Classification (HL) (DP IB Environmental Systems & Societies (ESS))

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

Awaiting image: Clades

Image caption: 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

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