Populations （2）
Population Growth and Types of Reproduction
Population growth is affected by species’ methods of reproduction. The two most important types of reproduction are asexual and sexual reproduction. Each type of reproduction has benefits and costs.
Asexual reproduction—such as that found in plants that reproduce by shoots or organisms that reproduce through parthenogenesis—requires less energy than its sexual counterpart. Because it requires less time and effort, asexual production allows a population to grow very quickly. For example, parthenogenesis occurs when an unfertilized egg develops offspring. Parthenogenesis creates female organisms that are identical to their mothers; the eggs of these female organisms undergo parthenogenesis and produce more females. By eliminating the necessity of males from the reproductive equation, parthenogenesis doubles the rate at which a population can grow. However, by eliminating males and sexual reproduction, populations that employ asexual reproduction limit their gene pool and the resulting diversity among members. In times when an environment is changing or competitive, the lack of variation damages these populations’ ability to survive.
Sexual reproduction exhausts more energy and therefore progresses slowly. A population that reproduces through sexual reproduction will not grow as rapidly as an asexually reproducing population, but the sexual population will maintain the diversity of its gene pool. A sexually reproducing population is therefore more fit to survive in a changing or competitive environment.
Sexually reproducing organisms have two reproductive substrategies. Organisms such as insects have many small offspring that receive very little or no parental care, reach sexual maturity at a young age, and reproduce only one or a few times. In an environment with abundant resources, this life-history strategy allows species to quickly reproduce and exploit opportunities for population growth. The disadvantage of this strategy is that it produces high mortality and great instability when resources dwindle. The alternative strategy is to bear fewer and larger offspring that receive intensive parental attention, mature gradually, and reproduce several times. Humans employ this strategy and are better suited to thrive in a competitive environment, exhibiting lower mortality rates and longer life spans. The disadvantage here is that the concerted investment of time and energy into a few individuals makes it difficult for a population to surmount large decreases in population size due to disasters or disease.