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
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
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)
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"
Typological Species Concept: species are a 'type' of organism
as many as were created in the beginning by the Infinite."
of the Creator, a special form of a general idea ( ~
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.
categories are logical sets with definitions
Ex.: "triangles" = "closed plane figures with exactly three sides"
Class Amphibia = tetrapods without scales
Homo sapiens = "featherless bipeds"
categories are arbitrary: multiple dichotomous keys are
Ex.: "bats + birds" are flying animals [Linnaeus grouped these],
"whales + fish" are swimming animals [Linnaeus separated these],
"lampreys + hagfish" are jawless animals [Agnatha]
Biological Species Concept (BSC): a reproductively isolated population
groups of interbreeding natural populations that are reproductively
isolated from other such groups." (Mayr, 1942; 1969:26)
the most widely used concept among ecologists
associated with Ernst Mayr (1904 -), a systematic ornithologist
=> 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)
Populations" an ecological unit
Organisms derive their properties from the group.
Variation among individuals is important.
Species must be understood wrt environment & other species.
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
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
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
maintain identity when
sympatric (occuring in the same place)
& synchronic (occuring at the same time)
O. virginianus (Artiodactyla: Mammalia) co-exist
in west Texas
differ in distribution, morphology, behaviour, ecology
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'
species are described from single 'skin & skull' or small series.
Most commonly used criterion: inference is often weak.
analyses show significant
(Fundamentals of Hypothesis
Ex.: latitudinal variation in weasels (Mustela erminea) evaluated by univariate F-test
Ex.: shape differences among Carnivora evaluated by multivariate
Principal Components Analysis (PCA)
studies indicate no gene flow between forms
Ex.: fixation for alternative alleles in Odocoileus
distinct DNA sequences in Martes (Carr & Hicks 1997)
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
(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
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
isolation arises without morphological differentiation:
Sibling species are morphologically identical species pairs
Ex.: treecreepers (Passeriformes: Aves) recognized only by close inspection
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
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
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
/ ape divergence ca. 4 ~ 5 MYBP
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]
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)
the most popular concept among paleontologists.
Associated with George G. Simpson (1902-1984), a mammalian paleontologist
Used extensively in comparative biology & phylogenetic systematics
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
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
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:
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
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"
event in Evolution is cladogenesis, not anagenesis
The study of speciation is key, not the study of microevolution.