Biology  5/e   Raven/Johnson
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Natural selection: Darwin proposed natural selection as the mechanism behind evolutionary change. Darwin's theory has been called "survival of the fittest" – that is, the best adapted or most fit individuals survive. The real key to understanding evolutionary mechanisms, however, is realizing that differential reproduction, not just survival, is the answer. Better adapted individuals reproduce more and pass their traits on to future generations. Because of the reproductive success of these individuals and their offspring, their genes become more and more common in future generations.

Mechanism of heredity: Gregor Mendel was the first person to quantify the patterns of inheritance. He discovered that inherited traits are determined in an individual by two discrete factors that assort independently during the formation of gametes. New combinations of factors are formed when two gametes come together at fertilization. Today we know that these factors are alleles, or versions of genes, and that the genes are located on chromosomes. During meiosis, each gamete receives one of the two alleles for each gene; thus when two gametes fuse at fertilization, the new offspring receives one allele for each gene from each parent. Meiosis produces haploid gametes from diploid parent cells, and fertilization brings together two haploid gametes to form a diploid zygote.

Alleles: Alleles are the alternative forms of a gene – a specific location and sequence of DNA nucleotides on a chromosome. Red and white, for example, are alleles or alternative forms of the gene for flower color in pea plants.

Heterozygosity and homozygosity: When an individual has two copies of the same allele, the individual is homozygous for that allele or trait. If the two alleles for a particular gene are not the same, the individual is heterozygous for that trait. If, for example, a pea plant has an allele for red flowers and an allele for white flowers, the plant is heterozygous for flower color. But if both alleles are for red flowers or both alleles are for white flowers, the plant is homozygous for flower color.

Mutation and recombination: A mutation is a change in the genetic composition of an organism. The change can be as small as the substitution, addition, or deletion of just one nucleotide in a gene. Mutations can be induced by many different factors. It is the ultimate source of all genetic variability; mutations create alleles – alternative forms of a gene. Recombination shuffles the genetic changes and is the driving force behind individual variation. Sexual reproduction results in extensive recombination because of chromosome assortment and crossing over. Other forms of chromosome alteration also contribute to recombination.

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