Reproduction and growth – type of reproduction
Living things grow and they reproduce. Growth is a way to generate the materials for reproduction. Reproduction is a way to make new organisms that can grow. Thus, the apparent “goal” of every organism is to fill the available world with its offspring, that is, with “self”. It has been suggested that each unit of inheritance itself, each gene, is selfish in this way. It acts in such a way as to increase its chances to spread to all available individuals of a population. If other genes are helpful in this, good. If not, don’t collaborate.
Reproduction is the process by which organisms generate new individuals of the same kind ensuring continuation of the species.
Types of reproduction
There are two major forms of reproduction: sexual and asexual
Sexual reproduction
Sexual reproduction is the process in which new organisms are created, by combining the genetic information from two individuals of different sexes. The genetic information is carried on chromosomes within the nucleus of specialized sex cells called gametes. In males, these gametes are called sperm and in females the gametes are called eggs. During sexual reproduction the two gametes join together in a fusion process known as fertilization, to create a zygote, which is the precursor to an embryo offspring, taking half of its DNA from each of its parents. In humans, a zygote contains 46 chromosomes: 23 from its mother and 23 from its father. The combination of these chromosomes produces an offspring that is similar to both its mother and father but is not identical to either.
Phenotype traits, such as physical adaptions to an organism’s environment and genotype traits, such as resistance to disease, are passed down from each parent during sexual reproduction. Natural selection, whereby individuals with favorable adaptions to their environment are able to survive and successfully reproduce, drives the evolution process. Sexual reproduction increases the diversity of genotypes and phenotypes within a population, allowing natural selection to select for the individuals best suited to an environment.
Sexual reproduction differs from asexual reproduction, which only requires one parent. In asexual reproduction, unlike sexual reproduction, there is no fusion of gametes, so the offspring are genetically identical to their parents and are therefore clones. Asexual reproduction does occur in some animals, although it is rare; most asexual reproduction occurs in bacteria, fungi, starfish, corals, hydras (jellyfish) and some flowering plants such as strawberries.
Types of Sexual Reproduction
Allogamy
Allogamy occurs when the gametes which join together during fertilization come from two different individuals. The female gamete is usually in the form of an egg or ovum while the male gamete takes the form of a sperm. Both egg and sperm are cells specialized to perform the task of reproduction; each sex cell contains only 23 chromosomes (these are called haploid cells) rather than the normal 46 chromosomes present in other cells of the body. The two haploid cells fuse together to create a diploid cell which then undergoes mitosis, in order to grow and form an individual organism. Mitosis is the division of one cell into two, after the DNA has been replicated within the nucleus.
Internal Fertilization
Internal fertilization is the fertilization of the egg by the sperm within the body of one of the parents, usually by means of sexual intercourse. Internal fertilization usually takes place within the female body, after the male implants sperm. However there are exceptionally rare examples, such as seahorses (Sygnathidae), where the female implants her eggs into the male and the zygote is formed within the male’s body.
External Fertilization
External fertilization occurs when a sperm cell and an egg cell join outside of the body. Most amphibians and fish and many invertebrates use external fertilization, producing anything from hundreds to billions of gametes at a time into close proximity. The quick release of gametes into aquatic environments this is called spawning. However, sometimes females will lay eggs on a particular substrate which are subsequently fertilized by males.
Autogamy
Autogamy, also known as self-fertilization or self-pollination, is the fusion of male and female gametes, which are produced by a single individual. Species which are able to produce both male and female gametes are called hermaphrodites.
Asexual Reproduction
Asexual reproduction occurs when an organism makes more of itself without exchanging genetic information with another organism through sex.
In sexually reproducing organisms, the genomes of two parents are combined to create offspring with unique genetic profiles. This is beneficial to the population because genetically diverse populations have a higher chance of withstanding survival challenges such as disease and environmental changes.
Asexually reproducing organisms can suffer a dangerous lack of diversity – but they can also reproduce faster than sexually reproducing organisms, and a single individual can found a new population without the need for a mate.
Some organisms that practice asexual reproduction can exchange genetic information to promote diversity using forms of horizontal gene transfer such as bacteria who use plasmids to pass around small bits of DNA. However this method results in fewer unique genotypes than sexual reproduction.
Some species of plants, animals, and fungi are capable of both sexual and asexual reproduction, depending on the demands of the environment. Asexual reproduction is practiced by most single-celled organisms including bacteria, archaebacteria, and protists. It is also practiced by some plants, animals, and fungi.
