5.4 Cladistics Flashcards
Essential idea:
The ancestry of groups of species can be deduced by comparing their base or amino acid sequences.
Cladogram
a tree diagram showing the similarities and differences between different species
The branching points on cladograms are called nodes.
Nodes denote a speciation event when a common ancestor splits into two, or more, species.
Cladograms are mostly based on DNA base sequences or the amino acid sequences in a protein.
Software applications combine multiple sequences* to find the most probable pattern of divergence.
5.4.U1 A clade is a group of organisms that have evolved from a common ancestor.
Species may evolve over time to form a new species. Consequently there are groups of species derived from a common ancestor that can be identified by looking at shared characteristics. Such groups are called Clades.
5.4.U1 A clade is a group of organisms that have evolved from a common ancestor.
Species may evolve over time to form a new species. Consequently there are groups of species derived from a common ancestor that can be identified by looking at shared characteristics. Such groups are called Clades.
5.4.A1 Cladograms including humans and other primates.
The DNA evidence used to build this cladogram of primates, including humans. can also be used to estimate when species diverged and hence when the common ancestor existed.
5.4.U2 Evidence for which species are part of a clade can be obtained from the base sequences of a gene or the corresponding amino acid sequence of a protein. 5.4.U3 Sequence differences accumulate gradually so there is a positive correlation between the number of differences between two species and the time since they diverged from a common ancestor.
Mutations in DNA that persist and are inherited* occur at a predictable rate#.
For example mitochondrial DNA from humans and primates has been completely sequenced and used to construct a cladogram between them.
The rate at which mutations occur at can be used as a molecular clock to calculate how long ago species diverged.
If the DNA base sequences or two species are similar - then few mutations have occurred - therefore the species only diverged relatively recently
The length of the lines separating species on cladograms is often used to represent the estimated time since they diverged.
* In species that reproduce sexually for mutations to be inherited they must occur duration the creation of sex cells.
#When considering multiple changes in the base sequence of DNA. The sample size is large enough to estimate a mean rate of mutation.
5.4.U4 Traits can be analogous or homologous.
Homologous structures are similar because of common ancestry and divergent evolution
e.g. pentadactyl limbs
Analogous structures are similar because of convergent evolution
e.g. the human eye and the octopus eye
Both have similar structures to provide similar function, but have evolved independently.
5.4.A2 Reclassification of the figwort family using evidence from cladistics.
DNA evidence identified the different common ancestors
The similarities among the members of the old Scrophulariaceae family (figworts) were superficial.
The general flower shape and form of the seed capsule evolved many times from different ancestors (convergent evolution). Different plant species adapted to similar pollinators and adopted similar seed dispersal strategies.
DNA evidence has cast doubt on some traditionally classified groups, but equally it has given support to others.
Though DNA evidence is very important, it must be remembered that it is not the only evidence used to decide evolutionary relationships.
