Final answer:
The likely explanation for characid fishes being found only in South America and Africa is that they evolved before the continents separated, a fact supported by biogeographical patterns related to historical continental drift. Adaptive radiation, while not the direct cause of their distribution, is a process of rapid species diversification seen in similar fish populations like the cichlids of Lake Victoria.
Step-by-step explanation:
The distribution pattern of characid fishes being found naturally only in South America and Africa, with fossils not found on other continents, is most likely explained by the historical division of landmasses due to tectonic plate movement. This distribution pattern is consistent with biogeographical evidence that suggests these fishes arose prior to the separation of the African and South American continents - when they were part of the ancient supercontinent Gondwana. This is in line with the principle that broad groups which evolved before the breakup of supercontinent Pangaea (around 200 million years ago) are now distributed worldwide, whereas groups that evolved subsequently are often unique to specific regions.
Moreover, the concept of adaptive radiation is relevant here, but not in the direct explanation of the characid fishes' distribution. Adaptive radiation refers to the process where a species rapidly evolves into many new species, filling different ecological niches. An example of this is the cichlid fishes in Lake Victoria, which underwent a spectacular adaptive radiation, diversifying into hundreds of species with specialized feeding strategies and morphologies within a relatively short period of time, as documented by biologists in the 1980s.