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Species Concepts and Speciation
Mark McGinleyAssociate Professor
Honors College and Department of Biological SciencesTexas Tech University
Species Concepts
Defining “a species” is not as simple as you might hope
There are many different “species concepts”.
Historical Species Concepts
Typological Species Concept species are a 'type' of organism
Lineaeus "Species are as many as were created in
the beginning by the Infinite.“ Systema Naturae, 10th ed. (1758)
establishes a catalog of 4,162 "types"
Historical Species Concepts Nominalistic Species Concept:
a name given for convenience Darwin
"I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other....“
philosophical EssentialismOnly individuals exist, not universal classes.
Historial/Modern Species Concepts
Morphospecies Concept Species traditionally have been described and
identified on the basis of morphological criteria According to this concept, species are “groups
of individuals that are morphologically similar and clearly distinguishable from individuals of other groups”
Modern Species Concepts
Biological Species Concept a reproductively isolated population
Mayr "Species are groups of interbreeding
natural populations that are reproductively isolated from other such groups."
Historically, the most widely used concept among ecologists
Biological Species Concept-Key Concepts "Interbreeding: => a genetic unit
species are gene pools: a coadapted gene complex
Members of the same species resemble each other because they have common ancestors
Biological Species Concept-Key Concepts "Natural Populations" => an ecological unit
Biological Species Concept-Key Concepts "Reproductively isolated" => a reproductive
unit Reproductive Isolating Mechanisms
features that prevent mating outside the species
Species Recognition Mechanisms features that allow recognition of potential
mates
Premating or prezygotic mechanisms
Ecological or habitat isolation Seasonal or temporal isolation Sexual or ethological isolation Mechanical isolation Isolation by different pollinators Gametic isolation
Habitat Isolation- Geese
Seasonal Isolation- Frogs
Behavioral Isolation- Crabs
Mechanical Isolation- Insects
Postmating or post-zygotic mechanisms Hybrid inviability Hybrid sterility
F1 fails to produce viable gametes Hybrid breakdown
F2s or backcrosses have reduced viability
Hybrid sterility- Mule
Reproductive IsolationSummary
Identifying Species Using the Biological Species Concept Biological Species Concept
suggests a research program that will allow scientists to identify species using this concept
the mark of a good theory
Identifying Species Using the Biological Species Concept (1) Experiment: test cross
A and B interbreed, but neither with C => A & B are conspecific
C is a separate species
Identifying Species Using the Biological Species Concept
(2) Field observations: a "species gap" exists Populations maintain identity when
sympatric (occuring in the same place) & synchronic (occuring at the same time)
Odocoileus hemionusMule Deer
Odocoileus virginianusWhite-tailed Deer
Odocoileus Distribution
Variation in Morphology
Variation in Behavior
O. hemionus O. virginianus
LacewingsChrysoperla plorabunda
Lightening BugsLampyridae: Hemiptera: Insecta
Firefly Flashing Patterns
Identifying Species Using the Biological Species Concept (3) Inference: forms 'look different' Many new species are described from
single 'skin & skull' or small series. Most commonly used criterion: inference is often weak.
Morphological Differences
Geographic Variation in Weasels Mustela erminea
Feloid (Panthera) and Canoid (Canis)
Identifying Species Using the Biological Species Concept4) Genetic studies indicate no gene flow
between forms - fixation for alternative alleles - distinct DNA sequences
- distinct karyotypes
Evolutionary Species Concept an evolving lineage
"An evolutionary species is a single lineage of ancestor-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate."
Historically, the most popular concept among paleontologists.
Used extensively in comparative biology & phylogenetic systematics
Evolutionary Species Concept (1) "Lineage": an
ancestor-descendent series genealogy is crucial:
members of a species have a common ancestor
Research program of paleontology is inferrence of genealogy Pelycodus
an early Eocene Primate
Evolutionary Species Concept (2) "Identity": a biologically
distinct entity Includes concepts
associated with Biological Species,
the Biological Species Concept is the broadest general case of the Evolutionary Species Concept
[i.e., a biological species is an evolutionary species at a partcular point in time]
Evolutionary Species Concept (3) "Tendencies & Fate": a
species is a historical entity Species have an origin
(by cladogenesis = 'splitting' of lineages)
undergo evolution (by anagenesis = change within lineages)
disappear (by extinction = termination of lineage).
Other Species Concepts
Phylogenetic Species Concept Recognition Species Concept Cohesion Species Concept Ecological Species Concept Internodal Species Concept
Why so many species concepts???
Problems with the Biological Species Concept (1) Reproductive isolation is a "typological
criterion" (an either / or rule). Does one successful hybridization invalidate species distinction? Extent & consequences of hybridization may vary: < 4% of Alberta deer show hybrid ancestry, including F1s (Hughes & Carr 1993) > 50% in West Texas (Ballinger et al. 1996): no F1s
Problems with the Biological Species Concept (2) Reproductive isolation evolves
gradually: species distinctions somewhat arbitrary Rassenkreis (''race circle'): a geographically convergent series of species adjacent forms are reproductively compatible
& morphologically similar ends of circle are reproductively isolated &
morphologically distinct
Ensatina - salamanders
Ring species in Gulls (Larus)
Problems with the Biological Species Concept Reproductive isolation arises without
morphological differentiation: Sibling species are morphologically identical species pairs
Tree CreepersSibling Species
Sibling SpeciesEmpidonax flycatchers
Problems with the Biological Species Concept (3) Reproductive criterion does not work
well with asexual species.
parthenogenesis is common in plants Plant species hybridize widely outside 'species' boundary:
Hybridization Raphanus radish X
Brassica cabbage => “Raphanobrassica" (radish leaves & cabbage root)
Problems with the Biological Species Concept (4) Genetic differences may not indicate
reproductive isolation or morphological divergence. Fixed allelic differences may indicate local adaptation or genetic drift.
Problems with the Biological Species Concept (5) What to do with paleospecies
(extinct or fossil species)? fragmentary ecological evidence: only by
inference
Problems with the Biological Species Concept
Are dinosaurs ? 'hot-blooded', 'maternal', ‘colorful'
fragmentary physical
evidence