Impact of demography on population genetics
To understand the processes occurring in populations, it is necessary to consider basic demographic indicators. These traditionally include the total population size, birth and death rates, as well as the sex and age composition of the group.
It is these parameters that shape the marriage structure within the community, which ultimately has a direct impact on the gene pool. At the same time, the most critical and determining factor for genetic stability or variability is the size of the population group. Depending on the human population size and how closed the community is (the level of isolation), two main categories of small populations are distinguished: demes and isolates.
Characteristics of demes
A deme is a small population characterized by a certain degree of isolation while still maintaining minimal contact with the outside world.
Main criteria of a deme:
- Population size: ranges from 1,500 to 4,000 individuals.
- Natural growth rate: remains low at about 20% per generation.
- Marriage structure: the vast majority of unions are formed within the group—the frequency of such intra-group marriages reaches 80–90%.
- Migration: the influx of genetic material (individuals from outside) is extremely limited and amounts to only 1–2%.
Isolates: extreme degree of separation
An isolate is an even smaller and much more closed group of people. Strict population size limits and almost complete lack of migration lead to pronounced genetic consequences.
Main criteria of an isolate:
- Population size: does not exceed 1,500 individuals.
- Natural growth rate: also low, no more than 25% per generation.
- Marriage structure: the frequency of marriages within one's own group exceeds 90%.
- Migration: the influx of new individuals from outside is less than 1%.
Genetic consequence of an isolate's existence: Due to the extremely high frequency of intra-group marriages, genetic diversity drops rapidly. After just 4 generations (equivalent to about 100 years), absolutely all members of such an isolate will have common ancestors and will be related as at least third cousins (sibbs).
Types of marriages and assortative mating
How pairs are formed (mate selectivity) depends directly on demographic conditions and can radically alter the gene pool. There are two fundamentally different approaches to forming mating pairs:
- Non-selective marriages (Panmixia): A situation in which absolutely equiprobable, random mating occurs between any individuals in the population. No external or internal traits influence the choice.
- Assortative (selective) marriages: Purposeful selection of a partner based on the similarity of certain characteristics.
Selective mating can be based on a wide variety of criteria: intelligence level, religious beliefs, property status, and so on. Partner selection based on a similar phenotype is of particular importance in biology.
The most important rule of assortative mating is that it affects the number of genotypes in the population, but does not change the total number of alleles at all.
If assortative mating is positive (partners choose each other based on similar traits), this can significantly reduce overall variability within the group. A striking example of this phenomenon is associations of people with various pathologies. For example, individuals with hearing, vision, or speech impairments, or patients with achondroplasia, often form isolated social groups to facilitate interaction. In such communities (including specialized athletic competitions for people with achondroplasia), the probability of marriages between people with a similar phenotype increases manifold.
Briefly: answers to topical questions
What are some historical or geographical examples of classic human isolates?
Historical and geographical examples of classic human isolates are small populations of Eskimos.
- Polar Eskimos—a group characterized by a very high frequency of the I^0 allele (84%).
- Eskimo tribe (Thule region, Greenland)—a population of 271 individuals that existed under conditions of prolonged complete isolation; the frequency of the I^A allele in it is 9%.
In such isolated groups, sharp fluctuations in allele frequencies occur, up to their complete loss as a result of random events and genetic drift.
To easily remember the boundary between the terms, use the "1500 Rule": everything strictly UNDER 1500 people is a strict isolate. From 1500 to 4000 is a deme, where isolation is slightly weaker.
Class and exam questions
Which parameters are considered demographic in the context of population genetics?
These include population size, birth and death rates, as well as age and sex composition. These parameters determine marriage structure and influence the gene pool.
What happens to family ties in an isolate after 100 years?
Due to the high frequency of intra-group marriages (over 90%), after four generations all members of the isolate become relatives—at least third cousins (sibbs).
What is the main genetic difference between assortative marriages and panmixia?
In panmixia, mating is equiprobable, whereas in assortative mating, there is selective matching by phenotype. This changes the ratio of genotypes in the population, but does not affect the total number of alleles.
What else to study on this topic
- Mechanisms of demographic indicators' influence on gene pool structure
- Detailed analysis of panmixia and conditions for ideal equiprobable mating
- Positive and negative assortative mating in partner selection
- Social factors of selective mating (religion, property)
- Reduction of phenotypic variability in groups with hereditary pathologies
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