NOT as silly a question as first appears. If we’re assuming natural implies no human intervention, then we’re pretty much dealing with “unnatural” grapes. And it’s easy to see why.
First, let’s admit that the folks making Muscadinia based wines in the Southeastern part of the country really are working with truly natural grapes – but the rest of us, well, no.
Now that’s out of the way, back to human intervention. As far as I remember Dr Olmo’s lecture on the subject – ALL species of grapes occur with male and female plants with the singular exception of Vitis vinifera. Even there, he mentioned the European assumed progenitor species Vitis sylvestris, which has sexed plants.
In nature, sex rules. You accept the energy wasting production of two sexual forms to maintain / enhance genetic diversity. This occurs virtually everywhere. There is a reason. Diverse offspring have a better chance of thriving or at least surviving in an ever changing environment. We’re not necessarily talking about cataclysmic changes either. In most parts, grapes are understory plants using trees for structural support and reaching light. What happens if the trees go away? Or if the species prefers to grow in riverside locations and the river changes course? New pests, new diseases, new weather patterns all stress existing plants – and the more diverse the plant genes the better their survival.
In nature, female plants gain an advantage from well matured seeds, by the careful allocation of nutrients to guarantee the highest survival rate of the seeds produced. Small clusters are the rule – 10, 20, maybe 50 flowers / berries. Male plants on the other hand enhance their reproduction by maximizing pollen production. The bigger the flower cluster, the better. After all they don’t need to mature seeds, just create lots and lots of pollen and so the flower numbers are many multiples of the female and BTW why have a great number of flower clusters if just larger ones will do the trick?
It is thought that one of the mutations that expressed itself somewhere along the line was the occasional male plant became self-fertile and had fruit on it’s large clusters. This is immediately attractive to fruit consuming animals (including humans). Fewer clusters to get for roughly the same result. Quite probably not as sweet as the similar fruit off the small clustered female plants, but quick gathering. Now self-fertile plants would be expected to become less genetically diverse with time versus their sexed cousins and so less able to cope with the environmental changes alluded to above. However, if they have another advantage, human dispersion or cultivation, that could offset much of the limitation.
In point of fact, grapes appear to have a diverse genome and relatively small chromosomes, both factors which favor offspring diversity. We do not see grapes (or most other fruits) breeding “true to type”. Plant a grape seed, get a new variety – better, maybe; worse, generally. I’ll be really interested to see how Randall Graham’s grown from seed vineyard works out – a real mulch-generational endeavor with potential for pleasant surprises but one needing careful records and rouging out of less than desirable progeny. An aside – Something we don’t discuss in basic biology class is how genes really assort. The classic Mendelian random assortment of characteristics usually doesn’t happen. Indeed it is thought Mendel ‘faked’ his data to fit his theory since his experimental results are mathematically “too” perfect. “Genes” tend to assort as groups of varying length, taking multiple alleles together into the new seed. Hence the persistence of family traits in most species – including people…
So a mutation enhanced and arguably maintained by human intervention. Still subject to interbreeding with wild type vines generations of vines gathered additional genetic diversity into their seeded progeny. We then reach the point of today where virtually all vines are reproduced by vegetative means first developed by Egyptians thousands of years ago and which intentionally curtails the rate of variability.
And finally, note that most unnatural grape of all – the Thompson Seedless. Seedless? What possible reproductive advantage does that confer? Yet it’s pretty widespread and will persist that way so long as people are around to help it do so…