The evolutionary process of absolutely any living species on our planet went through both the heyday and increase in the number of its populations, and the reduction in the number of copies to several thousand, hundreds or less. In the latter case, it is customary to talk about the effect of a bottleneck. Let us consider in more detail what this means.
What is the essence of the bottleneck effect?
Imagine that there is some kind of living creature, which is represented by a hundred thousand or even several million copies. In such a huge population, one can find a variety of characters among individuals of this species. For example, there will be individuals with white, black, brown, spotty color; large, small, and medium-sized individuals; some will be fast, others will be slow, some will have long limbs, others will have a large eye size. This list of qualities and attributes can be continued for a long time. The conclusion suggests itself: in a population with a large number of individuals, the diversity of genetic information is great, that is, the gene pool is rich.
Now imagine that there was a cataclysm that led to a sharp extinction of this species. As a result, out of a million individuals, only a few tens or hundreds remain. Naturally, genetic diversity will be lost. Surviving individuals are carriers of only a few different alleles, from which subsequent generations will form. This reduction in gene pool is the effect of a bottleneck. The situation is literally similar to the fact that only a few of them were poured out of a wide variety of colored balls present in the bottle through a narrow neck.
Founder effect
The number of individuals that survived through the “bottleneck” stage gives rise to new generations. In relation to them, this reduced number of individuals is the founder, or parental population.
If the number of individuals of a species is reduced to 10 or less, then they talk about the extreme effect of the founder. In this case, there will be practically no allele diversity in the gene pool of subsequent generations, and the same morphological characters will be encountered quite often.
Thus, the effects of the founder and the bottleneck are linked together in the same evolutionary chain: the first follows the second.
What do these effects lead to?
In other words, is a reduction in the gene pool good or bad? The answer to this question is not as simple as it seems at first glance. Here are the positive and negative sides that follow from the definition of the effect of the bottleneck, that is, from the reduction of genetic diversity in this species:
- Pros. In subsequent populations, specific characters and mutations are fixed that may be useful to individuals in a given environment.
- Minuses. A low level of genetic diversity leads to a decrease in the ability of a species to adapt to environmental changes, that is, makes it vulnerable. In addition, in individuals defects that are inherited often begin to occur.
Cheetah example
A striking example of the bottleneck effect caused by evolutionary selection is the modern cheetah. Before the global icing of our planet took place (Quaternary), several types of cheetahs existed in Africa, Eurasia and North America, which were very different from modern ones in both size and speed. According to some estimates, the total number of cheetahs on the planet could reach hundreds of thousands of individuals.
During the Quaternary, when food became less accessible, there was a massive mortality of many species of living creatures, including cheetahs. It is believed that the number of the latter could be only a few hundred individuals. Moreover, only the fastest and smallest specimens survived, that is, there was a bottleneck effect for cheetahs.
The cheetah is currently a mammal with extremely low genetic diversity. These animals are poorly resistant to various kinds of diseases, and any attempts to implant organs with them end in failure. The cheetah's body is practically unable to adapt to changes in the environment.
Artificial population reduction
Based on the name, this effect of the bottleneck is already caused by human intervention in nature. There are several examples:
- Northern elephant seals. As a result of active hunting and extermination of these animals at the end of the 19th century, out of 150 thousand, only 20 individuals remained.
- European and American bison. At the beginning of the 20th century, European bison numbered only 12 individuals (out of 3,600), and American bison - 750 (out of 370,000).
- Giant turtles of the Galapagos Islands.
It should be noted that this effect is also used in the selection of new subspecies of plants and animals, in order to consolidate traits that are advantageous to humans.
Can genetic diversity be restored?
The answer to this question is positive. Yes, it can, but for this it is necessary to create the appropriate conditions. Even when the parental group of individuals was small and a strong bottleneck effect was observed in the past, genetic diversity can be restored during a long subsequent evolutionary process.
For this, the environment must provide various niches for the living of this species, that is, the environment itself must be diverse. Then, adapting to new conditions and gradually accumulating new mutations, the species can restore its gene pool.
What about human evolution?
Various cataclysms of the history we know constantly took tens and hundreds of thousands of human lives, which created the effect of a bottleneck for Homo Sapiens and other human species. Here are some examples:
- 75 thousand years ago, the Toba super-volcano exploded in Indonesia. Its blast power is estimated as such for 3000 volcanoes of St. Helena! According to some assumptions, this eruption could reduce the number of different species of people to several thousand individuals on the entire Earth.
- In the Middle Ages, about 1/3 of the population of Europe died as a result of the black plague.
- During the European colonization of the New World in the late XV - first half of the XVI centuries, about 90% of the indigenous population was destroyed.
- In 1783, an explosion of the Laki volcano occurred in Iceland. Subsequently, hunger and disease were added to it, as a result of which about 20% of the island's population died.
As for the current situation with humans, their genetic diversity is quite large, since the world's population is about 7.5 billion and it is distributed throughout the Earth (different environmental conditions).