They are just bees, they all the same… nothing more wrong than that. A large number of bees has been described so far as belonging to different species, with large differences in morphology, nesting, foraging for acquiring resources and so on. On the other hand, even within a single species, some degree of (reasonably small) variation in morphology, behaviour, ecology and in genetics can take place.
This is true not only for bees and, for example, consider humans, as different people could have different eye shape, or people of an ordinary village could be sorted into diverse family trees, although these people all belong to the same entity of being humans. In a recent study, some colleagues and I have discovered that the populations of a bee species from Europe, quite widespread all over the continent, and with negligible morphological variation (for what we know), is actually structured in a western and an eastern genetic lineage. This means that at some point during the history of the bee and of the continent, a common entity ended up as separated in two spatially isolated clusters. This isolation allowed some genetic difference between these clusters to sediment in the DNA, and nowadays the apparently continuous bee range is actually divided into two adjacent genetic sections. This is quite surprising, because what superficially seemed homogeneous, in fact was an heterogenous structure. Here is the link the published study this blog is reffering to: http://fragmentaentomol.org/index.php/fragmenta/article/view/414. Similar studies have piled up over time, especially in the last 20 years, with case studies from other animals and plants.
Although not discussed in the bee study itself, the authors and I have later brainstormed that the root factor causing the observed genetic structuring of this bee might be connected with the turbulent climatic history of Europe. During the most recent ice ages, the cooling of most Europe lead organisms to seek refugium in southern Europe, where populations remained isolated by the E-W distribution of mountains that worked as geographical barriers. This likely contributed to what we have observed in the bee we studied.
It would be very interisting to know if other bee species hide patterns of variation across their range.