What is Systematics?
- the theory and practice of classifying organisms based on evolutionary history (phylogeny)
- systematics attempt to construct evolutionary informative classifications.
What is a phylogeny ?
- depicted as a phylogenetic tree
- evolutionary history of a species, or group of related species.
What do phylogenetic trees show?
- show hypotheses for the evolutionary decent of different organisms, or genes from common ancestors.
Phylogenies are inferred from…
- Morphological data - the size, shape, and presence of different anatomical features.
- Molecular data - molecular systematics uses DNA, RNA, and protein structures to infer phylogenies.
A phylogenetic tree is…
a hypotheses about evolutionary relationships among a set of organisms called taxa.
- each branch point (node) represents the divergence of two species (speciation).
- each node represents the last common ancestor.
What is sister taxa?
Two descendants that split from the same node.
What does it mean when a phylogenetic tree is rooted?
One branch that corresponds to the common ancestor of all taxa on the tree.
What is polytomy?
a branch in a phylogenetic tree from which more than two groups emerge.
- typically represent unresolved patterns of divergence.
What is a basal taxon?
diverges early in a phylogenetic tree, and in the history of the group. Originates near the common ancestor.
What is a clade?
A piece of a phylogeny that includes an ancestor and all descendants (living and extinct) of that ancestor.
- a piece of a larger tree that can be cut away from the root with a single cut.
- clades form a nested hierarchy.
Can a single external branch be a clade?
Yes.
Because tips of external branches (descendent taxa) may represent a group group of taxa.
- it includes all descendants of a single ancestor.
What is a cladogram?
- 1 of 2 types of phylogenetic trees
- depict evolutionary relationships where only the pattern of branching (the topology) is important.
- branch length and the position of the descendant taxa convey no information.
- does not convey the passage of time.
What are phylograms?
- 1 of 2 types of phylogenetic trees
- also depict evolutionary patterns, but branch lengths are proportional to evolutionary change.
- phylogram branch length can represent chronological time, and may also reflect the number of character changes that took place between taxa in that lineage.
What do phylograms show?
- evolutionary relationships, not evolutionary progress.
- taxa that share a more recent common ancestor are more closely related.
- the number of nodes between a taxa does not indicate the relatedness of the taxa.
What do phylogenetic polytomies indicate?
- Lack of knowledge - polytomy indicates there is insufficient data to resolve patterns of divergence (do not make conclusions about lineages)
- Rapid speciation - multiple speciation events possibly happened at the same time. (ex: when a species rapidly expands its geographic range).
What is the conceptual difference between taxonomy and systematics criteria?
- taxonomy uses similarity and difference to define taxonomic ranks.
- systematics uses branching order (phylogeny) to define clades.
What are the two reasons that traits can be similar between different groups?
- the trait is similar because of decent from a common ancestor (shared ancestry) (homologous)
- the trait is similar due to independent adaptation (convergent evolution) (analogous)
What is homology?
- it is similarly due to shared ancestry.
- shared evolutionary origin
What is analogy?
- it is due to convergent evolution
- convergent evolution = the independent evolution of similar traits in different lineages.
- convergent evolution does not provide information about shared evolutionary history.
What is convergent evolution?
When similar environmental conditions and natural selection produce similar adaptations in organisms from different evolutionary lineages.
(anologous)
What is homoplasy?
- interchangeable with analogous
- analogous structures or molecular sequences that evolved independently
- a trait that is shared by a set of taxa, but the trait is not present in their common ancestor.
When building phylogenetic trees…
- only homologous traits should be used to build phylogenetic trees.
- only homologous traits reflect evolutionary history.
What is the ingroup in a phylogenetic tree?
the group of taxa whose evolutionary relationships you are interested in determining.
What is the outgroup of a phylogenetic tree?
one or more taxa that are related to the ingroup, but have diverged from the ingroup at an earlier time.
