It is common knowledge that natural selection favors the survival of those organisms better suited to their environment. Often times, being better suited to one’s environment means using resources more efficiently than others in the same environment, or being the fastest at completing a particular task required for survival, among other things, for example. At the cellular level, to be more specific, the progeny of prokaryotic cells that are able to reproduce fastest by taking up raw materials from their environment and replicating themselves most efficiently, at the maximal rate permitted by the available food supplies, are the most likely to survive. One way in which cells can achieve a level of efficiency similar to that described is by being of a small size. Small cells have a large ratio of surface area to volume. This means that the uptake of nutrients across the plasma membrane is maximized and that the reproductive rate of the cell is boosted (Alberts et al. 2014).
Because of this small size, for many prokaryotic cells, genome sizes are small. Having a small genome size means that genes are densely packed and regulatory DNA between genes is kept to a minimum. Most bacterial and archaeal genes have anywhere from 10^6 and 10^7 nucleotide pairs, encoding 1,000 to 6,000 genes. The analysis of DNA allows scientists to compare the three domains of the living world that we previously discussed. A complete DNA sequence can easily provide information as to what genes are common among all of the domains of the living world, thus leading scientists to understandably conclude that such genes were present in the cell that serves as an ancestor to all present-day living things. On another note, the same such studies can show which genes are unique to only one branch of the tree of life. All of this information provides us with the basis with which we can further understand how new genes arise and how genomes evolve (Alberts et al. 2014).
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