Answer :
Natural selection acts on those fishes that are able to survive the decreased oxygen levels. These fishes will breed and increase their numbers.
The process through which populations of living things adapt and change is known as natural selection. A population's members are naturally varied, which means that they are all distinctive in some ways. Because of this variety, some people have characteristics that suit their surroundings more than others.
Fish respiration will be impacted by the water's lower oxygen levels. The fish may relocate to a better area with more dissolved oxygen in order to adapt to the change. However, because the fish are still the same genetically and phenotypically, there is no connection to the process of natural selection. Fish that can survive the reduced oxygen levels are selected naturally. These fish will reproduce and multiply in quantity. Fish from the next generation will be tolerant of decreased oxygen levels. This novel phenotypic denotes a modification in the genetic makeup of the fishes.
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