Biology  5/e   Raven/Johnson
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Meiosis: Meiosis is the process in diploid organisms that results in the formation of haploid gametes: the egg cells and sperm cells. The pairing and crossing over that occur between homologous chromosomes during meiosis, plus the random assortment of chromosomes among the daughter cells, result in tremendous genetic diversity in the gametes and then individual variation among the offspring formed from the union of two gametes. Meiosis is the cellular mechanism behind sexual reproduction.

Evolution of sexual reproduction: Many organisms can reproduce asexually (without sex) and scientists have often wondered why sexual reproduction evolved since it involves definite costs to an individual as well as potential benefits. Many scientists now believe that meiosis with its synapsis of homologous chromosomes evolved initially as a mechanism to repair chromosomal damage rather than as a mechanism for sexual reproduction per se.

Adaptation: Features that promote the likelihood of survival and reproduction of an organism in a particular environment are called adaptations. Over many generations, changes in gene frequencies occur in populations as individuals with favored characteristics reproduce more than do individuals with less favored or unfavored characteristics. Gradually the favored characteristics become more common in the population. This process of adaptation results in the formation (evolution) of new species.

Amniotic egg: The evolution of the amniotic egg was critical in helping the vertebrates successfully colonize land. Amniotic eggs protect the developing embryo from drying out in the terrestrial environment and also supply the necessary nutrients, water, and oxygen while removing or storing waste materials. Amniotic eggs first evolved in the reptiles, and are also found in birds and monotremes.

Mammals: The mammals evolved from the reptiles approximately 200 million years ago and have become an extremely diversified and successful class of vertebrates. Evolutionary advances such as homeothermy, a four-chambered heart, efficient locomotion, a large brain, and extensive care of their young have contributed to their success.

Hormones: Hormones are the chemical messengers of the vertebrate body. They are produced by numerous glands and have a multitude of long-term effects. Most hormones are under the hierarchial control of the central nervous system; the chain of command goes from the hypothalamus to the anterior pituitary gland to various other endocrine glands. Acting in concert, the central nervous system and the hormonal system help maintain proper homeostasis, growth, and development (including sexual development and reproductive cycles).

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