Plant chemicals and evolutionary interaction with insects

Why does one plant taste or smell different from another? The opening gives a chemical explanation, then divides plant chemicals into two groups. Primary substances include familiar things such as proteins, carbohydrates and vitamins. They are found in all plants and support growth and proper functioning.

Secondary substances are different. They have no known role in the internal chemistry of growth or metabolism, and a particular plant species contains only a few of the many kinds that exist. Related plants tend to have similar ones. These substances produce distinctive tastes and smells. Keep that distinction clear: primary and secondary describe different roles, not simply a large amount of a chemical versus a small amount.

Does no known role in growth or metabolism mean no useful role at all? The rest of the explanation answers that question. These chemicals can matter enormously through how insects respond to them.

The second paragraph explains why many of today's secondary substances are present. New substances appear through genetic mutations. Natural selection then favors mutations that help the plant survive and reproduce. Those are two different steps: producing a new substance and having an advantage that allows it to persist.

One advantage is attraction. A scent brings a pollinating insect to a blossom by signaling nectar. The insect gets nourishment without harming the plant, while the plant benefits from pollination. Think of the scent as helping bring in a useful visitor.

Another advantage is defense. A chemical can make a feeding insect ill or kill it. A different substance may supply a warning smell or taste without itself being harmful. The warning signals another harmful substance in the plant. So the signal and the danger needn't be the same chemical. Either way, discouraging an insect from eating the plant can improve the plant's chances.

Imagine a mutation that gives a plant a taste that deters an insect from feeding. If that helps the plant survive and reproduce, the substance has an advantage. This is the useful connection between the chemistry and the insect: the insect's response helps determine which mutations are favored. Attracting a pollinator and discouraging a plant eater look like opposite effects, but both can benefit the plant.

The final paragraph gives the insects' side of the competition. When a food plant develops defenses, an insect can switch foods or develop a way around the defenses. It might detoxify a harmful chemical, store it where it does no damage, or otherwise avoid its effects. A substance dangerous to one insect may therefore be manageable for another with the right adaptation.

That creates an evolutionary back-and-forth. Plants gain defenses; insects develop ways of coping with some of them. Insects come to prefer plants whose defenses they can overcome and evolve the ability to identify those plants by taste or smell. The same kinds of chemical cues that can discourage feeding can also help an adapted insect find suitable food.

The result is increasing specialization: any one insect species has fewer suitable plants available and tends to become associated with a restricted group. An insect that can handle one plant's chemicals may still be unable to cope with the defenses of many other plants.

Our main point should connect the chemistry to this interaction. The distinctive secondary substances plants have today reflect an evolutionary relationship with insects: insect responses help select plant chemicals, and those chemicals in turn shape insects' food choices and adaptations.