5.4 Cladistics

Question 1

What is cladistics?

Answer 1

Cladistics is a method of classifying organisms into groups of species called clades (from Greek ‘klados’ = branch)

Question 2

What does each clade consist of?

Answer 2

Each clade consists of an ancestral organism and all of its evolutionary descendants

Question 3

What will members of a clade share?

Answer 3

Members of a clade will possess common characteristics as a result of their shared evolutionary lineage

Question 4

What is the purpose of organising clades?

Answer 4

Clades can be organised according to branching diagrams (cladograms) in order to show evolutionary relationships

Question 5

What are cladograms and what do they represent?

Answer 5

Cladograms are tree diagrams where each branch point represents the splitting of two new groups from a common ancestor

Question 6

What does each node represent?

Answer 6

Each branch point (node) represents a speciation event by which distinct species are formed via divergent evolution

Question 7

What do cladograms show in terms of divergence?

Answer 7

Cladograms show the probable sequence of divergence and hence demonstrate the likely evolutionary history (phylogeny) of a clade

Question 8

What does a fewer number of nodes show?

Answer 8

The fewer the number of nodes between two groups the more closely related they are expected to be

Question 9

What do cladograms show in terms of evolution?

Answer 9

Cladograms can show evolutionary relationships and demonstrate how recently two groups shared a common ancestry

Question 10

What evolutionary history do humans share with primates i.e. what common clade?

Answer 10

Humans, chimpanzees, gorillas, orangutans and gibbons all belong to a common clade – the Hominoids

Question 11

What larger clade are hominoids a part of?

Answer 11

The Hominoid clade forms part of a larger clade – the Anthropoids – which includes Old World and New World monkeys

Question 12

What 4 features do cladograms share?

Answer 12

root
nodes
outgroup
clades

Question 13

What is a root on a cladogram?

Answer 13

Root – The initial ancestor common to all organisms within the cladogram (incoming line shows it originates from a larger clade)

Question 14

What is a node on a cladogram?

Answer 14

Nodes – Each node corresponds to a hypothetical common ancestor that speciated to give rise to two (or more) daughter taxa

Question 15

What is an outgroup on a cladogram?

Answer 15

Outgroup – The most distantly related species in the cladogram which functions as a point of comparison and reference group

Question 16

What are clades on a cladogram?

Answer 16

Clades – A common ancestor and all of its descendants (i.e. a node and all of its connected branches)

Question 17

Based on what two factors can cladograms be constructed?

Answer 17

Cladograms can be constructed based on either a comparison of morphological (structural) features or molecular evidence

Historically, structural features were used to construct cladograms, but molecular evidence is now more commonly used

Question 18

How to construct a cladogram

Answer 18

https://ib.bioninja.com.au/standard-level/topic-5-evolution-and-biodi/54-cladistics/cladograms.html

Question 19

What chemical similarity do all organisms have?

Answer 19

All organisms use DNA and RNA as genetic material and the genetic code by which proteins are synthesised is (almost) universal

Question 20

Due to DNA almost being universal, what can be compared to ascertain evolutionary relationships?

Answer 20

This shared molecular heritage means that base and amino acid sequences can be compared to ascertain levels of relatedness

Question 21

What will accumulate in DNA over the course of time?

Answer 21

Over the course of millions of years, mutations will accumulate within any given segment of DNA

Question 22

What does the number of comparable base sequences demonstrate?

Answer 22

The number of differences between comparable base sequences demonstrates the degree of evolutionary divergence

Question 23

What does a greater difference between comparable base sequences suggest?

Answer 23

A greater number of differences between comparable base sequences suggests more time has past since two species diverged

Question 24

Therefore from base sequences, how can we determine whether organisms are evolutionarily related?

Answer 24

Hence, the more similar the base sequences of two species are, the more closely related the two species are expected to be

Question 25

What molecular sequences can scientists compare?

Answer 25

When comparing molecular sequences, scientists may use non-coding DNA, gene sequences or amino acid sequences

Question 26

Why is non-coding DNA compared?

