The diagram below shows an evolutionary process taking place in a population of salamanders in California. The process took place gradually, millions of years ago. Study the diagram and answer the questions that follow.

Here are the answers to your questions:

3.6.1 A biological species is a group of organisms that can naturally interbreed with one another and produce fertile offspring.

3.6.2 The process shown in the diagram is allopatric speciation.
This process begins when an original population is divided by a geographical barrier, in this case, the Central Valley. This barrier prevents gene flow between the separated populations. Over millions of years, the isolated populations experience different environmental conditions and selective pressures. As a result, natural selection acts differently on each population, favoring different traits. Additionally, mutations and genetic drift occur independently in each group. These factors lead to the accumulation of genetic differences between the populations. Eventually, these differences become so significant that the populations can no longer interbreed and produce fertile offspring, even if the geographical barrier is removed, thus forming two distinct species (Species A and Species B).

3.6.3 Natural selection can lead to the development of different traits in two isolated populations of the same species living in different environments through the following steps:

• Variation: Within each isolated population, there is natural genetic variation among individuals for various traits.
• Differential Environments: The two isolated populations are exposed to different environmental conditions, which present different challenges and opportunities.
• Differential Survival and Reproduction: Individuals in each population with traits that are better suited to their specific environment are more likely to survive, reproduce, and pass on those advantageous traits to their offspring.
• Inheritance: These advantageous traits are heritable and are passed down through generations.
• Accumulation of Differences: Over many generations, due to these different selective pressures, the frequency of advantageous alleles increases in each population, leading to the accumulation of different traits and the divergence of the two populations.