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Lecture 13

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University of Toronto St. George
Shawn Lehman

ANT333 Lecture #13 Adaptation & Phylogeny: How can we determine that a trait is an adaptation? “The Spandrels of San Marco & the Panglossian Paradigm: a critique of the adaptationist programme” S. J. Gould & R. Lewontin  Do not assume every feature of organism is an adaptation  Gould and Lewontin chide adaptationists for making up too many adaptive stories  Danger lies in making up stories and then accepting them based on their plausibility alone  Stories are implausible and fall apart with science Some Traits are not Adaptive  For example, there are beautiful mosaics in triangular spaces that occur where two rounded arches meet (Spandrels).  Spandrels had no function in & of themselves when cathedral was built 300 years ago, but somebody later put them to good use.  Umbilicus of Snail Shell (natural by-product that has no function) is Example of “Biological Spandrel” Exaptations  Traits that have some function but was not what they were originally used for  Gould proposed that we call traits that perform a current adaptive function but arose either for some other function or with no adaptive function at all as “exaptations.”  Both preadaptations and “spandrels” qualify as exaptations Some Traits are not Adaptive  Traits may be by-products of physical architecture of an organism, or they may be due to developmental or genetic constraints  Aquatic ape hypothesis – hair flows in a certain way due to ancestral aquatic life – correlation but not causation per se Preadaptation vs. Adaptation  When a feature is secondarily co-opted for a function other than the one for which it originally evolved, it has historically been known as a “preadaptation.”  Distinguishes between features that evolved due to natural selection for their current functions, and features that originally evolved for one function and then were co-opted for a later function Evolution of Long Necks in Giraffes  Hypothesis: long neck of giraffe is adaptation for feeding on leaves high in trees.  If long neck is adaptation for foraging where competitors can't reach, then giraffes should spend most of time foraging high in trees during times when food is scarce and competition most intense (i.e., during dry season). 1 Long Necks in Giraffes an Adaptation for Male Fighting?  But what about Female Giraffes?  If this scenario is true, then females would have evolved long necks just because long necks were selectively favored in males.  Males with long necks pass on a genetic tendency toward long necks to both their sons and daughters, even though adaptive function is relevant only to males.  Thus, long neck in female has not evolved for any function—it’s a by-product of genetic correlations between males & females Are Big Necks better than Long Necks?  One possibility is that males with longer, larger necks could have greater success in combat & mating not because their necks are longer but because they are just bigger.  Bigger males have bigger necks, so if there is selection for increased body size as a result of advantages in fighting & getting mates, then long neck is a by-product of development in males. The Comparative Method  The comparative method seeks to evaluate hypotheses by testing for patterns across species, such as correlations among traits, or correlations between traits and features of the environment Phylogeny:  Evolutionary relationships within and between taxonomic levels, particularly the patterns of lines of descent. Phylogenetics:  Taxonomical classification of organisms based on their degree of evolutionary relatedness. Phylogenetic tree:  Variety of dendrograms (diagrams) in which organisms shown arranged on branches that link them are according to
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