MOLECULAR CLOCKS: Divergence Time & Sequence Differences
Molecular Clock: 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.
If a piece of DNA mutates at a predictable rate like a molecular clock, it becomes a valuable tool for estimating the dates when two species split from a common ancestor and how ancient a species may be.
This is represented by the lines in the branched tree diagram and can be set to a timeline if further analysed.
There is a positive correlation between the number of difference between two species and the time since they diverged from a common ancestor.
Because 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.
Scientists usually determine cladistics and when species diverged by analysing molecular similarities and differences.
DNA sequence comparisons are more accurate. This type of analysis has also been used to look at recent human evolution.
Coding sequences and amino acid sequences are not usually analysed when two species are suspected to be closely related because mutations in these sections of DNA occur at a much slower pace.
CLASSIFICATION ERRORS
Classification based on traditional methods e.g. morphology, feeding patterns and other behavioural aspects has not always given accurate results for the relationships between organisms.
Linneus took it a step further and classified organisms into kingdoms, phyla, classes, orders, families, genera and species.
Aristotle once classified animals according to their environment and plants according to their size or shape.
New molecular evidence has shown that the classification of some groups did not correspond with the evolutionary origins of a group or species in previously made cladograms.
PHYLOGENETIC TREES
Cladograms set to time scales are more commonly known as phylogenetic trees.
These phylogenetic trees show evolutionary relationships between organisms, as cladograms do however phylogenetic trees have lines representative of the amount of time between organisms' ancestry.
DEFINITION: A diagram that represents evolutionary relationships among organisms based on physical and genetic similarities and differences.
Protein and DNA base-sequence technologies helped analyze biochemicals and improved the accuracy of cladistic analysis immensely. This technology has changed classification. Some groups have merged, and others have been split.
CLASSIFICATION OF FIGWORT FAMILY
The figwort family is a good example of how DNA sequence analysis has changed the classification landscape as plant are difficult to classify so DNA sequencing techniques made it easier.
The figwort family (Scrophulariaceae) included more than 275 genera with around 5 000 species, based on shared morphological features. They were the 8th largest of all of the angiosperm families.
After comparison of chloroplast DNA, now the figwort family has around 200 species and is the 36th largest family of all of the angiosperms.
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.