Classification and Phylogenetic Relationships

• Early natural historians classified organisms based on shared features

• Now we seek to understand how organisms features are related to phylogenetic relationships, descent from a common ancestor

• Classifications and relationships change over time, as we learn more details and test hypotheses

Fig. 11.1 pg 231

Species Concepts

• How to recognize species– Morphological species

– Biological species

– Evolutionary species

Morphological Species• Assemblages of individuals with morphological

features in common

• Separable from other assemblages by discontinuities in a number of features

• Numerical (phenetic) taxonomy– Incorporates statistical tools to identify taxonomic

distinctions

– High statistical correlation of large number of characters distinguishes groups

Biological Species• From Buffon

– A sexually interbreeding (or potentially interbreeding) group, separated from other species by reproductive isolation

• Sibling species– Nearly identical but do not

normally cross-fertilize

• Polytipic species– Multiple geographic subspecies,

can exchange genes if come into contact

• Difficulties– Allopatric populations

– Subjective scale of success for interbreeding

– Cannot test fossils

– Does not apply to plants

– Horizontal gene transmission between different species

Fig 11.2 pg 233Evolution in the Genus Geum, Evolution 13, 378-388 by W. Gajewski. Reprinted by permission.)

Evolutionary Species• Evolutionary isolation

• Not dependent upon sexual isolation

• A species concept that incorporates change over time– Stage when group has attained complete

separation subjective

Phylogeny• Origin of new species

• Phyletic versus Cladogenesis– Phyletic evolution

• Changes within a single lineage

– Phylogenetic branching, Cladogenesis

• Splits and divisions result in multiplication in number of species

• The more a group shares common inherited attributes, the more likely their descent from a common ancestor

Fig 11.4 pg 235

Phylogeny• Shared attributes can occur by several ways

– Homology• Feature is shared because species derived from

common ancestor

– Homoplasy• Common feature but ancestor was different

• Similar adaptation from different starting points– Parallelism

» Immediate common ancestor was different

– Convergence

» Lineage has been separated for considerable time

Homology• A phenotypic feature that is shared due to common

ancestry

• It is critical to specify the level at which homology is designated– Genetic > Development > Morphology > Function

– Serial homology

• Similarities among parts of the same organism

• E.g. vertebrae

– Homologous features, may arise by different developmental pathways

Homology, Partial Homology and Percent Similarity

• Homology as applied to DNA sequence similarity– Orthologous genes

• Genes duplicated in different species

– Paralogous genes• Genes duplicated within a species

• Partial Homology, percent similarity – Early molecular workers comparing two genes might have

discussed partial homology, percent similarity between to genes

– No longer accepted– Homologous or not homologous

Homology and Constructing Phylogenetic Trees

• Homology is the basis for constructing phylogenetic trees

• Monophyletic groups– Individuals share a

common ancestor

• Polyphyletic groups– Individuals in do all share

a common ancestor– Convergence

Fig 11.8 pg 240

Phylogenetic Trees• Phylogeny is something that happened

• Classification is an arrangement of its results

• Ideal multi-limbed phylogenetic tree would include all ancestral populations– Impossible

• Most species are unknown (fossils)

• Unable to compute, to complex

• Taxon (plural taxa)– Unit of classification,

whether species, genus, order,…

• Groups might not always be monophyletic in different classification schemes

Fig 11.9 pg 241

Phylogenetic Trees

Cladistics, Phylogenetic Systematics

• Willi Hennig (1913-1976)

• Separation of plesiomorphic (ancestral) characters from apomorphic (derived) characters

• Monophyletic groups– Defined by synapomorphic

(shared derived) characters

– Evolutionary steps marked by dichotomous branching of sister taxa

• Paraphyletic groups– Groups that includes some

descendents of a common ancestor, but not all

• Parsimony method– Preferred phylogenetic tree

has least number of steps

Cladistics, Phylogenetic Systematics

Fig 11.12 pg 243