Strain: in microbiology, strain represents all cells descended from a single ancestral cell. Thus, ideally, one strain is a population of clones of the ancestral individual with the same genotype. In reality, we may consider lineages without observable change to be the same strain, regardless of the possible neutral mutation in their genomes, because the speed of microbial evolution always exceed of our control.
Ecotype: an ecotype is an ecologically cohesive taxon within a species. Individuals may slightly differ in their genomes but share the same niche and live a same lifestyle.
Prototroph: the individual with normal metabolic ability and normal nutrient requirement, sometimes refer to as “wild type”.
Auxotroph: the individual with certain kind of metabolic flaws comparing with prototrophs so that it needs some extra nutrients to grow which are not necessary to the prototrophs.
Periodic Selection: the periodic renew of population gene pool. When an advantageous mutant emerges in a population and is able to oppress till eliminate all the other individuals in the same population. After certain time, all the old genotypes will be outcompeted and replaced by the new genotype (the clones) and this process will continue forever as long as the population still exist.