In biology, a “bottleneck” describes a sharp, temporary reduction in population size. These events, whether caused by environmental shifts or human activities, can dramatically alter a species’ trajectory.
What is an Ancestral Bottleneck?
An ancestral bottleneck refers to a severe reduction in the size of a population that occurred in the past, leaving a lasting mark on the genetic makeup of its descendants. This sharp decrease in numbers leads to a significant loss of genetic diversity within the population. The term “ancestral” emphasizes that the event happened historically, impacting the lineage that followed.
Even if the population recovers in numbers, the genetic diversity often remains lower than before the event. The smaller gene pool of the surviving individuals means that certain genetic variants may be completely lost, while others become more frequent by chance. This reduction in variation can influence the population’s future adaptability and resilience.
How Bottlenecks Occur
Ancestral bottlenecks can arise from various scenarios that drastically reduce population numbers. Natural disasters, such as volcanic eruptions, widespread floods, or prolonged droughts, can decimate populations. For example, a supervolcano eruption can create a global “volcanic winter,” severely impacting ecosystems and leading to population crashes.
Widespread diseases can also act as a bottleneck, as pathogens can quickly reduce a population if there is little immunity. Drastic climate change, like ice ages, leads to significant habitat loss and resource scarcity, forcing populations into smaller, isolated groups. Human activities, including excessive hunting, overfishing, or extensive habitat destruction, also contribute to these reductions in population size.
A specific type of bottleneck is the founder effect, where a small group of individuals separates from a larger population to establish a new colony. The genetic diversity of this new “founder” group is inherently limited to the genes carried by those few individuals, which may not represent the original population’s full genetic range. This can happen when a few members of a species colonize a new isolated island or when small groups migrate to new territories.
Impact on Genetic Diversity and Evolution
The consequences of an ancestral bottleneck on a population’s genetic makeup are profound. A primary impact is the significant loss of genetic variation, meaning fewer different versions of genes, or alleles, are present. This reduction in diversity makes the population more uniform genetically.
This decreased genetic diversity limits a population’s ability to adapt to new environmental pressures, such as changes in climate or the emergence of new diseases. When there are fewer genetic options, the population has less raw material for natural selection to act upon, which can hinder its long-term survival.
Following a bottleneck, genetic drift, the random fluctuation of gene frequencies, becomes more pronounced in the smaller population. This random process can lead to the accidental loss of some alleles and the increased frequency of others, purely by chance, rather than through any selective advantage. The reduced population size also increases the likelihood of inbreeding, where related individuals mate, which can expose harmful recessive traits and lead to a decline in the population’s fitness, known as inbreeding depression.
Notable Examples in Nature
The Northern Elephant Seal ( Mirounga angustirostris) provides a clear example of an ancestral bottleneck. During the 19th century, intense hunting for their blubber oil reduced their population to an estimated 10 to 20 individuals by the late 1800s. Despite a remarkable recovery in numbers to over 200,000 seals today due to protection, their genetic diversity remains extremely low compared to other seal populations, demonstrating the lasting impact of the bottleneck.
Cheetahs (Acinonyx jubatus) are another well-documented case, believed to have experienced at least two significant bottlenecks. One occurred approximately 100,000 years ago, possibly linked to their migration from North America to Asia, and another around 10,000 to 12,000 years ago, potentially coinciding with the Late Pleistocene extinction event. This has resulted in cheetahs having unusually low genetic variability, making them more susceptible to diseases and reproductive issues.
For human history, a severe ancestral bottleneck has been identified in our lineage, where the population of human ancestors was reduced to approximately 1,280 breeding individuals between 930,000 and 813,000 years ago, lasting for about 117,000 years. This event, which coincided with significant climate changes, including glacial periods, is thought to have caused an estimated 65.85% loss of genetic diversity in early human ancestors.