Species and Species Concepts

What evolves? Evolution = "descent with modification"
      Only species reproduce & undergo modification:
             species are the unit of evolution

"Species Problem": the term "species" is used simultaneously for
      the unit of evolution &
      the unit of classification: a taxonomic category below genus
       category: "King Philip Came Over For Good Sex"
                Kingdom, Phylum, Class, Order, Family, Genus, & Species
       taxon: a named group at some categorical level
           Ex.: Humans are classified as:
                        Animalia, Vertebrata, Mammalia, Primata, Hominidae, Homo, H. sapiens

Taxonomy is the study of names & naming
        'alpha taxonomy' identifies new form
            Ex.: A "Greenland seal" is distinct from a "Harbor seal"
        'beta  taxonomy' shows discontinuity from other forms
            Ex.: "Greenland seals" have harp-shaped markings
        'gamma taxonomy' applies formal name, according to rules
         International Code of Zoological Nomenclature (ICZN) (4th ed., 2000)
                        prescribes all aspects of formation & publication of names

       Ex.: Article 31 says genus & species names must agree in latin gender.
           Harbour Seals were described by Linnaeus in 1758:
                     Phoca vitulina L., 1758 [-a ending is feminine]
           Harp Seals described  in same genus by Erxleben in 1777:
                    Phoca groenlandica Erxleben, 1777
                            recognized as distinct genus Pagophilus by Gray in 1844:
                                  Pagophilus groenlandicus (Erxleben, 1777) [-us is masculine]
                                       (Carr & Perry 1997)

      Practice and Theory may conflict:
             Classification should be stable, but evolution is dynamic.
             Classification requires a practical definition, but
                   study of evolution requires a theoretical understanding.
             Ex.:  Merriam's taxonomy of Ursus bears (in Hall 1981)
                   "Lumpers" versus "splitters"

Two historical species concepts:

Typological Species Concept: species are a 'type' of organism

"Species are as many as were created in the beginning by the Infinite."
        (Linnaeus, 1758)

      An idea of the Creator, a special form of a general idea ( ~ Biblical 'kind')
      Ex.: Lacerta muralis L. = the lizard on the wall, named by Linneaus
      Systema Naturae, 10th ed. (1758) establishes a catalog of 4,162 "types"

Nominalistic Species Concept: (just) a name given for convenience

"I look at the term species, as one arbitrarily given for the sake of convenience
to a set of individuals closely resembling each other...." (Darwin, 1859:52)

      philosophical Essentialism: Only individuals exist, not universal classes.

      Taxonomic categories are logical sets with definitions
       Ex.: "triangles" = "closed plane figures with exactly three sides"
                Class Amphibia = tetrapods without scales
           Homo sapiens = "featherless bipeds"

      Taxonomic categories are arbitrary: multiple dichotomous keys are possible
      Ex.: "bats + birds" are flying animals [Linnaeus grouped these],
             "whales + fish" are swimming animals [Linnaeus separated these],
             "lampreys + hagfish" are jawless animals [Agnatha]

Two modern species concepts (see  Futuyma Table 15.1):

Biological Species Concept (BSC): a reproductively isolated population

"Species are groups of interbreeding natural populations that are reproductively
isolated from other such groups." (Mayr, 1942; 1969:26)

      Historically, the most widely used concept among ecologists
      associated with Ernst Mayr (1904 -), a systematic ornithologist

      (1) "Interbreeding: => a genetic unit
             species are gene pools: a coadapted gene complex
             conspecifics (members of the same species) resemble each other
                   because they are related (have common ancestors)

      (2) "Natural Populations" an ecological unit
               Population thinking:
                   Organisms derive their properties from the group.
                   Variation among individuals is important.
             Species must be understood wrt environment & other species.

      (3) "Reproductively isolated" a reproductive unit
            Reproductive Isolating Mechanisms (RIMs):
                   features that prevent mating outside the species
            Species Recognition Mechanisms (SRMs):
                    features that allow recognition of potential mates

Assessing species differences with the Biological Species Concept

Criterion of reproductive isolation can be tested, observed, and / or inferred:
      BSC suggests a research program (the mark of a good theory)

(1) Experiment: a test cross

      A and B interbreed, but neither with C
       A & B are conspecific, C is a separate species
      Ex.: Artificial insemination among Xenopus frogs
            Pen breeding experiments between Odocoileus deer

(2) Field observations: a "species gap" exists

      Populations maintain identity when
                sympatric (occuring in the same place)
      &  synchronic (occuring at the same time)

       Ex.: Odocoileus hemionus & O. virginianus (Artiodactyla: Mammalia) co-exist in west Texas
                   differ in distribution, morphology, behaviour, ecology

      Behaviours related to courtship, mating & reproduction differ
       Calling patterns in lacewings (Orthoptera: Insecta)
            cryptic species have distinct calls
       Emergence time in Periodic Cicadas (Orthoptera: Insecta)
                13-year &  17-year forms emerge simultaneously only every 13 x 17 = 211 yrs
       Flashing patterns in lightning bugs (Lampyridae: Hemiptera: Insecta)
                timing & duration are species-specific

(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.

      statistical analyses show significant differences (Fundamentals of Hypothesis Testing)
          Ex.: latitudinal variation in weasels (Mustela erminea) evaluated by univariate F-test
          Ex.: shape differences among Carnivora evaluated by multivariate
                        Principal Components Analysis (PCA)

