Home / 9.3: Concept of Polymorphism & Selection, Mendelian Population

9.3: Concept of Polymorphism & Selection, Mendelian Population

Different individuals of a populations & different populations in themselves, differ from one another on different counts. All these differences can be broadly categorized in three groups

  • Purely Environmental (Non genetic) – Such as person’s language,
  • Purely Genetic – Such as blood type of a person
  • Both Environment & Heredity – Intelligence

Population Genetics

Introduction – PG Is the study of gene & genotype frequencies in population of interbreeding organisms (small or large, natural or artificial) and predicting the way these frequencies are maintained or changed under the combined influence of various factors.

  • it is the specialization in understanding the behavior of genes & application of Mendelian principles to the population.

Why to study Population Genetics – Population Genetics is primarily interested to know how genes behave in a population since the population is the unit of evolution.

Features

  • Its primacy founders were Sewell wright, JBS Haldane & Ronald Fisher.
  • Its study describe the genetic structure of population & study the change in gene frequencies brought about by various forces which causes genetic diversity.
  • It is a logical extension of Mendelian genetics in which the focus is shifted away from the individual towards the population of which the individual is a member.
  • It is concerned with applying models of gene frequency change involving different factors in the context of Mendelian genetics to examinee evolution in quantitive manner 
  • In order to understand the pattern of allele frequencies we need to have a defined population, in this case a Mendelian Population

Mendelian Population

Introduction – Acc. to Mather & Gregg. Population signify a “spatial-temporal group of conspecific (i.e belonging to the same species) interbreeding individuals” However individuals may tend to mate with each other on the basis of caste & other factors. So the element of inbreeding is added to the genetic picture.

Definition – Mendelian Population may be defined as – A group / aggregate/ community of similar individual, living within a circumcised area, at a given time & capable of interbreeding. 

  • Population geneticists restrict the definition to a population of genes rather than to that of individuals & call it a Gene Pool.
  • Dobzhansky (1951) defined it as the “reproductive community of sexual & cross fertilizing individuals which share common gene pool.”
  • Sewell Right – Population whose individuals follow mendelian principle of inheritance.

Characterised by having individual who have similar genetic constitution or gene composition.

Nature of Mendelian Population

  • Mating at random, equal chance of mating (Panmixis)
  • Has continuity throughout time – kinds of genotypes & their frequency in one generation depends on the kind of genotypes & frequency in previous generation.
  • Has its own gene pool
    • Passage of gene pool through generation
    • Factors dividing gene pool
      • Geographic Barriers
      • Linuistic Barriers
      • Social Barriers
      • Economic Barriers
      • Educational Barriers
    • Smallest subdivision of gene pool ; Isolate or Panmictic unit – relatively limited no. of individuals who are potential mates
    • Biologically meaningful
      • Biological species : most inclusive Mendelian Population
      • Gene pool of mankind in entirety.
  • All aggregates of interbreeding population constitutes Mendelian Population.
  • Mendelian Population is not restricted by absolute size if individuals mate at random (Panmixis)
  • Mendelian population is dynamic. It expand & contract thr
    • Birth
    • Death
    • Contact with other population → Migrate
    • Selection → individual within the population will reproduce more than other, contributing a disproportionate quantity of the alleles to next generation. 

This dynamic nature, over time lead to change in the population gene pool.

In the study of Mendelian population we are concerned with 

  • The properties of the gene pool
  • The ways of Changing the composition of the gene pool

Conclusion – Thus mendelian population is an important tool to study laws of inheritance & process of evolution in mankind.

The largest natural population is the species, the member of which are potentially capable of successful reproduction among themselves but not with members of other species.

