Savant syndrom indentifies people with general intellectual impairment who, in one specific field, reach ordinary or even exceptional performance.

In The Psychological Unity of Humankind, Eliezer argues that

So you can't have the X-Men.  You can't have "mutants" running around with highly developed machinery that most of the human species doesn't have.  And no, extra-powerful radiation does not produce extra-potent mutations, that's not how it works.

Again by the nature of sexual recombination, you're very unlikely to see two complexly different adaptations competing in the gene pool.  Two individual alleles may compete.  But if you somehow had two different complex adaptations built out of many non-universal alleles, they would usually assemble in scrambled form.

The argument behind this makes formal sense, but it's applicability strongly depends on how well we can judge what does and doesn't require complex adaptation. Reports of savants provide an interesting test of this; some of them seem like they are not merely an exceptional level of human skill, but not reachable by ordinary people. For example, in a recent post here that reminded me of this, the author claims:

Example 3: Stephen Wiltshire. He made a nineteen-foot-long drawing of New York City after flying on a helicopter for 20 minutes, and he got the number of windows and floors of all the buildings correct.

Other things I remember hearing are someone seeing at a glance that there are 163 peas on a plate, or remembering every word he ever heard. If these kinds of abilities can develop as a consequence of individual genetic quirks or possibly even brain injuries, then clearly we just don't have a good intuition about what's "close" in brain design space.

Now that I've made clear what kind of ability I'm talking about, has anyone done the relevant digging?