The Biological Evolution of ConsciousnessFor example, as Darwin discovered, having beaks of different shapes allows different finches
to feed on different things. Sharp beaks allow some finches to pierce into cacti to get their seeds,
whereas blunt beaks allow other finches to better eat seeds on the ground. Since beak shapes are
heritable and have different functions, it is plausible that different beak shapes were selected for
thanks to the functions they performed. Another example is coloration, which can have different
functions in different organisms. Many species of geckos have remarkable coloration patterns
that allow them to blend in with their environment, providing camouflage, which allows them
to better avoid predators and capture prey. Saharan silver ants are a shiny silver color, but rather
than functioning as camouflage, this color helps them reflect sunlight, keeping them relatively
cool. By contrast, marine iguanas are nearly black, which helps them absorb heat from the sun.
This is especially important for reptiles that spend considerable time swimming in the cold
waters of the Pacific Ocean. Again, the colors of these different animals are heritable; they were
selected because of the functions they performed. In short, providing an evolutionary explana-
tion of traits that have been selected for requires a determination of their function within the
ancestral environment as well as the traits’ heritability.
One notable complication is that the same trait may either acquire new functions or lose
some of the old functions, or both. On one hand, vestigial traits such as the human appendix
used to have a function but no longer have any function at all. On the other hand, traits such as
feathers may initially be selected for because they perform one function—thermal insulation,
perhaps—and later acquire other functions, such as flying. Thus, developing an evolutionary
explanation of a trait requires sensitivity to how the functions of a trait may change over time.
Until now, we have discussed traits that were selected for. Other traits may well be products
of evolution, but not because their existence directly confers any adaptive advantage. Because
they lack a function, these traits cannot be selected for. Yet they can be byproducts of other traits
that themselves are adaptive. For example, the transparency of many insect wings might not itself
be adaptive. Transparency is a byproduct of being made of thin films of chitin (unless a pigment
is added during development), and chitin is the substance that arthropod exoskeletons are made
of. Of course, the wings themselves do confer an adaptive advantage.
Byproducts can be either contingent or necessary, a distinction that enriches our understand-
ing of how a trait that was not selected for came to be. Consider insect wings again. If the only
way that insects could have had wings is for them to be made of thin films of chitin, then given
that thin films of chitin are transparent, transparency is a necessary byproduct of insect wing
evolution. Alternatively, if insect wings could have been made of some other substance that is
not transparent, or if coloration can be added during development, then transparency is only a
contingent byproduct of insect wing evolution.
An obvious question is how to distinguish what counts as necessary and what counts as con-
tingent. How this distinction is made will be relative to different explanatory purposes. Relative
to the actual evolutionary history of insects on Earth, it may be necessary that insect wings be
transparent: for them to have been made of some other, non-transparent material would simply
require a different evolutionary history. Relative to all possible flying organisms, insect wings
need not be transparent: in fact, some insects add pigment to their wings during development, so
their wings are not transparent. On the other hand, relative to the laws of physics, it is necessary
that wings be solid at ordinary temperatures near the surface of the earth: there are no wings
that have this trait accidentally.
Yet other traits may be simple evolutionary accidents. If a population possesses different vari-
ants of a trait, and none of the variants confer an evolutionary advantage, different variants may
become distributed among the population simply due to random genetic drift. For example,
blood type has no function in humans, but because the population of humans is large enough,