Thursday, 23 June 2022

Evolution News - Many Human Groups Show Evidence of a Sharp Dip in the Population

Many human genomes shaped by past events that caused sharp dips in the population | Plos Genetics.

Distribution of the oldest founder events dated at different locations of the world using ancient and present-day samples. Smaller sized points with warmer color correspond to more recent founder events.
Credit: Rémi Tournebize and Priya Moorjani (CC-BY 4.0)

In a paper published today in Plos Genetics, the author report finding evidence that many human populations show evidence of founder effects with a small number of founders and a consequent genetic bottleneck with a high level of inbreeding.

The problem this presents for YECs is that some of their claimed 'evidence' for a young Earth is based on projected population growth using highly suspect models which assume the current rate of growth has been constant for all of history, and which take no account of highly variable infant mortality and the effects of plagues. So, starting with the answer they want they project the current population of the Earth backwards and end up with humans being created just a few thousand years ago. It's a mathematical sleight of hand designed to fool the statistically illiterate.

This research also shows that many human groups would have suffered from genetic defects and low genetic diversity, due to the inbreeding inherent in descent from a small founder population.

According to information provided by PLOS ahead of publication:
Representation of the impact of a founder event on genetic data with an initial population made of various genomic segments that were inherited identical-by-descent (IBD) from a common ancestor. Following a founder event, the diversity of IBD segments is reduced and due to recombination events, the length of these IBD segments shortens gradually—these two parameters are thus informative about the founder strength and age, respectively.

Credit: Rémi Tournebize and Priya Moorjani (CC-BY 4.0)

The genomes of many human populations show evidence of founder events, which occur when a small number of initial members start a new population, and can lead to low genetic diversity as well as increase the risk of certain genetic diseases in the new population. Rémi Tournebize and Priya Moorjani of the University of California, Berkeley, U.S. report these new findings June 23rd in the open access journal PLOS Genetics.

Founder events can occur when a population experiences a sharp decrease in numbers or when a few individuals colonize an isolated environment, such as an island. Despite the large impact that founder events can have on a population’s genetics, we know few details of how these events have shaped the evolution of humans and other species. To better understand these past events, researchers developed a new technique called ASCEND that uses genomic analysis to estimate the timing and strength of founder events. In the new study, they applied ASCEND to analyze about 460 human populations worldwide. The researchers found that over half of the populations that they analyzed had evidence of recent founder events, including most living hunter-gatherer, nomadic and indigenous groups that were sampled. These founder events are associated with geographic isolation, a hunter-gatherer lifestyle, or the cultural practice of marrying within your own group or religion.

Some human populations like Ashkenazi Jews or Finns have been extensively studied in population genetics and have helped researchers identify many disease-causing mutations. Hence, we wanted to study if other populations have a similar history that could enable further progress in medical genetics. We were surprised to see how widespread the history of founder events is in humans, both in present-day and ancient DNA samples, suggesting that investigation of disease-causing variants will be fruitful to identify and reduce disease burden among contemporary groups.

Rémi Tournebize and Priya Moorjani, co authors
The researchers also analyzed about 200 modern dog populations and found that most populations show signs of extreme founder events. These events occurred within the last 25 generations, coinciding with the start of dog breeding during Victorian times, and may be related to inbreeding and the use of a few highly prized males to sire numerous litters.

The new analysis technique will help scientists to identify groups that experienced strong founder events and that may be at high risk of certain genetic diseases. Ashkenazi Jews and Finns, who experienced strong founder events, often undergo genetic screening to learn about their predisposition to certain genetic diseases. The study found that several populations among Native Americans, Oceanians, and South Asians have experienced even more extreme founder events than Ashkenazi Jews, and so these populations may also benefit from genetic screening.
In their open access paper in PLOS Genetics, the authors say:
Founder events play a critical role in shaping genetic diversity, fitness and disease risk in a population. Yet our understanding of the prevalence and distribution of founder events in humans and other species remains incomplete, as most existing methods require large sample sizes or phased genomes. Thus, we developed ASCEND that measures the correlation in allele sharing between pairs of individuals across the genome to infer the age and strength of founder events. We show that ASCEND can reliably estimate the parameters of founder events under a range of demographic scenarios. We then apply ASCEND to two species with contrasting evolutionary histories: *460 worldwide human populations and *40 modern dog breeds. In humans, we find that over half of the analyzed populations have evidence for recent founder events, associated with geographic isolation, modes of sustenance or cultural practices such as endogamy. Notably, island populations have lower population sizes than continental groups and most hunter-gatherer, nomadic and indigenous groups have evidence of recent founder events. Many present-day groups––including Native Americans, Oceanians and South Asians––have experienced more extreme founder events than Ashkenazi Jews who have high rates of recessive diseases due their known history of founder events. Using ancient genomes, we show that the strength of founder events differs markedly across geographic regions and time––with three major founder events related to the peopling of Americas and a trend in decreasing strength of founder events in Europe following the Neolithic transition and steppe migrations. In dogs, we estimate extreme founder events in most breeds that occurred in the last 25 generations, concordant with the establishment of many dog breeds during the Victorian times. Our analysis highlights a widespread history of founder events in humans and dogs and elucidates some of the demographic and cultural practices related to these events.

Author summary

A founder event occurs when small numbers of ancestral individuals give rise to a large fraction of the population. Founder events reduce genetic variation and increase the risk of recessive diseases. Despite their importance in evolutionary and disease studies, we still only have a limited comprehension of their prevalence and properties in humans and other species, as most existing methods require large sample sizes or phased genomes. Here, we present a flexible method, ASCEND, to infer the timing and the strength of founder events that is suitable for sparse datasets with few samples or limited coverage. ASCEND provides reliable estimates across a wide range of demographic scenarios. By applying it to data from two species (humans and dogs), we document a widespread history of recent founder events in both species and provide insights about the demographic processes related to these events. Our analysis helps to identify groups with strong founder events that should be prioritized for future studies as they offer a unique opportunity for biological discovery and reducing disease burden through mapping of recessive disease causing genes and pathways, as previously shown in studies of Ashkenazi Jews and Finns.

Of course, this study wasn't undertaken to refute a YEC claim, but to help predict what sorts of genetic disorders can be expected in different populations, and so extend to these other groups what is already being done for Ashkenazi Jews and Finns. Like all other factual science papers of course, it refutes creationism quite incidentally simply by revealing the real-world facts - which is exactly what we would expect for a superstition that has no basis in reality.

Thank you for sharing!

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