Why evolution isn’t just about plants and animals, with Dr. Mike Lynch

Darwin’s finches. Orchids and hawk moths. Peacocks. Apes and humans. Thinking about evolution often evokes images of charismatic and iconic plants and animals that we learn about in high school biology or through popular media. Dr. Mike Lynch, our most recent guest on Big Biology and a professor at Arizona State University, has a different perspective. “Animals and vascular plants are the oddballs of evolutionary biology,” writes Lynch in his most recent book Evolutionary Cell Biology: The Origins of Cellular Architecture.

“Oddballs” might sound like an unusual way to describe plants and animals. And yet, in terms of the number of individual organisms on the planet, “land plants and animals would be out in the range of 0.001% of all individuals,” Lynch explains in the episode. “I mean, there’s something like 10³¹ viruses on the planet, and the number of just bacteria and archaea is about 10³¹ so that’s far, far beyond the numbers of most multi-cellular things.”

Lynch is quick to affirm that plants and animals have a significant ecological impact on the planet. “Not to say we shouldn’t study animals and land plants,” he adds. “But it’s kind of an interesting commentary on where we are in biology. We’re studying the things that aren’t most of biology.”

These numbers comparing total plants, animals, and microbes in the world aren’t just fun facts—they have an immense impact on how evolution works. This impact comes primarily from vastly different effective population sizes among plants, animals, and unicellular organisms. Effective population size is a “pretty complicated technical topic,” Lynch admits, but it is fundamental to understanding how the forces of selection and drift interact to drive evolution.

Effective population size is “sort of a composite way of putting everything together that influences the noise in the evolutionary process,” Lynch explains. “The way to think about it is, if you have some number you call the effective population size, the inverse of that is related to the noise in the evolutionary process.” In other words, having a high effective population size translates to less noise in a population’s evolution. Having a high effective population size makes natural selection a major force driving changes in a population’s allele frequencies. In contrast, having a low effective population size turns up the noise, meaning that genetic drift tends