BIOSCI101 Evolution - Lecture 7

[blank_start]Taxonomy[blank_end] is the discipline of naming and classifying (grouping) organisms.

Linnaeus classified species into a hierarchy of increasingly broader categories i.e. families, orders, classes, phyla, kingdoms and domains.

The placement of species into orders, classes and so on always reflects evolutionary history

TAXONOMY AND SYSTEMATICS These words have a similar history: over time these have been used as synonyms, as overlapping or as complementary terms. In today's usage, [blank_start]taxonomy[blank_end] deals with discovering, describing and naming organisms. [blank_start]Systematics[blank_end] deals with the phylogenetic relationships between taxa, especially at the higher levels. Todays systematics is greatly influenced by DNA data from nuclear, mitochondrial and chloroplast genes – [blank_start]molecular[blank_end] systematics.

The evolutionary history of a group of organism can be represented in a branching diagram call a phylogenetic tree.

HOW TO INTERPRET A PHYLOGENETIC TREE A phylogenetic tree represents a hypothesis about [blank_start]evolutionary[blank_end] relationships. Phylogenetic trees show the pattern of descent [blank_start]not[blank_end] phenotypic similarity. The sequencing of branching in a tree [blank_start]does not[blank_end] necessarily indicate the actual ([blank_start]absolute[blank_end]) age of the particular taxon. Unless specific information about what the branch lengths in a phylogenetic tree means – we should interpret the diagram solely in terms of its pattern. It [blank_start]should not[blank_end] be assumed that a taxon on a phylogenetic tree evolved from the taxon next to it.

CHARACTERS AND CHARACTER STATES Phylogenetic [blank_start]reconstruction[blank_end] is based on the analysis of characters and changes in character states (presence / absence or [blank_start]modifications[blank_end]). [blank_start]Characters[blank_end] can be m[blank_start]orphological[blank_end], b[blank_start]ehavioural[blank_end] or molecular and need to have a shared ancestry - [blank_start]homologous[blank_end]. A potential source of confusion when constructing a phylogenetic tree is similarity between organisms that is due to convergence – [blank_start]analogy[blank_end]. Shared ancestral ([blank_start]primitive[blank_end]) characteristics are inherited with little / no change from an ancestors. Shared derived characters are those that have undergone [blank_start]recent[blank_end] change and may be found only in closely related species.

Sequences alignment is necessary to determine molecular homology. In organisms that have very different sequences and are not closely related, the bases that they share may simply be coincidental matches – molecular homoplasies. Label these two scenarios accordingly.

Which one of the following statements about cladistics is FALSE?

Label each of these three scenarios on a phylogenetic tree. NOTE: paraphyletic does not include all descendants of the common ancestor; polyphyletic does not include the most recent common ancestor of its members.

INFERRING PHYLOGENIES [blank_start]All[blank_end] the characteristics of an organism arose at some particular point in time Therefore it should be possible to determine the [blank_start]clade[blank_end] in which each shared derived character first appeared. [blank_start]Shared derived characters[blank_end] are unique to a particular clade. This information can then be used to infer evolutionary [blank_start]relationships[blank_end]. However to do this we need an [blank_start]outgroup[blank_end] to serve as a point of [blank_start]reference[blank_end]. An outgroup is a species or group of species that i known to have diverged [blank_start]before[blank_end] the lineage that includes the species we are studying.

The principle of parsimony dictates that we should investigate the simplest explanation that is consistent with the facts.

For phylogenies based on DNA, the least parimonious tree requires the fewest base changes.