Refer to this portion of a dichotomous key for fish identification to answer the question.

1. (a) has a single dorsal fin ® 5
(b) has a double dorsal fin ® 2
5. (a) has small fin on back near tail ® 6
(b) has no fin on back near tail ® 7
6. (a) has barbs near the mouth ® catfish
(b) does not have barbs near the mouth ® 10

according to this dichotomous key, any catfish must have:

a.) a double dorsal fin, a small fin on the back near the tail, and barbs near the mouth.
b.) a single dorsal fin, no small fin on the back near the tail, and barbs near the mouth.
c.) a double dorsal fin, a small fin on the back near the tail, and no barbs near the mouth.
d.) a single dorsal fin, a small fin on the back near the tail, and barbs near the mouth.

The correct answer to the question: according to this dichotomous key, any catfish must have, is D. a single dorsal fin, a small fin on the back near the tail, and barbs near the mouth.

Explanation:

To begin with, in this dichotomous key we find ourselves in key number 1 in option a, (has a single dorsal fin), this option refers us to key number 5 a, (has small fin on back near tail) that finally leads us to key number 6, (has barbels near the mouth) that allows us to identify the fish as a catfish.

This small fin on the back, located between the dorsal fin and the caudal fin is known as adipose fin.

On the other hand, the barbels found in catfish are whisker-like sensory appendages near to the mouth.

Sometimes barbels are incorrectly referred to as barbs, which are found in bird feathers for flight and represent a series of branches fused to the rachis.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".