Transcript Lecture 23 (4-1-11)

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, 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.)
Problems with allopatry
Aspidoscelis burti stictogramma
A. burti burti
• Problems with morphological characters
• Morphological differences may not be effective in
distinguishing species under this concept.
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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
A. tigris and A. marmorata?
OR
Problem 2. Geographic variation
Fertile
hybrids
Problem 3: hybridization or intergradation
How much is permitted under the BSC?
The hybridization zone: SW New Mexico
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
uniparens
velox
Aspidoscelis velox (3n, parthenogenetic)
Problem 4: Cryptic species
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 ♂
• 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