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Fig. 1. Hypothetical evolutionary relationships among 17 species of organisms.
Vertical axis represents time in relative units, with the top of the
`phylogenetic tree' representing the present. Hence, species 1–7 and
8–13 are alive now (extant), whereas species e1–e4 are extinct.
Statements about phylogenetic relationships are based solely on recency of
common ancestry. For example, species 1 and 2 are each other's closest
relative because they share a more recent common history with each other than
with any other species depicted in this figure. The horizontal axis is
arbitrary, and note that nodes could be rotated for graphical convenience with
no implication for evolutionary relationships. `Clades' are hierarchically
arranged, `monophyletic' groups of species, including all species that have
descended from a common ancestor as well as that basal ancestor. All species
within a given clade are more closely related to each other (they share a more
recent common ancestor) than to any species in another clade. In the strict
sense, a clade includes all species that have ever existed within it. Thus,
Clade B includes species 7 as well as e1–e4. However, as it is
impossible to know of all extinct species within a given clade and as
physiologists rarely include extinct taxa in their studies, the term `clade'
is often used in a relative way with respect to a particular collection of
species that are included in a given study. Consider a comparative
physiological study of species 1–13. Species 1–6 might be referred
to as Clade 1, while species 7–13 might be referred to as Clade 2.
However, note that species 7 is relatively distantly related to the other
extant species in Clade 2 (i.e. species 8–13 shared a last common
ancestor much more recently than the last common ancestor of them with species
7). Hence, a researcher studying species 1–13 might prefer to write in
terms of Clades A, B and C in order to highlight the fact that, a
priori, she would expect species 7 to be somewhat different from species
8–13. (Importantly, a priori hypotheses about particular single
species can be tested with well-established phylogenetically based statistical
methods, although they may not be convincing to some regardless of the level
of statistical significance; see Garland
et al., 1993; Garland and
Adolph, 1994; Garland and
Ives, 2000.) Branch lengths in this figure are proportional to
divergence times. All phylogenetically based statistical methods use branch
lengths in their calculations, although some assume (arbitrarily) that each
branch segment is equal in length. Alternatively, under the commonly assumed
Brownian motion model of character evolution (see
Fig. 2), branch lengths are
assumed to be in units proportional to (relative) divergence times and hence
to the variance of character evolution along each branch segment (i.e. longer
branches imply greater variance of character change; see
Felsenstein, 1985).
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