Yeasts used to make mead are close to the baker's yeast and biologists do not necessarily make a difference between these yeasts which, although very close from a biology standpoint, are used for different purposes. Yet baker's yeast is not used to make alcohol. Baker's yeast produces alcohol and CO2 from starch contained in flour, Alcohol evaporates due to high baking temperature (higher than 400°F) and CO2 will form bubble, giving some volume to the bread.
To make drinks, one focuses mainly on alcohol and let CO2 go (except for the second fermentation of champagne). Although aims are different, the basic principle is the same.
Unlike grapes which have yeasts (on their surface), honey does not contain much yeast. A fermentation relying only on wild yeasts would require months to start and would not be reliable. F1 tells the story of « traditional method » rhubarb wine, that is without adding yeast. The wine had methanol (toxic) and acetaldehydes (15 times as powerful as ethanol to provoke drunkenness). The result could even be a coma even when the consumption was moderate. Therefore wine yeasts are generally added for more safety and control. Nitrogen-containing molecules are also added as well as vitamins for honey contains very little of them.
There are three important types of yeasts (from the point of view of their utilisation, not from the point of view of biologists' nomenclature): baker's yeast, beer yeast and wine yeast.
Some people (a minority) use beer yeast to make mead. They are of two kinds: ale (saccharomyces cerevisiae) and lager (saccharomyces uvarum).
Most wine yeasts are also saccharomyces cerevisiae (Champagne yeasts are saccharomyces bayanus). One distinguishes between white wine yeast and red wine yeast (and a few specialized yeast as Sherry yeast).
These yeasts come in two forms. 5 g packages of dry yeast (cheap) are sold by Lalvin (lallemandwine.com) and by Red star (redstaryeast.net/). White labs (whitelabs.com) and Wyeast (wyeastlab.com) sell their yeasts in liquid form (much more expensive but supposed to be purer). White labs commercializes a yeast dedicated to mead and Wyeast has two of them (dry and sweet) but there are not well distributed and so there are harder to find than usual wine yeasts.
Yeasts will give different flavors but they will also lead to different alcohol contents and different quantities of residual sugar. Some yeasts (not very numerous) will not ferment beyond 4 % of alcohol, others (bayanus for example) can reach around 35 proof. Some have a low attenuation (proportion of sugar transformed into alcohol): they leave a lot of sugar and the result is sweeter than with other yeasts. On the other hand, some yeasts (bayanus again -- Champagne yeasts) will lead to a dry mead, really dry. One has to take this into account when a yeast and a quantity of honey are to be chosen.
Anyway, after the half dozen years of recommended ageing, the difference between different yeasts will have died out [V2].