Fertilization

External and Internal Fertilization

External and internal fertilization are forms of reproduction that vary in method and embryo development.

External and Internal Fertilization

External Fertilization

External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. After the sperm reaches the egg, fertilization can then take place. Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. The release of the reproductive material may be triggered by water temperature or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers). Pairs of fish that are not broadcast spawners may exhibit courtship behavior. This allows the female to select a particular male. The trigger for egg and sperm release (spawning) causes the egg and sperm to be placed in a small area, enhancing the possibility of fertilization.

External fertilization in an aquatic environment protects the eggs from drying out. Broadcast spawning can result in a greater mixture of the genes within a group, leading to higher genetic diversity and a greater chance of species survival in a hostile environment. For sessile aquatic organisms such as sponges, broadcast spawning is the only mechanism for fertilization and colonization of new environments. The presence of the fertilized eggs and developing young in the water provides opportunities for predation, resulting in a loss of offspring. Therefore, millions of eggs must be produced by individuals. The offspring produced through this method must mature rapidly. The survival rate of eggs produced through broadcast spawning is low.

Internal Fertilization

Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization: oviparity, ovoviparity, and viviparity.

In oviparity, fertilized eggs are laid outside the female's body and develop there, receiving nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. These animals are classified as oviparous.

In ovoviparity, fertilized eggs are retained in the female, but the embryo obtains its nourishment from the egg's yolk; the young are fully developed when they are hatched. This occurs in some bony fish (such as the guppy, Lebistes reticulatus), some sharks, some lizards, some snakes (such as the garter snake, Thamnophis sirtalis), some vipers, and some invertebrate animals (such as the Madagascar hissing cockroach, Gromphadorhina portentosa).

In viviparity, the young develop within the female, receiving nourishment from the mother's blood through a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some cartilaginous fish, and a few reptiles, making these animals viviparous.

Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is isolated within the female, which limits predation on the young. Internal fertilization also enhances the fertilization of eggs by a specific male. Even though fewer offspring are produced through this method, their survival rate is higher than that for external fertilization.

The Evolution of Reproduction

Sexually-reproducing organisms have evolved specialized gonads, along with a variety of ways to transfer sperm during reproduction.

The Evolution of Reproduction

Several competing scientific hypotheses have been proposed to explain the evolution of sexual reproduction. All sexually-reproducing eukaryotic organisms derive from a common ancestor that was a single-celled eukaryotic species. Many protists reproduce sexually, as do multicellular plants, animals, and fungi. However, there are a few species which have secondarily lost this feature. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction.

Once multicellular organisms evolved and developed specialized cells, some also developed tissues and organs with specialized functions. The evolution of reproductive organs arrived with the development of gonads that produced sperm and eggs. These cells develop through meiosis, an adaption of mitosis, which reduced the number of chromosomes in each reproductive cell by half, while increasing the number of cells through cell division. The development of specialized gonads to produce sperm and egg was a major step in the evolutionary process.

An early development in reproduction occurred in the Annelids. These organisms produce sperm and eggs from undifferentiated cells in their coelom, storing them in that cavity. When the coelom becomes filled, the cells are released through an excretory opening or by the body splitting open. Further evolution of reproductive systems resulted in the development of reproductive systems that are sex specific. In these more advanced systems, sperm is made in the testes and then travels through coiled tubes to the epididymis for storage. Additionally, in these more advanced systems, eggs are matured in the ovary; when released, they travel to the uterine tubes for fertilization. These types of reproductive systems developed in insects (compared to annelids which have a coelom for storage). Specifically, in the insect reproductive system, a specialized sac developed, called a spermatheca, which is used to store sperm for later use, sometimes up to a year. This was a key development since fertilization in insects can be timed with environmental or food conditions that are optimal for offspring survival.

Vertebrates have similar structures (i.e., gonads that specialize in sex cell production) with a few differences in their reproductive systems. Non-mammals, such as birds and reptiles, have a common body opening, called a cloaca, for the digestive, excretory, and reproductive systems. Coupling between birds usually involves positioning the cloaca openings opposite each other for transfer of sperm. In mammals, there are separate openings for the systems in the female and a uterus for support of developing offspring. Depending on the type of species, there are differences in the uterus. In species that produce large numbers of offspring, the uterus has two chambers. In other species that produce one offspring, such as in primates, there is a single uterus.

Another development in the evolution of reproduction is the means by which sperm is transferred. During reproduction, sperm transfer from the male to the female ranges from releasing the sperm into the watery environment for external fertilization, to the joining of cloaca in birds, to the development of a penis for direct delivery into the female's vagina in mammals. All of these methods of sperm transfer represent the varying ways reproduction has evolved and become specialized to specific organisms.