The “trait evolution” is a persuasive phenomenon among different species in natural world. This quotidian process to a large extent associates with selection and competition of mating partners to formalize a better reproduction and survival from different generations. According to the conceptions of natural and sexual selection has been pointed by Pianka (2000), this paper aims at analyzing the sexual selection among bird species and nonhuman primates.

Natural selection as the first theory was advanced by Darwin, revealing a phenomenon that competitions among species to survive or to be eliminated in a relentless and rigorous environment. The survivorship principle manifests high proportion of species offspring can regulate and evolve the holistic species traits, and this conception was defined by Pianka (2000) that “reproductive rate, rather than death rate, drives natural selection.” For instance, the insect breed surmounts the human-produced calamity of pesticide and antibiotic, this sort of insect descendants are resistant to homologous chemical. In addition to the gene mutation, Pianka (2000) exemplify tomcat to demonstrate the relationship between species lifespan and reproduction. “…short-lived tomcat leave more descendants…the natural selection will favor a short life span.”

Similar to natural selection, sexual selection is considered as an “evolutionary process that increase an organism’s fitness”(Pianka, 2000) to elevate the reproduction rate and expand the species community. However, “sexual rivals and mates are the exclusive agents of the sexual selection” as Pianka (2000) referred. The example of physical attraction of male peacock circumstantiates that higher mating rate with female peacock (comparatively subordinated in figure and color), which simultaneously accentuates the offspring has higher inheritance from its ancestors, such as the striking tails and increased size.

The physical and external appearances among species spur on the distensible differences between male and female, and this two disparate sexually forms are labelled as sexually dimorphic as mentioned in the peacock example. Moreover, Plavcan (2012) indicates sexual appeal traits in his research about galliform birds. He points out the adult male galliform has a naked head with a skin tumor which is usually bright red, but turns white and bright blue when excited. This makes it easier to attract female galliform which generally weigh half as much as the males, and have smaller skin tumors and lobes on their heads. The research result testifies the egg size and clutch size have an inseparably association with male sexual traits. Especially, the female tendency of choosing male galliform with vivid red skin tumor are disadvantageous to the survival of other breeds of male galliform.

As Pianka (2000) mentioned the sexual selection is a particular situation in birds and mammals, except wolves and buffalos, nonhuman primates will be the most convictive instance for the competition traits of sexual selection. Zinner (2004) mentioned adult gorillas have obvious primary and secondary sexual characteristics. The back of the head of an adult male gorilla will be raised high, and the back will be silvery white. It’s a sign of power, and it’s common in orangutan populations that the bigger the male, the more likely it is to become leader. Female gorillas, on the other hand, are smaller and less hairy than adult gorillas. Zinner (2004) considers larger orangutans have more mating partners, which means their offspring are more similar, limiting the diversity of the population.

Generally speaking, specific bird species and nonhuman primates both have many similarities among the natural and sexual selection, and these selections are more or less generate positive and negative impacts on the reproduction and survival of species communities.