Classification (HL) (HL IB ESS OLD COURSE - IGNORE)

Revision Note

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

Diagram contrasting clades and non-clades. Clades share a common ancestor; non-clades lack recent common ancestry or exclude some descendants.
Example of groups that form clades and groups that do not form clades
  • 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

Diagram showing the evolutionary tree of primates, highlighting the common ancestors of humans, chimpanzees, bonobos, gorillas, and orangutans with speciation points.
An evolutionary tree for five species of primate

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

A vertebrate cladogram
Phylogenetic tree diagram showing the evolutionary relationships among vertebrates, including fish, amphibians, reptiles, birds, and mammals.

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

Taxonomic classification chart with Eukarya at the domain level, followed by kingdom, phylum, class, order, family, genus, and species. Arrows lead to other groups.
The classification system within the Eukarya domain (note there are missing groups at each rank)

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

Diagram of species evolution showing hybridization and introgression. Hybridization between species X and Y creates a new hybrid species. Species X and Y diverge from a common ancestor.
Several generations after an interbreeding event has occurred, the species X individuals with some genes from species Y do not fit perfectly into either species X or Y, but cannot correctly be considered a new species; this is introgression
  • 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

Last updated:

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Bridgette Barrett

Author: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.