Possibly even more confusing than the species concept is the demarcation of subspecies. Although advocated by Linneaus, the classification of subspecies was seldom used until the mid-20th century. The adoption of subspecies around this time was particularly widespread in birds. Reclassification was usually based on morphological characteristics, and as a result the current classification of bird
No. of monophyletic groups classified as subspecies
Figure 7.4 Number of monophyletic mitochondrial lineages per species compared with the number of these lineages that currently match subspecies classifications. The size of each circle is proportional to the number of comparisons in each category. The diagonal line indicates where the circles would be located if the monophyletic mitochondrial lineages in each species were in complete agreement with designated subspecies. Because all circles are above this diagonal line, all species contain monophyletic groups that are not classified as subspecies. Adapted from Zink (2004) and references therein subspecies does not agree with the distribution of monophyletic mitochondrial lineages. A review of the literature has shown that bird species contain on average around two monophyletic mtDNA lineages, but are subdivided into an average of 5.5 subspecies (Zink, 2004; Figure 7.4). The cactus wren (Campylorhynchus brunneicapillus), for example, has only two evolutionarily distinct mitochondrial lineages but has six named subspecies.
Discrepancies such as these may mean that conservation efforts are directed at genetically indistinct subspecies while distinct lineages receive less attention, and this has led Zink (2004) to call for the reclassification of subspecies. This is a somewhat controversial demand because there are a number of reasons why the morphology and genetics of recently diverged species may not agree, one of these being incomplete lineage sorting. Furthermore, as we learned in Chapter 4, quantitative trait variation may exceed the genetic differences that are revealed by neutral molecular markers. Subspecific status should therefore be revoked with caution because morphological differences, however slight, may reflect local adaptation even if neutral molecular markers show no differentiation.
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