Phylogenetic Trees

What phylogenetic trees illustrate

Evolutionary relationships among taxa and have transformed the study of evolution and related fields

Interpreting phylogenetic trees

Focus on the most recent ancestors rather than the position of tips or the number of nodes as those features are flexible

How phylogenetic trees are estimated

By analyzing shared, derived characters called synapomorphies that identify monophyletic groups

Homology

Traits shared due to common ancestry

Homoplasy

Occurs when traits are similar due to reasons other than common ancestry, commonly a trait that arose independently more than once (convergent evolution)

Monophyletic group

Includes a common ancestor and all of its descendants

Polyphyletic group

Consists of taxa that do not share and immediate common ancestor

Paraphyletic group

Includes a common ancestor but not all of its descendants

Mass extinctions

Short-term environmental catastrophes that have eliminated most species at least five times in history, reshaping evolutionary paths

The major historical mass extinctions

End-Perminian, end-Cretaceous, leading to the loss of major branches and rise of new ones

End-Permian extinction

Resulted in the disappearance of ninety percent of all species

The sixth mass extinction

Human activities drastically affecting earth’s biodiversity

Adaptive radiation

The rapid production, from a single lineage, of many descendants species

Three hallmarks of an adaptive radiation

A monophyletic group, speciated rapidly, diversified ecologically into many niches

Ecological opportunity

Extrinsic factor of a favourable new environment that can trigger adaptative radiations

Morphological, physiological, or behavioural innovation

Intrinsic motivation that can trigger an adaptive radiation event

Escape-and-radiate coevolution hypothesis

When a species evolves a new trait that helps it escape from predators, parasites, or competitors, it can rapidly expand into new environments and diversify into many new species

Cambrian explosion

Major adaptive radiation event that led to the emergence of all major animal phyla existing today. Animal size and morphology increased significantly, leading to diverse feeding and movement

Burgess Shale

Fossil evidence of Cambrian animal diversity and complexity found in Canada’s Rocky Mountains

Branch

A line representing a population through time

Root

The most ancestral branch in the tree

Tip/terminal node

Represents a living or extinct group of genes, species, families, phyla, or other taxa

Outgroup

A taxon that diverged prior to the taxa that are the focus of the study

Node/fork

Represents the most recent common ancestor of the descendant groups

Polytomy

A node that depicts an ancestral branch dividing into three or more descendant branches; usually indicates that insufficient data were available to resolve which taxa are more closely related

Synapomorphy

A shared, derived trait that is only shared in groups after a particular ancestor

Branch lengths are…

Arbitrary and emphasis is on the branching pattern, which estimates evolutionary relationships among populations

Branch lengths show…

The extent of genetic difference among populations and the extent of evolutionary time between nodes

Data matrix

The first step in inferring evolutionary relationships

What the data matrix does

Determines which taxa to compare and which characteristics to use

Outgroups in data matrices

Used to establish whether a trait is ancestral or derived

Cladistic approach

Transitioned the matrix to the tree by reconstructing relationships among species by identifying synapomorphies

Parsimony

Used to identify the most likely tree by the logic that the most likely pattern is the one that implies the lease amount of change

Background extinctions

Occur when certain populations are reduced to zero because of normal environmental change, emerging disease, predation, or competition with other species