[Current Research Interests]
[Volvocales Projects]
[Articles About Our Work]

Current Research Topics

Several current research projects are being led by graduate students in my lab are described here.

Our current research on the volvocine green algae are described here.

Adaptive Value of Sex in Microbial Pathogens

Explaining the adaptive value of sex is one of the great outstanding problems in biology. The challenge comes from the difficulty in identifying the benefits provided by sex, which must outweigh the substantial costs of sex. Here, we consider the adaptive value of sex in viruses, bacteria and fungi, and particularly the information available on the adaptive role of sex in pathogenic microorganisms. Our general theme is that the varied aspects of sex in pathogens illustrate the varied issues surrounding the evolution of sex generally. These include, the benefits of sex (in the short and long term), as well as the costs of sex (both to the host and to the pathogen). For the benefits of sex (that is, its adaptive value), we consider three hypotheses: (i) sex provides for effective and efficient recombinational repair of DNA damages, (ii) sex provides DNA for food, and (iii) sex produces variation and reduces genetic associations among alleles under selection. Although the evolution of sex in microbial pathogens illustrates these general issues, our paper is not a general review of theories for the evolution of sex in all organisms. Rather, we focus on the adaptive value of sex in microbial pathogens and conclude that in terms of short-term benefits, the DNA repair hypothesis has the most support and is the most generally applicable hypothesis in this group. In particular, recombinational repair of DNA damages may substantially benefit pathogens when challenged by the oxidative defenses of the host. However, in the long term, sex may help get rid of mutations, increase the rate of adaptation of the population, and, in pathogens, may infrequently create new infective strains. An additional general issue about sex illustrated by pathogens is that some of the most interesting consequences of sex are not necessarily the reasons for which sex evolved. For example, antibiotic resistance may be transferred by bacterial sex, but this transfer is probably not the reason sex evolved in bacteria. Published in Infection, Genetics and Evolution (PDF)

The Group Covariance Effect And Fitness Trade-Offs During Evolutionary Transitions In Individuality

Transforming our understanding of life is the realization that evolution occurs not only among individuals within populations but also through the integration of groups of individuals into a new higher-level individual, that is, through evolutionary transitions in individuality. During evolutionary transitions (such as during the origin of gene networks, bacteria-like cells, eukaryotic cells, multicellular organisms, and societies), fitness must be reorganized; that is, it must be transferred from the lower- to the higher-level units and partitioned among the lower-level units that specialize in the fitness components of the new higher-level individual. We have been studying the role of fitness trade-offs in fitness reorganization, the evolution of cooperation, and the conversion of a group into a new individual during the origin of multicellular individuals. Specifically, our work shows that the fitness of the group is augmented over the average fitness of its members according to a covariance effect. This covariance effect appears to be one of the first emergent properties of the group and a general aspect of groups with multiplicative properties that are themselves averages of properties of lower-level units. In this way cooperation may emerge from existing life-history trade-offs and the group may break through the trade-off constraints that govern the lives of their members. The covariance effect likely applies to group dynamics in other fields.

Michod, R. E., Y. Viossat, C. A. Solari, A. M. Nedelcu, and M. Hurrand. 2006.  Life history evolution and the origin of multicellularity.  Journal of Theoretical Biology. 239:257-272. http://eebweb.arizona.edu/michod/Downloads/Life%20history%20evolution%20and%20individuality%203.pdf

Michod R.E. (2006). The group covariance effect and fitness trade-offs during evolutionary transitions. Proceedings of the National Academy of Sciences, USA. 103:9113-9117. http://eebweb.arizona.edu/michod/Downloads/fitness%20tradeoffs.pdf

Cooperation and Conflict in Evolution

Thirty years ago, the study of cooperation received far less attention than the other forms of ecological interaction (competition, predation and parasitism). Scholars generally viewed cooperation to be of limited interest, of special relevance to certain groups of organisms to be sure, but not of general significance to life on earth. All that has changed with the study of evolutionary transitions in individuality. What began as the study of animal social behavior, has now embraced the study of interactions at all biological levels. Cooperation is now seen as the primary creative force behind ever greater levels of complexity and organization in all of biology. While it may be easy to agree on the basic role played by cooperation in the diversification of life, it remains a difficult interaction to understand. Cooperation has traditionally been approached from two different perspectives-within species and between species. Our work aims to bridge these different traditions. The benefits of cooperation lead to the emergence of new functions among the collective. Yet the costs of cooperation also set up the opportunity for defection and conflict within. We have explored the evolution of cooperation and conflict at several different levels, including replicating molecules within networks of molecules, cells within organisms, and among organisms within societies. We have been especially interested in developing a general theoretical framework within which the evolution of cooperation at different levels can be understood. My interests in cooperation and conflict have led me to investigate the evolution of individuality of units of selection and the factors leading to transitions between evolutionary units (genes, cells, multi-cellular organisms, and societies), that is new evolutionary individuals.

To see our work on the evolutionary origin of altruistic genes in the volvocine green algae click here.

Individuality

The organization of the living world is hierarchical-lower level units group together and cooperate to form higher level units of organization (genes, chromosomes, bacteria-like cells, eukaryotic-like cells (cells in cells), multicellular organisms and societies). How this came about is far from clear. What is clear is that the major landmarks in the diversification of life have involved transitions between these hierarchical levels and that cooperation (among lower level units in the emergence of a higher level) has been a dominant force. The evolution of multicellular organisms is probably the premier example of the integration of lower evolutionary levels into a single, higher-level individual. Explaining the transition from single cells to multicellular organisms is a challenge for evolutionary theory. Our work shows how and why heritability of fitness--the defining characteristic of an evolutionary individual--emerges at the new higher level out of the need to regulate cooperation and conflict among lower level units. Current work focuses on life history approaches to increasing complexity and individuality in volvocalean green algae and on epigenetic change during development. My recent book on the topic of individuality describes the background cooperation and conflict framework we are using: Darwinian Dynamics: Evolutionary Transitions in Fitness and Individuality.

