Biodiversity – Sept. 17/12
- Phylogenetics: Phylogeny is evolutionary history, the evolutionary history of descent of a group of
taxa (a group of species, genera, families, etc.). A lineage is an ancestor and its descendants. All taxa
are lineages – they include the ancestor and its descendants. Phylogenetics is the pursuit of
determining the evolutionary relationships of groups of taxa and the building of natural
classifications based on those relationships.
- Homology/homologous features: Traits or features that different lineages inherited from their
common ancestor. The presence of homologous features in two different organisms defines
relatedness or common ancestry.
- Analogy/analogous features: Traits or features that appear to be similar, but evolved independently.
They are not similar due to relatedness or common ancestry, but due to convergent evolution, or
independent evolution of similar forms.
- Characters are traits/features/structures/characteristics. Character states – the character or trait
can occur in more than one condition. Ex. In Homo sapiens, a character is hair colour, and the
character states are brown, black, red, blond.
- Basal (primitive): At or pertaining to the base. The unmodified condition. Ancestral condition.
- Derived (advanced): Away from the base. Modified relative to the basal or ancestral condition.
Having evolved from the basal condition.
- The cladistics method (cladistics): The cladistics method has its beginnings with publications in the
1950s and 60s by Willi Hennig, a German entomologist. Cladistics is a method of classifying species of
organisms into groups called clades, which consist of an ancestor organism and all its descendants
(and nothing else). Prior to Hennig, any morphological resemblance equaled a degree of
phylogenetic relatedness. If two taxa looked similar, they were assumed to be related and were
classified together in the same taxon. This was called phonetics, which means prior to cladistics.
There was no attempt to resolve phylogeny in phonetics. Hennig broke up the concept of
morphological resemblance by proposing three different categories of morphological resemblance,
or three different ways in which taxa can morphologically resemble each other.
- Type 1 – Morphological resemblance due to convergent evolution.
- Type 2 – Morphological resemblance due to a symplesiomorphy (shared primitive or basal
- Type 3 – Morphological resemblance due to a synapomorphy (shared advanced or derived
- Hennig’s second major contribution to systematics was saying that Type 3 is the only type of
morphological resemblance that can be used to define a taxon, or natural group. Types 1 and 2 are
not evidence to define a taxon, or natural group.
- Type 1: Convergent evolution is the independent evolution of similar features in different
evolutionary lineages. Example – Hind jumping legs have evolved in the kangaroo and the
grasshopper. They serve the same function (jumping type of locomotion) and they are structurally
similar (large, highly muscular), but they evolved independently. We know that the common ancestor
of the kangaroo and the grasshopper did not have hind jumping legs. The hind jumping legs evolved
independently in each lineage. These are analogous structures – they are analogues. Analogous
structures look similar and have a similar function, but this is due to convergence and not to common
ancestry. Another example is insect and bird wings. They have similar form and function, but they are
- Type 2: A symplesiomorphy is a basal or ancestral character or character state that is shared by
members of a taxon. Problem – How do we define mammals from all other terrestrial vertebrates with four locomotory appendages? Mammals h