Natural Selection and Sexual Selection in Evolution


The terms ‘Natural selection’ and ‘sexual selection’ are important in understanding the process of evolution in different species and how they adapt to different environments. In evolution, the ideology of selection remains one of the predominant theories in explaining the origins and development of organisms. This argues that it is necessary for organisms, including humans, to fit in their environment in order to survive. To argue this thesis, the discussion will develop an in-depth analysis of the mechanisms of natural and sexual selection, factors affecting the mechanisms and the implications of adaptation and mating practices among humans.

Mechanisms of natural and sexual selection

Charles Darwin coined the term ‘natural selection’ in reference to a biological process most common in populations of organisms where differentiation is evident. According to Jay (2002), this is what accounts to evolution- where variation caused by random gene mutations exists. In nature, it has been observed that successive generations acquire some different traits not similar to the original members of the same species. Through interaction with the environment, some traits in species are subject to change, especially where there is either survival of other species or reproduction of better traits with different variations and high likelihood of adaptation. According to Mitchell and Skinner (2009), with the emergence of a new species, natural selection is an observable phenomenon in which a population of organisms emerges from reproductive advantages. Moreover, an emergence of a new type of species creates a particular ecological niche ( Jay, 2002). This type of selection is heritable on any phenotypic trait and is attributable to the environmental aspect in which a competition among members of the same species is evident.

Despite its interactivity with the environment, natural selection is not always directional, nor does it necessarily result to adoptive evolution. Rather, it considers the potentials to survive in a competitive environment, as conferred by physical and biochemical advantages over the other organism. Biologically, natural selection happens in the life process of every individual. In this case, the survival of an individual from conception to adulthood is necessary. According to the law of evolution, the adult organism must have the ability to reproduce a viable offspring. Viability of an organism is important in sexual selection, as it determines the continuation of the species from one generation to another. It is at the stage of viability that human beings, inclusive of other species, compete for mating partners. This is ‘sexual selection’, where the most successful individuals in obtaining mating partners form the basic population that determines the next generation.

Sexual selection, therefore, means a mechanism in which species compete for individual mating partners. These partners are biologically known as the intersexual partners. In some cases, a specific sex controls the process of reproduction through selection of the fittest mate from a large population. Intrasexual selection occurs when male individuals in a given population of species are competing for females. On the other hand, intersexual selection takes place when adult female members of a given population of species fight to have sex with the most.

In both cases, the aim is to obtain an offspring with the best characteristics for survival. According to Mitchell and Skinner (2009), to maximize on reproductive success, intersexual selection is realised by species making an attractive image towards the opposite sex while Intrasexual selection is realised by fighting other sexes in order to defend some territory or their species. There exists a relationship between natural selection and sexual selection because sexual selection later becomes a mechanism for natural selection. Though there are some criticisms challenging the explanation of how animals evolve, it is evident that different species have different ways of survival as determined by the environment in which they are raised.

Factors that influence natural and sexual selection

Many factors contribute to natural and sexual selection. For instance, the mode of reproduction and survival are two common factors that influence selection. However, factors influencing natural selection are limitless because species have different strengths of immunity and different adaptations towards different environments. The specific factors will include gene flow that is determined by migrations of species into new environment, bringing new genes and thus new populations that can comfortably interact with each other (Cosmides & Tooby, 1999). Genetic drift is another factor of natural selection where existing populations reproduce new offspring with minimal variations in their behaviour patterns.

In addition, gene Mutation is an important factor that tends to improve the survival capabilities of mutants species, conferring them with an easy way of adopting to any environment, hence surviving in the environment over several generations.

Darwin’s theory and organism survival

Biologically, Darwin’s theory of evolution applies when attempting to explain how species survive and reproduce in a hostile environment. In this theory, an organism is said to employ a tactic called ‘survival for the fittest’ in which only those that are better adapted to the environment will have the ability for survival. The weaker ones are naturally eliminated. This ensures that only the ‘good’ genes are passed on to the next generations, while the weak traits are eliminated.

Implications of human adaptations and human mating practices

According to Boyd and Silk (2011), sex has to be involved in reproduction in some way. In humans, individuals have the choice of looking for mates that they feel sufficiently comfortable to have sex with in order to reproduce. This is quite different from most other animals because humans have achieved higher levels of brain development and are conscious of their welfare. Through different adaptations, human beings produce stronger genes that confer them with the potential for survival. To achieve the best survival characteristic, individual genes must flow from one generation to the next. According to Gaulin and McBurney (2004), it is through mating in humans that genetic conflict gives rise to different characteristics, which in turn promotes fitness (sexually antagonistic adaptation). This form of adaptation leads to differences in mating and the patterns of fertility.


It is evident that natural selection is heritable on any phenotypic trait. Environmental aspects play a major role in this process, with members of the same species competing for survival. Despite its interactivity with the environment, natural selection is not always directional and does not necessarily result to adoptive evolution. Rather, it considers the aspect of environmentally determined survival strength and fitness. Sexual selection is a mechanism in which species compete for individual mating partners. In addition, it is evident that this type of selection can be either intersexual or intersexual. Humans have also not been left out in natural and sexual selection. With the support from Darwin’s theory, this discussion provides evidence to support the thesis that even in humans, there has to be better traits for a population to survive in a competitive environment.


Boyd, R., & Silk, J. B. (2011). How Humans Evolved. New York, NY: W. W. Norton & Company.

Cosmides, L., &Tooby, J. (1999). Toward an evolutionary taxonomy of treatable conditions. Journal of Abnormal Psychology, 108 (3), 453–464.

Gaulin, C., & McBurney, D. (2004). Evolutionary Psychology. New York, NY: Prentice Hall.

Jay, G. S. (2002). The Structure of Evolutionary Theory. London: Harvard University Press.
Mitchell, G., & Skinner, J. D. (2009). Sexual selection is not the origin of long necks in giraffes. Journal of Zoology, 278(4), 281–286.

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