by Carolyn Lee

Created on: March 26, 2008

All living thing need to reproduce in order to exist as a species. What is interesting, however, is how different species achieve reproduction. Reproduction can be divided into two categories, asexual and sexual. Both types can be further divided based on the different reproductive mechanisms.

Asexual reproduction is the creation of offspring whose genes all come from one parent without the fusion of an egg and sperm. Asexual reproduction relies entirely on mitotic cell division and has several potential advantages. For example, it allows animals that live in isolation to reproduce without having to find a mate. Asexual reproduction is best suited for stable environments because it perpetuates favorable genotypes.

Sexual reproduction, on the other hand, is the creation of offspring by the fusion of two haploid gametes (egg and sperm, usually each from a different parent), called fertilization. Sexual reproduction relies on meiosis, which forms the haploid egg and sperm. As opposed to asexual reproduction, sexual reproduction increases the genetic variability among offspring due to the unique genetic combinations of genes inherited from each parent.

The mechanism of fertilization (fusion of egg with sperm) plays an important role in sexual reproduction. Fertilization can either be external or internal, each with specific requirements. In external fertilization, eggs are shed by the female into the external environment, and then fertilized by the male. Internal fertilization occurs when sperm is deposited in the female reproductive tract where the egg and sperm unite inside the female body.

External fertilization requires an environment free from heat stress and dry conditions and occurs almost exclusively in wet habitats. There are many factors that lead to animals shedding eggs and sperm externally including temperature cues, day length or courtship behavior. The resulting fertilized zygotes are usually numerous, but the survival rate is low.

Internal fertilization requires cooperative behavior, leading to copulation between the parents. Internal fertilization also requires sophisticated reproductive systems. Copulatory organs for the delivery of sperm and the storage and transport to the waiting eggs must be present. Internal fertilization usually produces few zygotes, but there is a high survival rate due to the greater protection of the embryos.

As with most rules, there are exceptions. Animals may reproduce exclusively asexually or sexually, or they may alternate between the two modes. The freshwater crustacean Daphnia will reproduce asexually during seasonally favorable conditions and reproduce sexually during times of environmental stress. This asexual reproduction is also called "parthenogenesis". The adults produced by parthenogenesis are usually haploid, and their cells do not undergo meiosis.

Sexual reproduction is very complex, with many different mechanisms depending on the species. Regardless of the mechanism however, sexual reproduction produces offspring with varying phenotypes, likely enhancing the reproductive success of parents in a changing environment.

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