Bird guides once listed the myrtle warbler and Audubon's warbler as distinct species that lived side by side in parts of their ranges. However, recent books show them as eastern and western forms of a single species, the yellow-rumped warbler. Most likely, it has been found that these two kinds of warblers
a. Live in similar habitats and eat similar foods.
b. Interbreed often in nature, and the offspring are viable and fertile.
c. Are almost identical in appearance.
d. Have many genes in common.

Verified step by step guidance

1. Step 1: Understand the biological species concept, which defines a species as a group of organisms that can interbreed and produce viable, fertile offspring in nature. This concept is key to solving the problem.
2. Step 2: Analyze the options provided in the question. Option (a) refers to similar habitats and diets, which does not necessarily indicate that two organisms are the same species. Option (c) refers to appearance, but physical similarity alone does not define a species. Option (d) refers to shared genes, but many species share genes due to common ancestry.
3. Step 3: Focus on option (b), which mentions interbreeding in nature and producing viable, fertile offspring. This aligns directly with the biological species concept and is a strong indicator that two organisms belong to the same species.
4. Step 4: Consider the context of the problem. The reclassification of the myrtle warbler and Audubon's warbler as forms of a single species suggests that evidence was found supporting their ability to interbreed and produce viable, fertile offspring.
5. Step 5: Conclude that the most likely reason for the reclassification is the discovery that these two warblers interbreed in nature and produce viable, fertile offspring, as described in option (b).

Key Concepts

Species Concept

The species concept refers to the criteria used to define and distinguish species from one another. Traditionally, species are defined as groups of organisms that can interbreed and produce fertile offspring. This concept is crucial for understanding why the myrtle and Audubon's warblers were once considered separate species and how they are now viewed as forms of a single species based on genetic and reproductive evidence.

Hybridization

Hybridization occurs when individuals from two different species interbreed, producing hybrid offspring. In the context of the yellow-rumped warbler, if myrtle and Audubon's warblers interbreed and produce viable, fertile offspring, it suggests that they share a close genetic relationship. This phenomenon can lead to the reclassification of species based on their ability to exchange genes and maintain genetic continuity.

Genetic Similarity

Genetic similarity refers to the degree of genetic overlap between different populations or species. When two forms of a species, like the myrtle and Audubon's warblers, have many genes in common, it indicates a close evolutionary relationship. This genetic evidence supports the idea that they are variations of a single species rather than distinct entities, influencing how biologists classify and understand biodiversity.