why an evolutionary image merits a ‘fail’





Last year I commented that the following image, while funny, was a ‘fail’ in scientific terms:

evolution of the cat.jpg

A recent commenter asked, so is this image scientifically correct or incorrect? (My first thought was that teh lolcat at the end should be a clue…) 

But no, it’s not scientifically correct (lolcats aside). It’s another in the long line of images of ‘evolutionary iconography’ that portray evolution as an inexorable march towards some sort of progress – a generalisation that isn’t particularly helpful in explaining how evolution actually works.

It’s not good on the particulars of feline evolution, either…

The word ‘cats’, in its broadest sense, encompasses 38 different living species, which fall into 8 major groups comprising 11 genera (Johnson et al., 2006). All the extant species have evolved relatively recently: a combination of fossils & DNA analyses suggest their radiation began no more than 11 million years ago (mya) in the late Miocene (ibid.). The earliest divergence (10.8 mya) was between the lineage leading to the ‘big cats’ (lion, tiger, leopard, jaguar, snow leopard & clouded leopard) and ‘the rest’. In other words, domestic cats are not particularly closely related to lions, despite the iconography above.

Taxonomists have found classifying the various felids a difficult problem, due to the paucity of recent fossils (notwithstanding the classic sabre-toothed cats of the Pleistocene), a shortage of distinctive skeletal features, & some confusing distirbution patterns. Johnson & his team obtained sequences from autosomal and X- and Y-linked genes, plus mitochondrial DNA, for a total of 39 gene segments, which they then compared across all living cat species. A group of 7 distantly-related species – including hyaenas, which are more closely related to cats than to dogs – made up the ‘outgroup’, something that’s used in a phylogenetic analysis in order to distinguish between ‘ancestral’ & ‘derived’ features. (Basically, if a feature is found in the outgroup as well as the group of interest, then it’s likely to be ancestral & so won’t be particularly informative about patterns of evolution in your study group.) And the molecular dates were calibrated using 16 sets of fossil remains. 

The team found that the 8 major cat lineages evolved relatively quickly, over about 4.6 million years. Between6.4 & 2.9 mya these lineages in turn underwent a fair bit of adaptive radiation, at a time when sea levels were around 100m higher than they are at the moment. There was another burst of divergence 3.1-0.7 mya which produced 27 of the extant cat species. This was at a time when sea levels were on average relatively low.

The sea level part is important, because during periods of low sea level it would have been possible for species to migrate via land bridges into previously inaccessible areas. Based on their molecular data & available information on sea level changes, Johnson et al. suggest that modern cats evolved in Asia with that divergence between the big cats (Panthera) and all other feilds.Somewhere between 8.5 & 5.6 mya the ancestors of caracals, servals & golden cats arrived in Africa. Then, between 8.5 & 8.0 mya felids arrived in North America for the first time via the Bering Strait land bridge. This immigrant group seems to have been the common ancestor to ocelots, puma, leopard cats, lynxes – and the domestic cat. When the Panamanian land bridge formed 2.7 mya this opened up more new ecological opportunities for the feline explorers.

Subsequently there were other migrations back from the Americas to Eurasia & then further west. Cheetahs, for example, are now found in Africa, but the genetic analyses by Johnson’s team indicate that their closest relatives are the North American pumas. Similarly members of the genus Felis must have crossed back into Eurasia at least once, given that the domestication of the common moggy seems to have occurred in the Near East, at about the same time that agricultural settlements were developing in the Fertile Crescent (Driscoll et al., 2007) (Other American species moved across the Bering land bridge to Eurasia, & hence Europe, at various times – most notably the various horse species. The fossil remains of this particular sequence of species migrations were interpreted by T.H.Huxley as evidence for a European origin of the horses, a view he rapidly & happily relinquished when presented with evidence of the horses’ long evolutionary history in America.)

Once more – that simple linear iconography is not a scientific representation of feline evolution, and a long way from the much more complex and fascinating reality.

C.A.Driscoll, M.MenottiRaymond, A.L.Roca, K.Hupe, W.E.Johnson, E.Geffen, E.H.Harley, M.Delibes, D.Pontier, A.C.Kitchener, n.Yamaguchi, S.J.O’Brien & D.W.Macdonald (20008) The Near Eastern origin of cat domestication. Science  317: 519-523

Johnson, W., E.Eizirik, J.Pecon-Slattery, W.J.Murphy, A.Antunes, E.Teeling & S.J.O’Brien (2006). The Late Miocene Radiation of Modern Felidae: A Genetic Assessment Science, 311 (5757), 73-77 DOI: 10.1126/science.1122277

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