A Hydrodynamics Approach to Motility of Volvocalean Green Algae

In collaboration with Cristian Solari, John Kessler and Ray Goldstein, we are studying the hydrodynamics of motility (as the primary determinant of the viability component of fitness) in volvocalean green algae. This research is described in more detail here.

Evolution of Sex

The living world is sexual, why? While accepting the importance of variability in the evolutionary process and the role of sex in creating variation, my research has led me to explore alternative explanations for the ubiquity of sex, especially the role of sex in coping with genetic error. The most obvious consequences of sex are its risks and costs. I believe that individuals would not undertake such an inherently risky and costly process as sex, unless they were in trouble to begin with. Genetic error, both mutation and damage are a constant threat to the integrity of the organism and the vitality of life. Sex efficiently repairs damage while masking the effects of mutation. The role of sex in coping with genetic error explains the rejuvenating effects of sex and the immortality of DNA. In addition, the nonlinearities involved in mating, help explain the existence of species as relatively distinct entities. These themes were developed in my book Eros and Evolution, a Natural Philosophy of Sex. According to the repair hypothesis developed in collaboration with colleagues here at the University of Arizona, sex originated as a cooperative interaction between cells-- the benefit of these interactions being damage repair. Outcrossing sex is maintained in modern organisms by the need to simultaneously manage genetic errors, both damages and mutations. Ongoing projects involve both laboratory and theoretical investigations of this hypothesis. I have two books on the topic of the evolution of sex.

Recently Aurora Nedelcu has been studying the role of oxidative stress as the impetus for sex in the facultatively sexual volvocalean green algae lineage. This work is described in more detail here and at Aurora's web site.

Mutation and Multi-level Selection

Coping with mutation is a critical factor during evolutionary transitions, since mutation enhances the scope for selection both within and between evolutionary units leading to conflict between levels of selection. We have studied the evolution of the basic reproductive modes used in multicellular organisms from the point of view of coping with mutation. The way in which mutational variation is organized by sex and other aspects of reproductive biology is a fundamental problem for the emergence of new evolutionary units. In spite of nearly a century of work on the population genetics of mutation in various respects, there has been no systematic treatment of mutation in a multi-level selection context. We are in the process of developing such a treatment. We model mutation in a multi-level context from a variety of points of view (including epigenetic and genetic systems of inheritance) and using a variety of techniques (computer, mathematical, stochastic and deterministic). The evolutionary significance of mutation in a multilevel context is investigated by studying the variation and scope for selection both within and between populations, by studying the genetic load at the different levels, and by studying the evolution of modifiers of development and other aspects of the biology of the organism (or other evolutionary individual) as these modifiers shape the organism by modulating the opportunity for within and between group change. Furthermore, we developing a systematic means of organizing gene function according to their distribution of mutational effects at two levels. For example, mutations in genes involved in cell cycle control are more likely to be selfish (advantageous at the cell level but disadvantageous at the organism level), while mutations in genes involved in housekeeping are more likely to be uniformly deleterious at both levels.

Fitness

Lewontin once remarked that evolution by natural selection should explain "fitness." In my recent book Darwinian Dynamics: Evolutionary Transitions in Fitness and Individuality, I take the approach that to explain fitness we need to understand three things: (i) how fitness originated in the transition from the non-living to the living, (ii) how new levels of fitness are created during evolutionary transitions to greater levels of complexity, and (iii) the role of fitness in the theory of natural selection. I begin by considering the dynamical view of fitness that emerges from our studies of evolutionary transitions and individuality (summarized above). When the consequences of Darwin's principles are fully understood as embodied in the dynamics of natural selection in a multi-level setting, there is no meaningful concept of the individual as a maximizing agent, especially when the units of selection are changing. Frequency-dependent fitness effects frustrate the creation of new evolutionary units and prevent the application of simple maximization arguments. The tension between lower and higher level units is never completely resolved in any evolutionary transition. Units of selection do not exist in isolation, nor are they completely interdependent. Instead, they exist in a hierarchy of nested but partially decoupled levels, and any focal level provides both the context for lower level units as well as the components of higher level units. Because evolutionary units (genes included) play the roles of both context and component at the same time, the dynamics of natural selection at any level involves an interplay between the dynamics at all levels. After considering the meaning of fitness that emerges from these studies, I consider touchstone cases of natural selection using a dynamical view of fitness: the so called "tautology problem," the evolution of sex and reproductive systems, the immortality of the germ line in contrast to the mortality of the soma, such old standbys as sickle-cell anemia and heterozygote superiority, and the existence of species as distinct entities. These case studies all recommend a dynamical view of fitness and on this basis I offer a general framework for the role of fitness in evolutionary explanation.

Origin of programmed cell death and the eukaryotic cell

Programmed cell death (PCD) is an evolutionarily conserved from of cell suicide that enables metazoans to regulate cell numbers and control the spread of selfish mutants (cancerous cells) that threaten the organism. We are studying the origin of PCD as a means of coping with deleterious mutation. In addition, we are studying the evolution of interactions of cells-within-cells, that is, the endosymbiotic origin of the eukaryotic cell (and other kinds of mutualisms such as bacteria-insect endosymbiosis). The origin of multicellularity and the origin of the eukaryotic cell are related-PCD is an important means of conflict mediation in metazoans and likely had its origins in the levels of selection conflicts that were inherent in the origin of the eukaryotic cell (between protomitochondria and their host cells).