Gene Pool – A gene pool is the total collection of all the allelic forms that are carried by all the gametes of a population. A gene pool has been variously termed as

  • Panmixia by Sewell Wright,
  • Mendelian Population by Dobzhansky
  • Gamedeme by Glimour & Gregor
  • Deme –  term that is most common today

Local Mendelian Population – Species can be broken down into smaller reproductive populations or smaller gene pool, which are to some degree, & often temporarily isolated from one another. In humans this could be on basis of Race, Geography, Language, Religion, Socio-economic conditions, education etc. 

These smaller gene pool is called by different names Panmitic Unit / Local Mendelian Population/ Gamodene / Deme

Deme – A deme represents a local population of interbreeding or potentially interbreeding individuals i.e., any gamete in this population has an equal chance to unite with any gamete of the opposite sex in the same population and result in a viable zygote

  • Successful reproduction in a population however are prevented by a number of factors
  • An ideal deme is, of course, a panmictic one.

Just like in the case of an individual, we can also calculate the phenotype & genotype of a population through statistical methods (Genotype of population is genepool.) ex – Avg stature of a Population can be calculated & the variation from that average can be noted.

Successful reproduction of populations requires

  • sexual behavior culminating in copulation, fertilization, normal development of the fetus, & offspring  that are normal & healthy, capable of reproducing in turn.
  • rate of reproduction sufficient to sustain the population.
    • Successful reproductive rate is one that maintains a balance between population size & the potential & limitations of the environment

Gene Frequency – It refers to the proportion of an allele in the gene pool as compared with other alleles at the same locus.

Organic Evolution & Gene Frequency – Evolution is gradual change. Biological evolution can be defined as change in the gene frequency of a population.

Evolving Population : A population is said to be evolving if the frequency of its alleles are changing. It is not the situation is known as Genetic equilibrium. (Hardy & Weinberg Equilibrium)

Genotypic Frequency – It is the proportion of the genotypes in the population.

Genetic Property of Mendelian Population is influenced by

  1. Size of Population
  2. Difference in fertility among parents & viability among offspring generation.
  3. Migration
  4. Mutation
  5. Selection
  6. Mating system

Genetic Polymorphism

Introduction – As per Ford, Polymorphism is defined as “The Occurrence together in the same habitat of two or  more discontinuous morphs (or gene loci or supergene) of a same species in such as way / proportions that the rarest of them can’t be maintained by recurrent mutation”.

Eg. a population with individuals who have, & who don’t have the ability to taste phenylthiocarbamide (PTC) is a polymorphic population.

Genetic Polymorphism is the variation in genetic material & is the occurrence of more than one gene for a particular trait

Genetic Polymorphism vs Phenotypic Polymorphisms

  • The genetic variations far outnumber phenotypic variations.
  • Phenotypic variations are only the proverbial tip of the iceberg of the genetic variation.
  • Many genetic variations are concealed at the DNA, Chromosomal & cellular levels & are not expressed as phenotypic variations which amenable to direct observation.

Polymorphic traits

  • are discreet traits & discontinuous genetic characters that are controlled by single genes or closely liked genes (super genes) acting together as unit and are less influenced by environment.
  • Criteria to call GP A trait is called a polymorphic trait only when the frequency of the gene is more than 1% in the population b/c only then we are sure that some selection is involved in its maintenance in the population & it is not b/c of recurrent mutations.
    • if present in below this frequency then it is supposed that it is arising due to mutation & selection is not operation on it.
  • Ex – Ability to taste PTC & ability to roll the tongue longitudinally
  • The characters that are present in continuous, graded series of phenotypic trait such as skin color, height in man are not polymorphic traits
    • Graded & continuous variations are controlled by many genes( Polygenes) & are strongly modified by environment

Distinguish  b/w Polymorphic & Monomorphic Traits

Polymorphic TraitsMonomorphic Traits
DiscreteGraded
Discontinuouscontinuous
controlled by single genecontrolled by many genes
Mendelian inheritance (Unifactrorial inheritance)Doesn’t follow mendelian inheritance
less modified by environmentstrongly modified by environment
e.g Abolity to roll the tongue longitudinale.g Heigh, weigh etc

