A3.2 Classification and Cladistics Flashcards
Difficulties classifying organisms into the traditional hierarchy of taxa
Traditional hierarchy of kingdom, phylum, class, order, family, genus and species does not always correspond to patterns of divergence generated by evolution
NOS: fixed ranking of taxa (kingdom, phylum and so on) is arbitrary because it does not reflect the gradation of variation. Cladistics offers an alternative approach to classification under unranked clades
Example of the paradigm shift that sometimes occurs in scientific theories
why is it difficult to reclassify organisms
Sometimes very difficulty to reclassify organisms after they have been put under a certain genus because one would wonder if the rest of the group should be moved as well
other difficulties with using taxa
Another common challenge with classifying hybrids plants of two species that can produce fertile offspring
Difficult to fit all organisms into such a simple system
dendrogram
Classification by grouping together of organisms that look most alike, to produce a dendrogram
cladogram
Classification by the grouping together of organisms with a more recent common ancestor, to produce a cladogram
Clade
A clade is a group of organisms that have evolved from a common ancestor
what do members of clade possess
Members of a clade possess common characteristics because they share the same evolutionary lineage
what is the most objective evidence for placing organisms in the same clade
The most objective evidence for placing organisms in the same clade comes from base sequences of genes or amino acids sequences of proteins. Morphological traits can be used to assign organisms to clades.
The evolutionary clock
The DNA sequences differences between different organisms accumulate gradually as they diverged from a common ancestor
There is a positive correlation between the number of different DNA sequences between two species and time due to random genetic mutations
the use of the evolutionary clock
Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. 1 change per million years)
If this rate of change is reliable, scientist can calculate the time of divergence according to the number of difference
E.g. if a gene which mutations at rate of 1bp per 100,000
limitations of evolutionary clock
Different genes or proteins may change at different rates
The rate of change for particular genes may differ between different groups of organisms
Over long periods, earlier changes may be reversed by later changes, potentially confounding the accuracy of predictions
Cladogram
A (tree) diagram showing the similarities and differences between different species
The branching points on cladograms are called nodes
Nodes denote a speciation even when a common ancestor splits into two or more species
Cladograms are mostly based on DNA base sequence or the amino acid sequences in proteins (in the organisms)
what are cladograms used for
Deducing evolutionary relationships, common ancestors
use of cladistics
investigate whether the classification of groups corresponds to evolutionary relationships
A case study of transfer of plant species between families could be used to develop understanding, for example the reclassification of the figwort family (Scrophulariaceae).
NOS: Similarities in morphology due to convergent evolution rather than common ancestry suggested a classification that by cladistics may be wrong
Reclassification of figworts
(Family Scrophulariaceae)
DNA evidence suggested that plant species in this family do not have common ancestors
The general flower shape and form of the seed capsules evolved many times from different ancestors (convergent evolution)
Different plant species adapted to similar pollinators and adopted similar seed dispersal strategies
