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