In this article, the author summarizes hypotheses and studies related to the evolution of variation in nose shape. You were asked to read it and summarise any ONE of these hypotheses with any supporting evidence for that hypothesis. Perhaps 2-3 students did that; most of you summarised the article (i.e. reproduced all hypotheses), while some of you talked about other adaptations and other creatures (not asked).
Two major hypotheses are presented to explain evolutionary variation of nose shape in humans.
One, the variation is adaptive*; natural selection acts to promote narrow nasal cavities in cold climates and broad cavities in warm climates. This are based on functional arguments
- This hypothesis is supported by the facts that Siberian and Alaskan populations have large sinuses, while African populations have small sinuses–varying geographically as expected, and supporting the functional argument.
- However, it is not supported by the fact that Neanderthals might have had broad noses with not relatively large sinuses.
- One possibility is that body size affects the form of the nasal cavity
- Another possibility is that Neanderthals never faced selection in the cold
Two, the variation is due to genetic drift, as inferred from the geographic patterns of variation in nose shape.
Not explicit stated is the following default set of assumptions:
Selection and/or Drift act on these Phenotypes
/ \
narrow cavity broad cavity
\ /
Variant 1 Variant 2
Molecular genetic variation
Whether selection or drift, there HAS to have been genetic variation!
Five genetic loci determine variation in nose shapes of modern humans.
Many of you discuss genetics as a separate explanation, possibly because the article (and quotation) was poorly worded:
‘Although these nasal differences in temperate zones may not be driven by environmental variables, geographically related traits are still evident. “There is wide variation across continents, and that sort of tells you that there are underlying genetic reasons,”…’
Some of you seem to have taken this to mean that there are two different, mutually exclusive possibilities: One that the variation is due to “environmental factors” (??PLASTICITY??) or to “natural selection.” Think about this.
Now think back to our discussion (lecture on 3 Aug) of how adaptive hypotheses are developed and tested. Try to relate those principles to the current example*.