The evolutionary process of absolutely any living species on our planet went through both the stages of flourishing and increasing the number of its populations, and reducing the number of specimens to several thousand, hundreds or less. In the latter case, it is customary to speak of the bottleneck effect. Let's take a closer look at what this means.
What is the bottleneck effect?
Let's 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, a wide variety of characters can be found among individuals of this species. For example, there will be individuals with white, black, brown, spotted color; large, small and medium-sized individuals; some will be fast, others slow, some will have long limbs, others will have large eyes. This list of qualities and attributes can be continued for a long time. There is only one conclusion: in a population with a large number of individuals, there is a great variety of genetic information, that is, the gene poolis rich.
Now imagine that some cataclysm occurred, which led to a sharp extinction of this species. As a result, out of a million individuals, only a few tens or hundreds remained. Naturally, genetic diversity will be lost. Surviving individuals carry only a few different alleles, from which subsequent generations will form. This reduction in the gene pool is a bottleneck effect. The situation is literally similar to the fact that out of a wide variety of colored balls present in a bottle, only a few of them were poured through a narrow neck.
Founder effect
The number of individuals that survived, having passed through the "bottleneck" stage, gives rise to new generations. In relation to them, this reduced number of individuals is the founder or parent population.
If the number of individuals of a species is reduced to 10 or less, then one speaks of an extreme founder effect. In this case, there will be practically no diversity of alleles in the gene pool of subsequent generations, and the same morphological characters will occur quite often.
Thus, the effects of the founder and the bottleneck are connected with each other in one evolutionary chain: the first follows the second.
What do these effects lead to?
In other words, is gene pool reduction good or bad? The answer to this question is not as simple as it seems at first glance. Here are the positives and negatives thatfollow from the definition of the bottleneck effect, i.e. the reduction in genetic diversity in a given species:
- Pros. Subsequent populations acquire specific traits and mutations that may be beneficial to individuals in that environment.
- Cons. 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, individuals often begin to have defects that are inherited.
Cheetah example
A vivid example of the bottleneck effect caused by evolutionary selection is the modern cheetah. Before the global icing of our planet (Quaternary period), there were several species of cheetahs in Africa, Eurasia and North America, which were very different from the modern ones both in size and in speed abilities. According to some estimates, the total number of cheetahs on the planet could reach hundreds of thousands of individuals.
During the Quaternary period, when food became less available, there was a mass 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.
Currently, the cheetah is a mammal with extremely low genetic diversity. These beasts are weakresistant to various kinds of diseases, and any attempts to implant organs in 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 bottleneck effect 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. European bison at the beginning of the 20th century, there were only 12 individuals (out of 3600), and American - 750 (out of 370 thousand).
- Giant tortoises of the Galapagos Islands.
Note that this effect is also used in the selection of new subspecies of plants and animals, in order to consolidate traits beneficial to humans.
Can genetic diversity recover?
The answer to this question is yes. Yes, it can, but for this it is necessary to create the appropriate conditions. Even when the parental group of individuals was small and there was a strong bottleneck effect in the past, genetic diversity can be restored in a long subsequent evolutionary process.
For this, the environment must provide various niches for the habitat 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 known history constantly claimed 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 supervolcano exploded in Indonesia. Its explosive force is estimated as such at 3,000 Saint Helena volcanoes! According to some assumptions, this eruption could reduce the number of different types of people to several thousand individuals throughout the Earth.
- During the Middle Ages, about 1/3 of the population of Europe died as a result of the black plague.
- During the colonization of the New World by Europeans at the end of the 15th - the first half of the 16th centuries, about 90% of the indigenous population was destroyed.
- In 1783, the Lucky volcano exploded 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 planet's population is about 7.5 billion and it is distributed throughout the Earth (different environmental conditions).
