Variation

Variation Sexual reproduction promotes variation because offspring inherit genes from two different parents. In addition, when an individual produces gametes, crossing-over and independent assortment mix genes from that individuals parents. Gametes are haploid; they contain some genes from the individuals mother and some from the father. Variation is advantageous to species in fluctuating environments. Reproductive cycles Reproductive cycles in many vertebrates and invertebrates are related to changes in day length. Day length is a reliable indicator of season for timing circannual events such as reproductive behavior and migration. In many species, the pineal gland secretes melatonin in the dark. Increasing day length (decreased melatonin levels) triggers reproductive behavior. Fertilization External fertilization During external fertilization, many gametes are released into the water by each sex at the same time and place. This type of fertilization requires water because animal sperm must swim to the eggs. Water also protects the gametes from drying out. Species which have external fertilization are either aquatic or return to water for reproduction. Internal fertilization Internal fertilization is practiced by species that lay shelled eggs or have a period of internal embryonic development. This type of fertilization enables animals to reproduce in a terrestrial environment because it enables sperm to swim to the egg and it prevents gametes from drying out. The male often has a copulatory organ (a penis) for transferring sperm. Penis The penis evolved independently in insects. It is found in all mammals. Males of some aquatic animals (ex: sharks, skates, rays) have specialized pelvic fins which allow the sperm to be passed to the female. Some reptiles and birds have a penis but most have a single opening for the reproductive, excretory, and digestive systems called a cloaca. Parthenogenesis is most simply defined as reproduction without fertilization. More specifically, it occurs when a female gamete develops a new individual without being fertilized by a male gamete. It is often called a form of "asexual reproduction," but it is more accuratley defined as an incomplete form of sexual reproduction. This is because it involves the production, activation, and development of a female egg which is a specialized reproductive cell Asexual reproduction in AnimalsIn the animal kingdom there are very few examples of asexual reproduction. The fact that animals have complex bodies makes it difficult for them to break into two or grow a bud. There are some examples amongst the simple animals. One good example is the Hydra which lives in freshwater ponds and streams. The Hydra shows budding similar to that of yeasts. An adult Hydra will develop a swelling on the side of its body. This will grow into a daughter bud. Eventually this daughter will

grow tentacles and start to feed itself by catching small water animals. At this point it breaks off from the mother Hydra and floats freely until it lands on a support. 1 The hydra develops a "bud". 2 The bud develops a mouth and tentacles 3 When it is fully formed the daughter hydra detaches from its parent. 4 The daughter hydra is now fully independent.