Answer 26

Non-coding DNA provides the best means of comparison as mutations will occur more readily in these sequences

Question 27

Why do gene sequences mutate at a slower rate?

Answer 27

Gene sequences mutate at a slower rate, as changes to base sequence may potentially affect protein structure and function

Question 28

What sequence has the slowest rate of change?

Answer 28

Amino acid sequences may also be used for comparison, but will have the slowest rate of change due to codon degeneracy

Question 29

Therefore what are amino acid sequences used to compare?

Answer 29

Amino acid sequences are typically used to compare distantly related species (i.e. different taxa)

Question 30

What are DNA/RNA base sequences used to compare?

Answer 30

while DNA or RNA base sequences are often used to compare closely related organisms (e.g. different haplogroups – such as various human ethnic groups)

Question 31

Can mutations accumulate at a constant rate?

Answer 31

YES
Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. 1 change per million years)

Question 32

What can constant mutation be used for?

Answer 32

If this rate of change is reliable, scientists can calculate the time of divergence according to the number of differences

E.g. If a gene which mutates at a rate of 1 bp per 100,000 years has 6 bp different, divergence occurred 600,000 years ago

Question 33

What is using mutations at a constant rate to date divergence called?

Answer 33

This concept is called the molecular clock

Question 34

What are 3 limitations of a molecular clock?

Answer 34

Different genes or proteins may change at different rates (e.g. haemoglobin mutates more rapidly than cytochrome c)

The rate of change for a particular gene may differ between different groups of organisms

Over long periods, earlier changes may be reversed by later changes, potentially confounding the accuracy of predictions

Question 35

Historically, what was classification based off of?

Answer 35

Historically, classification was based primarily on morphological differences (i.e. structural characteristics)

Closely related species were expected to show similar structural features, indicating common ancestry

Question 36

What are two key limitations of using morphological differences as a basis for classification?

Answer 36

Closely related organisms can exhibit very different structural features due to adaptive radiation (e.g. pentadactyl limb)

Distantly related organisms can display very similar structural features due to convergent evolution

Question 37

What is convergent evolution?

Answer 37

Convergent evolution is the independent evolution of similar features in species with distinct lineages

Question 38

When may convergent evolution occur?

Answer 38

It may occur when different species occupy the same habitat and are thus subjected to the same selection pressures

Question 39

What is an example of convergent evolution?

Answer 39

The shared conditions cause common adaptations to be selected in different species, resulting in structural similarity

An example of convergent evolution is the development of wings in birds, bats and insects

Question 40

Why are structural features not commonly used to determine clades?

Answer 40

Structural traits are not commonly used to determine clades as such features may not necessarily indicate shared heritage

Question 41

What are homologous structures?

Answer 41

Traits that are similar because they are derived from common ancestry are termed homologous structures

Question 42

What are analogous structures?

Answer 42

Traits that are superficially similar but were derived through separate evolutionary pathways are termed analogous structures

Question 43

By what process do homologous structures arise?

Answer 43

arise via divergent evolution

Question 44

by what process do analogous structures arise?

Answer 44

arise via convergent evolution

Question 45

What evidence has debunked many evolutionary similarities (based on morphology)

Answer 45

Using molecular evidence, scientists have discovered that many species thought to be closely related based on shared structural characteristics actually demonstrate distinct evolutionary origins

Question 46

Give two examples of molecular evidence debunking previous evolutionary relationships

Answer 46

Crocodiles have been shown to be more closely related to birds than lizards, despite closely resembling lizards in structure

Many species of plants previously classified as figworts have been reclassified based on molecular evidence

Question 47

What are figworts?

Answer 47

Until recently, figworts were the 8th largest family of flowering plants (angiosperms), containing 275 different genera

Question 48

Why was the figwort group not perfect?

Answer 48

This was problematic as many of the figwort plants were too dissimilar in structure to function as a meaningful grouping

Question 49

How was the figwort split into different clades?

Answer 49

Taxonomists examined the chloroplast gene in figworts and decided to split the figwort species into five different clades

Question 50

How has reclassification affected the figwort family?

Answer 50

Now less than half of the species remain in the figwort family – which is now the 36th largest among angiosperms