Polymorphic vs Monomorphic Population

  • Populations composed of individuals with discrete discontinuous genetic traits are said to be polymorphic
    • populations including some taster & some nontaster.
  • Where as those lacking such individuals are monomorphic.
    • populations with only tasters or with only non tasters etc. are  naturally monomorphic

Source of Genetic Polymorphism – is in fact the manifestation of evolutionary process. – like Mutation & Migration

  • Mutation supplies the genetic material ; Not entirely depend on mutation, though it is the ultimate source.
    • small rate of mutation can’t maintain the observed genetic polymorphism in a population.
  • On this selection operates & continues in the population.
  • Other sources like migration also introduces new genes or leads to genetic recombination

Evolution is never dependent solely upon mutation. Thus, genetic polymorphism can be brought about by activity of many factors.

Levels of Genetic Polymorphism – Genetic Polymorphism exists at multiple levels of the heredity material of the population., These levels based on site of occurrence include

  • cells  – polymorphism at antigenic molecules – ABO antigens, Rh typing, MN System, HLA, Histocompatibility
  • DNA & gene  – str. difference in DNA – due to change in bases sequences ; types
    • All inherited differences in relation to colour, height etc represent the changes in the structure of gene.
    • Such polymorphism is called RFLP – Restriction Fragment length Polymorphism. Where VNTR are seen 
    • At gene level, for example , Sickle cell Anemia (HbS), Interleukins (IL-1, IL-6, IL-10) etc
  • chromosomes (X and Y) – due to numerical & structural abnormalities in chromosome
  • Proteins (Hba & Hbs) etc & Enzymes – str. differences in the molecules of proteins.

The Particular type of variant is maintained by selection in that population. 

Types

  • Single Nucleotide Polymorphism(SNP) – change in a single base pair in the genomic DNA disease – like sickle cell anaemia & cystic fibrosis
  • Variable Number of Tandem Repeat (VNTR) – it is a location in a genome where a short nucleotide sequence is organised as a tandem repeat. These can be found on many chromosomes, and often show variation in length (no of repeats) among individuals.
  • Restriction Fragment Length Polymorphism (RFLP) – When DNA of different individual is cut with restriction endonuclease, different restriction fragment lengths of DNA is found in different individuals, meaning that sequence of bases is different in different individual.
  • InDel – Insertion or deletion (InDel) of bases in the genome of an organism are a major source of genetic variation within species.

Advantage of Polymorphism

  1. Provides population with alternative set of traits of a character. In stable environment it may be of little significance, but in certain conditions of environmental flux, population with different polymorphic traits can explore environment for an adaptive success.
  2. The different polymorphic traits are thus, not only suitable for the present environment but also potential solution for the future environment.
  3. DNA polymorphism has only beginning to show : only future can assess the potentialities of these vast polymorphisms.

Significance of Genetic Polymorphism Study – Many variations are present at the genetic level which are not seen as phenotypic expression. The polymorphic or discontinuous traits are controlled by single genes or closely linked genes (super genes) acting together as a unit. These are less modified by environment. Hence by examining the discrete, discontinuous variations we can study teh genetic polymorphism existing in the population without examining their DNA or chromosome. Ex

  • 13-14 Human blood cell system ABO, Rh, MN
  • 160+ Red cell antigens
  • 30+ serum protein – haptoglobins, transferrins
  • Haemoglobin molecule 

Genetic Polymorphism & Selection

Introduction – The role of natural selection & its relation to Genetic polymorphism is unclear. Debate exist on whether all the polymorphism exhibited by man has any selective value in evolution of this species – like ABO ?

