The error catastrophe states that when selection is weaker than mutation, the population will do a random walk in genotype space. A similar version can be formulated for environmental changes: when selection is weaker than the rate at which the environment changes, the population will again do a random walk in genotype space. I will show that an outcrossing population can overcome this version of the error catastrophe - no matter how weak the selection pressure, an outcrossing population can track the environment, if the population size is sufficient.
In unicellulars, epigenetic inheritance systems, such as DNA methylation, transmit information across cell division. During the evolution of multicellularity, these systems were taken over to be used to control differentiation. In this transition, the boundary between transmission within the organism and transmission across generations was less well defined than it is now. I will show that this can have many interesting effects, one of which is the evolution of a germline. In plants the germline-soma distinction is not as strong as it is in animals, and therefore we might still be able to observe phenomena that arise when epigenetic information is transmitted across generations.
There have been several transitions during which individuals of the same species began to cooperate, forming higher levels of organization, and sometimes losing their independent reproductive identity. Two examples are multicellularity and insect societies, both of which evolved multiple times independently. Several factors that confer evolutionary advantages on higher levels of organization have been proposed in the past. I will highlight an additional factor: the sharing of information between individuals. Information sharing is not subject to the intrinsic conservation laws that characterize the sharing of physical resources. A simple model will illustrate how information sharing can result in aggregates in which the individuals both receive more information about their environment and pay less for it. This may have played a role in the evolution of higher levels of organization.
The understanding of the principles underlying the complicated spatial-temporal processes during organismic development, the maintenence of tissues or perturbations of these require a model tool or a class of models capable to consider adequately the many degrees of freedom of these biological systems.
This includes the particular ability of biological cells to differentiate according to a program determined on the time scale of evolution. A differentiation often changes the physical properties of a cell and hence the way it is represented to its environment.
A successful model approach to multicellular systems must be able to include the spatial and temporal dynamics of cells as physical objects capable of changing their physical properties under certain stimuli. It must also be able to consider cell-type specific differences on small length scales of the order of the cell diameter.
Nevertheless, since it is hopeless to gain any insight from models with an endless number of parameters, a model approach must focus on those parameters that are essential to the effects of interest. To find an appropriate balance between these requirements is the challenge in modeling multicellular systems.
Starting from a (simple) single cell-based approach for cells in tissue culture it will be shown that this approach can easily be extended to describe (i) the growth of non-vascular tumor spheroids, (ii) blastula formation and gastrulation of sea urchin, and (iii) the buckling of one layered tissues as observed e.g. during the fission of intestinal crypts (pear-shaped pockets in the intestinal wall responsible for maintenence of the intestinal epithelium) after x-ray radiation.
Depending on the particular system studied cells are considered similar to Brownian particles with elastic interactions between them and the ability to divide and change their material or kinetic parameters by differentiation. >From these systems simple generic properties and effects can be extracted and analyzed separately by simple analytical models that allow to link the single-particle picture with a continuum description.
