Lecture 17

• Carolus Linnaeus


  • The 10th edition of his Systema Naturae (1758) was the basis of modern taxonomy for animals
  • Developed binomial nomenclature
  • Popularized hierarchical taxonomy: Kingdom–Phylum–Class–Order–Family–Genus–Species
• Homo sapiens


  • The generic name is always capitalized
  • The specific epithet is never capitalized
  • The whole binomial name must be italicized
  • Or underlined if handwritten, but the space must not be underlined: e.g., Homo sapiens
  • A specific epithet cannot be used alone: "Homo sapiens" and "H. sapiens" are okay; just "sapiens" is not
• Species


  • Fundamental units of biological organization
  • A species can be generally defined as "a population or group of interbreeding populations that evolves independently of other populations"
  • There are dozens of different species concepts; the "best" way to define a species is a matter of intense philosophical debate
  • For instance, species can be identified by distinctive morphological traits, reproductive isolation, and/or phylogenetic independence
  • Having reliable criteria for species identification is essential for cataloguing and preserving biodiversity
  • Each species concept has advantages and disadvantages
  • Under all species concepts, the defining qualities (such as reproductive isolation) usually evolve gradually
  • Therefore, some populations cannot be classified unambiguously as the same or different species
• Typological species concept


  • A typological species (or "morphospecies") is a group of individuals that differ from other groups by possessing constant, diagnostic morphological characteristics
  • The typological species concept gets its name from the process, initiated by Linnaeus, of collecting a type specimen for each species and preserving it in a museum
  • The type specimen is the official representative of the species to which its Latin name is formally attached
• There is usually conspicuous variation within populations, so how can we identify diagnostic characters?
• There is also conspicuous variation among populations, so should all populations that differ by just one diagnostic character really be classified as different "species"?
• There are also cryptic species that cannot easily be distinguished morphologically, but which differ genetically because they do not interbreed
• Phylogenetic species concept


  • A species is the smallest diagnosable monophyletic group of populations
  • Can be applied to asexual species and fossils
  • Most populations can be diagnosed by some character(s)
  • Therefore, its widespread use would greatly increase the total number of species recognized
• Biological species concept


  • Species are groups of actually or potentially interbreeding organisms that are reproductively isolated from other such groups
  • This concept stresses reproductive isolation and the concomitant lack of gene flow
  • These are important because when two populations become reproductively isolated, they become evolutionarily independent units
  • Thus, it defines speciation as the evolution of reproductive isolation
• The biological species concept is inapplicable to asexual species and fossil species
• It is difficult to diagnose allopatric populations using the biological species concept
• Hybridization commonly occurs in nature and can result in introgression
• Ring species are a chain of interbreeding populations that loop around, such that terminal populations coexist without interbreeding
• General lineage species concept

  • Recognizes a species as a group of populations that exchange alleles sufficiently to form a single gene pool
  • Also considers other factors such as morphology, reproductive isolation, phylogenetic history, ecology, etc.
  • Suggests that all other species concepts do not define what species are; each is only a partial description of a species
  • No one species concept is necessary to define a species; satisfying a species criterion is a line of evidence
  • More evidence means greater corroboration
  • During the speciation process, various species criteria are satisfied over time; in the grey zone, they will conflict
• In the past, conservation strategies for African elephants have consistently been based on the consensus that all populations belong to the single "species" Loxodonta africana

  
• Comprehensive morphological comparison of skull measurement from 295 elephants noted appreciable distinctions between forest and savannah specimens

  
• Based on sequence divergence for 1732 nucleotides from four nuclear genes (BGN, CHRNA1, GBA, and VIM), there is a deep genetic division between the forest and savanna populations of African elephants

  
• The following lines of evidence suggest extremely limited interbreeding between forest and savannah elephants: different habitats, different morphology, large genetic distance, multiple genetically fixed nucleotide site differences
• This supports the recognition and management of two species: African savanna elephant (Loxodonta africana) and African forest elephant (Loxodonta cyclotis)