CLASSIFICACTION
Molecular clocks: divergence time and sequence differences - 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.
There is a positive correlation between the number of differences between two species and the time since they diverged from a common ancestor. That is, the longer the time period since the two species separated, the more differences there will be when the DNA of the two species is compared.
click to edit
The conversion of differences in base sequence to units of geologic time usually needs to be checked against the fossil record.
Classification errors
Cladograms
A clade is a group of organisms that consists of a common ancestor and all its lineal descendants, and represents a single 'branch' on the 'tree of life'.
A node is a point on a cladogram where divergence occurs. Also called a branching point.
click to edit
Analysis of cladograms
The analysis of a cladogram relies on a number of assumptions and definitions.
The branching pattern is assumed to represent the evolutionary relationship between species.
If extinct species are included, information must be given on whether the cladogram is based on morphology only because DNA generally is not available for the analysis, although there are some exceptions to this.
The more nodes there are between species, the more distant their relationship.
Mutations at the DNA and protein level are assumed to occur at a constant rate. For example, 1 base change per 109 years – this may seem like a long time, but remember there are billions of bases in genomes.
Some cladograms are drawn to scale, that is the length of the branches is proportional to the time since divergence.
click to edit
The root of a cladogram is the point prior to any divergence where the organisms share certain characteristics.
An outgroup is the most distantly related organism on a cladogram. It will be on the longest branch of the tree and will be set apart from the other branches.
Phylogenetic tree is a diagram that represents evolutionary relationships among organisms based on physical and genetic similarities and differences.
The order Artiodactyl contains mammals such as cows, sheep and camels. These are often commonly called hoofed mammals.
Classification of the figwort family
The figwort family is a good example of how DNA sequence analysis has changed the classification landscape. Plants are notoriously difficult to classify, and only since the arrival of modern DNA sequencing techniques has it been possible to assess the relationships between some plants with confidence.
The figwort family (Scrophulariaceae) included more than 275 genera with around 5 000 species, based on shared morphological features. The figwort family was the 8th largest of all of the angiosperm families.
However, when scientists compared chloroplast DNA, it became apparent that what was once thought to be one clade, i.e. all descendants from one ancestor, was actually five clades. Until this analysis, it was impossible to distinguish between the five clades, because their morphology was so similar due to convergent evolution. Now the figwort family has around 200 species and is the 36th largest family of all of the angiosperms.
click to edit
click to edit
The five clades that contain the taxa traditionally assigned to Scrophulariaceae are: Calceolariaceae, Orobanchaceae, Scrophulariaceae, Stilbaceae and Veronicaceae. But, it should be noted that several figwort genera have also been moved to other families such as plantain and broomrape. In fact, less than half of plants classified as figworts still remain in the figwort family now that we know more about their DNA similarities and differences.