Cladistics

• Biologie
• Middelbare school
• vwo
• Leerjaar 6

Intro to cladistics

Question: what does it mean for groups of organisms to be related?

How are some organisms more closely related than others?

How can we research, describe and visualise these evolutionary relationships?

Clade: A group of organisms consisting of a common ancestor and all of its descendants (both extant and extinct). From Ancient Greek κλάδος (kládos): "branch”

Cladogram: A diagram that shows the evolutionary relationship of a group of organisms.

Cladistics: A system of classifying organisms according to shared characteristics, based on ancestry.

Which of these is a clade/are clades?

Cladograms can be rotated at any node without changing the relationships between sister taxa

cladograms can be drawn in a variety of different styles, without changing their meaning

How are cladograms constructed?

Which data is needed/can be used?

Clades can be based on anatomical characteristics such as the composition of skeletons, but most clades nowadays are based on the molecular similarity of DNA base sequences or amino acid sequences. Not all similarities are easy to see by anatomical characteristics alone.

Question: Which gives more accurate information about closely related organisms, DNA base sequences or amino acid sequences?

Cladograms set to time scales are more commonly known as phylogenetic trees. These phylogenetic trees show evolutionary relationships between organisms, as cladograms do, but also have lines representative of the amount of time between organisms' ancestry.

The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged.

Sometimes new data from cladistics leads to the reclassification of species into different taxa.

For example: The figwort family (Scrophulariaceae) used to be the 8th largest of all of the angiosperm families (275 genera, 5,000 species).

However, based on chloroplast DNA, it became apparent that this family was not one clade, but five.

Now, the figwort family is the 36th largest angiosperm family (200 species).

Two phylogenies of the same set of six organisms. The left one is often misinterpreted as showing an evolutionary series from left to right.

Root – The initial ancestor common to all organisms within the cladogram (incoming line shows it originates from a larger clade)

Nodes – Each node corresponds to a hypothetical common ancestor that speciated to give rise to two (or more) daughter taxa

Outgroup – The most distantly related species in the cladogram which functions as a point of comparison and reference group

Clades – A common ancestor and all of its descendants (i.e. a node and all of its connected branches)

Cladogram: groep met gemeenschappelijk kenmerk