A Mendelian population is, in Dobzhansky's definition, "a reproductive community of sexual and cross-fertilizing individuals which share in a common gene pool". It is the group within which people find their marriage partners, also called a population isolate. Its boundaries may be geographical, such as an island or mountain valley, or cultural, such as an endogamous caste or tribe. The breeding population is the part of it, the actual parents, that passes genes to the next generation.
This note explains the Mendelian population, the gene pool, isolates and the breeding population, with examples, for the UPSC Anthropology Optional. It falls under Paper I, topic 9.3 (Mendelian population, Hardy-Weinberg law) of the Anthropology Optional syllabus.
Population: the general and the genetic sense
| Basis | Demographic population | Mendelian (genetic) population |
|---|---|---|
| Definition | The number of people in an area at a given time | A group of interbreeding individuals sharing a common gene pool |
| Boundary | Territory or administrative unit | The limit of mating |
| What holds it together | Residence | Reproduction: the web of genetic relationships |
| Described by | Size, density, age and sex structure | Allele and genotype frequencies, mating pattern, gene flow |
| Example | The population of a district | An endogamous tribe within that district |
The two need not coincide. A single village may contain several Mendelian populations, each an endogamous group, and one Mendelian population may be spread over many villages.
Defining the Mendelian population
A population isolate is the group of persons within which individuals choose their partners. Such an isolate is a Mendelian population. For evolutionary genetics its importance lies not in where it lives but in the genetic relationships within it and between it and others: allele frequencies, consanguinity, mating patterns, gene flow and natural selection.
Characteristics
- Sexual reproduction. The concept applies only to organisms that reproduce sexually and cross-fertilise.
- Interbreeding. Members mate with one another more often than with outsiders.
- A common gene pool. All the genes carried by the members form a single pool from which the next generation is drawn.
- Continuity in time and space. It persists over generations, although its individual members die.
- It is the unit of evolution. Individuals do not evolve; the gene frequencies of populations do.
- Its boundaries are relative. No human population is completely closed.
Related terms
| Term | Meaning |
|---|---|
| Gene pool | The sum of all alleles at all loci in the members of a Mendelian population |
| Deme | The smallest local Mendelian population, within which mating is nearly random |
| Panmixia | Random mating: every individual has an equal chance of mating with any other of the opposite sex |
| Isolate | A population separated from others by barriers to mating |
| Breeding population | The individuals who actually reproduce |
| Effective population size | The size of an ideal population that would show the same amount of drift or inbreeding as the real one |
A hierarchy of Mendelian populations
Dobzhansky pointed out that Mendelian populations are nested within one another.
- The deme or local breeding group: a village cluster or band.
- The endogamous group: a tribe, sub-caste or religious community.
- The regional population: groups linked by occasional intermarriage.
- The species: the most inclusive Mendelian population. All living humans share one gene pool in the sense that any two groups can interbreed.
Isolates: geographical and cultural
Ideally a population isolate inhabits an island, a mountain valley, a peninsula, a forest tract, or a wide area of several villages within which marriage is confined. Although all human gene pools are open to some degree, panmixia does not take place across the whole species. Today the more important mechanisms that keep human populations genetically separate are cultural, not geographical.
| Type of isolate | Barrier | Examples |
|---|---|---|
| Geographical | Sea, mountains, desert, forest, distance | Tristan da Cunha in the South Atlantic; Pingelap atoll in the Pacific; Himalayan valley communities; the Jarawa, Onge and Sentinelese of the Andaman Islands |
| Religious | Rules of marriage within the faith | Old Order Amish and Hutterites of North America; the Samaritans; the Parsis of India |
| Social | Caste and sub-caste endogamy | The thousands of endogamous jatis of India |
| Ethnic and linguistic | Tribe endogamy; language | The Toda of the Nilgiris; most Indian tribes; the Basques of Europe |
A cultural barrier can be as effective as an ocean. Two castes may live in the same street for centuries and exchange almost no genes.
The Mendelian population and equilibrium
If a Mendelian population is not changed by natural selection, mutation or migration, if it is large, and if its members choose partners at random with respect to the trait in question, it is said to be in equilibrium: its genotype frequencies stay the same from one generation to the next. These are the assumptions of the Hardy-Weinberg law, and they are fundamental to all population analysis.
| Assumption | What happens when it fails |
|---|---|
| No mutation | New alleles enter the pool |
| No selection | Some genotypes leave more offspring; frequencies shift |
| No migration | Gene flow brings in or removes alleles |
| Large population | Genetic drift causes random change |
| Random mating | Inbreeding or assortative mating changes genotype frequencies |
No real population satisfies all five. The model is useful because departures from it reveal which force is at work.
The breeding population
The problem of definition
Defining a human Mendelian population accurately is difficult for two reasons.
- Biological openness. Every human group receives some genes from outside.
- Social groupings. People cluster in villages, castes, classes and nations, which may or may not be the units within which they actually breed.
The first task of the population geneticist is therefore to identify and describe the biological population as accurately as possible, before analysing its gene pool and the forces acting on it.
