5.4 ~ Cladistics

Essential idea of Cladograms

The ancestry of groups of species

Cladograms

• Are tree diagrams based on similarities or differences between species in a Clade

  • Nowadays, these are almost exclusive based on amino acid sequences

Clades

• a group of organisms that consist of a common ancestor and all its lineal descendants and represents a single ‘branch’ on the ‘tree of life’

• Principle of parsimony in that species with fewer differences are closely related and those with more differences are more distantly related

• Branching points are called nodes

  • Represent ancestral species where characteristics branch and species diverge

Parts of a cladogram

• The root of a cladogram is the point prior to any divergence where the organism share certain characteristics

• An outer group is the most distantly related organism on a cladogram

  • It will be on the longest branch of the tree and will be set apart from the other branches

Species over time

• Evolve over time to form new species

  • This can happen repeatedly so there are groups of species all with a common ancestor

• These groups of species can be identified by looking for shared characteristics

• A group of species evolved from a common ancestor is called a clade

• Clades include all of the species alive today with any ancestral species and species that evolved from that are now extinct

Evidence for clades

• All living materials use DNA as their genetic material

• As such, with the development of DNA sequencing and proteomics, scientist have been better able to determine which species belong in a clade together

• Species that have recent common ancestor will have few differences in the base or amino acid sequence

  • However, species that diverged from a common ancestor millions of yers ago are likely to have many differences

• Therefore, the more similar the DNA base or amino acid sequences between two species, the more closely related they are, the more recently the species diverged

  • And vice-versa

Differences in DNA

• Differences in DNA base sequences arise due to mutations

  • These accumulate over long periods of time

• Evidence suggests that mutations occur at a relatively constant rate

  • As such, the number of mutations can be used to approximately deduce how long ago two species diverged

  • Called the molecular clock

Sequence differences

• If the DNA base sequences of two species are similar, then few mutations have occured - species only diverged relatively recently

• There is a positive correlation between the number of differences and the time since they diverged from a common ancestor

  • The longer the time period since the two species separated, the more differences there will be when the DNA of the two species is compared

Principle of parsimony

Species with fewer differences are closely related and those with more differences are more distantly related

Homologous structures

Similar because organisms have evolved from a common ancestor (e.g. the pendactyl limb)

Analogous structures

Similar because of convergent evolution (organism evolving similar structures due to commonalities in the environment they are in)

E.g. the white coat of the arctic fox and polar bear

Analogous and homologous structures

• Historically, it has been very difficult to distinguish between homologous and analogous structures and this has led to mistakes in classification

  • Nowadays, morphology (forms and structures) of organisms is rarely used for classification purposes, and, instead, DNA bases or amino sequences are deemed more reliable

Cladistics

• The construction of Cladograms and identification of Clades is called cladistics

  • Their constriction based on DNA bases and amino acid sequences only become possible toward the end of the 20th century

• Cladistics has caused many revolutions in the classification of animals and plant species

  • It is now clear that traditional classification based on morphology were not always very reliable and did not match the evolutionary origins suggested by DNA analysis

Classification of groups

• Many groups have been reclassified: some have merged, some have been divided and some have transferred from one group to another

• Whilst reclassification of organisms is disruptive for biologists, it utilises a far more reliable evidence which have much higher predictive power than previous classification

  • These reclassification are far more likely to represent the actual evolution of a species

• It has also revealed some differences between species previously thought to be similar, and identified similarities between species previously thought to be different

Reclassification of the figwort family

• Scrophulariceae were, until recently, the 8th largest family of angiosperms

• Composed of 16 genera and was proposed by de Jussieu in 1789 based on morphological similarities

• As more plants were discovered, the family grew to 275 genera and over 5000 clades

  • Researches compared the base sequences of three different chloroplast genes in genera assigned to the figwort family and closely related families

• Many species in the family were not a true clade and 5 clades had been incorrectly combined into one family

• DNA analysis show the similarities in flower shape to be a product of convergence evolution

• Less than half off the species that were originally in the figwort family still remain and it is now the 36th largest family of angiosperms

Skill: analysis of Cladograms to deduce evolutionary relationships

• Branching pattern is assumed to represent the evolutionary relationship between species

• If extinct species are included, information must be given on whether the cladogram is based on morphology only because DNA is generally not available for the analysis, although there are some exceptions to this

• The more nodes there are between species, the more distant their relationship

• Mutations at the DNA and protein level are assumed to occur at a constant rate

• Some Cladograms are drawn to scale, the length of the branches is proportional to the time over divergence

• Cladograms assume that the smallest number of mutations occured to account for the differences in base/amino acid sequence

  • However, sometimes this assumption is not correct and evolution is a lot more convoluted

  • Comparison of different versions of Cladograms using different genes is an effective way to assess the reliability of a hypothetical evolutionary relationship