What is Evolution?
For our purposes, evolution is stripped down to its bare essentials. It requires a population of imperfect replicators with heritable variation in an environment with limited resources. Evolution occurs when the replicating population changes over time in its 'genetic' composition - the genome of heritable elements - and (usually) in the features of critters that are influenced by the heritable elements, the phenotype.
Imperfect Replicators:
Replicators" are entities that can reproduce themselves, that can produce offspring. Replicators are not necessarily biological critters. Some computer programs can replicate themselves. We will be using Avida, a research platform in which computer programs evolve.
"Imperfect" means that occasional errors -- mutations - occur when the replicators reproduce themselves. Even in asexual (clonal) organisms (e.g., bacteria) offspring are sometimes different from their parent due to errors in the reproduction/copying process. In addition, various mutagens -- things like chemicals, cosmic rays, and other similar stimuli -- can alter germ line DNA, generating mutations in offspring.
Heritable Variation:
"Variation" means that the replicators differ from one another in ways that are relevant to their survival and replication capability.
"Heritable" means that at least some of the variation among replicators is transmitted to offspring when a replicator reproduces itself. The inherited variation is typically carried via a coded representation, DNA in biological organisms or computer instructions in artificial replicators.
Limited Resources:
"Limited" means that the resources provided by the environment cannot sustain an indefinitely large population. Evolution occurs when there is competition for resources among replicators. That occurs when the reproductive rate of a population outstrips at least one of the environment's resources (a Malthusian system).
"Resources" include such things as living space, food, or whatever other environmental features sustain the core functions -- metabolism and reproduction -- of replicators.
In Avida the default limiting resource is world size: there is a limited number of spaces for critters in the Avida world, so there is competition for living room. Performing certain actions in their world earn Avidian critters extra computer cycles that enable them to reproduce faster and thus compete (as lineages) more effectively with other lineages. In addition, in version 2.0 of Avida one can define particular amounts of specific resources that can be consumed by the critters under specified circumstances, allowing experiments with multiple resource variations.