Someone asked me recently to explain the "founder effect," an idea presented to him as somehow explaining how life evolves following a bottleneck (population decrease).
The founder effect generally is used to describe a situation in which a small population of a given species becomes isolated from the much larger portion of it. (It could also occur if the majority of a population were wiped out, leaving only a small portion remaining.) Evolutionists believe that in such cases, the genetics in this "founding" population may be a subset different enough from that of the overall population (from which it came) that a new species might result. This assumes that this small population is able to survive and reproduce despite their low numbers.
This is primarily a theoretical idea, although there have been those who claim to have seen this in action (in one or more species of plants).
Although the founder effect may be a legitimate concept, and new species might arise through this phenomenon, this would do nothing to support the grand claims of neo-Darwinism. For the resulting new species represents a significant loss of genetic variability. This means (among other things) that it is likely less able to cope with change (natural selection) and more likely to go extinct. But even if this new species survives, it has not undergone mutational advance (the sort of thing evolutionists require for the making of new body plans, for macroevolution); rather, the new species represents a step backwards.
The founder effect--if a valid biological concept--provides no evidence that macroevolution is an accurate portrayal of the history of life on Earth.