topic 4: systematics and phylogeny

systematics definition

theory and practice of classifying organisms based on evolutionary history (phylogeny)

phylogeny definition

shown as a phylogenetic tree, evolutionary relationships among organisms

• shows hypotheses for evolutionary relationships between extinct and extant organisms as lines of evolutionary descent of different species, organisms, or genes from common ancestors

biologists use phylogenies to:

• organize biological diversity: group organisms into groups based on shared evolutionary history

• visualize evolution: highlight events like speciation, adaptive radiations, and convergent evolution

• structure classification: align taxonomic classification to reflect evolutionary relationships

• guide research: generate hypotheses on traits and processes

data used to infer phylogenies

1. morphological data

2. molecular data

morphological data definition

physical traits such as size, shape, presence/absence of anatomical features

molecular data definition

uses DNA, RNA, and protein structures to infer phylogenies, applicable to all extant species, but not extinct species

speciation definition

divergence of two species from a common ancestor

sister taxa definition

two descendants split from the same node; shares an immediate common ancestor not shared with other taxa

basal taxon definition

diverges early in the history of a group and originates near the common ancestor of a phylogeny

polytomy definition

a branch from which more than two groups emerge; tends to indicate a lack of data, and/or rapid speciation

clades definition

groups that include an ancestor and ALL its descendants

cladograms

type of phylogenetic tree; depicts evolutionary relationships where only the branching pattern (topology) is important

• branch length and position of descendant taxa do not convey information about time or evolutionary change

phylograms

type of phylogenetic tree; depicts evolutionary patterns, branch lengths represent evolutionary change

• evolutionary difference between two taxa is the sum of branch lengths from their common ancestor

homologous traits definition

traits that are similar because of descent from a common ancestor are homologous; only these traits are used to construct phylogenetic trees

analogous traits definition

independent adaptation (convergent evolution) of similar traits in unrelated taxa due to similar environments

homology definition

similarity between organisms due to shared ancestry

analogy defintion

similarity between organisms due to convergent evolution

convergent evolution definition

when unrelated organisms independently evolve similar traits or adaptations in response to similar environmental pressures and conditions; does not provide information about shared evolutionary history

information used to differentiate homology from analogy

• corroborating evidence

• trait complexity

steps for building a phylogenetic tree

1. select the taxa: choose the taxa, including an ingroup and an outgroup

2. identify homologous traits

3. construct the phylogenetic tree: organize taxa into a tree based on shared derived characters (cladistics)

4. validate the tree: compare the tree with multiple lines of evidence, including fossil records, molecular data, or existing trees

ingroup definition

the group of taxa whose evolutionary relationships you are studying

outgroup definition

one or more taxa related to the ingroup but that diverged earlier

monophyletic definition

a group of taxa that consists of:

• an ancestor taxon

• ALL its descendants

• NO other unrelated taxa

(a plain old clade)

paraphyletic definition

a group that consists of:

• a common ancestor

• NOT ALL descendants of the common ancestor

(almost a clade)

polyphyletic definition

a group that consists of:

• several distantly related taxa

• NOT including the common ancestor of all group members

shared derived characters

traits that evolved in the most recent common ancestor of a clade and are unique to that group

shared ancestral characters

ancestral traits shared by taxa of a clade but are also present in taxa of earlier clades

synapomorphy definition

derived character (apomorphy) present in an ancestral taxon and its descendants but is absent from more distant ancestors

symplesiomorphy definition

ancestral character (plesiomorphy) present in several groups but inherited from a distant common ancestor

maximum parsimony definiton

selects the tree with the fewest evolutionary changes (simplest explanation), minimizing the number of trait changes across the tree

maximum likelihood definition

considers the statistical probability of specific evolutionary changes, such as genetic mutations