Classification & Cladistics

why is classification needed

• diversity on Earth is vast

• to make sense of the huge amount of species, scientists organise them into logical groups, which helps understand relationships between organisms

• organisms are placed in groups according to their

  • similar characteristics

  • evolutionary origins

taxon

a group of organisms that have been given a group name by scientists based on their shared features

taxonomic hierarchy of taxa

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

difficulties with traditional hierarchy of taxa

• unclear how populations should be grouped into species

• even if a species is agreed on, taxonomists disagree over what rank the group should have

• these uncertainties are the result of the gradual divergence of species and large groups over time

why is a fixed ranking of taxa arbitrary

• doesn’t reflect the gradation of variation

• set up arbitrarily by humans because it’s a neat way of organising life into groups

• cladistics offers an alternative approach to classification (example of paradigm shift of scientific theories)

paradigm shift

a fundamental change in approach

cladistics

the branch of science where scientists put organisms into clades

clade

• a group of organisms that have all descended from a common ancestor

• can be large or small, not all members have to be alive

advantages of classifying using cladistics

• ensures that groups of organisms are close evolutionary relatives, rather than arbitrary groups that happen to look similar

• characteristics within a clade are often inherited from a common ancestor, so likely to be shared and can be predicted

how to place organisms into clades

• most objective method is using sequencing data

  • the more similar the sequences, the closer related the species

• morphology

  • species that share a more recent common ancestor are more likely to share similar morphologies

  • this method is more subjective

  • more closely related species share large numbers of derived traits

sequencing data & the molecular clock

• the number of differences between sets of sequence data provide information on how closely related the species are

• they can also provide a quantitative estimate on when the species diverged

  • differences in sequence data are due to mutations

  • the number of mutations give an indication of the amount of time passed since the species diverged

  • the constant rate of mutation is called the molecular clock

why the molecular clock is only an estimate

• mutation rate can be affected by factors such as:

  • population size

  • generation time

  • selection pressures

cladograms

• evolutionary trees that show probable order of divergence and likely evolutionary history

• information used to build cladograms come from base or amino acid sequence data

parsimony analysis

• states that the simplest explanation is the most likely

• cladograms are constructed using available evidence on the basis of parsimony

analysis of cladograms

• the point at which 2 branches separate is a node, nodes on a cladogram represent common ancestors

• a node immediately adjacent to a pair of clades shows that these 2 clades share a recent common ancestor

• the root of a cladogram is found at its base, represents the common ancestor of all the organisms within the cladogram

• the terminal branch represents the most recent species in evolutionary lineage

reclassification & cladistics

• cladistics can be used to investigate whether the classification of groups corresponds to evolutionary relationships

• e.g the reclassification of the figwort family

why organisms may be reclassified

• organisms used to be classified based on shared physical characteristics

• shared physical characteristics are not due to common ancestry

• those characteristics may be analogous (a result of convergent evolution)

theories and other scientific knowledge claims can eventually be falsified

• when new evidence that no longer supports the theory is found, it needs to be changed

• e.g the figwort family, similarities in morphology due to convergent evolution rather than common ancestry suggested a classification that by cladistics has proven to be false

domains

• the science of taxonomy has frequently changed to match the latest discoveries about features of organisms

• recent (1977) rRNA analysis has shown that there are 2 distinct groups of prokaryotes, leading to a shift in taxonomic thinking, and the beginning of a 3 domain system

• largest taxonomic group is now known as a domain

3 domains

archaea, eubacteria, eukaryota