Herbivorous forms can and do evolve from carnivorous forms

There are many examples of animals that have branched off from their carnivorous relatives to become herbivorous or at least more herbivorous. There are herbivorous canivorans (belonging to the mammal order Canivora). The fundamental variability and flexibility of animal form and behavior we see in nature is what will enable the directed evolution toward herbivory to proceed smoothly without wiping out species.

Though most raccoons are omnivorous, two species are are almost completely frugivorous, the South American Olingos and kinkajous.

Many of the palm civets, which belong to the typically carnivorous viverrid clade, are mostly frugivorous.

While most bears are omnivorous, the panda bear eats almost exclusively bamboo and the spectacled bear is mainly frugivorous.

The only living red panda species evolved from their omnivorous ancestors to eat primarily bamboo.

While most of the extinct borophagines dogs, or "bone-cracking" dogs, were carnivorous, some forms were mostly frugivorous. So even within a single clade, we see a broad range of feeding behavior, from hypercarnivory to herbivory. Herbivory can and does evolve from carnivory and vice-versa (Wang et al. 1999).

A type of jumping spider found in Central America, Bagheera kiplingi, feeds primarily on acacia leaves.

Although not a case of a herbivorous shift, the aardwolf, a hyaenid, evolved to have a completely different diet than that of other hyenas, eating termites almost exclusively. Their teeth, tongue, saliva, and body size, are very different from their hypercarnivorous ancestors and closest living relatives. Similarly, some species of stinkbadgers, have evolved to eat primarily insects, unlike their omnivorous skunk and stinkbadger relatives. Likewise, the sloth bear mainly eats ants and termites.

In the other direction, carnivory has evolved from herbivory in some marsupials.

Wang, X.M, Tedford, R.H., and Taylor, B.E. (1999). Phylogenetic systematics of the Borophaginae (Carnivora: Canidae). Bulletin of the American Museum of Natural History, 243, 1-391.

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