Evolution, monarch adaptation to toxic milkweed (Evolution)

by David Turell , Thursday, October 03, 2019, 20:33 (1666 days ago) @ David Turell

A poisonous plant is the only food for their caterpillars. It took three different mutations to do it:

https://www.scientificamerican.com/article/how-monarch-butterflies-evolved-to-eat-a-poi...

"The monarch butterfly’s colorful caterpillars, for example, devour milkweed with gusto—in fact, it is the only thing they ever eat. They can tolerate this food source because of a peculiarity in a crucial protein in their bodies, a sodium pump, that the cardenolide toxins usually interfere with.

"All animals have this pump. It’s essential for physiological recovery after heart muscle cells contract or nerve cells fire—events that are triggered when sodium floods into the cells, causing an electrical discharge. After the firing and contracting is done, the cells must clean up, and so they turn on their sodium pumps and expel the sodium. This restores the electrical balance and resets the cell to its usual state, ready again for action.

"Cardenolides are noxious because they bind to key parts of these pumps and prevent them from doing their job. This makes animal hearts beat stronger and stronger, often ending in cardiac arrest.

***

“'They needed to get the mutations in the right order,” Whiteman says. First, a mutation of small effect would have altered the structure of the sodium pump to provide some resistance, but also some neurological problems. The second mutation would have amended the pump structure slightly, thereby fixing that problem. By so doing, it would have prepared conditions for the third mutation—the one with the heftiest antitoxin effect. By itself, that third mutation would have created intolerable neurological issues. But with the second mutation already in place, all would be well, or at least much better.

“'Biologists call this a constrained adaptive walk,” says Whiteman, “where one mutation is followed by another, in a predictable order, setting a species, or more than one, on a trajectory to higher fitness.” (my bold)

***

"The monarchs’ evolutionary innovation had an ecological ripple effect. Not only did resistance to the toxin open up a whole new source of food, but it also allowed the butterflies to repel predators by storing the toxins in their bodies."

Comment: three specific mutations are needed, and note my bold, must appear in specific order to evolve. Not by chance; only a designer can do this. And for survival all three mutations had to be present for the caterpillars to survive. And if the taste is very noxious to most insects, it must taste good for these guys, which means more mutations must be present or the three mutations also make it taste good.