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Message 1 of 5 (857797)
07-11-2019 1:07 PM
Faith often makes the argument that when species divide into daughter populations some amount of genetic diversity - alleles - is always lost.
This argument is often countered with mutations adding new alleles or modifying old ones to increase genetic diversity.
Let's pursue this argument in greater detail.
First off, let's introduce the scientific term zygosity
Zygosity (the noun, zygote, is from the Greek zygotos "yoked," from zygon "yoke") (/zaÉªËˆÉ¡É’sÉªti/) is the degree of similarity of the alleles for a trait in an organism.
Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same loci on each of their two sets of homologous chromosomes except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they are different, the organism is heterozygous at that locus. If one allele is missing, it is hemizygous, and, if both alleles are missing, it is nullizygous.
In population genetics, the concept of heterozygosity is commonly extended to refer to the population as a whole, i.e., the fraction of individuals in a population that are heterozygous for a particular locus. It can also refer to the fraction of loci within an individual that are heterozygous.
Typically, the observed (Ho) and expected (He) heterozygosities are compared, defined as follows for diploid individuals in a population:
where is the number of individuals in the population, and are the alleles of individual at the target locus.
where m is the number of alleles at the target locus, and is the allele frequency of the allele at the target locus.
Just to be clear, the value for the observed heterozygosity function is 0 when the individual in question is homozygous, and ai1 = ai2. The function just counts up the number of individuals that are heterozygous at each locus i.
What we would expect from a sub-population separating off from a parent population is that it would have the same zygosity as the parent population ... or less zygosity (depending on whether or not the sub-population does or does not not have all the alleles from the parent population).
We would also expect evolution to continue in each population, likely increasing the zygosity of each population over time. Thus after a series of splits of population we could see a general trend of less zygosity in populations furthest from the original parent population.
What we see from Humans spreading around the world is just such a pattern:
heterozygosity of Humans
|Heterozygosity values of 51 worldwide human populations. Sub-Saharan Africans|
have the highest values in the world.
|By David LÃ³pez HerrÃ¡ez , Marc Bauchet , Kun Tang , Christoph Theunert,|
Irina Pugach, Jing Li, Madhusudan R. Nandineni, Arnd Gross, Markus Scholz,
Mark Stoneking - LÃ³pez HerrÃ¡ez D, Bauchet M, Tang K, Theunert C, Pugach I,
Li J, et al. (2009) Genetic Variation and Recent Positive Selection in Worldwide
Human Populations: Evidence from Nearly 1 Million SNPs. PLoS ONE 4(11): e7888.
d=10.1371/journal.pone.0007888, CC BY 2.5, https://commons.wikimedia.org/
Here we see Humans (Homo sapiens) first appearing in Africa, then emigrating to central/south Asia, Europe, and the Middle East, then to East Asia and the Americas and Oceania.
The heterozygous data does not tell us whether the heterozygous alleles are due to new mutations or pre-existing alleles in the parent population/s, or whether or not all the populations have increased in heterozygosity over time or stayed the same since separation.
Nor do the differences in heterozygosity say anything regarding incompatibilities in interbreeding. We do know, however, from history that all humans can (and frequently do) interbreed without any apparent loss in viability of the offspring: there is no sign of any impending speciation in humans.
Edited by RAZD, : deleted duplicated sentence
Edited by RAZD, : No reason given.
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