Followup: why did certain surname-first names become exclusively associated with women (e.g. "Allison") whereas others became exclusively associated with men (e.g. "Jackson") whereas others remained neutral (e.g. "Jordan")?
William Camden, in Remaines of a Greater Worke, Concerning Britaine, 1605, mentions this as a new fashion, which many consider unsuitable. Every society has its list or rules for what is considered properly a name, normally called the personal onomasticon. Bringing in family names was just ugly and wrong to these people, who did not have our mania for novelty in naming. Camden thought it was a passable way to immortalize family connections or benefactors. But then, he was living through the New Names, all those Puritan curiousities like Tribulation & Fear-not - but also Grace, Charity, & Hope that survive in our onomasticon.
You may notice that the old patronymics like -son tend to remain masculine. In Iceland to this day people are -son & -dottir, so that Erik's children are Gunnar Eriksson and Bjork Eriksdottir.
"Allison" is not one of these. It has been feminine for a couple thousand years. It's from the medieval French Allisoun, a diminutive of Allicia, the fem of the Gaulish Allicio, meaning "the other one."
This was used entirely for boys until the middle 20th C, as far as I can find (love to find earlier ones). Like any use of the male onomasticon for a girl, it's claiming more strength & toughness for her. 4x, Billy or Billie was very popular for girl babies in the interwar. But by the Sixties, Roger Price, in What Not to Name the Baby, points out that girls with family names for given names "can swear and nobody minds": they can get away with a male behavior.
Most of these names, besides the patronyms, are unisex, like Taylor, Tyler, Jordan, Kelly. Some are on their way to feminine: I haven't met a boy named Kelly or Courtney in decades. That could change tomorrow.
Which they become is pretty arbitrary. A long-term celebrity can swing it one way or the other. Feminizing ones tend to be the diminutives ending in Y or its sound.
BTW, something similar is happening in Chinese. The one-child policy has created a new habit of using the mother's family name fot the child's personal name, so the child of Wang Humei & Ze Shibai is Wang Ze.