From a reader:
Hello Dr Hawks I am a reader of your blog and respect your expertese so I thought you would be the right person to ask this question to. I was debating a creationist about human genetic history the creationist is a literal believer in Noah's ark andi was saying to the creationst that one of the reasons we know the story of the global flood is nor true is because if it were all species including humans would have a bottleneck of two individuals dating to the exact same time. The creationist then cited this article as proof that humans could have been bottlnecked to 2 or six individuals"However, the global extent of [beta]-globin divergence has at first sight some startling demographic implications because the hunter-gatherers who migrated from Africa. Europe and Asia have rather similar haplotype frequencies. Hence, the emigrants must have undergone the major change in haplotype frequency in the interval between leaving Africa and dispersing throughout the rest of the world. Assuming--and this is little more than an informed guess--that this interval was 20,000 years, population-genetics theory tells us that the mean effective size of the ancestral population for all non-Africans throughout this period must have been 600 individuals; or alternatively ;that ;the bottleneck was 6 individuals for 200 years, or even a signle couple for 60 years. (The expected time for the loss of a neutral gene present in thepopulation at frequency p is E(T) = -4N plnp/1-p, where N is the population size. We assume a generation interval of 20 years and that the 4 common haplotypes were present at equal frequencies in the ancestral African population.) If this is the case, much of mankind was an endangered species during an imporant part of its evolution." ~ J.S. Jones and S. Rouhani, "How Small was the Bottleneck?" Nature, 319, Feb. 6, 1986, p. 450What is this article actually saying? Is it saying that it really is possible for every human alive today to have sprung from only 2 or 6 people? Because that contradicts everything Ive read that says genetics shows that our population could neevr have been bottlnecked below at least a few thousand individuals. Can you explain it to me. kind regards
A single gene can never provide evidence showing such a bottleneck, it requires every gene in the genome to show a consistent pattern. In this case, the most obvious genes to examine are those with the most variation. For example, the human HLA genes have hundreds of allelic variants in human populations that have existed for thousands of years. Each of these genes (including HLAA, HLAB, HLA*C, DRB1, DRB2, DQB) has old variations, the oldest alleles have been retained from our common ancestors with chimpanzees and gorillas. These could never have been retained for so long if we had undergone a bottleneck to two or a few individuals.
It is true that human genetic variation is low relative to some other mammals, but it is not indicative of a bottleneck to a handful of individuals. When geneticists today refer to bottlenecks, they are estimating many hundreds of individuals at the least, and 10,000 individuals as a more likely value.