Evolution can occur in just one full generation
Strong selection will rapidly reduce the gene frequencies of genes which cause negatively selected phenotypes. This reduces the likelihood of unfavourable genotypes occurring in the next generation.
(I regard generation here as the complete cycle of one individual being born to the point at which they successfully give birth/sire young).
Population genetics explanation:
Imagine a population in which LOC1 is a locus affecting a phenotypic marker allowing perfect identification of genotype at that locus. There are two alleles at LOC1, A and a. This gives rise to three different genotypes, AA, Aa and aa which occur in the population in equal proportions. Those individuals with AA exhibit a certain trait, whilst Aa and aa individuals do not. If only the AA genotypes are allowed to mate then the following generation will only contain AA genotypes at LOC1 (assuming no mutation).
For more population genetics read Principles of Population Genetics and then Introduction to Quantitative Genetics.
Genetic drift caused by using 5 Pairs
The new population was started with just 5 pairs of individuals. This means there is huge potential for fixation of alleles via drift with in the very first generation. I ran a (basic) genetic drift simulation in R just now. Out of 5000 replicates 4709 went to fixation (Single locus, two alleles, using 30 generations of 10 individuals).
Environmental change can cause hidden genetic variation to be exhibited
Molecular regulators which affect the expression of traits can respond differently depending on environmental influences. This point is summarized nicely by the opening lines of this paper- thanks to @leonardo for this point.
Hsp90 is a molecular chaperone for many signal transducers and may
influence evolution by releasing previously silent genetic variation
in response to environmental change. In fungi separated by 800
million years of evolution, Hsp90 potentiated the evolution of drug
resistance in a different way, by enabling new mutations to have
immediate phenotypic consequences.
Evolution and speciation are different things
These are just the simple wikipedia definitions, for better ones then consult the literature:
Evolution is the change in the inherited characteristics of biological
populations over successive generations.
For a good thorough introductory text on evolution I recommend Evolution by Mark Ridley (not to be confused with Matt Ridley - popular evolution science writer).
Speciation is the evolutionary process by which new biological species arise.
Speciation, thanks to a long fought debate over species concepts, is also an ambiguous process. What some species concepts would define as two distinct species, another would call one species, and another may call them 10 species! Seemingly everyone who specializes in the study of speciation would have there own twist on a species concept so don't expect that argument to resolve any time soon! It's not advanced much since Dobzhansky wrote this.
The species concept is one of the oldest and most fundamental in
biology. And yet it is almost universally conceded that no
satisfactory definition of what constitutes a species has ever been
When does evolution occur? and when is change not evolution in such a short time?
I've put this bit in because what I find is a strange, but common, question people ask is
"species X has changed/adapted to novel selection very rapidly, has
evolution really occurred?"
The way people ask this makes it sound like they want a certain amount of evolution to occur before we can say something has evolved. To me saying evolution has occurred means that some change has occurred, via any of the mechanisms of evolution, which results in one subset of individuals being different from another it is compared to (spatially or temporally separated). You could ask if such strong evolution can really occur over a short time period, but the answer is obvious - yes it can. If it has occurred then why do you think it is exceptional? what is your alternative explanation for the differences seen?