Why Cheetahs are Screwed
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Cheetahs have faced extinction in the past after their population was forced into inbreeding because of the lack of potential mates. Today, with the help of conservationists and laws, their numbers are rising; yet, geneticists argue that the threat of extinction is still very much real.
Before we get into the fun stuff about cheetahs and why they are one eyelash wish away from being gone forever, let’s give some background information for our readers that prefer to read Pride and Prejudice to Nature.
Gene Flow
Gene flow is simply the exchange of genes. As animals migrate, genes flow easily, and species evolve and adapt with the introduction of new/different genes. Small populations feel the effects of new genes much more, as these new genes have a greater effect on the genome of the population. Imagine you have one big island with a built-in geographic barrier in the middle of it like a mountain. On one half of the island, the population of mice is 100, and the other has only 10 mice. If 5 mice from the smaller population migrate to the side with more mice, it is not going to affect the genome of the population much because it is only 4.8% of their new population. However, if 5 mice from the big population migrate to the smaller half, the effect on that population will be more noticeable because it is now 33.33% of their gene pool.
Genetic Drift and Population Size
Genetic drift is the random fluctuations in frequencies of alleles (a different form of a specific gene) from generation to generation. It can cause traits to be dominant or disappear from a population. In the simplest terms, genetic drift is an evolutionary process that is all down to chance (although it is important to note that we can predict potential outcomes).
The effects of genetic drift are most pronounced in small populations, where infrequently occurring alleles face a greater chance of being lost. Once it begins, drift will continue until the involved allele is either lost by a population or until it is the only allele present in the population. Both possibilities decrease genetic diversity, which is important to the success and longevity of the population. Genetic drift is common after population bottlenecks events.
Bottleneck Effect
When an event causes a drastic decrease in a population, it causes a type of genetic drift called a bottleneck effect. It can occur by natural disasters like the ice age, a volcano eruption, or an earthquake; however, today, it is often happening by humans like over-hunting, deforestation, and pollution. When most of the population dies suddenly, genetic variation decreases.
A practical example of this is that you have a bag full of beads—40 red, 40 blue, and 20 yellow. Each color represents alleles for a gene. The beads are all living peacefully in their habitat (the bag), and suddenly, disaster strikes! You take out 5 of the beads at random and without looking into the bag and are left with 3 red, 2 blue, and 1 yellow. When this happens, the few remaining individuals end up inbreeding because of the reduced size of the mating pool, leading to decreased genetic variability. In this example, after the first five generations, the majority of the individuals are red, and only a couple of them are blue, and none are yellow. As more time occurs, the population will be all red because of inbreeding. The population is now very different from the one before the disaster happened.
Acinonyx jubatus - Cheetah (Africa)
The cheetah population has faced significant reductions in the past due to two separate bottleneck events. The first event that scientists predict that cheetahs may have undergone occurred around 100,000 years ago when they expanded their range into Asia, Europe, and Africa. This range expansion is believed to have occurred rapidly, dispersing the cheetahs over a large area and restricting their exchange of genes. The lack of mates in the pool decreased genetic variability. The second bottleneck event occurred around the end of the last ice age--about 10,000 to 12,000 years ago. During this event, North American and European cheetahs went extinct, leaving the only extant species in Asia and Africa. As large mammals died out worldwide, the number of surviving cheetahs dwindled, leading to inbreeding.
Although the number of cheetahs grew significantly in the 19th century, their genetic variability remained low due to these two bottlenecks. Recently, there have been studies done to investigate the evidence of low genetic variability due to inbreeding. First, scientists have performed skin grafts that have shown they have lost genetic diversity. Then, it was noticed that cheetahs have developed an asymmetrical skull, which is a sign of inbreeding. The third evidence for inbreeding comes from their high homozygosity. Homozygosity is a measure of the genetic similarity between individuals in a population. Cheetah genomes typically exceed 90% homozygosity. To put things into perspective, domestic dogs that are commonly inbred have a significantly lower rate of homozygosity--which shows how long the inbreeding has occurred and compounds this idea of the lack of genetic diversity.
So, inbreeding led to low genetic variability? Why does that matter? Well, for all the reasons listed in the other sections. With the decreased levels of genetic variability, mutations are more likely to have a major effect on the population’s gene pool because there are not enough individuals in the population with unique, non-homozygotic genes to flush out the mutation, meaning that one random bad mutation from genetic drift could easily wipe out the entire species. And if the cheetahs aren't affected by a mutation, another bottleneck event will eventually occur. They cannot survive a bad case of poaching, another ice age, or illogical migration, and we as humans don’t really have control over poaching most days, much less natural disasters or migration.
The moral of the story is that cheetahs are doomed. It is not a question of if they are going to go extinct, only a question of when. Even without humans invading their habitats, global warming, and the disease that they are catching from domestic cats, they are going to eventually die off because of their lack of genetic variability. So, what do we do about this? I would suggest that you take pictures while you can at your local zoo or maybe even make a special trip to the San Diego Zoo. This way, you can pass photos down in your family so that your great-grandchildren can see the fastest animal on the planet because, as I said before, cheetahs are one eyelash wish (or bad mutation) away from being gone for good.
