An overview of speciation theory reveals an increasingly held view that many events leading to the origin of new species occur in transient, unstable populations. A transient, unstable population should be under stood as a fast episodic phase in a population subjected to genetic and environmental factors that tend to disrupt its cohesive, balanced genome architecure, thus enhancing its probability to produce a new species. Striking the core of Darwinian thought, some authors claim that these· processes may be non-adaptive. Among the environmental factors one may cite biotic (e.g. resource availability) and abiotic (e.g. temperature) stress conditions that break up the population stability producing random, unpredictable changes in population size, population trait distribution, breeding structure, inter- and/or intrapopulational hybridization, etc. Genetic factors consist of those events that induce rapid changes in genetic expression and/or that determine reproductive isolation, such as substitutions, insertions, deletions, duplications, transpositions, gross chromosomal rearrangements, recombination and, in general, any mechanism that changes the regulatory pattern of the organism or the balance of its meiotic system. Both kinds of factors are often intertwined in a complex net and may influence each other.

A Founder, Colonizing and Bottleneck Populations

A. 1. Theoretical Framework
Phenotypically Plastic Characters in Isolated Populations
Multivariate Morphometrics of Bottlenecked Populations
Niche Overlaps and the Evolution of Competitive Interactions
Marginal Populations in Competitive Guilds
A. 2. Experimental
Flush-Crash Experiments in Drosophila
Founder Effects in Colonizing Populations: The Case of Drosophila buzzatii
Mating Probability, Body Size and Inversion Polymorphism in a Colonizing Population of Drosophila buzzatii
Colonization and Establishment of the Paleartic Species Drosophila subobscura in North and South America
Short Range Genetic Variations and Alcoholic Resources in Drosophila melanogaster
The Variance in Genetic Diversity Among Subpopulations is More Sensitive to Founder Effects and Bottlenecks Than is the Mean: A Case Study
B Evolutionary Mechanisms
B. 1. Molecular
Mobile Genetic Elements and Quantitative Characters in Drosophila: Fast Heritable Changes Under Temperature Treatment
The Potential Evolutionary Significance of Retroviral-like Transposable Elements in Peripheral Populations
Paradoxes of Molecular Coevolution in the rDNA Multigene Family
B. 2. Chromosomal
Two Ways of Speciation
Karyotypic Repatterning as one Triggering Factor in Cases of Explosive Speciation.