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This set contains the content understandings, applications, skills and nature of science syllabus statements for IB Biology topic 5.4: Cladistics.
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Mutations in the nucleotide base sequences of DNA, and therefore differences in the amino acid sequence of proteins, accumulate gradually over time.
Therefore, the more differences there are in biological sequences, the more time has passed for the differences to accumulate. The more time that has passed since organisms have shared a common ancestor, the less evolutionarily related the organism are to each other.
Visa versa: the less differences there are in biological sequences, the less time has passed for the differences to accumulate. The less time that has passed since organisms have shared a common ancestor, the more evolutionarily related the organism are to each other.
Therefore, the more differences there are in biological sequences, the more time has passed for the differences to accumulate. The more time that has passed since organisms have shared a common ancestor, the less evolutionarily related the organism are to each other.
Visa versa: the less differences there are in biological sequences, the less time has passed for the differences to accumulate. The less time that has passed since organisms have shared a common ancestor, the more evolutionarily related the organism are to each other.
The DNA base sequences of the same gene in different species is compared. Because mutations (differences) in the DNA sequence will accumulate over time, the species with more similarities in the base sequence are likely more closely related (meaning they have a more recent common ancestor; have diverged from their common ancestor more recently) than species with more differences in the base sequence. The percent similarity between species can be used as the basis for creating a cladogram of hypothesized evolutionary relationships.
The amino acid sequences of the same protein in different species is compared. Differences in the amino acid sequences are due to mutations of the DNA. Because DNA mutations (and in turn amino acid sequences) will accumulate over time, the species with more similarities in the amino acid sequence are likely more closely related (meaning they have a more recent common ancestor; have diverged from their common ancestor more recently) than species with more differences in the amino acid sequence. The percent similarity between species can be used as the basis for creating a cladogram of hypothesized evolutionary relationships.
Outline the use of a “molecular clock” to determine time since divergence between two species.
Understanding: Sequence differences accumulate gradually so there is a positive correlation between the number of differences between two species and the time since they diverged from a common ancestor.
Understanding: Sequence differences accumulate gradually so there is a positive correlation between the number of differences between two species and the time since they diverged from a common ancestor.
The "molecular clock" is a technique that uses the rate of mutation of DNA sequences (or amino acid sequences of proteins) to estimate the time when two species diverged from a common ancestor. The molecular clock hypothesis states that DNA and protein sequences change at a rate that is relatively constant over time and among different organisms. A consequence of this constancy is that the genetic difference between any two species is proportional to the time since these species last shared a common ancestor. 
Different species share molecular homologies as well as anatomical ones. Roundworms, for example, share 25% of their genes with humans. These genes are slightly different in each species, but their similarities are evidence of common ancestry.
The universal genetic code is a homology that links all life on Earth to a common ancestor. DNA and RNA possess a simple four-base code that provides the information for making proteins in all living things.
The universal genetic code is a homology that links all life on Earth to a common ancestor. DNA and RNA possess a simple four-base code that provides the information for making proteins in all living things.
Cladograms are branching diagrams where each branch represents an evolutionary lineage (a clade). Cladograms are used to show the hypothesized sequence of divergence between organisms and common ancestor(s). The names of specific taxa are put at the end of each branch and the names of the clade to which those taxa belong is often placed at the intersection of branches (nodes).
A cladogram does not really have axes, but implied in one direction (x axis in this example) is some sort of evolutionary distance from each other and implied in the other direction (y axis in this example) is relative time (the first branch at the bottom happened long ago the latter branches higher up happened more recently).
A cladogram does not really have axes, but implied in one direction (x axis in this example) is some sort of evolutionary distance from each other and implied in the other direction (y axis in this example) is relative time (the first branch at the bottom happened long ago the latter branches higher up happened more recently).
To build a cladogram, heritable traits are compared across organisms, such as physical characteristics (morphology), genetic sequences, and behavioral traits.
Homologous traits can be used to group organisms into clades. Traits shared among the species or groups in a dataset tend to form nested patterns that provide information about when branching events occurred in the lineage.
For example, amphibians, turtles, lizards, snakes, crocodiles, birds and mammals all have (or had) four limbs. Four limbs is a homologous trait inherited from a common ancestor that helps set apart this particular clade from other vertebrates.
Homologous traits can be used to group organisms into clades. Traits shared among the species or groups in a dataset tend to form nested patterns that provide information about when branching events occurred in the lineage.
For example, amphibians, turtles, lizards, snakes, crocodiles, birds and mammals all have (or had) four limbs. Four limbs is a homologous trait inherited from a common ancestor that helps set apart this particular clade from other vertebrates.
With sequencing technology we can compare the sequences evolutionarily related genes or proteins. Each nucleotide of a gene or amino acid of a protein can be viewed as a separate characteristic. The amount of data available from sequence comparisons is much higher than when using physical traits. To analyze sequence data and identify the most probable cladogram, biologists typically use computer programs and statistical algorithms that are able to rapidly and accurate compare sequences, compute differences between sequences and analyze most likely divergence patterns between sequences. 
This cladogram tells you that all the animals listed are in the mammalia clade. The koala and kangaroo are more closely related to one another than to anything else on the diagram. Their branches intersect each other before they intersect any other lineage. This indicates that they diverged from each other more recently than they diverged from any of the other animals on the diagram. Furthermore, they both belong to the Marsupialia clade. Similarly, the bat and the lion are more closely related to each other than they are to either of the marsupials. They are both in the placental mammals (eutherians) clade.