Answer:
A. allopatric
Step-by-step explanation:
Allopatric speciation consists of the geographic separation of a continuous genetic background -population-, giving place to two or more new geographically isolated populations. These separations might be due to migration, extinction of geographically intermediate individuals, or geological events. In this type of speciation, some barriers impede genetic interchange or flow. The populations are separated by a barrier and can not get together and mate anymore. These barriers might be geographical or ecological.
When a geographical barrier emerges, this is known as a vicariant event. Vicariance is the geographical separation of a population imposed by discontinuities in the physical environment that divides populations that originally were continuous. The process of vicariant allopatric speciation involves different steps:
• The emergence of the barrier.
• Interruption in the genetic interchange
• The occurrence of new mutations and their accumulation in time in each population. Slow and gradual differentiation.
• Genetic divergence by natural selection and reproductive isolation makes it impossible for the two groups to mate even if the barrier disappears.
• Prezigotic isolation mechanisms favored by selection once a secondary contact between the new formating species occurs.
So in the exposed example, the original population was composed of fishes living in a pond under the same environmental conditions and selective pressures.
However, with time, the pond dried, causing a decrease in water level and the emergence of a new physical barrier. The original population got separated by a piece of ground, and the original pond turned into two smaller ponds. The physical barrier is now interrupting the genetic flow between both groups.
Animals on each pond suffer from different environmental pressures to which they need to adapt. New mutations occur, leading to genetic divergence between the two separated groups. Two species emerged, red fishes and blue fishes.