Definition of 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 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.
Because genes of individuals are passed down through sexual reproduction, and survival of genes is controlled by natural selection, individuals are driven to choose mates based on their ability to produce offspring likely to survive and live to reproduce themselves. It is therefore in each individual’s best interests to find a mate with qualities such as good health, aggression, speed and agility to survive fights, and qualities that will help the offspring attract future mates. The genetic and phenotypic diversity produced by sexual reproduction allows individuals to choose mates based on best display of these characteristics. The choices given to sexually reproductive species causes competition between individuals and means that usually only those individuals who display desired traits are able to pass on their genes. This is known as sexual selection.
Sexual selection also leads to sexual dimorphism, whereby males and females of the same species differ greatly in appearance. As the females are usually responsible for protecting the offspring after birth, they are often camouflaged with dull colors, comparative to males, which often display bright colors and exaggerated body parts (such as large horns or antlers). Often these sexually selected characteristics can conflict with the survival ability of the animals. For example, bright colors that attract females may also attract predators. Nonetheless, the desire to mate is strong and so these characteristics persist and increase within populations.
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.
The next step in internal fertilization depends on the species. Some creatures, such as birds, insects and reptiles, then lay an egg containing the cells, which are undergoing mitosis, and a reserve of yolk to feed and support growth of the embryo. After a period of time (often having been incubated) a fully formed individual will hatch from the egg. This is known as oviparity.
The embryos of most mammal species grow and develop within the body of their mother, resulting in the live birth of a fully formed offspring: this is called viviparity. Embryos are supported by the placenta, which provides nutrient uptake, waste removal and thermo-regulation in placental organisms (most mammals). Alternatively, marsupial offspring (for example, koalas and kangaroos) are removed from inside the mother’s body after a short gestation period and complete their development within an external pouch on the front of the mother’s body.
A third form of development is ovoviviparity, in which embryos develop in eggs stored within the body of the parent until they are ready to hatch, giving the appearance of a live birth.
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.
The sex cells of creatures which reproduce through external fertilization often have special adaptions for movement, such as the addition of strong flagella for independent movement.
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.
Although autogamy is similar to asexual reproduction, in that there is no input of genetic diversity from a partner, the recombination of chromosomes from the male and female gametes results in offspring with slightly altered genetic information, which can therefore look phenotypically different from their parents. Most plants and earthworms reproduce by autogamy. It is sometimes possible for hermaphrodites to reproduce with other hermaphrodites. In this case, genetic diversity does increase within the population.
Advantages of Sexual Reproduction
Deleterious mutations accumulate within DNA over time, through cell mitosis. Organisms which produce asexually simply pass these mutations on to their offspring, while organisms which combine their DNA through sexual reproduction allow only a portion of their deleterious mutations to pass to their offspring, increasing their chances of survival. This effect is increased through natural selection, where individuals with exceptionally harmful mutations are unable to pass on their genes through sexual reproduction.
Increased diversity of genes within a population also allows natural selection to improve an organism’s ability to adapt to environmental changes. Random mutations, which are of no use to one generation, may become key to survival in future generations under a change of environmental pressures. This is the driving force behind speciation.