BIO153 Leture 6 notes

Overview of Cladistics

  • Cladistics is a method of classifying organisms based on evolutionary relationships.

  • Focuses on the grouping of organisms (clades) and their shared ancestry.

Definitions

  • Clades: Groups of organisms that include a common ancestor and all its descendants.

  • Monophyletic Groups:

    • Defined as a grouping that includes the most recent common ancestor of all members.

    • For instance, if clade ABC includes all its descendants, it is monophyletic.

  • Paraphyletic Groups:

    • Groups that share a most recent common ancestor but exclude some descendants.

    • A paraphyletic group is an incomplete representation of organisms with close evolutionary relatedness.

    • Example: Clade DEF includes a common ancestor but does not include all its descendants (such as G).

  • Polyphyletic Groups:

    • This includes organisms from different lineages that do not share the most recent common ancestor.

    • Caution: Para/ and polyphyletic clades are also relative.

    • We usually use clades to infer immediate relatedness.

    • Example: Clade ABCD can be polyphyletic if members A, B, and C share a different ancestor than D.

Importance of Cladistics in Classification

  • Cladistics helps avoid common classification mistakes like paraphyletic and polyphyletic groups.

  • Historically, categories like 'Reptile' were considered paraphyletic until birds, which were excluded, were included due to new evidence regarding their relation to dinosaurs.

Algae as an Example

  • Algae were traditionally labeled as eukaryotic photoautotrophs but now we know are a polyphyletic group, meaning they derive from multiple evolutionary lines independent of plants.

Guidelines for Classification Using Cladistics

  1. Track shared ancestral and shared derived characters as they are passed from ancestor to descendants.

  2. Avoid classifying based on traits that may arise from convergent evolution, which can mislead.

Classification Process

  • Define Ingroup: The species being studied group (e.g., frog, cow, dog).

  • Define Outgroup: A closely related species without shared traits (e.g., fish).

  • Utilize a character table to identify homologous traits.

  • Traits are marked as 1 (present) or 0 (absent).

  • Build a phylogenetic tree assuming these traits arouse only once during evolution.

Phylogenetic Tree Construction

  • Start by separating the outgroup from the ingroup based on shared traits.

  • The tree develops by dividing the ingroup into relevant lineages based on trait presence, ensuring the simplest evolutionary path is followed.

    • Example: Evolution of traits like four legs or mammalian characteristics occurs through singular events in lineages.

Maximum Parsimony Principle

  • When multiple explanations exist, the simplest one is typically preferred.

  • For classification, this methodology allows clear distinctions in evolutionary relationships.

Genetic Classification

  • Genetic data can also employ cladistics and parsimony.

    • Analyze mutation pathways to establish relationships between species based on genetic traits.

    • Example: Determine if mutations occurred once or multiple times to deduce the most likely evolutionary pathway.

Computer Use in Cladistics

  • With increasing species numbers, classification complexity escalates, requiring computational support to evaluate evolutionary scenarios effectively.

Horizontal Gene Transfer (HGT)

  • HGT complicates taxonomy since it allows genes to move between unrelated lineages, making it difficult to discern true relationships. This can challenge traditional views of evolutionary distance based solely on traits.