4: Variation: A Key Requirement for Evolution in .NET framework

Creation QR Code JIS X 0510 in .NET framework 4: Variation: A Key Requirement for Evolution

increased or decreased in the different populations could be the result of natural selection (discussed in 5), random factors (the topic in 6), of both If two populations continue to become genetically different, they can become too different to interbreed, a topic addressed in 8 In evolution, the relevant variation occurs at the group or species level You cannot talk about variation unless you have more than one individual If you have variation in the genetic composition of the group or species, the next generation can be different That s evolution A variety of mechanisms can cause the genetic makeup of the next generation to differ from the genetic makeup of the current generation Natural selection ( 5) and random factors ( 6) are two immediate examples These mechanisms change the genetic makeup of a population from one generation to the next: natural selection, because some genetic types have better reproductive success than others, and genetic drift, just because sometimes stuff happens a fire kills many blue-eyed deer, leaving fewer blue-eyed deer to make more blue-eyed deer

The two kinds of variation are phenotypic (changes in outward physical traits) and genotypic (changes in the organism s underlying genetic makeup basically, the DNA sequence of its genes) Individuals with the same phenotype can have different genotypes, and individuals with the same genotype can have different phenotypes Height, for example, has both a genetic component and an environmental component You d think that two people with identical genetic makeup (such as identical twins) would be the same height, but if only one received a healthy diet, the result could be two adults of different heights In this example, two people with the same genotype have different phenotypes Conversely, two people with different genetic make-ups can be the same height In this case, the two individuals have different genotypes but the same phenotypes Keeping this distinction in mind is important when you think about how natural selection can cause evolutionary change Because natural selection doesn t see the different genotypes, it acts only on the phenotypes the physical features of the organism that can influence its survival and reproduction But evolution can occur only if phenotypic differences are correlated with genotypic differences

Imagine that being taller carries a selective advantage that is, taller folks are better at surviving and reproducing In the case of the identical twins with different heights, the taller of the two identical twins would be more likely to contribute genes to the next generation, but because the height difference was due to environmental factors and not genetic makeup, the genetic composition of the next generation doesn t change In the case of the two unrelated people of the same height, the two individuals have different genotypes, but because natural selection acts only on phenotypes, both individuals are the same height, so they have an equal chance of contributing genes to the next generation

You can partition variation by whether it s heritable (or not) or has fitness consequences (or not): Heritable variation: This type of variation is genetic; these are the differences at the DNA level that are passed from parent to offspring Non-heritable variation: This type of variation is the result of the environment factors, things like diet, amount of sunlight, and so on not genes Variations that have fitness consequences: These variations impact, for better or worse, how well an organism is able to survive and reproduce Variations that don t have fitness consequences: These variations don t impact either positively or negatively an organism s ability to survive and reproduce The type of variation determines in what manner (or even whether) it has an impact on the genetic makeup of future generations: If a variation has a fitness consequences but the variation isn t heritable, it won t lead to evolution For example, people with really cute tattoos may be more likely to pass along their genes, but there s no genetic component to getting a tattoo Their genes are, on average, no different from anyone else s genes Remember: Without heritable variation, there is no evolution (or evolution books, for that matter) If a variation is heritable but it doesn t cause any fitness differences, then the frequencies of these differences in a population will change only as a result of genetic drift, not natural selection If variation is both heritable and has fitness consequences, well then, evolution by natural selection can occur