idea of what kind of entity is represented by the word species....

Species Concepts• Species Concept: An idea of what kind of entity is

represented by the word SPECIES.

• There are several concepts recognizing the special reality of species (entities that exist independent of definition).

• To be avoided: concepts considering species as classes of objects (entities that exist by definition).

• Species criteria = standards used for species recognition = operationalism.

• Most non-biologists recognize non-dimensional species.

Species defined by a species conceptSpecies diagnosed by unique differences

Holbrookia maculataCophosaurus texanusCallisaurus draconoides

Species levellineages

Biparental organismsOrganisms are unitedto form species-levellineages by sexualreproduction

Uniparental organismsConstant divergencebecause lineages arenot linked togetherby reproduction

Group held together byecological adaptations

• The Biological Species Concept• Ernst Mayr: 1942, 1963. Animal Species and Evolution.

• Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.

• Species ranking criterion = absence of interbreeding

• Ancestral population may persist after speciation.

• Most widely used concept

• Used to make decisions re the Endangered Species Act, in anthropology, and many other biodiversity issues.

• Why don’t species interbreed?

BSC: isolating mechanisms (barriers)• Premating mechanisms (barriers): favored by selection

• A. Potential mates do not meet– Ecological mechanisms

– Temporal; habitat; for plants--species specific pollinators

• B. Potential mates meet but do not mate– Behavioral isolation

• Postmating mechanisms

• C. Potential mates meet and mate– Prezygotic isolation

• Gametic incompatibility

– Postzygotic isolation

– Developmental problems

– Reduced hybrid fitness (sterility, lower viability, etc.)

Aspidoscelis burti stictogramma A. burti burti

Problems with allopatry

• Problems with morphological characters• Morphological differences may not be effective in

distinguishing species under this concept. – 1. Individual variation (e.g., ontological variation)

– 2. Geographic variation

– 3. How much hybridization is permitted?

– 4. Cryptic species

Problem 1: Ontological variation Aspidoscelis stictogramma

A. tigris punctilinealis and A. tigris marmorata ORA. tigris and A. marmorata?

Problem 2. Geographic variation

Fertilehybrids

The hybridization zone: SW New Mexico

Problem 3: hybridization or intergradationHow much is permitted under the BSC?

Three concordantstep-clines

• Barriers to interspecific hybridization

• Reproductive isolating mechanisms

• A. Prereproductive (favored by natural selection)

– 1. Ecological

• Temporal (phenological)

• Habitat segregation

– 2. Behavioral

– 3. Mechanical

• B. Postreproductive

– Gametic wastage

Aspidoscelis velox (3n, parthenogenetic)

1. A. gularis stictogramma ♀ x A. inornata ♂ 2. F1 diploid parthenogenetric ♀ x A. inornata ♂

Aspidoscelis uniparens (3n, parthenogenetic)

1. A. inornata ♀ x A. gularis stictogramma ♂ 2. F1 diploid parthenogenetic ♀ x A. inornata ♂

Problem 4: Cryptic species

velox

uniparens

• Phylogenetic Species Concept(s)

• Phylogenetic species: the smallest aggregation of individuals diagnosable by a unique combination of character states.

• Characters and character states• Organisms are grouped into species because of shared

derived character states (SDC).• SDC states inherited from a common ancestor• Ancestors and derived species form a branching pattern of

divergence• Ancestral populations typically do not (by definition)

persist past a speciation event.

• The Evolutionary Species Concept– E. O. Wiley. 1978,1981, 2001.

• • A species is an entity composed of organisms

– maintaining its identity from other such entities through time and space

– and having its own independent evolutionary fate and historical tendencies.

• Operationalism absent• Use fixed diagnostic differences