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A Brief Introduction to Tree Thinking

Seen in the light of evolution, biology is, perhaps, intellectually the most satisfying and inspiring science. Without that light it becomes a pile of sundry facts some of them interesting or curious but making no meaningful picture as a whole.

Dobzhansky, T. (1973). Nothing in biology makes sense except in the light of evolution. The American Biology Teacher 35(March): 125-129.

... nothing in evolution makes sense except in the light of phylogeny ...

Society of Systematic Biologists <>.

Evolutionary biologists think about biological phenomena in terms of how those observations fit within the branching structure of genealogical relationships between species. They recognize the roles of descent from common ancestry and evolutionary modification in establishing patterns of similarity and difference among groups of organisms. In short, they see biology through the perspective of phylogeny.

The adoption of a tree thinking perspective is not compatible with some informal ways of thinking about biological groups and the relationships among them. The figures below represent three very different sets of assumptions about the nature of species. In the first panel species are represented as fixed and separate types. To a first approximation, this is what we experience in our everyday interactions with organisms. The second panel shows species as different steps along an evolutionary (developmental) trajectory. Given our human-centric experience it is easy to see why we would place our species at the top of this ladder. The third panel emphasizes both the historical relationships among species and their non-linear, or branching, divergence. Tree thinking involves being able to incorporate these historical and non-linear features into one's biological sense making.


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