A population of mice in Mexico that has large ears relative to the size of its head. A population of mice in Alaska that has small ears relative to the size of its head. If you travel from Mexico to Alaska, you find that ear size relative to head size decreases gradually. Individuals from all populations can and sometimes do interbreed with populations farther north or farther south. The two kinds of oak sometimes interbreed in natural populations to produce healthy, fertile offspring.
All forms are capable of interbreeding with each other and produce healthy, fertile offspring.
Each kind of flicker has several unique derived characteristics that allow it to be clearly distinguished from the other. Between California and Tennessee, there is a region of less than 200 miles wide within which the two forms both occur and produce healthy, fertile hybrids. Despite this, the different forms of maple retain their own unique characteristics. State one advantage to grouping groups such as this as a single species, and one advantage to splitting groups such as these into separate species. Discuss the role of gene flow, genetic drift, and natural selection in speciation.