5.4 Clades Flashcards
What is a clade?
A clade is a group of organisms that have evolved from a common ancestor. Species evolve over time to form new species, this happens a lot and so there are now loads of species derived from the same ancestor these are in a clade.
How do we identify members of a clade?
Evidence for which species are part of a clade can be obtained from the base sequences of a gene or the corresponding amino acid sequence of a protein. Species that have a recent common ancestor can be expected to have a few differences in base or amino acid sequence. Conversely, species that might look similar in certain respects but diverged from a common ancestor tens of millions of years ago are likely to have many differences.
What is the relationship between number of differences and time since they diverged?
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.
Differences occur due to mutations, there is evidence that mutations occur at a roughly constant rate so they can be used as a molecular clock. The number of differences in a sequence can be used to deduce how long ago different groups split from a common ancestor.
How are molecular clocks used?
Differences occur due to mutations, there is evidence that mutations occur at a roughly constant rate so they can be used as a molecular clock. The number of differences in a sequence can be used to deduce how long ago different groups split from a common ancestor.
What are homologous traits?
Homologous structures are similar because of similar ancestry; for example the chicken wing, human arm and other pentadactyl forelimbs.
What are analogous structures?
Analogous structures are similar because of convergent evolution. The human eye and the octopus eye show similarities in structure and function but they are analogous because they evolved independently.
How do we classify clades?
Because of analogous structures looking like they came from each other and not, the form and structures are rarely used for identifying members of a clade and evidence from base or amino acid sequences is trusted more.
What are the branching points on a cladogram called?
Nodes
Give an example of reclassification of organisms because of amino sequencing revealing the order of evolution or clades and not just looking and structures and seeing what seemed to evolve from what?
Classification of the figwort family:
- There are more the 400 families of angiosperms
- Until recently the eighth largest was the Scrophylaria, more commonly known as the figwort family.
- Taxonomists recently investigated the evolutionary origins of the figwort family using cladistics. One important research project compared the base sequences of three chloroplast genes in a large number of species in close related families.
- It was found that species in the figwort family were not a true clade and that five clades has incorrectly been combined into one family.
- Major reclassification was carried out and now loads of plants have been removed from the family. It is now only the 36th largest.
