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Old 01 Jan 2018, 10:42 AM   #682489 / #1
lpetrich
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Default Could only two people repopulate our planet?

BBC - Future - Could just two people repopulate Earth?

A problem with too few people is a strong risk of deleterious mutations getting fixed. That has indeed happened in some places, like Pingelap island in the western Pacific Ocean, where an 18th-cy. typhoon had only 20 survivors, one of them with achromatopsia, the inability to see colors. Today, about 1/10 of the island' population has that disease.

European royalty has also been very inbred. Consider the Spanish Habsburgs with their numerous marriages of close relatives, all to keep assets in the family.
Quote:
Charles II was the family’s most famous victim. Born with a litany of physical and mental disabilities, the king didn’t learn to walk until he was eight years old. As an adult his infertility spelled the extinction of an entire dynasty.

In 2009 a team of Spanish scientists revealed why. Charles’ ancestry was so entangled, his “inbreeding coefficient” – a figure reflecting the proportion of inherited genes that would be identical from both parents – was higher than if he had been born to siblings.
Charles II lived 1661 - 1700 -- only 39 years.

This has been a consideration for protecting endangered species -- too few individuals and the "founder effect" can happen, fixing variants that can be deleterious.
Quote:
So how much variety do you need? It’s a debate that goes right back to the 80s, says Stephens, when an Australian scientist proposed a universal rule of thumb. “Basically you need 50 breeding individuals to avoid inbreeding depression and 500 in order to adapt,” he says. It’s a rule still used today – though it’s been upped to 500-5,000 to account for random losses when genes are passed from one generation to the next – to inform the IUCN Red List, which catalogues the world’s most threatened species.
Then,
Quote:
But before you write off our couple, as one scientist pointed out, we’re living proof of the concept’s inherent flaws. According to anatomical and archaeological evidence, our ancestors wouldn’t have made our own population targets, with 1,000 individuals in existence for nearly a million years. Then between 50,000 and 100,000 years ago, we hit another rough patch as our ancestors migrated out of Africa. As you would expect, we’ve been left with astonishingly low genetic diversity. A 2012 study of the genetic differences between neighbouring groups of chimpanzees found more diversity in a single group than among all seven billion humans alive today.
It's unlikely that our breeding population was that small over that time, though we likely have had population bottlenecks as small as that.

Close Calls: Three Times When Humanity Barely Escaped Extinction

1.2 Million Years Ago: Humanity Before We Were Exclusively Homo Sapiens
Quote:
They read humanity’s history on its genes, and it seems that 1.2 million years ago, things weren’t looking good. Homo sapiens, Homo ergaster, and Homo erectus had, worldwide, a breeding population of about 18,000 people—no more than 26,000 people. This means that all over the world, every human-like species that could possibly contribute genes to a current human added up to a smaller population than that of gorillas today. Considering gorillas are only on one continent, and humans occupied Africa and Eurasia, that’s a very small population indeed.

This result came as a surprise because other evidence indicated that humans were doing very well. They occupied a great deal of territory. Scientists found stone tools in Turkey that date back 1.2 million years. In 2008 archaeologists found the jaw of a thirty-something human who lived in Atapuerca, Spain about 1.2 million years ago. We were all over the place. Why were we so very near extinction?
I don't see how they worked out the existence of an earlier bottleneck, since a later one would make it seem like earlier populations were always small.

But such bottlenecks could be due to speciation events, the origin of new species from earlier ones. That's a part of punctuated equilibrium.

150,000 Years Ago: Homo Sapiens and the Big Chill

Back then, there was an ice age -- and the warmer parts of our planet got drier. Our ancestors may have gotten down to a breeding population of 600. They could have lived in South Africa, living off of roots and shellfish.

70,000 Years Ago: The Toba Explosion

Another bottleneck due to cold weather, this time reducing humanity's population to 1,000 to 10,000.

-

After that, humanity's population expanded to as much a 4 - 6 million people (Genetic study pushes back timeline for first significant human population expansion -- ScienceDaily), with agriculture expending it even further, to 40 - 60 million by 4000 BCE.

But even then, there were some genetic bottlenecks.
| Proceedings of the Royal Society of London B: Biological Sciences
Quote:
There is a strong consensus that modern humans originated in Africa and moved out to colonize the world approximately 50 000 years ago. During the process of expansion, variability was lost, creating a linear gradient of decreasing diversity with increasing distance from Africa. ...

We find evidence of two primary events, one ‘out of Africa’ and one placed around the Bering Strait, where an ancient land bridge allowed passage into the Americas. These findings agree well with the regions of the world where the largest founder events might have been expected, but contrast with the apparently smooth gradient of variability that is revealed when current heterozygosity is plotted against distance from Africa.
So the colonists of the rest of the world had a genetic bottleneck, as did the colonists of the Americas. For the latter, I recall a population figure from somewhere of as low as 75
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Old 01 Jan 2018, 07:09 PM   #682508 / #2
Loren Pechtel
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It's not a binary state. Also, it depends on their reproductive rate.

In animal breeding inbreeding tiny populations is actually used as a technique to improve the line. Have lots of offspring, keep only the best for the next generation. You can improve the line still further by doing this for some generations and then crossing the result with other such lines.

Obviously, Adam and Eve won't have any other lines to cross with, you get very narrow genetic diversity which means any weakness could be species-wide.

Lets assume Eve has a recessive, lethal mutation on chromosome 1--50% of the population have the mutation. They have 4 kids. Statistically, two of them get the mutation but none die of it. For the sake of keeping the argument simple I'm assuming an equal sex ratio and I'm assigning who gets the mutation. We have M, M, F, F.

Next generation each possible pairing has four kids.

MF => MMFF
MF => MMFF
MF => MMFF
MF => MMF and a F that dies. They try again, F

Note that at this generation only 37.5% of the population carries the deadly mutation. While Eve cries over her dead granddaughter the population has actually been improved.

Of course, if you can identify the genes you can ensure the MF pairing never happens but very often we don't know the genes.

Where this becomes really problematic is scum-of-the-Earth breeders who breed for appearance and ignore health. They cull the genes they don't like but often end up with a very high level of serious but not immediately lethal defects.
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