Evolution__3rd_Edition

62 PART 1 / Introduction
(a) α-hemoglobin
(b) β-hemoglobin
(c) Fibrinopeptide A
(d) Fibrinopeptide B
(e) Cytochrome c
HAROMDPCSEK HARMDOPSCEK HARMODPSCEK HARDOMPSCEK HARMDOESCPK HARODMSCPEK
HARDOSCPEMK HARDOMSCPEK HARDOMSCPEK HARDSCPEOMK HADROMSCPEK HARDOMSCPEK
HARMOCPSEDK
HARMOSPCEDK HAMROSPCEDK HARMOEPCSDK HARMOPCSEDK HARMOPCSEDK HARMOSCPEDK HARMOSCPEDK HARMOSPECEDK
HARPSCEDMOK HAROCSPEDMK HAROMSCPEDK HAROMSCPEDK HAROSCPEDMK HARMOSCPEDK
Figure 3.10
Penny et al. constructed the best estimate of the phylogenetic
tree for 11 species using five different proteins. The “best
estimate” of the phylogenetic tree is the tree that requires the
smallest number of evolutionary changes in the protein. For
(a) α-hemoglobin, and (b) β-hemoglobin there were six
equally good estimates of the tree for the 11 species. All six trees
in each case require the same number of changes. (c) For
fibrinopeptide A there was one best tree; (d) for fibrinopeptide
B there were eight equally good trees; and (e) for cytochrome c
there were six equally good trees. The important point is how
Species that are more similar in one
protein are also more similar in
other proteins ...
similar these trees are for all five proteins, given the large
number of possible trees for 11 species. A, ape (Pan troglodytes
or Gorilla gorilla); C, cow (Bos primogenios); D, dog (Canis
familiaris); E, horse (Equus caballus); H, human (Homo
sapiens); K, kangaroo (Macropus conguru); M, mouse (Mus
musculus) or rat (Rattus norvegicus); O, rabbit (Oryctolagus
ainiculus); P, pig (Sus scrufa); R, rhesus monkey (Macaca
mulatta); S, sheep (Ovis amnion). Redrawn, by permission of
the publisher, from Penny et al. (1982). © 1982 Macmillan
Magazines Ltd.
five proteins are very similar (Figure 3.10). For 11 species, there are 34,459,425 possible
trees, but the five proteins suggest trees that form a small subclass from this large number
of possible trees.
The similarities and differences in the amino acid sequences of the five proteins are
correlated. If two species have more amino acid homologies for one of the proteins,
they are also likely to for the other proteins. That is why any two species are likely to be
grouped together for any of the five proteins. If the 11 species had independent origins,
there is no reason why their homologies should be correlated. In a group of 11 separately
created species, some would no doubt show more similarities than others for any
particular protein. But why should two species that are similar for, say, cytochrome c,
also be similar for β-hemoglobin and fibrinopeptide A? The problem is more difficult
than that, because, as Figure 3.10 shows, all five proteins show a similar pattern of
..