Ecology and Evolution

Adaptation, Phylogenetic Inertia, and Trait Independence

A Conceptual and Methodological Investigation into the Use of Comparative
Data to Test Adaptive Hypotheses in Evolutionary Biology

Steven Orzack, Senior Research Scientist
(with Elliott Sober, of the University of Wisconsin and Thomas Hansen, University of Oslo)

In recent years, a number of biologists have proposed methods for using comparative (cross-species) data to test adaptive hypotheses. The most widely used of these methods is Felsenstein's method of independent contrasts. One important objective of these methods is to control for the possibility of phylogenetic inertia -- to take account of the fact that closely related species may have similar traits, not because these traits are adaptive, but simply because they were inherited from a common ancestor. In this project, we evaluate these methods; we also construct, justify, and apply a new method (which we call the method of controlled comparisons) that we think avoids the defects found in existing methods. This task will require careful conceptual analyses of what phylogenetic inertia actually means and of how inertial hypotheses are related to optimality models. One analytic tool that we will use is the work by philosophers on probabilistic explications of the concept of cause. This project is part of the larger program of showing how adaptationism can be tested; it supplements our previous analyses of how optimality models should be tested by using within-species data.

This project is partially supported by the National Science Foundation.


Molecular evolution and sex ratio evolution in the genus Nasonia

Steven Orzack, Senior Research Scientist

This project is an investigation of molecular evolution and sex ratio evolution in two species of parasitic wasp, Nasonia giraulti and N. vitripennis. The second species has long served as a model system in the experimental analysis of sex ratio evolution, with a critical assumption of this work being that the female-biased sex ratios of this species have evolved as a response to population subdivision. Despite this attention, there is no reliable information about population subdivision in this species. What is known is that at least some local populations appear to be highly polymorphic for sex ratio traits (as would be expected without population subdivision) and that females of the two species produce average sex ratios that differ from one another in a way that is unexpected given our present very incomplete understanding of the species difference in population structure. These discrepancies stem from an important gap in our understanding of sex ratio evolution in these species. By extension, given the importance of N. vitripennis as a model system, this is an important gap in our general understanding of sex ratio evolution.

This study helps to alleviate this gap. For both species, a single-copy locus (coding for a LIM protein) has been amplified from single males and the geographic structure of a G, A SNP polymorphism has been assessed with denaturing high performance liquid chromotography (DHPLC). More than 2000 males have been assayed from more than forty collection sites at the High Ridge Wildlife Refuge in Gardner, Massachusetts. Analysis of the data is ongoing.

This project is partially supported by the National Science Foundation.


Evolutionary Dynamics of Life Span

Steven Orzack, Senior Research Scientist (with Shripad Tuljapurkar, of Stanford University)

The broad goal of this research is to extend our understanding of the determinants, patterns, and constraints that shape mortality within and across species. This project aims to extend evolutionary theories of mortality and to bring hypotheses generated by this extended theory into confrontation with accurate longitudinal data on a range of species. Our aims are to review and analyze existing empirical data and also develop new theory, as follows: (1) Assemble a database of demographic data that will be unique in including (a) high-quality time series of observations on individuals in natural populations of many different species; (b) high quality data on captive breeding primates; (c) estimated age or size specific demographic rates (many published, some not) on many species of plants and animals, (2) Analyze the database to estimate mortality hazards and age patterns of fertility. We will analyze individual level covariation between survival and the timing and level of reproduction. We will exploit long time series of data to estimate temporal variance in survival and reproduction, and covariances between life history components. These analyses will be carried out on data sets for individual species and on related groups of species, (3) Extend the theory of life history evolution in age- and stage-structured populations to develop testable hypotheses about age pattern and correlation structure of mortality and fertility (and other life history components) and the maintenance of variation in these vital rates, and (4) To use the theoretical results to formulate testable hypotheses appropriate to the different populations in our database. Use our database to test and evaluate the theoretical hypotheses.

This project is partially funded by the National Institute of Aging as part of the PO1 project Biodemographic Determinants of Life Span.


Evolutionary Game Dynamics and the Evolution of Strategy Variation: Will an ESS evolve?

Steven Orzack, Senior Research Scientist (with Gordon Hines, of the University of Guelph)

Evolutionarily stable strategy (ESS) models are widely viewed as predicting the trait of an individual that when monomorphic or nearly so prevents a mutant with any other trait from entering the population. In fact, the prediction of some of these models can be ambiguous when the predicted trait is “mixed”, as in the case of, say, a sex ratio, which may be regarded as a mixture of the subtraits “produce a daughter” and “produce a son”. Some models that predict such a mixture predict only that it be manifested by the population as a whole, instead of necessarily by each individual as an ESS. This population manifestation is known as an evolutionarily stable state. The well-known Hawk-Dove game and the sex ratio game in a panmictic population are models that make such a “degenerate” prediction. We analyze the consequences of incorporating population finiteness into degenerate models. Population finiteness has effects for and against the evolution of an ESS that are of equal order in the population size. Therefore, we used Monte Carlo simulations in order to determine the probability that the population evolves to an ESS (as opposed to an evolutionarily stable state) in populations of various sizes. In contrast to previous analyses, we also examined the influence on this probability of the type of competition among individuals, of differing numbers of genotypes, and of random initial distributions of genotypes. We show that the probability that an ESS will evolve is generally much less than has been reported and that the tendency to do so depends upon on the population size, on the type of competition, on the number of genotypes in the initial population, and on their initial frequency distribution.

This project is partially supported by the National Science and Engineering Research Council of Canada.



Human Population Genetics     Census 2000 and Demography