How do we define Dialect? (Dialect versus SE)

A 'Non-Standard English' seminar on wednesday made me think, so I thought I'd share.
We were asked which of the three following descriptions best describes dialect:

• 'One of the subordinate forms of varieties of a language arising from local peculiarities of pronunciation and idiom.' [OED]
• 'Variety of a language spoken by a group of people and having features of vocabulary, grammar, and/or pronunciation that distinguish it from other varieties of the same language. Dialects usually develop as a result of geographic, social, political, or economic barriers between groups of people who speak the same language. When dialects diverge to the point that they are mutually incomprehensible, they become languages in their own right' [Encyclopedia Britannica]
• 'A dialect is a complete system of verbal communication (oral or signed but not necessarily written) with its own vocabulary and/or grammar.' [Wikipedia]

Most chose the Encyclopedia Britannica version because it was more specific and we felt it encapsulated more about what dialect was. However, the issue was raised on further examining the OED *cough*crappy*cough* definition: can dialect be defined without explaining, comparing to or having some firm idea of the Standard?

Rather humourously (in a forced ha-ha type way i guess for uni), one of the guys in the class raised the 'very important' point, that in the animal kingdom, there isn't a standard form of species from which we define the other varieties (i.e. a mallard duck isn't described as a deviant form of the standard duck), so what makes English language any different?
(oh- apart from the fact that it's less visual seeing as it's the way we speak you buffoon? Guffaw, guffaw, you're hilarious.)

The OED obviously presents dialect as 'subordinate' and 'peculiar' in relation to SE; is it possible to understand dialect without having the idea of how lanuguage 'should' be in the prescriptive sense? Are we agreeing more with the Ency'Brit' definition because it's more descriptive and we think we should be descriptive more than judgemental? Think about it.

Without thinking about or using the phrase Standard English, define dialect. Doable or no?

Have fun..