SUMMARY
■■Modern biology has affirmed Darwin’s hypothe-
sis that all the organisms we know of, present and
past, have descended from one ancient common
ancestor. Current understanding of the history
by which diverse groups have originated reveals
fascinating events, such as the symbiotic origin of
eukaryotic cells and the multiple origins of multi-
cellular organisms from single-celled ancestors.
■■A phylogeny is the history of the events by
which species or other taxa have successively
originated from common ancestors. it may be
depicted by a phylogenetic tree, in which each
branch point (node) represents the division of
an ancestral lineage into two or more lineages.
Closely related species have more recent com-
mon ancestors than distantly related species. The
group of species descended from a particular
common ancestor is a monophyletic group, or
clade; a phylogenetic tree portrays nested sets of
monophyletic groups.
■■The phylogeny of a focal group of species can be
readily estimated by using characters that change
so rarely that those species that share a derived
(“advanced”) character state can safely be as-
sumed to have inherited it from their common
ancestor. A character state that occurs within the
group of species can be judged to be derived
rather than ancestral if it does not occur among
other lineages (outgroups) that are related to the
focal group.
■■i n some cases, a phylogeny is not strictly dichoto-
mous (branching), but may include reticulation
(joining of separate lineages into one). This can
occur if some species have originated by hy-
bridization between different ancestral species
or if genes have moved “horizontally” between
organisms.■■Phylogenetic methods can be used to describe
the history not only of species, but also of DNA
sequences, gene families, tumors and other cell
lineages, and cultural traits such as languages.
■■Phylogenetic analyses have many uses. An im-
portant one is inferring the history of evolution
of interesting characters by “mapping” changes
in a character onto a phylogeny that has been
derived from other data. Such systematic studies
have yielded information on common patterns
and principles of character evolution.
■■The rate of evolution of DNA sequences can be
shown in some cases to be fairly constant (pro-
viding an approximate molecular clock), such
that sequences in different lineages diverge
at a roughly constant rate. The absolute rate of
sequence evolution can sometimes be calibrated
if the ages of fossils of some lineages are known.
The rate of sequence evolution can then be used
to estimate the absolute age of some evolution-
ary events, such as the origin of other taxa.
■■New features almost always evolve from pre-
existing characters. Homologous characters in
different organisms are those that have been
inherited from their common ancestors, with or
without evolutionary change.
■■Different characters commonly evolve at differ-
ent rates (mosaic evolution).
■■Homoplasy, including convergent evolution and
reversal, is often a result of similar adaptations in
different lineages.
■■i n an adaptive radiation, numerous related lin-
eages arise in a relatively short time and evolve in
many different directions as they adapt to differ-
ent habitats or ways of life. Radiation, rather than
directional trends, is perhaps the most common
pattern of long-term evolution.TERMS AND CoNCEPTS
adaptive radiation
anagenesis
character
character state
clade
cladogenesis
common ancestor
conservative
character
convergence
(convergent
evolution)
divergence
(divergent
evolution)evolutionary
reversal (reversal)
gene duplication
gene family
gene tree (gene
genealogy)
haplotype
higher taxon
homology
(homologous)
homoplasy
(homoplasious)
horizontal (lateral)
gene transfer
(HGT)hybrid speciation
ingroup
lineage
molecular clock
monophyletic
mosaic evolution
most recent
common ancestor
(MRCA)
orthology
outgroup
parallel evolution
paralogy
paraphyleticparsimony
phylogenetic tree
phylogeny
polyphyletic
root
sister group
species tree
taxon (plural: taxa)02_EVOL4E_CH02.indd 52 3/23/17 8:59 AM