Reading – Reproductive Strategies

CP Biology

Asexual Reproduction

Reproduction is a marvelous culmination of individual transcendence. Individual organisms come and go, but, to a certain extent, organisms "transcend" time by reproducing offspring. Let's take a look at reproduction in animals.

What Is Reproduction?

In a nutshell, reproduction is the creation of a new individual or individuals from previously existing individuals. In animals, this can occur in two primary ways: through asexual reproduction and through sexual reproduction. Let's look at asexual reproduction.

Asexual Reproduction

In asexual reproduction, one individual produces offspring that are genetically identical to itself. These offspring are produced by mitosis. There are many invertebrates, including sea stars and sea anemones for example, that produce by asexual reproduction. Common forms of asexual reproduction include:

Budding: In this form of asexual reproduction, an offspring grows out of the body of the parent. Hydras exhibit this type of reproduction.
Fragmentation: In this type of reproduction, the body of the parent breaks into distinct pieces, each of which can produce an offspring. Planarians exhibit this type of reproduction.
Regeneration: In regeneration, if a piece of a parent is detached, it can grow and develop into a completely new individual. Echinoderms exhibit this type of reproduction.
Parthenogenesis: This type of reproduction involves the development of an egg that has not been fertilized into an individual. Animals like most kinds of wasps, bees, and ants that have no sex chromosomes reproduce by this process. Some reptiles and fish are also capable of reproducing in this manner.

Advantages and Disadvantages of Asexual Reproduction

Asexual reproduction can be very advantageous to certain animals. Animals that remain in one particular place and are unable to look for mates would need to reproduce asexually. Another advantage of asexual reproduction is that numerous offspring can be produced without "costing" the parent a great amount of energy or time. Environments that are stable and experience very little change are the best places for organisms that reproduce asexually. A disadvantage of this type of reproduction is the lack of genetic variation. All of the organisms are genetically identical and therefore share the same weaknesses. If the stable environment changes, the consequences could be deadly to all of the individuals.

Asexual Reproduction in Other Organisms

Animals are not the only organisms that reproduce asexually. Yeasts, plants and bacteria are capable of asexual reproduction as well. Bacterial asexual reproduction most commonly occurs by a kind of cell division called binary fission. Since the cells produced through this type of reproduction are identical, they are all susceptible to the same types of antibiotics.

Sexual Reproduction

In sexual reproduction, two parents donate genes to their young, resulting in offspring with a mix of inherited genes. These genes are donated through a process called fertilization. There are two mechanisms by which fertilization can take place. The first is external (the eggs are fertilized outside of the body), and the second is internal (the eggs are fertilized within the female reproductive tract). While fertilization is necessary for organisms that reproduce sexually, individuals that reproduce asexually do so without the need for fertilization. These organisms produce genetically identical copies of themselves through the process of mitosis.

Gametes

In animals, sexual reproduction encompasses the fusion of two distinct gametes to form a zygote. Gametes are produced by a type of cell division called meiosis. Gametes are haploid (containing only ½ set of chromosomes) while the zygote is diploid (containing one full set of chromosomes).

In most cases, the male gamete, called the spermatozoan (sperm), is relatively motile and usually has a flagellum. On the other hand, the female gamete, called the ovum (egg), is nonmotile and relatively large in comparison to the male gamete.

External Fertilization

External fertilization occurs mostly in wet environments and requires both the male and the female to release their gametes into their surroundings (usually water). An advantage of external fertilization is that it results in the production of a large number of offspring. One disadvantage is that environmental hazards such as predators greatly reduce the chance of surviving into adulthood. Amphibians and fish are examples of animals that reproduce this way.

Internal Fertilization

Animals that use internal fertilization specialize in the protection of the developing egg. For example, reptiles and birds secrete eggs that are covered by a protective shell that is resistant to water loss and damage. Mammals, with the exception of monotremes, take this idea of protection a step further by allowing the embryo to develop within the mother. This extra protection increases the chances of survival because mom supplies everything that the embryo needs. In fact, most mammalian mothers continue to care for their young for several years after birth.

Patterns and Cycles

Reproduction is not a continuous activity and is subject to certain patterns and cycles. Oftentimes these patterns and cycles may be linked to environmental conditions which allow organisms to reproduce effectively.

For example, many animals have estrous cycles that occur during certain parts of the year so that offspring can typically be born under favorable conditions. Humans however, do not undergo estrous cycles but menstrual cycles.

Likewise, these cycles and patterns are controlled by hormonal cues. Estrous can also be controlled by other seasonal cues such as rainfall.

All of these cycles and patterns allow organisms to manage the relative expenditure of energy for reproduction and maximize the chances of survival for the resulting offspring.