From phenotypes to the gene pool
A gene pool cannot be examined directly. All conclusions about its composition are inferences, made from the phenotypes, and today the genotypes, of individuals. To infer the composition of the gene pool at one point in time, the geneticist must first count and describe the actual progenitors, the parents. These progenitors constitute the breeding population.
Why the breeding population is smaller than the total
- Children and the old do not reproduce.
- Some adults never marry or have no children.
- Unequal numbers of breeding men and women reduce it further.
- Families differ widely in the number of children they leave.
- Population size fluctuates; a past bottleneck has lasting effect.
Sewall Wright introduced the effective population size to allow for these. Where breeding males and females are unequal in number, it is given by 4NmNf / (Nm + Nf). In human groups the effective size is commonly about a third of the census size or less. It is this figure, not the head count, that decides how strongly drift and inbreeding act.
How anthropologists identify a breeding population
| Measure | What it shows |
|---|---|
| Rate of endogamy | The proportion of marriages contracted within the group |
| Marital distance | How far apart the birthplaces of spouses are; the geographical extent of the mating circle |
| Pedigrees and genealogies | Actual descent and the frequency of consanguineous marriage |
| Admixture rate | The proportion of genes entering from outside in each generation |
| Surname analysis (isonymy) | An indirect estimate of inbreeding where surnames are inherited |
| Genetic markers | Allele frequencies and genetic distance from neighbouring groups |
This is the point at which social anthropology becomes necessary to genetics. Rules of endogamy and exogamy, preferred marriages and marriage distance, described by ethnographers, define the breeding population.
Genetic consequences of isolation
- Genetic drift. In a small breeding population allele frequencies change by chance, and alleles may be lost or fixed.
- Founder effect. A population started by a few people carries only a sample of the parent gene pool. The Old Order Amish of Pennsylvania have a high frequency of Ellis-van Creveld syndrome traced to their founders. On Pingelap a form of colour blindness became common after a typhoon reduced the population to a handful of survivors.
- Inbreeding. In a small isolate, spouses are often related even without a preference for kin marriage, which raises homozygosity and the frequency of recessive disorders.
- Divergence. Isolates come to differ from their neighbours in allele frequencies.
- Usefulness for research. Isolates, with their known genealogies and uniform environment, have been valuable for mapping disease genes.
The breaking of isolates
Gunnar Dahlberg drew attention to the "breaking of isolates" in the modern world. Roads, schooling, migration and urban life widen the circle of marriage. The effects are a rise in heterozygosity, a fall in the frequency of recessive disorders, and a reduction of genetic differences between groups.
Mendelian populations in India
- A mosaic of isolates. With several thousand endogamous castes, tribes and religious communities, India is one of the most finely subdivided populations in the world. Each endogamous group is a Mendelian population in its own right.
- Small tribal isolates. The Toda of the Nilgiris and the Onge and Jarawa of the Andamans have very small breeding populations, in which drift is strong.
- Founder effects. Many Indian communities carry particular recessive disorders at raised frequency, the mark of long endogamy from a limited number of founders.
- Marriage rules matter. Clan and gotra exogamy lowers inbreeding within the endogamous group; cross-cousin and uncle-niece marriage in the south raise it.
- Change. Urbanisation and marriages across community lines are slowly breaking isolates.
For the communities named, see tribal India today.
Using this topic in a UPSC answer
- Begin with Dobzhansky's definition and underline "common gene pool".
- Distinguish the Mendelian population from the demographic population.
- Explain isolates, stressing that in humans the barriers are mostly cultural.
- Define the breeding population and effective population size, with Wright's formula.
- List the consequences of isolation: drift, founder effect, inbreeding.
- Use India: the endogamous caste or tribe as a Mendelian population.
For presentation, see our guide to anthropology answer writing with diagrams, thinkers and case studies.
Frequently asked questions
What is a Mendelian population?
A Mendelian population is a reproductive community of sexually reproducing, interbreeding individuals who share a common gene pool. The definition is Dobzhansky's.
What is a gene pool?
A gene pool is the total of all the alleles at all loci carried by the members of a Mendelian population. The next generation is drawn from it.
What is a population isolate?
A population isolate is a group within which people find their marriage partners, separated from other groups by geographical or cultural barriers to mating.
What is a breeding population?
The breeding population consists of the individuals who actually reproduce, the progenitors or parents. It is always smaller than the total population.
What is effective population size?
Effective population size, a concept of Sewall Wright, is the size of an ideal population that would experience the same degree of genetic drift or inbreeding as the actual population. It is usually well below the census size.
How is a Mendelian population different from a demographic population?
A demographic population is defined by the number of people living in an area. A Mendelian population is defined by interbreeding and a shared gene pool, and its boundary is the limit of mating.
Why are Indian castes and tribes called Mendelian populations?
Because they are endogamous. Their members marry within the group, so each caste or tribe has its own gene pool, largely separate from those of its neighbours.
What is meant by the breaking of isolates?
It is the widening of marriage circles through migration, communication and social change, which increases gene flow, raises heterozygosity and reduces the frequency of recessive disorders.