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 concept Species diagnosed by unique differences Holbrookia maculata Cophosaurus texanus Callisaurus draconoides

Species level lineages Biparental organisms Organisms are united to form species-level lineages by sexual reproduction Uniparental organisms Constant divergence because lineages are not linked together by reproduction Group held together by ecological adaptations

The Biological Species Concept Ernst Mayr: 1942, 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 stictogrammaA. 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 OR A. tigris and A. marmorata? Problem 2. Geographic variation Fertile hybrids

The hybridization zone: SW New Mexico Problem 3: hybridization or intergradation How much is permitted under the BSC?

Three concordant step-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. F 1 diploid parthenogenetric ♀ x A. inornata ♂ Aspidoscelis uniparens (3n, parthenogenetic) 1. A. inornata ♀ x A. gularis stictogramma ♂ 2. F 1 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, 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