5.4 Syllabus
U1. A clade is a group of organisms that have evolved from a common ancestor.
-Over time species evolve and split to from new species. This process can occur repeatedly with some highly successful species leading to a large group of organisms that share a common ancestor.
-These groups of species evolved from a common ancestor, that have shared characteristics is called a clade is method of classifying organisms into groups of species called clades.
-These groups of species evolved from a common ancestor, that have shared characteristics is called a clade is method of classifying organisms into groups of species called clades.
- Each clade consists of an ancestral organism and all of its evolutionary descendants
- Members of a clade will possess common characteristics as a result of their shared evolutionary lineage
- Branch points in the tree represent the time at which two taxa split from each other
- The degree of divergence between branches represent the differences that have developed between the two taxa since they diverged
U2. 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.
-All organisms use DNA and RNA as genetic material and the genetic code by which proteins are synthesized is universal. This shared molecular heritage means that base and amino acid sequences can be compared to ascertain levels of relatedness.
-Over the course of millions of years, mutations will accumulate within any given segment of DNA.
-Sometimes determining which species are part of certain clade is difficult. The most accurate evidence is derived from amino acid sequences of certain proteins.
-Over the course of millions of years, mutations will accumulate within any given segment of DNA.
-Sometimes determining which species are part of certain clade is difficult. The most accurate evidence is derived from amino acid sequences of certain proteins.
- Phylogeny = the evolutionary history of a taxonomic group, often shown in a phylogenetic tree
- Mutations in DNA occur with predictable rates
- Differences can be used as a molecular clock to develop phylogeny
- DNA nucleotide sequences
- Protein amino acid sequences
U3. 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.
-Differences in nucleotide base sequences in DNA, and therefore amino acid sequences in proteins, accumulate gradually over long periods of time
- Differences accumulate at roughly constant and predictable rate
- Therefore, the number of differences can be used as a clock
- To measure the time since two divergent groups shared a common ancestor
- However, variations are partly due to mutations
- Which are unpredictable chance events
- So there must be caution in interpreting data
U4. Traits can be analogous or homologous.
-Analogous characteristic: Structures with a common function, but a different evolutionary origin
- Example: Dolphin fins and Shark fins
- Example: Dolphin forelimbs and human arms
U5. Cladograms are tree diagrams that show the most probable sequence of divergence in clades.
-Cladograms are tree diagrams where each branch point represents the splitting of two new groups from a common ancestor. Each branch point (node) represents a speciation event by which distinct species are formed via divergent evolution.
-Cladograms show the probable sequence of divergence and hence demonstrate the likely evolutionary history of a clade. The fewer the number of nodes between two groups the more closely related they are expected to be.
-The strength of cladistics is that the comparisons are objective, relying on morphological and molecular homologies
-The weakness of cladistics is that molecular differences are analyzed on the basis of probabilities.
-Cladograms show the probable sequence of divergence and hence demonstrate the likely evolutionary history of a clade. The fewer the number of nodes between two groups the more closely related they are expected to be.
-The strength of cladistics is that the comparisons are objective, relying on morphological and molecular homologies
-The weakness of cladistics is that molecular differences are analyzed on the basis of probabilities.
- Improbable events occasionally occur, making the analyses wrong
U6. Evidence from cladistics has shown that classifications of some groups based on structure did not correspond with the evolutionary origins of a group or species.
-Historically, classification was based primarily on morphological differences (Structural characteristics). Closely related species were expected to show similar structural features, indicating common ancestry. However, there are two key limitations to using morphological differences as a basis for classification:
-In many cases, cladograms have confirmed existing classification, as expected, since both are based on phylogeny
-In some cases, cladograms can be difficult to reconcile with traditional classifications
-In some cases, cladograms radically alter existing classifications
- Closely related organisms can exhibit very different structural features due to adaptive radiation (Pentadactyl limb)
- Distantly related organisms can display very similar structural features due to convergent evolution
-In many cases, cladograms have confirmed existing classification, as expected, since both are based on phylogeny
-In some cases, cladograms can be difficult to reconcile with traditional classifications
- Nodes can be placed at any point
- Making the fit of taxa to the cladogram arbitrary
-In some cases, cladograms radically alter existing classifications
- For example, birds are grouped within a clade including dinosaurs