Cladistics
is a method of classifying organisms into groups of species called clades (from Greek ‘klados’ = branch)
Clades
- Each clade consists of an ancestral organism and all of its evolutionary descendants
- Members of a clade will possess common characteristics as a result of their shared evolutionary lineage
Cladograms
Cladograms are tree diagrams where each branch point represents the splitting of two new groups from a common ancestor
- Each branch point (node) represents a speciation event by which distinct species are formed via divergent evolution
- *The fewer the number of nodes between two groups the more closely related they are expected to be
Cladograms and evolution
Cladograms can show evolutionary relationships and demonstrate how recently two groups shared a common ancestry
- As each node represents a point of divergence, closely related species will be separated by fewer nodes
Constructed cladograms key features:
- 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)
Evidence for which species belong in a clade
All organisms use DNA and RNA as genetic material and the genetic code by which proteins are synthesised is (almost) universal
- This shared molecular heritage means that base and amino acid sequences can be compared to ascertain levels of relatedness
Evolutionary divergence factors
- mutations will accumulate within any given segment of DNA
- The number of differences between comparable base sequences demonstrates the degree of evolutionary divergence
- A greater number of differences between comparable base sequences suggests more time has past since two species diverged
- Hence, the more similar the base sequences of two species are, the more closely related the two species are expected to be
Comparing species/organisms
Amino acid sequences are typically used to compare distantly related species (i.e. different taxa), while DNA or RNA base sequences are often used to compare closely related organisms (e.g. different haplogroups – such as various human ethnic groups)
molecular clock (the time of divergence)
If this rate of change is reliable, scientists can calculate the time of divergence according to the number of differences
E.g. If a gene which mutates at a rate of 1 bp per 100,000 years has 6 bp different, divergence occurred 600,000 years ago
molecular clock limitations
- Different genes or proteins may change at different rates (e.g. haemoglobin mutates more rapidly than cytochrome c)
- The rate of change for a particular gene may differ between different groups of organisms
- Over long periods, earlier changes may be reversed by later changes, potentially confounding the accuracy of predictions
limitations to using morphological differences
- Closely related organisms can exhibit very different structural features due to adaptive radiation (e.g. pentadactyl limb)
- Distantly related organisms can display very similar structural features due to convergent evolution
Convergent evolution
- It may occur when different species occupy the same habitat and are thus subjected to the same selection pressures
- The shared conditions cause common adaptations to be selected in different species, resulting in structural similarity
- *An example of convergent evolution is the development of wings in birds, bats and insects
Structural traits
- Traits that are similar because they are derived from common ancestry are termed homologous structures
- Traits that are superficially similar but were derived through separate evolutionary pathways are termed analogous structures
molecular evidence
Using molecular evidence, scientists have discovered that many species thought to be closely related based on shared structural characteristics actually demonstrate distinct evolutionary origins
- Crocodiles have been shown to be more closely related to birds than lizards, despite closely resembling lizards in structure
- Many species of plants previously classified as figworts have been reclassified based on molecular evidence
Reclassification
- Until recently, figworts were the 8th largest family of flowering plants (angiosperms), containing 275 different genera
- This was problematic as many of the figwort plants were too dissimilar in structure to function as a meaningful grouping
- Taxonomists examined the chloroplast gene in figworts and decided to split the figwort species into five different clades
- Now less than half of the species remain in the figwort family – which is now the 36th largest among angiosperms
