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Slow worms are considered lizards as opposed to snakes, both are reptiles. Now I get that there are traits that distinguish them (eye lids, ears ...). But snake species themselves vary already quite a lot.

The Wikipedia article on snakes states:

Based on comparative anatomy, there is consensus that snakes descended from lizards.

But how comes slow worms are considered lizards despite the loss of their limbs during evolution, whereas the same evidently has happened to snakes?

What's the defining factor here? The time frame when it happened, that it happened independently (like the evolution of, say, an eye)?

Note: I couldn't find any detailed descriptions about the alleged evolution of slow worms, whereas for snakes it is easy to find material.

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the defining factor is that the species definition comes from where they species has come from, not only its body plan. In fact the slow worm has features in its body plan that more resemble lizards than snakes. The species name is as much a reflection of the history of that animal as it is a description of what may look like. –  shigeta May 14 '13 at 20:30
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2 Answers

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You are correct that the reason is similar to that of convergent evolution of the eye.

Both snakes and legless lizards are lizards (Squamata) that have lost their legs. However they have done so entirely separately much as octopus and human eyes have evolved entirely separately. An easier analogy is between whales and sea cows. Both are legless mammals but as they have evolved separately the must be given separate names.

Unfortunately it seems that the exact position of the snakes in the tree is unknown which makes this explanation less clear.

http://tolweb.org/Squamata/14933

However, the legless lizards (subfamily Anguinae) are within the family Anguidae, most of which have legs. This family is within the Anguimorpha, which includes even more species with legs. Just one small tip of the tree, with around 20 species is legless. On the other side of the tree, separated by many millions of years is the snakes (Serpentes).

A related question is why are snakes given their own name while legless lizards are just considered lizards without legs. This is partially due to the fact that there are so few species of legless lizard while there are thousands of species of snake. It is also probably due to cultural history. As snakes are dangerous to humans and prevalent in most inhabited areas of the world words for snakes are much older than the study of taxonomy. A western example is the snake in the garden of Eden. It is also important in Hinduism and many other cultures/relegions.

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thanks a lot for your answer. I am reading through all the links still, but I think it answers what I was asking. –  0xC0000022L May 21 '13 at 10:59
    
I'd dispute the premise that octopus and human eyes evolved separately! Too many of the molecular mechanisms involved in development are shared in all animals known to have photosensitive organs. –  Chinmay Kanchi May 22 '13 at 23:07
    
I'd agree that the visual system of cephalopods and vertebrates likely evolved in one common ancestor. However, this was a rudimentary "eye". The modern lens-type eyes of both groups almost certainly evolved independently from this common ancestral organ. –  single_digit May 23 '13 at 17:29
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Slow worms share many characteristics with other lizards, such as the presence of eyelids, which no snake has. Leglessness in slow worms evolved separately than in snakes, as can be seen by the fact that other lizards in the same family, Anguidae (and therefore, more closely related to them than snakes) possess legs (see Gerrohonotus and Abronia).

Things are complicated a bit because work by Fry et al. (2006) shows that the Anguimorpha (slow worms), along with Varanids (monitor lizards), Iguanids (iguanas), Helodermatids (gila monsters and beaded lizards) and snakes form an evolutionarily related clade, since all of these possess some or all of the components of snake venom.

However, for anatomical and phylogenetic reasons, it is clear that the slow worm is more closely related to other lizards than it is to snakes.

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The argument in the first paragraph is circular because you justify something based on an artificial categorization. Animals have often been re-categorized later into another family etc. Also some snakes do have remnants of legs (pythons), so what now? –  0xC0000022L May 23 '13 at 10:08
    
Yes, pythons and some boas have remnants of legs. All this proves is that the common ancestor of anguimorphs and snakes was likely to have had legs. So what? The argument is not circular at all. I'm using established phylogenetic relationships to make an argument about evolutionary relationships between taxa. –  Chinmay Kanchi May 23 '13 at 14:46
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