      Genetic studies indicate no gene flow between forms
        Ex.: fixation for alternative alleles in Odocoileus
                   distinct DNA sequences in Martes (Carr & Hicks 1997)
                   distinct karyotypes
                    polytene chromosomes in Drosophila
                    Robertsonian fissions / fusions in Perognathus deer mice (Rodentia)
                                            2N = 8 versus 46  in Muntiacus deer (Artiodactyla)
                    'cryptic' polyploids of Xenopus (2N, 4N, 8N)

Difficulties with application of the BSC

(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

(2) Reproductive isolation evolves gradually: species distinctions somewhat arbitrary
      Rassenkreis  (''race circle'): a geographically convergent series of species
        Ex.  Ensatina salamanders (Anura) are continuously distributed in California
              adjacent forms are reproductively compatible & morphologically similar
              ends of circle are reproductively isolated & morphologically distinct

      Reproductive isolation arises without morphological differentiation:
       Sibling species are morphologically identical species pairs
       Ex.: treecreepers (Passeriformes: Aves) recognized only by close inspection

      Morphological differences arise without reproductive isolation
       Species flocks: multiple closely-related species in same place
       Ex. Haplochromis (Cichlidae) fishes in high African lakes
                Different 'species' hatch out of same brood

(3) Reproductive criterion does not work well with asexual species.
      parthenogenesis  is common in plants
          Mendel studied hawkweed (Hieracium) after peas: didn't work.
      Plant species hybridize widely outside 'species' boundary:
        Raphanus radish X Brassica cabbage  "Raphanobrassica" (radish leaves & cabbage root)

(4) Genetic differences may not indicate reproductive isolation or morphological divergence.
      Fixed allelic differences may indicate local adaptation or genetic drift.
       Ex.:  DNA sequences of Pan & Homo  are  ~ 99% identical (King & Wilson 1975)
                Genetic differences reflect time of separation, not degree of morphological difference
                Chimps & humans are less differentiated than sibling species of Drosophila
       Molecular Clock estimates timing of evolutionary events:Allan C. Wilson (1935-1991)

(5) What to do with paleospecies (extinct or fossil species)?
     fragmentary ecological evidence: only by inference
             Are dinosaurs 'hot-blooded', 'maternal', 'colourful'?
      fragmentary physical evidence: scattered bones or teeth
             scattered in space: three specimens of Archaeopteryx
             scattered in time: origin & evolution of Homo

       Ex.: Human Evolution has a relatively typical fossil record for mammals

       Human / ape divergence ca. 4 ~ 5 MYBP (Million Years Before Present)
       Australopithecus ("southern ape") extends from 3.8  1.0 MYBP
         A. afarensis: "Lucy" first bipedal hominid
         A. africanus & A. robustus co-exist: separate species?
         A. africanus (500 cc brain)  Homo habilis (650 cc) at 1.8 MYBP
                        Are they transitional or co-extant?
       Homo: extends from 2.5 MYBP  present
        Homo habilis (650 cc)   H. erectus (850 ~ 1200 cc) from 1.5  0.3 MYBP
           H. sapiens neanderthalensis 700,000  30,000 YBP
                H. s. sapiens (1400 cc) diversified 250,000 ~ 50,000 YBP
                           Do we have single or multiple origins? [07 Dec 2000 "Nature" article]

Evolutionary Species Concept (ESC): 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." (Wiley, 1981:25 ; cf. Simpson, 1961:153)

      Historically, the most popular concept among paleontologists.
      Associated with George G. Simpson (1902-1984), a mammalian paleontologist
      Used extensively in comparative biology & phylogenetic systematics

(1) "Lineage": an ancestor-descendant series
      genealogy is crucial: members of a species have a common ancestor
      Research program of paleontology is inferrence of genealogy
        This can now be done by molecular methods
        Phylogenetic Species Concept : a recognizable monophyletic group
       "A cluster of organisms that is diagnosably distinct from other such clusters,
                  and within which there is a parental pattern of ancestry and descent." (Cracraft 1989)
           Discuss this as Molecular Systematics in Lab #5

(2) "Identity": a biologically distinct entity
      Includes concepts associated with Biological Species,
             the BSC is the broadest general case of the ESC
                [i.e., a biological species is an evolutionary species at a partcular point in time]
       Species Recognition Mechanisms (SRMS)
                   permit recognition of conspecifics as mates

(3) "Tendencies & Fate": an historical entity
      Species have an origin (by cladogenesis = 'splitting' of lineages),
              undergo evolution (by anagenesis = change within lineages),
                        & disappear (by extinction = termination of lineage).

Some (radical) implications:

      New species arise as "species-level" taxa, not as higher categories:
             First "bird" did not create a new "Class".
                   [Archaeopteryx resembles a coelosaur (Archosauria), with feathers]
            Aves raised to class-level only retrospectively

      New species arise only by "splitting" from other forms:
             Change within lineages does not create a new species.
             Ex.: If Homo ergaster  H. heidelbergensis  H. sapiens
                          then the series is a single, evolving species.
                 "species" behave like "individuals": they undergo birth, growth, & death
             chronospecies are successive temporally differentiated "species"

      The fundamental event in Evolution is cladogenesis, not anagenesis
            The study of speciation is key, not the study of microevolution.

Text material © 2010 by Steven M. Carr