Final answer:
Morphological differences such as size, body coloration, presence of sex combs in males, and patterns of X-linked trait expression like eye color are key characteristics that enable the distinction between male and female flies. These traits are a result of sexual dimorphisms that have evolved through natural and sexual selection.
Step-by-step explanation:
Morphological differences between male and female flies, such as Drosophila melanogaster, can be quite distinct. Sexual dimorphisms are traits that help distinguish between the sexes and are a result of differential reproductive strategies that arise due to natural and sexual selection pressures. For example, in many species, males tend to be larger, more colorful, or possess elaborate ornaments to attract females and compete with other males for mating opportunities, while females often have a more subdued appearance. In the case of fruit flies, males are generally smaller and may have darker body coloration compared to females.
Additionally, male fruit flies have a unique structure on their abdomen called the sex comb, which is a row of bristles that they use during mating. These bristles are not present in females. Female fruit flies tend to be larger and lack the distinguishing sex combs. Moreover, male Drosophila have hemizygous sex chromosomes (XY), while females are XX, which can affect the expression of X-linked traits, such as eye color. Eye-color in Drosophila was the first X-linked trait to be identified, with males having only one allele for any X-linked characteristic due to their XY chromosome pair.