UWCSEA 2023 - IB Biology A3.2 Classification and Cladistics

A3.2.1 Why do we need to classify organisms?

There are millions of different species that have been named and described;

placing organisms into groups based on the evolutionary origins helps to organise this information;

A3.2.2 What are the traditional taxa (groups) into which taxonomists place organisms?

From the largest to the smallest: Domain, kingdom, phylum, class, order, family, genus and species

Darn, Katy, Perry, Crashed, Our, Family, Game, Savagely!

A3.2.2 Why are there difficulties classifying organisms into traditional taxa (groupings)?

It can be difficult to work out the precise taxonomic group to place organisms into;

in one persons opinion there might be enough difference and traits between two species to place them in different family, but someone else think that they are similar enough to be in the same genus;

changes occur slowly over a long time; the exact point at which they move from one grouping to another cannot be identified;

A3.2.3 What are the advantages of classification according to evolutionary origin?

Classification according to evolution origin is where all organisms in group evolved from the same common ancestor;

if this is fulfilled, all organisms will share similar traits;

sharing morphology;

e.g. all bats share features of the group above, mammals

A3.2.4 What are clades?

A clade is a group of organisms that have evolved from a common ancestor;

sharing common characteristics;

and sharing similar DNA sequences;

they can be very large, or very small, depending on how many of species have survived;

e.g. birds form one large clade of 10,000 species;

every species is in multiple different clades, with smaller clades having more similarities;

A3.2.5 How can differences in sequences between organisms be used to work out when they divereged/had a common ancestor? What is a 'molecular clock'?

as mutations in DNA occur at a roughly constant rate;

the number of mutations and therefore differences in a base sequence can be used to estimate time taken;

therefore DNA is acting like a molecular clock that tells the time since the sequence was identical; ie the species shared a common ancestor;

e.g. between two species if there are small number of mutations, it means they split recently and share a more recent common ancestor;

A3.2.6 What are cladograms? How can base sequences of genes and/or amino acid sequences be used to construct cladograms?

Cladograms are tree diagrams that show the most probable sequence of divergence in clades;

branching diagrams that show when species are likely to have split;

they are based on DNA base sequences and amino acid sequences;

using computer software, sequences are aligned and compared;

the more closely related species are, the more similar their DNA sequences are; they are then placed in the same clade;

the smallest number of changes in bases required to account for the difference is usually accepted as the most likely;

A3.2.7 How can we analyse cladograms? How are they arranged and what can you work out from them? Describe the parts of them

a cladogram is tree diagram with branches; the length of the branches is the estimated time due to changes in the DNA sequence of organisms;

the terminal branches (ends) represent individual clades/species;

nodes are the point where two branches meet going back in time;

they represent the point of the common ancestor of one or more species;

the root is the base of the cladogram and is the proposed common ancestor;

A3.2.8 How can cladistics be used to investigate whether organisms are correctly classified? Give and example

DNA sequencing allowed scientists to analyse the relationships in the figwort family;

through analysis, they found that not all organisms in the family actually had a common ancestor; as they had significantly different DNA sequences;

so some species were moved out of the family to the correct taxonomic groups;

A3.2.9 What are the domains that all life is classified into? What evidence was used to do this?

the three domains are Bacteria, Archaea and Eukaryota;

previously there were only 2, but evidence from rRNA (ribosomal RNA) showed that there were two distinct groups of prokaryotes, eubacteria and archaea;

How cladograms be used to to deduce evolutionary relationships? Why should we be cautious over their use?

branching pattern is assumed to match the evolutionary origin;

it is assumed that mutations have occurred at a roughly constant rate; and the smallest number necessary has occurred;

there is a positive correlation between the number of changes in DNA and time;

therefore if two species have few mutations between them, they are very closely related and diverged recently;

they are not proof of when the divergence happened;

and perhaps the smallest number of mutations possible to create the two new species did not actually occur;