Larson 10

light and dark forms of the peppered mothtest will be Wed:
The Peppered Moth (above ) was seen at the time as an example of evolution, but could be explained by different theories of how evolution worked (the paragraph at the bottom of p. 221 and the top of 222 is a very useful summary of that)

The resolution of this is called The Modern Synthesis and was developed by J.B.S. Haldane, Ronald A. Fisher and Sewall Wright beginning in the 1920s.  It was needed because Mendelian genetics didn't instantly solve the problems with understanding the detail mechanism of evolutionsummary of
                modern synthesis
Population biology: Haldane showed mathematically that a given frequency of mutation and level of selection (say black moths have a 50% higher survival rate than speckled moths) can cause the formation of a new variety (Haldane was an arrogant type and wrote that only the best one out of one thousand of the human population should reproduce)

Fisher (motivated by very narrow eugenics concerns) showed how gene frequencies would shift under natural selection.  The more benefit provided by a variation (allowing the animal to reproduce more), the more quickly it would spread.  Even though his motivations were deplorable, his science was good.

 Wright translate the statistics into an understanding of how new species could form from genetic drift in isolated populations. 
Theodosius Dobzhansky published Genetics and the Origin of Species in 1937 pulled this together with real world examples to show that small scale genetic shifts can lead to evolutionary change on a large scale

The computer models could actually be matched to population studies in isolated situations such as the Galapagos islands

finsh beaks on different islands
On islands where different species of finches co-exist, their characteristics are more separate

There is an interesting modern popular book on the finches The Beak of the Finch

Present day examples