LC: I think a lot of people know about Darwin’s work about 150 years ago, but little about what’s happened in evolutionary biology since. What have been the accomplishments of the field since, what are the major landmarks?
SS: The answer to that is probably a bookshelf about five or six feet long, so you want the elevator version. The elevator version is that Darwin didn’t know about genetics and even after Mendel and Morgan and Johanssen and Vavilov all of the other great early pioneers of genetics, and even after Ronald Fisher and J. B. S. Haldane and Sewall Wright put together population genetics as a way of thinking about evolutionary change, we still didn’t know what a gene really was or how a gene worked. I would say that clearly one of the big revolutions in evolutionary biology has been our understanding of the nature of the gene and of gene action and the way that genes are structured, inherited, controlled, and expressed.
Then of course the great technological advance in sequencing capacity has allowed us to recreate the history of life through phylogenetic analysis to a degree of detail we never had before and allowed us to understand the relationships among organisms with a striking degree of logical certainty. At least two major developments allowed that. One was the technological revolution of DNA sequencing, but there was also the logical revolution – called cladistics – led by a german phylogeneticist called Wili Hennig. Hennig’s ideas spread in the 60s and gained momentum in the 70s and 80s. The DNA sequencing revolution built from Watson and Crick through Sanger but did not arrive in full force until the 90s. It has given us a great deal of information about the history of life and the relationships among all the organisms on the planet that we hadn’t had before, and that has changed a lot of things. There are some wonderful examples.
Did you know that the Water Lily is more closely related to the Sycamore tree than the Sycamore tree is to the Maple Tree? If you just look at the Sycamore the leaves look like Maple leaves, but in fact Sycamores are more closely related to Water Lilies. Surprising discoveries like that now go on and on.
So that’s part of it. Another landmark conceptual revolution in evolutionary biology in the twentieth century that was not phylogenetic was Bill Hamilton’s insight into kin selection and George Williams’ insistence on focusing on genes in evolution. Both were inspired by Ronald Fisher. In the popular press that view is encapsulated in Richard Dawkin’s The Selfish Gene (1976) and The Extended Phenotype (1982).
They saw that we can look at an evolutionary process from the point of view of a gene, taking a “gene’s eye view” of evolution. It allows us to express things in terms of the interests of the gene and its abilities to control the replicators, us, in which it sits. That led to not only Bill’s insights into cooperation and altruism, which are big general issues, but to many insights into the role of conflict in evolution. Pollyanna might take the adaptationist view that everything is perfectly adapted to the environment in which it sits, but what Bill and his successors have shown us is that there is a lot of conflict among various genetic interests in evolution. The replication vehicles, the phenotypes, can contain genes that have different interests. The interests of the father are not always the same as the interests of the mother, the interests of the mother are not always the same as the interests of the infant. This leads to parent-parent and parent-offspring conflict theory, it leads to the theory of parent-of-origin genomic imprinting, it leads to evolutionary explanations for mental disorders. It has opened up a whole new way of looking at the world, and we are far far from having explored all of the consequences. It’s a very intriguing part of evolutionary biology right now. That’s another big conceptual step in evolutionary biology.
Another one, where I sit mostly on the sidelines but to which I’ve contributed a bit, is eco-evolutionary dynamics. Here the idea is that the ecology of a population of organisms causes natural selection that changes the properties of the population in such a way that they alter their ecology, and then the changed ecology again changes selection.
This creates a feedback loop that generates eco-evolutionary dynamics. That’s something that is very actively explored right now. We have at least two faculty in EEB who make that one of the things that they hang their hats on. It’s interesting, it’s complicated, and it’s an area where to do the analytical mathematics you need a lot of power and where nature rapidly gets so complex you often need to go beyond analysis into simulation to understand what is going on.
I’m curious, just because this sounds like an interesting field, do you have an example of something off the top of your head that represents eco-evolutionary dynamics well?
When anadromous fish come into a pond in Connecticut they change its community structure by eating the large plankton, which are then replaced by small plankton that the fish can’t feed on. That shapes the feeding apparatus of the fish so that they can feed on smaller plankton but the fish disappear and go back to sea if they continue to be anadromous. If you want to understand the state of fish in the state of Connecticut right now, you have to realize that 300 years ago the colonists built dams and in the three-hundred year period since then the land-locked populations of those anadromous fish that got trapped by the dams have evolved along a different path and have actually structured the whole community assemblage of the lakes in a whole different way than if they had been anadromous. That’s David Post’s research, and it’s pretty convincing.
Another has to do with the evolution of aging, but you’d be surprised–the model system is the aquarium guppy, and it’s investigated in the mountains of Trinidad, where it’s native. You can only understand the evolutionary dynamics of these guppies by combining an understanding of the age-specific selection pressures – are the predators primarily eating the juveniles or the adults? – which affects their rates of aging, with an understanding of how the guppies are changing their population density and their food supplies by eating things in their environment. When you put those two things together and realize that the change in population density has an age-specific impact on the guppies, then you can understand why it is that we see the patterns of life span and aging that we do see, which are not the ones predicted by theory that does not take such eco-evolutionary dynamics into account. If one just thought that it’s a simple problem in age-specific mortality, that wouldn’t work. You have to put in the coupling of evolution to the ecology to understand the system. That one has been pretty well worked out by David Reznick and his colleagues, and it took them from 1979 until about last year, so about forty years, to put all the pieces in place.