Many individuals of the platyfish, Xiphophorus maculatus, have spots on their dorsal fins. A closely related species, the swordtail, X. helleri, lacks spots. In some backcross hybrids of these species, enlarged spots develop into malignant tumors, which reduce the organisms' life spans. This could be an example of .a. the Dobzhansky-Muller model of speciation.
b. beneficial hybridization.
c. the morphological species concept.
d. gene flow.
e. genetic compatibility.