5. Cladistics Flashcards
Define a ‘clade’.
If you’re interested or it helps you learn it: where is the term ‘clade’ derived from?
a group of organisms that evolved from a common ancestor:
- can be large groups with a common ancestor far back in evolution
- or can be smaller groups with a more recent common ancestor
‘clade’ derived from Ancient Greek “klados” - a branch
How can evidence for which species are part of a clade be obtained?
- by looking at any characteristics anatomical features are now rarely used because it is sometimes hard to distinguish between homologous traites
Evidence for which species are part of a clade can be obtained by looking at any characteristics. Is there an exception to this statement? Why?
anatomical features are now rarely used:
- because it is sometimes hard to distinguish between homologous traits derived from a common ancestor, and analogous characteristics that have developed by convergent evolution
Evidence for which species are part of a clade can be obtained by looking at any characteristics. What is usually used to identify which clade a species should be in? Why?
base sequences of a gene are used, or the corresponding amino acid sequence of a protein:
- sequence differences accumulate gradually, so there is a positive correlation between the number of differences between two species and the time they diverged from a common ancestor
Why is classifying humans particularly difficult?
because the differences between humans and other species seem so huge to us
Give an example of how can cladistics be used to produce an objective classification of humans?
sequencing mitochondrial DNA and then constructing a cladogram (p.70)
How long ago did humans and chimpanzees split into separate classifications (according to sequence differences in mitochondrial DNA)?
5mil years ago
How long ago did Africans and European/Japanese split into separate classifications (according to sequence differences in mitochondrial DNA)?
140,000 years ago
How long ago did European and Japanese split into separate classifications (according to sequence differences in mitochondrial DNA)?
70,000 years ago
What are cladograms?
tree diagrams that show the most probable sequence of divergence in clades
What are branching points on cladograms called? What do they show?
- branching points called nodes
- show groups of organisms which are related, and therefore presumably had common ancestry
What two things related to cladograms grew towards the end of the 20th century? What is significant about this?
1 amount of base and amino acid sequence data
2 analytical power of computers
grew exponentially.
Cladograms could therefore be produced showing the probable evolutionary relationships of large groups of species.
Better (more accurate and larger) cladograms could be made towards the end of the 20th century. What did this lead to?
a re-evaluation of the classification of many groups of organisms
What is ‘cladistics’?
the method of classification using cladograms
What has evidence from cladistics as a new method of classification shown us?
that classifications of some groups based on structure did not correspond with the evolutionary origins of a group or species
Give an example of an organism that has been re-classifed following new evidence generated by cladistics.
the figwort family of plants (Scrophulariaceae)
What did cladograms of the figwort family of plants (Scrophulariaceae) reveal?
- species in the family did not all share a recent common ancestor
How did this new evidence - that species in the Scrophulariaceae family did not all share a recent common ancestor - change our classification of Scrophulariaceae?
1 some genera were therefore moved to:
- the plantain and broomrape families
- two newly created families - lindernia and calceolaria families
2 two existing families, the buddleja and myoporum families, were found to contain species that shared common ancestry with the figwort (Scrophulariaceae) family so they were merged with it
Is cladistics the definitive way of classifying plants?
no - just as the traditional classification was falsified and replaced as a result of cladistics, evidence may be discovered that shows further reclassification is needed
What two things can analysing cladograms show us?
- how closely organisms are related to each other
- the probable sequence in which groups split
Practice reading cladograms.
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