Classification and Evolutionary Relationships

• Lab 7 focuses on classification and evolutionary relationships.
• The activity involves creating classification systems and trees using living and extinct organisms.

Classification

• Humans classify almost everything, which can be very useful.
• Biologists classify organisms based on shared characteristics.
• Organisms with a backbone are classified as vertebrates, serving as a defining characteristic for the group.
• Taxonomy is the field dedicated to the classification of organisms. It is derived from the Greek words taxis (arrange, put in order) and nomos (law).
• Carolus Linnaeus (1707-1778) developed the modern taxonomic system.
• The taxonomic system is hierarchical, organizing organisms into inclusive categories from species to kingdom.
• Genera and species names are underlined or italicized to indicate their Latin origin.
• Formal scientific names are capitalized at each level, except for species.
• If the scientific name is used as an adjective in a sentence, it is not capitalized.

Linnean Hierarchy

• Domain: Eukarya
• Kingdom: Animalia, Plantae
• Phylum: Chordata, Arthropoda, Anthophyta
• Class: Mammalia, Insecta, Liliopsida
• Order: Primate, Carnivora, Hymenoptera, Liliales
• Family: Hominidae, Canidae, Apidae, Alliaceae
• Genus: Homo, Canis, Apis, Allium
• Species: Homo sapiens, Canis lupus, Apis mellifera, Allium cepa
  • Human, Wolf, Honeybee, Onion

Evolutionary Relationships

• In the 18th century, scientists believed the Earth and its organisms were created suddenly around 4004 BC.
• Linnaeus' classification system was seen as a way to catalog life's diversity, with some suggesting it revealed the Creator's mind.
• Charles Darwin's "On The Origin of Species" (1859) introduced the concept of evolution via natural selection.
• Organisms are descended from a common ancestor, diversifying over 3.5 billion years.
• Taxonomic classifications should reflect evolutionary history.
• Humans and wolves are in Mammalia and Chordata because of shared traits like a backbone, hair, and mammary glands.
• Shared traits are inherited from a common ancestor (homologous).
• Homology is similarity resulting from common ancestry.
• Greater resemblance suggests a more recent divergence from a common ancestor.
• Humans and wolves share a more recent common ancestor than either does with honeybees.

Phylogenetic Trees

• Phylogenetic trees show evolutionary relationships among organisms, using the following Greek roots: phylon (stock, tribe) and genus (birth, origin).
• The vertical axis represents time.
• A branch point/node indicates lineage split.
• Mammals diverged from reptiles about 150 million years ago.
• The most recent common ancestor is indicated by the branch point labelled A.
• The horizontal axis represents the amount of divergence, with greater distance indicating greater difference.
• Species within the same taxonomic group tend to be closer on the tree, sharing a recent ancestor.
• Evolutionary relationships are determined through:
  • Taxonomic classification
  • Phylogenetic trees
  • Comparing morphology among living species.
• Greater differences suggest longer divergence.
• Pitfalls of comparing morphology are:
  • Some species resemble each other due to similar environments/lifestyles, not recent common ancestry (convergent evolution).
• Convergent evolution: distantly related species converge in appearance.
  • Examples: wings of bats/birds/insects, streamlined shape of dolphins/fish.
• Though dolphins might appear similar to fish at first, they are mammals.
• Analogous structures: resemblance is superficial, resulting from adaptation to the same environment.
• Taxonomists must look at a whole range of characteristics for phylogeny reconstruction.
• The fossil record helps construct phylogenetic trees.
  • Recently discovered fossils show bears actually diverged from the Canidae (wolves, etc.) fairly recently.
• Problems with fossils are:
  • An Incomplete fossil record, with gaps or lacking fossils altogether.
  • Organisms with shells/bony skeletons are more likely to be preserved.

Systematics and Monophyletic Groups

• Systematics is an active field where biologists classify organisms and determine their relationships.
• Goal: organismal classification matches evolutionary history.
• Taxa (named groups) comprise a single common ancestor and all its descendants (monophyletic group).
• A named group is a hypothesis of set of species that are monophyletic.
• Systematists study morphology/molecular data of a named group (ingroup) and relatives (outgroup) through phylogenetic analysis.
• A phylogenetic tree confirms or rejects the monophyly of a group.
• If rejected, the group is termed paraphyletic or polyphyletic.
• Paraphyletic groups contain a common ancestor and some, but not all, descendants.
• Polyphyletic groups contain taxa with different ancestors.
• If a systematist hypothesizes that there are two separate genera, “A” with two species and “B” with one species, but the result of a phylogenetic analysis shows that genus A is not monophyletic.
• The systematist must rename the species A1, A2, and B to either be in three separate genera, or make A1, A2 and B the same genus or make B and A2 the same genus and A1 its own genus.
• The scientific name is changed and this happens at the species level as well as other levels in the hierarchy.