chapter 23

• One common misconception about evolution is that organisms evolve, in a Darwinian sense, during their lifetimes.
• Natural selection does act on individuals. Each individual’s combination of inherited traits affects its survival and its reproductive success relative to other individuals in the population.
• However, the evolutionary impact of natural selection is only apparent in the changes in a population of organisms over time.
• It is the population, not the individual, that evolves.
• Consider the example of bent grass (Agrostis tenuis) growing on the tailings of an abandoned mine. These tailings are rich in toxic heavy metals.
• While many bent grass seeds land on the mine tailings each year, the only plants that germinate, grow, and reproduce are those that possess genes enabling them to tolerate metallic soils.
• These plants tend to produce metal-tolerant offspring.
• Individual plants do not evolve to become more metal-tolerant during their lifetimes.

Anopheles mosquitoes, which carry the malaria parasite, cannot live above elevations of 5,900 feet. In addition, oxygen availability decreases with higher altitude. Consider a hypothetical human population that is adapted to life on the slopes of Mt. Kilimanjaro in Tanzania, a country in equatorial Africa. Mt. Kilimanjaro's base is about 2,600 feet above sea level and its peak is 19,341 feet above sea level. If the incidence of the sickle-cell allele in the population is plotted against altitude (feet above sea level), which of the following distributions is most likely, assuming little migration of people up or down the mountain? - Correct Answer B

If global warming permits mosquitoes to live at higher altitudes than they currently do, then in which direction should the entire plot in the correct distribution below be shifted?

A) to the right
B) to the left
C) upward
D) downward