The milkweeds are well known for their cardenolides, which make them unpalatable to most animals besides the specialist insects that seek out these chemicals for self defense, such as the monarch butterfly and milkweed tussock moth. Cardenolides are found in almost all parts of the plant, including the nectar. This makes it dangerous for pollinators to consume too much milkweed nectar, since a large enough dose would contain enough cardenolides to poison and kill them. This study discusses how some bumblebee species have evolved adaptations in order to regulate how much milkweed nectar they can safely consume, while other species simply choose to avoid these flowers in order to avoid poisoning themselves. Milkweeds are self-incompatible and predominantly rely on bees for pollination, so killing the insects they rely on for reproduction seems counter productive.
Asclepias tuberosa contains some of the lowest levels of cardenolides among its genus. In fact, it contains so little that monarch butterflies will avoid laying their eggs on this plant if there are other milkweed species available, since the caterpillars would need to consume a huge amount of A. tuberosa leaves in order to sequester enough cardenolides. A. tuberosa's leaves are also rough and thinner than most of its congeners. A. tuberosa is common in eastern North America, but it also has populations in the southwestern part of the continent. It thrives in the arid regions of the continent, where it is sympatric with many other milkweeds that possess a similar morphology, but it is quite unique in the east, where it finds itself among species more suited for wetter and shadier areas.
According to a study from Rutgers University, Asclepias tuberosa and A. fascicularis attract more pollinators than other species, but are not as attractive to cardenolide specialist insects. This makes sense when considering that lower cardenolide levels make it safer for bees to feed on their flowers. According to the same study, the swamp milkweed (A. incarnata), which is known for having high cardenolide levels, attracted plenty of monarchs but was less desirable to bees.
It appears that milkweeds are forced to make a choice on what they want to do in order to guarantee their survival and repr
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Hey all
I was hoping you could either answer, or point to resources, that answer something I have yet to understand about evolution. Basically, my understanding of evolution is that environmental pressure and random mutation combine to allow species to adapt to their environment over a long period of time. What I struggle with is that surely the environmental advantage of any random mutation on a generational level would be so small as not to correlate with increased reproductive success?
I think this is best summarized via example:
- Consider a species of terrestrial lizard that get rehomed within a coastal/aquatic environment.
- Presumably after a long enough time we would observe adaptive changes such as strengthened/flattened tails or webbed feet.
- But consider within the context of a single generation. There would presumably be both adaptive, and maladaptive, mutations in reference to the previous generation. And these would most likely to be very minor.
- Given how minor these changes are, I fail to see how they would correlate with increased reproductive success - for example I don't see how the smallest hint of foot webbing, or a slightly flatter tail, would result in mating success?
I think natural counter to my point is that if even the slightest reproductive benefit is introduced by any change, over sufficient time, we would observe adaption. And that I am simply underestimating the timeframe. But I don't quite buy this - surely one has to first prove how the most minor adaptions increase reproductive success at all? Am I perhaps underestimating the change that can occur in a single generation?
Prove me wrong.
