Discuss the Comparative Evolutionary Costs and Benefits of Parthenogenesis vs. Sexual Reproduction.

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This essay will discuss the comparative benefits and costs of two forms of reproductions; parthenogenesis and sexual reproduction. To narrow the scope of the piece, the area of parthenogenesis is going to be largely limited to organisms that are obligately parthenogenetic. Parthenogenesis is the “production of an embryo from a female gamete without any genetic contribution from a male gamete” (Mittwoch, 1978). Sexual reproduction is the process of cross-fertilisation in which the genomes of two parents via gametes are brought together in one cell forming a zygote (Hörandl E, 2009). It is necessary to clarify that parthenogenesis is a different mechanism to self-fertilisation and asexual reproduction. In these mechanisms offspring either do not originate form an egg or incorporate paternal. It should be established that offspring produced by parthenogenesis are called parthenogenones (Mittwoch, 1978). The implications of both reproductive mechanisms will be compared in this essay.

The Cost of Sexual Reproduction
Many costs of reproduction compared to parthenogenesis are derived from the presence of males. The cost of males is associated with the fact that males do not directly produce offspring. Thus in a population containing males and females (sexually reproducing) the birth rate per capita is lower than in a population solely made up of parthenogenetic females (Maynard Smith, 1978). This means parthenogenetic populations can produce twice as many progeny compared to populations with males (assumed 1:1 sex ratio), as each parthenogenetic female in the population can bare progeny. This is called the “twofold cost” of sexual reproduction (Maynard Smith, 1978, Morran, Parmenter & Phillips, 2009, Tobler & Schlupp, 2010). With the ability to reproduce in greater numbers parthenogenetic species can spread into areas where conditions are favourable more quickly that sexual species (Hörandl, E. 2009). Maynard (1978) also suggested that without males, populations will have an increase in reproductive resources that can be used to the species advantage. Though parthenogenesis beneficial combinations of genes can be passed though a lineage, an unachievable outcome in sexual species due to genetic dilution. Genetic dilution is caused by only 50% of each parent’s genes are passed to the offspring (Peck & Waxman, 2000, Green & Noakes 1995, Tobler & Schlupp, 2010). This dilution can cause the immediate breaking up of beneficial gene combinations, presenting a short-term evolutionary disadvantage. According to Hörandl (2009) there are two more implications to sexual reproduction. Fertilisation increases the chances of transferring viruses and parasites within a population due to sexual interactions. This provides a means for the spread of disease posing a risk to the overall population’s health. The other implication is that there is density dependency as each individual must be able to find a partner of the opposite sex (Green & Noakes 1995). This restricts the population dynamics of the species reducing birth rates per capita. Comparatively parthenogenesis has the evolutionary advantage over sexual reproduction as it has lower costs in reproduction due the lack of males, the ability to retain beneficial combinations of genes, allows for a relatively greater supply of reproductive resources and gives the population the ability to produce more offspring with the same number of individuals.

Sexual reproduction promotes far greater genetic variation compared to parthenogenesis. It should be noted that parthenogenones aren’t identical to the mother as chromosomal crossing-over can still occur (unless meiosis is completely suppressed in which case parthenogenesis is similar to asexual reproduction). Variation can occur though failure of cells to divide resulting in polyploidy or diploidy (Mittwoch, 1978).Shown figure 1 there are multiple paths to producing a parthenogenone (Mittwoch, 1978).

However there is little...
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