Linearizing the labyrinth
The progressive co-evolution of large body size, grass-grinding teeth, and single-hoofedness in horses over-simplifies equine evolution to the point of modern myth. There was a whole bush of horse ancestors, all possessing different combinations of these traits. The five living equid species are but paltry remnants of a once-diverse lineage. How is that progressive? or exemplary? And of what use is such an over-simplification, when it does nothing to explain the current diversity of bovids (cows, bison, sheep, goats, antelopes), arguably much more successful in terms of species numbers, sheer quantity of individuals, variety of traits, and habitats occupied? Very little, and Gould has these choice (if numerous) words to say on the subject:
“If we define evolution as anagenetic [non-branching evolution] trending to a “better” place, how can we depict a successful group with copius modern branches extending in all directions within the cladal [geneological] morphospace? Instead . . . our conventions lead us to search out the histories of highly unsuccessful clades - those now reduced to a single surviving lineage - as exemplars of triumphant evolution. We take this only extant [living] and labyrinthine path through the phyletic bush, use the steamroller of our preconceptions to linearize such a tortuous route as a main highway, and then depict this straggling last gasp as the progressive thrust of a pervasive trend.” [my emphasis] (SET, pg 908).
It’s hard to generate accurate depictions of complicated phenomena using “linear thought.” Really hard. And even if you get it right, you have to work to not misrepresent your subject material. Linearized thinking very quickly generates false dichotomies, about which I’ve already ranted :o). Best to think in trees of possibilities, and take your time about it, and do a good job. That’s when all the interesting stuff happens.
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