Question

characters and sexual dimorphism of fresh and marine fishes​

Answer 1

Answer:

Naahi btayenge

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Answer 2

Explanation:

Sexual Dimorphism

Sexual dimorphism in trophic structures is a common phenomenon in most animal phyla, and has attracted considerable interest from evolutionary biologists. In most animals, males and females differ in size, sometimes substantially. The usual cause cited for sexual dimorphism in animals is sexual selection acting through female choice and/or male–male competition.

Natural selection acting on the fitness advantages of reduced resource competition between the sexes, however, is also an important alternative evolutionary scenario that can produce sexual dimorphisms. This alternative idea is that sexual differences in body size or morphology may evolve due to ecological causes – that is, the ecological benefit of the sexes occupying different ecological niches, which reduces intersexual food competition and expands the species’ overall feeding niche. Adaptation to different diets may also be due to sex-specific nutritional requirements, as for example females may have additional dietary components compared to males – this directly relates to nutrients needed for the production of eggs. However, sexual selection and natural selection for sexual dimorphisms are not incompatible; for example, it may well be that the direction of dimorphism in body size is determined by sexual selection, but the degree to which the sexes diverge is either constrained or amplified by ecological factors.

Sexual Dimorphism and Sexual Selection

Sexual dimorphism in body size, coloration, and a variety of morphological characteristics is well known in amphibians and reptiles. Male bullfrogs, Lithobates catesbeianus, and green frogs, Lithobates clamitans, for example, have larger tympana than females because male calls are critical for territory maintenance (Fig. 9.25). In many instances, competition among individuals of the same sex (usually males) for access to individuals of the other sex is believed to be the driving force behind the evolution of sexual dimorphism. These interactions between individuals of the same sex determine reproductive success and result in intrasexual selection. Male competitive ability is at a selective advantage. Because size often dictates a male’s success in contests with other males, intrasexual selection can drive the evolution of increased body size in males and result in sexual dimorphism with males larger than females. In other instances, females may choose males for mates based either on size or some other overt male trait. This female choice also can result in males being the larger sex. Sexual selection, however, is not the only factor that determines body size within each sex. For example, large female body size can be selected because size and fecundity are linked in many species. Sexual size differences can arise from differing growth trajectories, age at sexual maturity, and patterns of energy use. Differences in size between males and females more often represent a combination of the effects of sexual selection and natural selection. Moreover, if all members of a given clade show the same or similar sexual dimorphism, then the origin of that dimorphism my lie deep within the evolutionary history of the clade and contemporary selection pressures maintain it.