Classification (HL) (HL IB ESS OLD COURSE - IGNORE)
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Clades
The development of DNA sequencing technology means that classification can now be carried out on the basis of evolutionary relationship
Organisms that are grouped together using this method of classification form groups known as clades
Every member of a clade shares a common ancestor
A common ancestor is a shared ancestor, e.g.
The most recent common ancestor of siblings is their parents
The most recent common ancestor of cousins is their grandparents
Clades are monophyletic groups
This means they contain all of the descendants of a common ancestor
In the evolutionary tree below:
Chimpanzees and bonobos share a recent common ancestor
Chimpanzees are therefore most similar to bonobos (more similar than they are to any other primate species)
Chimpanzees and bonobos form a small clade
Humans share a more recent common ancestor with gorillas than they do with orangutans
This means we are closer to gorillas than we are to orangutans
Humans and gorillas do not form a clade
Humans, chimpanzees, bonobos, and gorillas do form a clade
All five primate species shown here share a common ancestor (from the distant past)
Humans, chimpanzees, bonobos, gorillas, and orangutans form the biggest clade
Advantages of classification by evolutionary relationship
Historically, organisms would have been classified on the basis of morphology
This often led to organisms being classified into groups that were not all close relatives
Classifying organisms correctly according to their clade ensures that groups of organisms are close evolutionary relatives (rather than groups that happen to look similar)
The characteristics within a clade are often inherited from a common ancestor, so are likely to be shared
The use of DNA sequencing has allowed some organisms to be reclassified into more accurate groups
Some species have been reclassified into different groups of organisms
Some groups of organisms have been split
Some groups have been merged
Cladistics
Cladistics is the branch of science in which scientists put organisms into clades
Clades can include both living and extinct species
Some of the descendants of a common ancestor may have gone extinct
The common ancestor species itself may have gone extinct
Clades can be large or small depending on the common ancestor being studied
While taxonomy is about classifying and naming organisms, cladistics is about identifying evolutionary relationships between organisms
A taxon is a group of organisms that have been given a group name by taxonomists on the basis on their shared features
A clade is a group of organisms classified together on the basis of their shared descent from a common ancestor
If taxonomy is carried out correctly, then all of the members of a taxon should form a clade
Cladograms
Evolutionary relationships between species can be represented visually using a diagram called a cladogram
Cladograms are evolutionary trees that show:
Order of divergence from ancestral species
Relationships between species
The point at which two branches separate is known as a node
Nodes represent common ancestor species
Analysis of a cladogram can provide several important pieces of information:
A node immediately adjacent to a pair of clades indicates that these two clades share a recent common ancestor
This shows that the two clades are more closely related to each other than they are to any other clade in the cladogram
If several nodes need to be traced back before two clades can be joined, this indicates a more distant relationship between two clades
The root of a cladogram is found at its base
This represents the common ancestor of all of the organisms within the cladogram
The root of a cladogram will represent organisms that were present a long way back in evolutionary history
The terminal branch (i.e. the final branch) represents the most recent species in an evolutionary lineage
Some cladograms have a time scale to show how many millions have years have passed
Difficulties in Classifying Organisms
The traditional hierarchy of taxa
Biological classification involves putting organisms into groups, or taxa (singular taxon)
The taxa form a hierarchy
A hierarchical system is one in which larger groups contain smaller groups with no overlap between groups
The taxonomic hierarchy contains the following taxonomic groups in descending order of size:
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
Difficulties with classification
There are multiple challenges when it comes to accurately classifying organisms into the hierarchy of taxa described above
These difficulties include:
Morphology
Historically, organisms have been classified on the basis of their morphology, but this can lead to errors
Similarities in observable characteristics do not always mean that organisms share a recent common ancestor
E.g. dolphins and sharks could in theory be grouped together as they are both groups of aquatic animals that share a similar body shape
However, they belong to different classes
Dolphins are mammals and sharks are fish
Their streamlined body shapes evolved separately rather than originating in one common ancestor
One solution to this difficulty is to use genome sequencing data
This helps to avoid the difficulties with misleading morphology
Taxonomic rank
In the hierarchy system described above, each level of classification fits into an established taxonomic rank
i.e. kingdom, phylum, class, etc.
Classification can be complicated if:
A group of organisms falls across taxa
Or organisms need to be moved from one taxon to another
For example, plant species in distant taxa can sometimes breed together to produce fertile hybrids
The resulting offspring will technically be a new species, but will be very difficult to classify under the hierarchy of taxa
Moving a group of organisms between taxa can risk of needing to move all of the groups currently in a taxon into a different rank to make room for the new grouping
Species
The point at which two populations are classified as different species can be highly subjective
The fertile offspring of a cross between two species may go on to only breed with members of one parent species
This is known as introgression
Introgression demonstrates how difficult it can sometimes be to neatly apply species classification
The resulting offspring after several generations do not fit completely into either species
But neither does it seem to make sense to classify them as a new species
E.g. hundreds of thousands of years ago, an early human bred with a Neanderthal and the offspring of this cross then went on to breed only with early humans
The result of this is that some groups of modern humans have some Neanderthal genes in their genomes
A fixed ranking of taxa may not be logical because it does not reflect the gradation of variation
The hierarchy of taxa described here has been arbitrarily set up by humans
This is mainly because it is a neat way of organising life into groups, not because it fits with the patterns that we see in the natural world
In order to fall neatly into the taxonomic ranks of the traditional classification system, species need to:
Be clearly distinct from each other
Obey reproduction rules, e.g. not interbreeding with other species
Produce fertile offspring
This is far from the reality of the biological world, where the differences between organisms are on a gradual scale and neat breeding rules do not always apply
A successful classification system needs to follow the evidence rather than seeking to fit the natural world into a human-designed system
New evidence comes from genome sequencing, and often leads to the reclassification of species
The newer system of cladistics uses unranked groups based on evolutionary relationships alone to produce evolutionary trees
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