The term speciation refers to the process by which two or more different species are produced from a single original population by different speciation mechanisms. Speciation occurs in three main steps: separation of the original population, divergence (trophic divergence, reproductive divergence, genetic divergence, etcetera), and isolation.
The most related option to speciation is B. Two populations of humans living on separate sides of the ocean. After a catastrophic event, travel across the ocean is not possible. After millions of years of separation, the two groups are no longer able to produce viable offspring together, but can still produce within their isolated groups.
Speciation occurs when the original population is divided into two groups. They inhabit different places and are exposed to varying environmental conditions. With time, each group experience many genetic changes. These mutations depend on each group´s environment and its ecological pressures, and they accumulate in time. Pressures act on individuals and make them adapt to the new environmental conditions. Generation after generation, the genotype of individuals will increase the fitness to survive and reproduce. Fixation of mutations will increase the difference between the two groups, up to the diverging point into two new species. Even is they get together again, these species will not be able to mate and produce offspring, and if they do, the new generations will not be viable or fertile.
The exposed case of two human population living on different sides of the ocean, is an example of allopatric speciation.
Allopatric speciation refers to the geographic separation of a continuous genetic background, originating two or more new geographically isolated groups. During these events of speciation, emerging new barriers impede genetic flow between groups, as the two new populations that are separated can not get together and mate anymore. These barriers might be geographical or ecological.
Different stages are involved in the allopatric speciation process:
• The emergence of the barrier.
• Interruption in the genetic flow
• Slow and gradual differentiation due to the occurrence of mutations in each population and their accumulation in time.
• Genetic divergence by natural selection and reproductive isolation, impeding the two groups to mate even if the barrier disappears.
• Prezigotic isolation mechanisms favored by selection a secondary contact between the new species occurs.
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