Hanson, Thor. 2015. The Triumph of Seeds. pg. 128-143. Basic Books, New York, United States of America.
In many early evolution courses, the development of traits over time is often described as a linear progression. For instance, I was taught a few years ago that the development of insect wings was initially due to an extension of the body (flaps, if you will) on their backs. This increased surface area of the insect, allowing them to heat up faster in the sun. Beneficial, to be sure, but it is believed that the “flaps” eventually thinned out (again, to maximize surface area) and became functional wings. We believe this process generally followed this route, but in many cases the species evolves traits that are most beneficial, and we tend to organize this process linearly.
Linear evolution is not even an acceptable model for seed plants.
Seeds are a remarkable structure, and given their incredible ability to increase reproductive fitness, there is also substantial selective pressure that acts on them as a consequence. Plants have remarkable biochemistry, able to produce and release a tremendous variety of compounds, only being outdone in sheer diversity by bacteria. Their biochemistry yields useful compounds such as zingerone (gravol) and salicylic acid (aspirin), and these compounds are often involved in a process or defense mechanism which increases the plant’s fitness. Due to the potency of these compounds, they influence the plant’s relationships with other organisms quite noticeably. While a prospective herbivore might not enjoy eating a plant laced with toxic or otherwise distasteful compounds, birds or rodents which would spread their seeds by consuming their fruit will also be deterred. This leads to a distinct trade-off: does the plant increase toxic compound content to confer protection, or does it reduce the concentration to facilitate better dispersal? The answer is never a clear one, and it’s why linear models of evolutionary change, while easy to understand, often oversimplify a much more complex problem.
Hanson discusses his experience with a chile pepper specialist in chapter 10 of “The Triumph of Seeds.” In particular, Hanson describes the incredible “race” that these peppers have undergone with a fungus. There is apparently a fungus that will rot pepper seeds, but the capsaicin content in the peppers helps to protect them from fungal attack. However, it makes the fruit less palatable for prospective herbivores in many cases, and the selective pressure of dispersal vs. protection has a discerning impact on the capsaicin content in the seeds. For instance, if there is a period where the fungus thrives, then the successful plants will have higher capsaicin content to facilitate their survival. It’s an interesting interaction, and definitely not the only one like it in plants. Due to the diverse symbiotic relationships in the plant kindgom, co-evolution is incredibly common. Hanson also discusses that there is a bird species that seems largely invulnerable to capsaicin’s intense heat, and as such it is the ideal disperser of the seeds, allowing it to maximize protection of its seeds while not having to trade dispersal range.
Hanson’s writing on this subject is nicely laid out and organized, he articulated the process by first mentioning how valuable spices have been to us historically, and then discusses his interaction with the specialist on the capsaicin content in peppers. While my science background would appreciate some additional depth in the chemistry/metabolism of capsaicin, I appreciate his ease of transitions and simple language. Hanson does a great job illustrating how the active components of plants have influenced us through the ages (in the form of spice), but are also integral to their survival as well. In fact, the network of evolutionary relationships regarding plants and their active constituents only gets more confusing when considering human influence.
It’s a strong selective force; but it has no distinct direction.