Question
I know of a tree near my place of work that has both crabapples and real apples growing on adjacent branches.  I see it, but is this possible?  How is it possible?

Thanks,  

Answer
jeff
nature has produced many different plants on its' own via gene mutation on the plant while it is developing. perhaps this is really darwin's theories at work short term vs over millions of years.
plants can have different traits(in this case fruit - in other cases flowers, smells, etc.) via grafting (& now w/new science test tube gene combination techniques) on the same plant. actually new rose hybrids have been developed for centuries by grafting(making a slice in the cambium of an older rose root stock and attaching the section of new plant cambium stem to it via grafting). many new varieties of ornamental plants(bradford pear trees, weeping cherry trees, etc.) are regularly produced that way. this allows us to transfer desireable traits-genes (flower color, weeping ability, etc.) to new trees. it is possible (and has been done numerous times) to graft a stem from an apple tree, a stem from a peach tree, and a stem from a pear tree to a fruit tree root stock and end up w/a peach, pear, apple tree. of course w/apple and crabapple trees you are working w/the same genus MALUS. staying in the same genus makes it easier(new hybrid roses are grafted onto ROSAE genus rootstock). most hybrids w/different species fruit on the same root stock are not real hardy and therefor their success is not good and they turn out to be not so hardy. in many cases the new flowers are not abundant or healthy(black roses, etc.).
so your apple/crab apple tree is very possible and can easily be accomplished w/grafting.
with our new knowledge of genetics and gene splicing we will no doubt see many new plants be produced w/many different un-thought-of combinations in the near future. i think the whole plant production sytem we now use(mainly grafting) will be obsolete in 10-20 years. somewhere down the line you will just pop a stem or seed in the ground, water, fertilize and w/in a week or so have a 20' tree producing multiple type fruits on a regular basis.
hope this helps explain your question.
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happy labor-day
rick