A dam is built that separates a river into two sections—a deep upstream section and a shallow downstream section. When the dam is first built, a fish species is able to move from the upstream section into the downstream section, but not vice versa. Predators can more easily catch the fish in the shallow downstream section than they can in the deep upstream section. Which scenario is most likely to lead to speciation?

A. The fish population contains individuals with effective predation evasive traits and eventual changes to the dam prevent fish from moving downstream.

B. The fish completely lack any effective predation evasive traits across the entire population, move infrequently from upstream to downstream locations, and reproduce with low frequency.

C. The fish reproduce with high frequency, move with high frequency from upstream to downstream locations, and have a robust set of predation evasive traits across the entire population.

D. The rate of upstream fish entering the downstream section decreases over time without being completely shut off, and many of these fish carry predation evasive traits.

A fish species can transfer from the upstream section into the downstream section when the dam is originally constructed, but not the other way around.

What is Nam Ngum River?

A dam is constructed to divide a river into two sections: a deep segment upstream and a shallow section downstream. Cross-river buildings can cause two different forms of damage. They initially act as obstacles to most species' upstream and downstream movement. Just west of Ban Long Cheng, on the Nam Ngum River, NN3 plans to construct a 220 m high dam. A little amount of household fish is caught upstream and downstream.

Our first chapter in the first book, titled Fish Biology, examines the biological foundations of fisheries management. A dam is constructed to divide a river into two.

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