470 Natural Resources Systems Copyright Goodheart-Willcox Co., Inc. The Loss of Genetic Variation As populations decline, the genetic variation available also declines. The genetic variation of new generations of organisms within the population becomes limited to the point that all members are closely related. Organisms that inbreed leave the species vulnerable to genetic mutations, lessen their dis- ease resistance, and often develop physical disabilities. Members of the current cheetah population (Acinonyx jubatus), for example, have little genetic variation because of a minor mass extinction that occurred about 11,000 years ago. Over 90% of the cheetah population was lost and the cheetahs that survived and reproduced have little genetic variation, Figure 19-6. A Sixth Mass Extinction A mass extinction is a period during which a large number of species become extinct. It has been determined that mass extinctions, which are caused by major ecological changes, have occurred five times on Earth. Experts explain that the sixth mass extinction, which we are currently witnessing, is not caused by natural evolutionary forces it is caused by human beings. The sixth extinction is also referred to as the Anthropocene extinction or the Holocene extinction. According to the Center for Health and the Global Environment, we are losing approximately 1000 species each year, many of which have not been identified. This is much more than losses caused by the natural evolutionary process, which STEM Connection is the loss of approximately one to five plant and/ or animal species each year. This great loss of biodiversity is the result of human activities, including habitat loss, the introduction of invasive species and disease, and global climate change. Sadly, many scientists believe that in the next five centuries, nearly 75% of all life forms on Earth may become extinct. Consider This 1. Do you agree or disagree that we are experiencing a sixth mass extinction? Explain your answer. 2. Which species do you think will survive the next five centuries? 3. How will the species that survive change to ensure they continue to survive? Figure 19-6. The limited genetic pool may be the ultimate cause of the cheetah’s extinction. Excessive inbreeding may one day eliminate the cheetah’s ability to birth healthy offspring. Vaganundo_Che/Shutterstock.com