Ch 19 Evolution and Classification
Determining evolutionary relationships btw organisms and deciding if similar structures or
similar-looking structures are grounds for grouping species together
Convergent (or parallel) evolution: tendency among organisms living under the same conditions
to develop similar body forms
o Parallel or convergent depending on evolutionary relatedness of organisms involved
o Convergent evol. refers to more distantly related organisms
o Parallel evol. refers to more closely related ones
Similarity(resemblance) in appearance does not always mean relatedness
biologists classify (and name) organisms based on features
o morphological traits, chromosomal anatomy, gene sequences, physiological functioning,
morphology of sub-cellular structures and behaviour patterns
19.2 Systematic Biology: An Overview (Pg 422)
Systematics : analytical approach for studying the diversity of life and evolutionary relationships
o identify, describe, classify and name organisms based on evolutionary relationships
Two goals of systematic: reconstruct phylogeny and taxonomy
Phylogeny: evolutionary history of a group of organisms
o Phylogenetic tree
Formal hypotheses identifying likely relationships among species
Allows us to distinguish similarities inherited from a common ancestor from
those that evolved independently in response to similar environments
Taxonomy: identification and naming of species and their placement in a classification
o ordered division of organisms into categories based on a set of characteristics used to
assess similarities and differences
o uses binomial nomenclature
two-part format of the scientific name of an organism
developed by Carolus Linnaeus (mid-1700’s)
binomial name of an organism or “scientific epithet”
is the genus and species
is italicized or underlined
o Classification: arrangement of organisms into hierarchical groups that reflect their
o Most Systematists want classifications to mirror Phylogenetic history and adaptive
radiation (evolutionary history) of group of organisms
Gynogenetic: the process whereby a sperm fertilizes an ovum, but does not contribute genetic
material to the resultant zygote. Androgynous: Partly male and partly female in appearance; of indeterminate sex.
o Having the physical characteristics of both sexes; hermaphrodite.
19.3 The Linnaean System of Classification (Pg 424)
Established by Carolus Linnaeus
Taxonomic hierarchy: arranging organisms into ever more inclusive categories.
o Species, genus, family, order, class, phylum, kingdom, domain
King Philip came over for great spaghetti.
o Life on earth is classified into 3 domains.
o Taxon: organisms included within any category of taxonomic hierarchy
Next step is to determine which species are most closely related to one another.
o Want to know which characteristics some species have in common and can be used to
place the species in the same genus and family
19.4 From Classification to Phylogeny (Pg 425)
for at least 200 years (before Darwin), biologists relied on organismal traits (mainly morphology)
when analyzing and classifying organisms - taxonomy after Darwin – biologists wanted to include evolutionary history of a species = phylogeny
biologists now use systematics
o an analytical approach to understand the diversity and relationships of organisms
After Linnaeus, they developed phylogenies based on chromosomal anatomy, details of
physiological functioning, morphology of subcellular structures, cells, organ systems and whole
organisms, and patterns of behavior
Today systematists also use molecular sequences of nucleic acids and proteins as additional
characters when deriving phylogenies
19.5 Evaluating Systematic Characters (Pg 425)
systematics - provides essential information for all of the biological sciences
o provides data for testing for homologous vs. analogous relationships
systematists study traits in which phenotypic variation reflects genetic differences.
o Exclude diff caused by env. Conditions
o Must be genetically independent, reflecting diff parts of organisms’ genomes
Necessary because diff organismal characters can have the same genetic basis.
Want to use each genetic variation only once in an analysis
Example: two foots, toes (pads on lizards). Both right and left toe coded by same
genes, therefore choose only one as systematic character
Homologous characters: useful in preparing phylogenies
o Emerge from comparable embryonic structures and grow in similar ways during
Systematists have put great stock in embryological indications of homology on
the assumption that evolution has conserved the pattern of embryonic
development in related organisms.
o Such phenotypic similarities btw organisms reflect underlying genetic similarities o Systematic analyses rely on the comparison of homologous characters as indicators of
common ancestry and genetic relatedness
o Homologous structures where their function has changed can differ considerably among
Analogous characters: homoplasious (homoplasies): phenotypic similarities that evolved
independently in different lineages.
o Similar function in diff species
o Systematists exclude homoplasies from their analyses because homoplasies provide no
information about shared ancestry. Mosaic evolution
o Refers to reality that in all evolutionary lineages, some characteristics evolve slowly,
whereas others evolve rapidly.
o It is pervasive: (esp. of an unwelcome influence or physical effect) Spreading widely
throughout an area or a group of people.
o Every species displays a mixture of ancestral characters (old forms of traits) and derived
characters (new forms of traits).
o A "derived character" is a trait that arose in the most recent common ancestor of a
particular lineage and was passed along to its descendants.
Among a given group of organisms, the shared derived characters are generally
the less common characters.
The evolutionary interpretation is that these characters of organisms are more
recently evolved. They are contrasted with primitive characters. Shared derived
characters should have the same structure and function.
o Derived characters provide the most useful information about evolutionary
relationships because once a derived character becomes established, it is usually
present in all of that species’ descendants.
Unless they are lost or replaced by newer characters over evolutionary time,
derived characters can serve as markers for entire evolutionary lineages. Systematists score characters as either ancestral or derived only when comparing them among
o Any particular character is derived only in relation to what occurs in other organisms
Older version of same character or an entirely new trait, absence of it altogether
o Example: most species of animals lack a vertebral column (backbone)
Systematists compare vertebrates with all animals lacking vertebral column
Absence of vertebral column = ancestral condition
Presence of vertebral column= derived
Fossils of earliest animals lack backbones.
o Systematists distinguish between ancestral and derived characters to ascertain in which
direction a character has evolved.
Outgroup comparison: comparing the group under study with more distantly related species
not otherwise included in the analysis
o Used to identify ancestral and derived characters
o Example: butterflies mostly have 6 walking legs, two families have 4 walking and 2
Outgroup comparison with other insects (most which have 6 walking legs as
adults) show that 6 legs is the ancestral character and 4 walking legs and 2 non
walking legs is a derived character.
19.6 Phylogenetic Inferences and Classification (Pg 427)
Phylogenetic trees portray the evolutionary diversification of lineages as a hierarchy that
reflects the branching pattern of evolution
o Each branch rep the descendants of a single ancestral species
When converting Phylogenetic tree into a classification, Systematists use the principle of
Monophyletic taxa: those derived from a single ancestral species
Polyphyletic taxa: includes species from separate evolutionary lineages
Paraphyletic taxon: includes an ancestor and some but not all of its descendants
o Eg class reptiles
Most Systematists strive to create parsimonious Phylogenetic hypothesis.
o Include fewest possible evolutionary changes to account for diversity with