Why do Black and Honey Locust trees still have thorns?

I've read claims that these trees evolved thorns to repel browsing animals back in the days when North America still had megafauna. The sweetish pods of Honey Locust might even have been intended to induce mammoths to eat them (something like Osage Oranges). But it's been thousands of years since those animals existed, and at least in Honey Locusts, the allele that removes the thorns is dominant. So why haven't the trees lost their thorns?

I can't help but wonder if the popularity of thornless varieties of these trees is missing something important, and possibly contaminating the gene pools of these species.

Our native Devil's Claw Proboscidea also evolved during the time of the megafauna, the large claws on the seed pods gripped the legs of the megafauna. No megafauna exist here today, yet I have a large Devil's Claw plant in my garden. If there is no disadvantage to having a characteristic, it will tend to remain. I think evolutionary biologist Stephen J Gould called these characteristics "spandrels." I'm not sure thorns and claws fall into the spandrel category, because they did at one time have a function; though it may not be useful anymore, it is no disadvantage.

http://www.sibleynaturecenter.org/essays/moseying/exploringnature/080702_devilsclaw.html

Because "thousands of years ago" is relatively recent for evolutionary changes. It hasn't been that long.

Eleven thousand years is more than enough time for certain kinds of evolutionary change to take place. And again, the gene for thornlessness in Honey Locust is dominant - so even if having or losing thorns has no evolutionary benefit, we should expect thornlessness to be a common trait. But it isn't. Why not?

My sister was a researcher in genetics in a past career, I'll ask her.

Cal Edon wrote:Eleven thousand years is more than enough time for certain kinds of evolutionary change to take place. And again, the gene for thornlessness in Honey Locust is dominant - so even if having or losing thorns has no evolutionary benefit, we should expect thornlessness to be a common trait. But it isn't. Why not?

Perhaps it helps prevent deer from browsing them while young?

Cal, my sister responded to the question with this:

"Is this simple dominance, or complex? If there are a number of genes that regulate whether a Honey Locust has thorns, then it depends on the relationship of all of those genes. And do all the thornless Honey Locusts get eaten because they don't have thorns?"

Don't forget, it's impossible to breed out a recessive gene so there will always be locust trees with thorns.

It is quite possible that the gene for thorns is also a gene for some other traits such as drought tolerance, temperature tolerance or even pod sweetness. So by still having a beneficial outcome from one trait, you get the thorns too. If having the thorns isn't a detriment then there is no reason to favor those trees without thorns. It's all about the total population, so as long as there is genetic diversity in that population these traits will continue to express themselves in some cases, dominant or otherwise.

Think about it like this: Say that in the next few years, this drought in the US continues to a point where there is very little for critters to eat in the wild. Those trees with the thorns will have a lower chance of being eaten while the thornless ones will be eaten out of existence (for the most part). After the drought is over and things go "back to normal" the thornless trees make a resurgence to some degree, until another forest food shortage. This would cause the population, over time, to favor the thorny trees even if the trait for thorns is recessive.

Tyler Ludens wrote:And do all the thornless Honey Locusts get eaten because they don't have thorns?"

This is true. No locust would get more than 6" tall here if thornless. The black locust here start as a thorn--NASTY if you aren't looking where you are walking.