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Sexual Reproduction
Sexual reproduction, the union of the genes of two individuals in their offspring, is a source of variation in species. Each parent can contribute numerous combinations of alleles -- different forms of genes -- to their sex cells. When individuals produce sex cells during the process of meiosis, genetic material in the form of DNA may become altered. Chromosomes, structures carrying the DNA, duplicate and yield two identical sister chromatids connected together. Crossover happens during meiosis when pieces on each chromatid break off and rejoin the other chromatid. The two chromatids then separate, and each chromatid is distributed to a sex cell. When male and female sex cells unite, the resulting offspring carries half of each parent̵7;s genes in varying combinations.
Mutation
Spontaneous mutations arise frequently in populations, becoming significant sources of variation within species. Individuals succumb to the effects of certain mutations, but others flourish because of their mutations. Mutations often happen when an error occurs during the process of copying DNA. Strands of DNA contain genes, which provide the code for the different proteins made within a cell. When a gene mutates, it results in a totally different protein, a variation of the correct protein or a non-functional variant. Mutations are responsible for the different alleles seen in populations. These errors can be passed along to offspring if they occur in the sex cells, sperm or eggs. Scientists estimate that in humans, one gene out of every 10,000 in each generation contains a new mutation, meaning every human should possess an average of two or three mutations.
Adapting to Environments
Changes in a species̵7; environment may elevate mutants that would have declined in ordinary circumstances. In England before the 1800s, most peppered moths sported light colors, with darker-colored variants spotted only rarely. With the advent of the Industrial Revolution during the 19th century, the incidences of the darker mutants became more pronounced. The chemical wastes given off by factories and industries killed off the light-colored lichens that clung to trees. The light-colored moths declined in numbers because they no longer blended with the trees on which they alighted and were picked off easily by birds. The darker mutants now had the advantage because they were closer in color to the tree bark, which became darker because of the layers of soot coating the trees.
Niche Adaptations
Members of the same species sometimes are forced to explore different ways of life. Upon landing on the Galapagos Islands, Darwin observed the population of finches on the different islands. He noted that the finches on each of the islands were of different species, but he also observed that they actually diverged from one species not too long ago. Darwin hypothesized that when populations of the original finch species migrated to different islands, those birds that were best-adapted to each island̵7;s food source would live comfortably. The environment of each island molded the finch̵7;s bill to fit the prevalent food source. The birds on each island eventually varied enough from each other to become different species.
Antibiotic Resistance
Humans sometimes provide the impetus for evolution. Normal bacterial populations exhibit a mutation rate from 1 in 10,000 to 1 in 1,000,000. Under normal conditions, these mutants have decreased chances of survival. When humans introduce a detrimental situation such as antibiotics, the mutant̵7;s characteristics may allow it to survive the adverse condition. A greater number of these mutants survive and reproduce in contrast to the normal strain. Some bacteria can even transfer their antibiotic-resistant genes to other bacteria of the same species or even of different species. The infamous bacteria, the methicillin-resistant Staphylococcus aureus, had developed resistance to several antibiotics, including penicillin; the most dangerous MRSA strains have become resistant to all but a couple of antibiotics.