state two different ways to generate a bimodal distribution

state two different ways to generate a bimodal distribution in finch beaks that persists for 100 years when there are no

geographic barriers separating distinct habitats. please give a brief explanation about each.

Solution

Isolation-by-Distance and Outbreeding Depression Are Sufficient to Drive Parapatric Speciation in the Absence of Environmental Influences.

This is extensively discussed in the following paper:
http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1000126

Another possibility is Disruptive selection of Darwin\'s finches. The process starts with divergent/disruptive selection causing adaptive divergence between conspecific groups that occupy different environments or use different resources.

The actual paper can be found here:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2660944/