Classification Assumptions: Nature is Lawful, Uniform & Intelligible
Classification based on utility: edible vs inedible, poisonous vs non poisonous, where found, etc. • Natural Classification: reflects the order we assume exists in nature independent of us (e.g. western thinkers prior to 1860’s thought their system reflected the order of God’s creation, modern systems reflect the concept of evolutionary relationships)
A Classification by Utility • Habitats where found: • Marine • Estuarine • Freshwater • Terrestrial
Pelagic = swimming or floating in water columns. nekton = capable of swimming independent of currents plankton = carried by water currents • Benthic = living on (epifaunal) or in (infaunal) the substrate, or between particles in the substrate (interstitial or meiofaunal [smaller than 0.5mm]
Marine • Thus a sea anemone, and a sea squirt would be classified as: marine sublittoral & epifaunal organisms because they are both found as sessile organisms living on the surface in the marine habitat just below the level of low tide.
Freshwater • The terms pelagic and benthic and associated terms can also be applied to freshwater organisms. Additional terms for designating habitats: • Lentic = still waters (ponds, lakes, etc.) • Lotic = running water (rivers, creeks, etc.)
Estuarine • Estuaries are areas where freshwater and marine meet – include coastal swamps and marshlands as well as the mouth of rivers. They provide unique habitats but are not easily subdivided except by salinity which may change relative to tides and freshwater run-off. Thus estuarine animals may be classified in terms of the salinity they can tolerate as well as whether they are pelagic or benthic
Terrestrial • With the exception of flying insects and some extremely small insects and spiders subject to the wind the term pelagic and thus its opposite, benthic, are not useful. Most terrestrial inverts are either epifaunal, infaunal, or interstitial. • Further utilitarian classification involves association with dominant plant communities and/or moisture content.
Though useful, there are problems with utility as basis for classification: • Very different organisms may be lumped together – sea anemone, sea squirt, sponge (sublittoral, epifaunal) – snail, millipede, spider (epifaunal, deciduous woodland). • Similar organisms in different habitats separated – marine sea anemone & freshwater Hydra – terrestrial snail & marine conch.
Natural Classification based on Ancestor-Descendant Relationships • Clades vs Grades A clade is a group of organisms lumped together because they share an ancestor – descendant relationship. e.g – Annelida, or Cladocera. A grade is a group of organisms lumped together because they share a level of functional or structural complexity, e.g – Pseudocoelomates, carnivores In some cases a grade may also be a clade – one of the debates we’ll consider later in the semester is whether or not Arthropoda is a ‘natural taxon’ (a clade) or a grade resulting from convergent evolution.
Systematics - terms • Monophyletic group – all the taxa and only the taxa descendant from a single common ancestor plus that ancestor. • Paraphyletic group – a grouping of organisms in which some descendants of the common ancestor are missing. • Polyphyletic group – a grouping of organisms that arose from two or more recent ancestors.
Systematics • The aim of modern systematics is to organize our experience of organisms into a hierarchical nested set of monophyletic groups: Kingdom phylum class order family genus species (see page 25 for the various sub groupings etc.) • Currently the most widely accepted method for doing this is Cladistics or Phylogenetic Systematics.
A word about Modern Evolution Theory • Note: The discussion of evolution in your text on pages 15 – 19 is very misleading and in some cases just plain wrong! Please ignore it. • Speciation is a part of the process known as microevolution – the Modern Synthetic Theory of Evolution developed in the 1930s & 40s and the dominant view of the evolutionary process until recently includes the role of isolating mechanisms and the process of speciation by the development of reproductive isolation. • Macroevolution deals with the evolution of higher (above species) taxa, differences in the amounts and rates of speciation among different lineages and at different historical periods, and the causes and consequences of extinction events. • We will consider the debates over macro & micro evolution later this semester