Evolution__3rd_Edition

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Distance can be measured in more
than one way
CHAPTER 16 / Classification and Evolution 479
used in Figure 16.2 is Euclidean distance: the straight line between two points; in two
dimensions it is measured by Pythagoras’ theorem. But other distance measures exist,
such as mean character distance (MCD). MCD is the average distance between the
groups for all characters measured. Thus, in two dimensions, if species 1 and 2 differ by
x units in character A and y units in character B, then MCD = (x + y)/2 and Euclidean
distance =√(x 2 + y 2 ). The different measures of distance can give different hierarchies
and the pheneticist is again faced with a subjective choice of which to use.
Phenetic classification, therefore, even in its modern numerical form, is not objective.
It can produce classifications, but classifications that lack a deep philosophical
justification. Let us see how the introduction of evolution into classification can help
with the problem.
16.6 Phylogenetic classification uses inferred phylogenetic
relations
16.6.1 Hennig’s cladism classifies species by their phylogenetic
branching relations
1 2 3 4 5 6 7
Phylogenetic classifications group species solely according to recency of common
ancestry. When a species splits during evolution it will usually form two descendant
species, called sister species, and in a cladistic classification sister species are classified
together. The branching hierarchy of ancestral relations is a unique hierarchy, extending
back to the beginning, and including all, of life. The phylogenetic hierarchy is easy
to convert into a classification (Figure 16.3). (I say it is “easy,” but in Section 16.6.3
below we look at problems that arise in moving from a phylogenetic hierarchy to a
Linnaean classificatory hierarchy.)
(a) Phylogeny (b) Cladistic relations
(c) Classification
Figure 16.3
The phylogenetic (cladistic) classification of a group is simply
related to their phylogenetic tree. (a) The evolutionary history
of seven species. (b) Their cladistic classification. (c) The
1
2
3
4
5
6
7
Order
Suborder
Species 1–7
Family
Species 4–7
Genus
Species 5–7
Species
Species 5–6
Species 5
formal Linnaean classification, for species 5 as an example.
This particular classification is an example only; it could be,
depending on the detail in a particular case, that different
Linnaean levels should be used.