Final answer:
Speciation is typically caused by a reduction in gene flow, and genetic drift can increase genetic diversity.
Step-by-step explanation:
The statement that is true among the options provided is c) Speciation is caused by a reduction in gene flow. Speciation, the process through which new species arise, can indeed occur when there is a reduction in gene flow between populations due to geographic, behavioral, or other barriers. This can lead to allopatric speciation, where populations are physically separated, leading to the formation of new species over time as genetic differences accumulate. Option d is also true as genetic drift, a random change in allele frequencies, can indeed increase genetic diversity within a population by chance.
Current rates of extinction are indeed higher than background extinction rates due to human activities, and this profoundly affects biodiversity. An increase in human population can lead to habitat destruction, pollution, and overexploitation of resources, causing a decrease in biodiversity. It is also true that Earth has experienced more than one mass extinction. In fact, it has gone through five mass extinctions, each having a significant impact on global biodiversity by dramatically affecting the number of species on Earth.