Systematics and Phylogeny

Evolutionary Taxonomy

names and classifies things using Linnaean classification in the context of evolution

Systematics

classifies organisms based on evolutionary history

Phylogenetics

a specialized field within systematics that is focused on evolutionary relationships among organisms

Cladistic

method of phylogenetics representing evolutionary relationships with a tree of diverging branches

• species are grouped based on shared derived features

• direct ancestors are never specified, and are treated as branching points

Phylogeny/Phylogenetic Tree

evolutionary hypothesis of a group of biologically related species that depicts lineages descending from common ancestors

Parts of a Phylogeny/Phylogenetic Tree

• The root is the base

• The tips are the terminals/ends

• The nodes/branch points are the points where branches meet, representing a hypothetical last common ancestor and divergence of two taxa

  • Internal nodes connect between nodes

  • External nodes connect between a node and a tip

Sister taxa

two closest branches from a node

Basal taxa

lineages that diverge relatively early

Derived taxa

lineages that diverge relatively late

Phylograms

type of phlyogeny that depicts hypotheses of evolutionary history that require fossils for calibration

• Include scale bars as measures of time

Cladogram

hypothesise on the relative relationships among taxa

Monophyly/Monophyletic group

a clade that includes the most recent common ancestor and all of its descendants

Paraphyletic group

includes the most recent common ancestor and some of its descendents

Polyphyletic group

includes some descendants without their common ancestor

Validity of type of groups

• In evolutionary taxonomy, both monophyletic and paraphyletic were valid classifications

• In phylogenetic systematics, only monophyletic groups are valid classification

Dichotomy vs Polytomy

A Dichotomy occurs when only two branches diverge from a common ancestor; A polytomy occurs when more than two branches diverge from a common ancestor

Soft Polytomy

represents uncertainty about which of the taxa arising from the node are most closest related to each other, implying that more data, observation, and experiments are needed

• Majority of polytomies are soft polytomies

Hard Polytomy

represents a hypothesis that more than two taxa arose from the same common ancestor, implying that an evolutionary event is likely taking place

• More difficult to prove

• Evolutionary events usually take place very quickly

Morphological data

records anatomical features such as qualitative data and quantative data that can be used in fossils

Molecular data

records information relating to the molecule such as DNA sequences and protein structure

Character

a heritable feature of a given organism (anatomical, physiological, molecular)

Character State

a specific observable condition/state of the character

• A character can have two or more states

• Character state is largely equivalent to traits

Analogy

character traits that independently evolved from convergence evolution

• Analogous traits are not used for building phylogenies

• Appearance may not display similarity

Convergence Evolution

occurs when organisms from separated lineages adapt to similar environment

Homology

character traits that evolved from a common ancestral structure

• Appearance may not display similarity

Identifying Homology

    - topology (the way body parts are arranged)

    - detailed morphology (form, shape, structure)

    - neural innervation (innervated by the same nerve)

      - supplying an organ with a nerve

    - embryonic development

Synapomorphy

a derived feature shared by more than two taxa and their last common ancestor

Autapomorphy

a derived feature found only in one taxa

Symplesiomorphy

an ancestral feature shared by at least some members of a taxonomic group

Maximal Parsimony

an idea, developed by William of Ockham, that the simplest hypothesis consistent with the facts is one most likely to be correct

• In cladistics, the cladogram that implies the fewest evolutionary changes is considered the most likely

Role of Embryonic Development in distinguishing between traits

finds that traits developed at relatively early stages are more likely to be ancestral

• However, ancestral traits don’t always appear in embyonic development and processes like metamorphosis can occur, such as in insects and frogs

Role of Fossil Evidence of distinguishing between traits

can serve as indisputable evidence of a taxa that existed in time long passed, whose traits are likely ancestral, depending on how early in Earth’s history the fossil orginated

• However, the fossil record is incomplete and molecular data is very rare in fossils