Evolution Lecture No. 12: Phylogenetics II
Friday October 5 , 2012
Examples Of Hybrid Zones:
-Throughout Western and Eastern Europe, 2 particular species of crow preside in specific habitats where
they are most successful. The hooded crows prefer farmland territory, while carrion crows prefer alpine
valleys. As these are more transitional habitats, hybrid offspring arising from these crow species have
similar or higher fitness than their parental forms. Thus, a stable hybrid zone is established.
-Studying the hybrid zones present in sage brushes, 3 unique indigenous habitats were examined: the
basin zone (lowest elevation), the hybrid zone (intermediate elevation) and the mountain zone (highest
elevation). Among these three habitual zones, there is a gradual change in morphology/phenotype and
the parental forms are to be found in the two extremes (basin and mountain ranges). Ultimately,
through the reciprocal transplant of species into experimental gardens, the hybrid fitness was much
higher than the parental forms in the intermediate zone, resulting in a stable hybrid zone.
Alternate Hypotheses Of Whales As Hoofed Animals:
-A pulley-shaped astragalus bone is a synapomorphy that defines artiodactyl (even-toed) ungulates such
as moose and hippos. Recall that a synapomorphy is a shared, derived and homologous trait. In this
example, two alternate hypotheses arise in terms of explaining the phylogenetic relationship between
modern whales and artiodactyl ungulates:
a) Whales are a sister group of today’s artiodactyl ungulates that diverged from a common ancestor.
b) Whales are a sister group of the hippopotamus species and are most closely related to them.
Mapping Traits Of Hypothetical Phylogenies:
-By outlining specific traits in the suggested species of this example, we find that all ungulates are
distinct from the out-group and whales as they all possess the pulley-shaped astragalus bone, while the
latter does not. By parsimony, it would suggest that one change (the artiodactyl ancestor gained the
pulley-shape of the astragalus bone) resulted in the phylogeny of hypothesis a). However, the same can
be said for hypothesis b), where one change (whales lost the pulley-shape of the astragalus bone)
resulted in the respective phylogenetic tree. Homoplasy must also be taken into consideration under
such circumstances as a possible explanation for this phylo