Geographical variation refers to differences among populations in genetically based traits across the natural geographic range of a species. ... In the simplest case, we divide these factors into purely genetic versus environmental components to tease apart their relative contributions to observed phenotypic variation.
The randomness in the alignment of recombined chromosomes at the metaphase plate, coupled with the crossing over events between nonsister chromatids, are responsible for much of the genetic variation in the offspring. To clarify this further, remember that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Thus, any maternally inherited chromosome may face either pole. Likewise, any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.
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B
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Animals derive energy by eating plants or animals and using cellular respiration to make ATP. Plants capture energy from the sun via photosynthesis and make sugar and oxygen. They can then use the sugar to perform cellular respiration to derive energy.
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To survive in drought conditions, plants need to decrease transpiration to limit their water loss. Some plants that live in dry conditions have evolved to have smaller leaves and therefore fewer stomata. Extreme examples are plants with leaves that resemble spiky thorns or spines.
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Nucleotides are building blocks of nucleic acids (DNA and RNA). A nucleic acid contains a chain of nucleotides linked together with covalent bonds to form a sugar-phosphate backbone with protruding nitrogenous bases. For example, DNA contains two such chains spiraling round each other in the famous double helix shape.
