Evolution of Sexual Reproduction
There are different forms of reproduction, which includes vegetative, asexual, sexual reproductions. Naturally, the question arises, what are the benefits of sexual reproduction in terms of fitness of a species, and how does it compare to other forms of reproduction? Logically, females reproducing asexually are more likely to have twice as many grandchildren and can increase their fitness two folds(Hurst and Peck). On the other hand, by reproducing sexually, both sexes have to invest copious amounts of time in selecting mates, making nests, bringing up offspring etc. In this paper we will try to understand the benefits and disadvantages associated with sexual reproduction and how it translates into a better evolutionary strategy.
MITOSIS VS MEIOSIS
Mitosis is a process by which a single cell creates two replicates of itself; these daughter cells contain the same genetic characteristics of the parent cell. On the other hand, meiosis is a more complex form of cell division where a single cell with diploid chromosome (2n) divides to form four daughter cell with half chromosome (n). Meiosis is the process by which sexually reproducing organisms form gametes (n) that fertilize to for the zygote (2n), therefore the offspring receives a combination of genetic material from both parents. Most evolutionary biologists agree that meiosis is the basis for complex cell and tissue formation. This type of cell division not only halves the chromosome number but also creates unique genotypes through crossing over of sister chromatids. Meiosis is one of the most vital steps for sexual reproduction. However, scientists are not sure how organisms transitioned from asexually reproducing mitosis to a sexually reproducing meiosis based system.
SEXUAL VS ASEXUAL REPRODUCTION
In asexual reproduction the organism creates a copy of itself without any genetic variation though the process of mitosis. Asexual reproduction is found across a diverse assemblage of taxa, where the offspring is created from the diploid cells of a single parent (Hurst and Peck). Asexual reproduction has its own advantages, which includes less energy expenditure to seek and choose partners. Furthermore, asexual organisms propagate faster than sexually reproducing organism and therefore can colonize more suitable habitats faster. The lack of genetic variability in asexual reproduction, places a significant risk of extinction (local or global) on the species.
On the other hand, in sexual reproduction (meiosis) due to crossing over of parent’s chromosomes during meiosis, the offspring has a slightly different genotype compared to parents, which in turn promotes diversity in gene pool and protects the species from the impact of natural selection. Moreover, due to the creation of unique genotypes through sexual reproduction, the offspring are more likely to have small genotypic variations that may help them adapt better to changing environmental conditions. Thus sexual reproduction ensures the survivability of the species by creating many variations of the existing genotype(Williams). The ‘Red Queen’ hypothesis proposed by Leigh Van Valen proposes that, species are constantly fighting a battle to stay ahead of competition and therefore more variability ensures a better chance of survival. Sexual reproduction could also be a causative factors behind speciation, which leads to greater biodiversity on the planet.
Scientists believe that somatic mutations occur in both sexually and asexually reproducing organisms, some mutations can be deleterious in nature(Cartwright). Therefore, in asexually reproducing organisms, somatic mutations get accumulated from one generation to the next and can have extremely deleterious effects on the fitness of the species. On the other hand, in sexually reproducing organisms, due to recombination and variation in genotypes the effects of somatic mutations are often suppressed. In fact according to some biologists, DNA repair can be one of the probable reasons for sexual reproduction.
Therefore in conclusion we can say that as organisms evolved from simple unicellular organisms to more complex multicellular systems, the need for more variability became apparent. Asexual reproduction although energy efficient has its limitations in genetic variability and reduces adaptability of a species. Mutations, natural selection, genetic drift and other evolutionary forces can have significant impacts on asexually reproducing organism. On the other hand, sexual reproduction increases the chances of survival of a species by bringing in genetic variations that may increase the fitness of the species. Therefore, it can be assumed that sexual reproduction balances between creating variation in gene pool and conservation of vital genetic information by performing DNA repairs to increase the fitness of a species.
Works Cited
Cartwright, John. Evolution and Human Behavior: Darwinian Perspectives on Human Nature. MIT Press, 2000. Print.
Hurst, LD, and JR Peck. “Recent Advances in Understanding of the Evolution and Maintenance of Sex.” Trends in ecology & evolution II.2 (1996): n. pag. Web. 20 Nov. 2014.
Van Valen, Leigh. “The Red Queen.” American Naturalist (1977): 809–810. Print.
Williams, George Christopher. Sex and Evolution. Princeton University Press, 1975. Print.
