My Essays Immortalised (BIO1022: Biology 2)

Question 4:

Compare the advantages and disadvantages of sexual reproduction in animals.

Almost every organism in the animal kingdom reproduces by sexual reproduction. Hence, it should have a significant advantage in terms of reproductive success and survivability of the organism over asexual reproduction, for it to be such a popular form of reproduction. If it does not, it would have been phased out over the course of evolution. However, the advantage of sexual reproduction is not immediately obvious and sexual reproduction presents its own sets of disadvantage as well. This essay will discuss the disadvantages of sexual reproduction and various theories about the advantages of sexual reproduction.

Disadvantages of sexual reproduction

The most commonly cited disadvantage of sexual reproduction is the “two-fold” cost of males. Both male and female is needed for sexual reproduction whereas in asexual reproduction any individual is capable of reproducing. To understand the two-fold cost of males, one must assume that a sexual pair and a single asexual individual can only produce the same amount of young. The population size of asexual organisms would be double of the previous generation, because each individual can produce, whereas in sexual reproduction, males do not reproduce. Thus, there is a 50% cost of having males (Bernstein, Byers & Michod 1981; Campbell et al. 2008; Crow 1994; Daly 1978). Moreover, recombination in meiosis also disallows cytological changes like triploidy and other complex chromosomal arrangements that would lead to aneuploidy in sexual organisms, not a problem for asexual organisms (Crow 1992; Crow 1994).

Another disadvantage of sexual reproduction is that it requires more energy and resources. The process of meiosis and syngamy, both not present in asexual, require energy (Crow 1992). In animals, sexual reproduction brings about some maladaptive traits. Some animals develop physical structures that serve no purpose but to advertise its fitness to potential mates (e.g. peacock tails) and some animals have courtship rituals that are elaborate and arduous. Both of these traits are maladapted as they expend resources and may attract unwanted attention from predators instead of mates (Crow 1992; Daly 1978). Animals that do not live in a pack would also need to spend energy to travel to look for mates (Crow 1992; Daly 1978). Female individuals of some animals must also expend energy to escape avoid unwanted sexual attention from males and/or recover from injuries inflicted by aggressive males (Daly 1978). Males of many sexual species would also need to compete and some cases fight with other males and get injured in the process (Crow 1992; Daly 1978).

Animals that reproduce sexually also risk the transmittance of sexually transmitted disease (Crow 1992; Daly 1978). Most sexually transmitted disease is lethal and can be inherited to offspring. This can be especially disadvantageous in times of plague (Daly 1978). Asexual species do not risk catching parasites from close proximity.

Advantages of sexual reproduction

Sexual reproduction is advantageous because it produces genetic variability through meiotic recombination, random fertilisation and mutation whereas asexual reproduction only gets variability through mutation (Campbell et al. 2008). Variability is important in changing environments as the certain phenotypes can be advantageous in a changed environment and change is unpredictable, thus having multiple alleles in preparation for change is beneficial (Crow 1994).

When a changed environment reverts back to its previous state, the direction of selection is reversed. Individuals with higher fitness in the changed environment now have a lower fitness. In such a situation, sexual reproduction is more advantageous than asexual reproduction (Crow 1992). This is so because asexual populations continue to produce offspring that has adapted to the changed environment; its variability continues to decrease. Conversely sexual populations have an increased variability because the population is selected towards the direction of equilibrium and are able to respond to the situation much readily than asexual populations (Crow 1992).

Sexual reproduction is also more efficient in incorporating multiple advantageous alleles. Through recombination, advantageous mutations from two different individuals can be brought together. Asexual reproduction can incorporate two different beneficial mutations if only the first mutation is previously developed in the same lineage (Bernstein, Byers & Michod 1981; Crow & Kimura1965; Crow 1994). This would take a long time to occur since mutations are rare. The speed of incorporation in sexual reproduction is faster because two mutations could have occurred in two different individuals at the same time and brought together by recombination (Bernstein, Byers & Michod 1981; Crow & Kimura1965). Sexual species generally has a faster rate of change in variable conditions than asexual species.

Besides that, sexual populations are also better at eliminating harmful mutations. Assuming all mutations are slightly deleterious in nature and the build up of deleterious mutations decreases the organism’s fitness, recombination will act by rearranging the genotype, creating more lethal mutations (Bernstein, Byers & Michod 1981; Crow 1992; Crow 1994). In essence, recombination will decrease the number of mutations and at the same time making it more deleterious, thus allowing those individuals to be selected against and die out and benefiting the population (Crow 1992; Crow 1994).

Conclusion

In conclusion, while there are numerous disadvantages of sexual reproduction, the advantages may cancel out or even outweigh the disadvantage. As a caveat, most of the advantages are theories created by leading biologists that are highly probable and explainable with calculations but near impossible to experiment. There is still plenty of research on the evolution of sexual reproduction.

(794 words)

Reference

Bernstein, H, Byers, G.S, & Michod, R.E 1981, ‘Evolution of sexual reproduction: importance of DNA repair, complementation, and variation’, The American Naturalist, vol. 117, no. 4, pp. 537- 549.

Campbell, N.A, Reece, J.B, Urry, L.A, Cain, M.L, Wasserman, S.A, Minorsky, P.V & Jackson, R.B 2008, Biology, 8^th edn, Pearson Benjamin Cummings, San Francisco.

Crow, J.F & Kimura, M 1965, ‘Evolution in sexual and asexual populations’, The American Naturalist, vol. 99, no. 909, pp. 839-450.

Crow, J.F 1992, ‘An advantage of sexual reproduction in a rapidly changing environment’, Journal of Heredity, vol. 83, no. 3, pp. 169-173.

Crow, J.F 1994, ‘Advantages of sexual reproduction’, Developmental genetics, vol. 15, pp. 205-213.

Daly, M 1978, ‘The cost of mating’, The American Naturalist, vol. 122, no. 986, pp. 771-774.

Mark: 8 /10

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