So what fascinates me now is the great variation in fruit flavors, such that even closely related fruits taste different. An apple tastes different from a pear; a peach tastes different from a plum.
Part of this can be explained by human selection, e.g., why a Braeburn apple tastes different from a Gala apple. But even then, the range of flavors of apple varieties is different from the range of flavors of pear varieties, presumably because of some species-level difference between the two.
So what I am wondering is: is the difference in flavor simply the result of random mutations and genetic drift accumulating over time since the species diverged? Or is there in fact selective pressure for related fruits ā with, presumably, a similar dispersal strategy ā to taste different?
Thereās definitely some selective pressure based on the disperser - for example, things that want to attract mammals may become sweeter, while those that attract birds become more sour because birds donāt taste that (I donāt know if thatās their actual preferences, just a made-up example). Keep in mind that some of the flavors that some animals like are poisonous to others, like most things that taste bitter to us.
But also remember that wild fruits tend to be fairly bland, and a huge amount of fruit flavor is the result of selection for more intense flavor alongside less sour and bitter tastes. And that may be somewhat random in terms of what kind of compounds are there to be selected for, since completely new ones would be fairly rare.
Iāve often wondered as well if the same principle applies to wild fruit texture, i.e. juicy berries vs granular dry ones, based on mammal vs bird dispersers. In a university course we were encouraged to try black chokeberries and it was one of the worst fruits Iāve ever had because of the horrendous texture (and bitterness) that I canāt imagine would be palatable to any mammal that would be chewing or tasting the fruit, whereas birds simply swallow them whole.
But lots of people like Aronia fruits? For texture and bitterness. Not me, but itās clearly is working for many mammals! Same with Sorbus which is also clearly oriented on birds, but humans love it (and other mammals occasionally eat it too).
To be fair, humans eat a lot of plants that contain distasteful or toxic metabolites, so I feel like weāre not the best judge! The plant world must be frustrated and baffled that their deterrents arenāt working on usā¦
Selective pressure is certainly a prime factor in how fruits taste the way they do today. Go back several thousand years and fruits looked and tasted much different than they do now. Ancient peaches looked more like weird cherries. The flesh of ancient watermelons looked more like some weird cross section of a brain. These days what makes fruit appealing is their sweet flavor and bright, appealing colors, which only happened because we humans bred them over generations to be different.
If flavor was naturally selected for dispersal, then Iād expect there to be one or a few āwinningā flavors. But that doesnāt seem to be the case for wild apples, at least, where a stunning variety of flavors can be found in a small area, including liquorice, honey, and berry flavors: https://www.bbc.com/future/article/20160523-kazakhstans-treasure-trove-of-wildly-flavoured-apples
Maybe what we hone in on as āflavorā is just an incidental trait, better explained by neutral evolution than natural selection. We might be detecting small molecules that are byproducts of biochemical pathways with more important roles, selection-wise. Or maybe something much more complex and interesting is going on.
For what itās worth, bears seem to be a major dispersal agent in the last remaining apple forests. Maybe we have their gastronomical preferences to thank for the range of apple flavors. :)
āFlavorā is a complex mix of chemosensation mediated through perceptionāwhether flavors are similar, different, or non-existent can vary enormously from animal to animal. Furthermore, this need not be connected to the chemical similarity of the flavorants, so that relatively small shifts (from the perspective of the genome, metabolism, etc. of the plant) could produce a dramatic shift in the flavor perceived by an animal.
In this particular case, one might add that the cost in resources to produce the secondary metabolites of flavor is probably rather small compared to that of the carbohydrates within the fruit, so selection pressure on these molecules is likely to come mostly from the way in which they influence plant-animal interactions (the attraction of dispersers, the repulsion of parasites, and so on.)