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Biology IB HL - 5.4 Cladistics Part 2
This deck covers key concepts in cladistics, focusing on molecular evidence and its impact on understanding evolutionary relationships. It includes questions on DNA comparison, mutation rates, molecular clocks, and the reclassification of species.
Why is non-coding DNA compared?
Non-coding DNA provides the best means of comparison as mutations will occur more readily in these sequences
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Key Terms
Term
Definition
Why is non-coding DNA compared?
Non-coding DNA provides the best means of comparison as mutations will occur more readily in these sequences
Why do gene sequences mutate at a slower rate?
Gene sequences mutate at a slower rate, as changes to base sequence may potentially affect protein structure and function
What sequence has the slowest rate of change?
Amino acid sequences may also be used for comparison, but will have the slowest rate of change due to codon degeneracy
Therefore what are amino acid sequences used to compare?
Amino acid sequences are typically used to compare distantly related species (i.e. different taxa)
What are DNA/RNA base sequences used to compare?
while DNA or RNA base sequences are often used to compare closely related organisms (e.g. different haplogroups – such as various human ethnic groups)
Can mutations accumulate at a constant rate?
YES
Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. 1 change per million years)
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| Term | Definition |
|---|---|
Why is non-coding DNA compared? | Non-coding DNA provides the best means of comparison as mutations will occur more readily in these sequences |
Why do gene sequences mutate at a slower rate? | Gene sequences mutate at a slower rate, as changes to base sequence may potentially affect protein structure and function |
What sequence has the slowest rate of change? | Amino acid sequences may also be used for comparison, but will have the slowest rate of change due to codon degeneracy |
Therefore what are amino acid sequences used to compare? | Amino acid sequences are typically used to compare distantly related species (i.e. different taxa) |
What are DNA/RNA base sequences used to compare? | while DNA or RNA base sequences are often used to compare closely related organisms (e.g. different haplogroups – such as various human ethnic groups) |
Can mutations accumulate at a constant rate? | YES
Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. 1 change per million years) |
What can constant mutation be used for? | 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 |
What is using mutations at a constant rate to date divergence called? | This concept is called the molecular clock |
What are 3 limitations of a molecular clock? | 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 |
Historically, what was classification based off of? | 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 |
What are two key limitations of using morphological differences as a basis for classification? | 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 |
What is convergent evolution? | Convergent evolution is the independent evolution of similar features in species with distinct lineages |
When may convergent evolution occur? | It may occur when different species occupy the same habitat and are thus subjected to the same selection pressures |
What is an example of convergent evolution? | 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 |
Why are structural features not commonly used to determine clades? | Structural traits are not commonly used to determine clades as such features may not necessarily indicate shared heritage |
What are homologous structures? | Traits that are similar because they are derived from common ancestry are termed homologous structures |
What are analogous structures? | Traits that are superficially similar but were derived through separate evolutionary pathways are termed analogous structures |
By what process do homologous structures arise? | arise via divergent evolution |
by what process do analogous structures arise? | arise via convergent evolution |
What evidence has debunked many evolutionary similarities (based on morphology) | Using molecular evidence, scientists have discovered that many species thought to be closely related based on shared structural characteristics actually demonstrate distinct evolutionary origins |
Give two examples of molecular evidence debunking previous evolutionary relationships | 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 |
What are figworts? | Until recently, figworts were the 8th largest family of flowering plants (angiosperms), containing 275 different genera |
Why was the figwort group not perfect? | This was problematic as many of the figwort plants were too dissimilar in structure to function as a meaningful grouping |
How was the figwort split into different clades? | Taxonomists examined the chloroplast gene in figworts and decided to split the figwort species into five different clades |
How has reclassification affected the figwort family? | Now less than half of the species remain in the figwort family – which is now the 36th largest among angiosperms |