Theory of Genetic Load

  • Population geneticists had earlier concluded that the total amount of genetic variation maintained in natural populations could not be large.
  • B/c two much variation in a population would lead to serious departure from overall fitness b/c variants would have differing gene frequencies from the gene frequencies producing overall fitness, the optimal average population fitness.
  • Thus maintenance of too much of genetic variation would mean increase in genetic load, affecting overall avg. population fitness & population would be driven to extinction.
  • But Two things are now recognized
    • Genetic load is not detrimental to population but essential for thinning of population
    • fitness has a threshold effect, capable of absorbing a great amount of genetic variation.

Neutral Theory of Kimura

  • Acc. to Kimura, the vast genetic polymorphisms at the gene & protein level involve only alteration of str. being equivalent at functional level.
  • Such variants, being functionally equivalent, would respond similarly to selective forces.
  • So majority of polymorphic traits have no role to play vis a vis natural selection & such phenomenon is termed as “Selectively Neutral” polymorphisms.

Contrarily, for some, polymorphisms have selective basis

  • might be useful in some way for different shades of environment
  • also useful for inbreeding & prevents a gradual build up of homogeneity in population which is considered to be deleterious.
  • Acc. to these, genetic polymorphism is very advantageous to the population

Genetic Polymorphism & Natural Selection

Introduction – The role of natural selection & its relation to Genetic polymorphism is unclear. Debate exist on whether any relation exists at all b/w polymorphism & evolution. The various roles played by GP  

Monomorphism/ Permanent Polymorphism  & Stabilising / Normalising Selection

  • Some polymorphic traits are very important for the survival of the individual itself – like for example, sex, which is permanently polymorphic trait
  • Stable polymorphic features are maintained in the population by stabilizing selection & it removes individual from population which deviate from the population mean.
  • It operates in constant or unchanging environment for a long period of time so operates rarely b/c the env. is rarely constant.
  • Monomorphic features are maintained in a population by stable selection.
  • It favors average or normal individuals & eliminates over specialized as well as less specialized or less adapted individuals.
  • It checks accumulation of mutations that may lower the fitness of species in unchanging environment. Thereby, it tends to arrest variance & evolutionary  changes.
  • It introduces homogeneity in population
  • In stabilizing selection the frequency of the alleles of lower fitness decreases until they vanish
  • Examples
    • Thus individuals not perfectly male or female are removed from the population so that population shows only two discrete individuals with reference to sex -male & female
    • Optimum Birth weight in human is 7.3 pounds & newborns less than 5.5 & more than 10 pounds have highest morality rate

Transient Polymorphism & Directional Selection

  • it may be in a transitory phase in the sense that certain populations with sifting environment demand, shift their population mean for some character in the direction of environmental change.
  • Experimentally shown by Kettlewell in case of Industrial melanism in a moth.
  • It is a situation when there are two alleles in a gene pool & one allele is gradually replacing another under changing environmental conditions.
  • It operates when environment is changing in particular direction.
  • If favors accumulation of those mutations that increase fitness of population in changing environment
  • Polymorphism is in transiting phase when a certain population, with shifting environmental demands, shifts their population mean in the direction of environmental change for some characters
  • it favors non average i.e specialized phenotypes & eliminates normal or average individuals.
  • If brings about progressive evolution.
  • In directional selection, the allele frequency for a trait continuously shifts in one direction, thus favoring a particular phenotype
  • Examples –
    • Mosquito resistant to DDT
    • Resistance of Bacteria to certain drugs
    • Biston betularia / Melanic moth  – Directional selection was shown by Kettlewell in case of Industrial melanism in a moth betularaia. When env got polluted, mutation for blank coloration spread in the population of white moth that protected if from being sighted by the predators. At a time when one form has not replaced the other, a polymorphic population may exist. But one from has not replaced the other, a polymorphic population may exist. But gradually over time a transient polymorphism is replaced by monomorphism

Balanced Polymorphism or Superiority of Heterozygotes & Balanced Selection

  • Balancing selection refers to the process by which multiple alleles are actively maintained in a population at a frequency above that of gene mutation & Polymorphisms maintained through this process are called Balanced Genetic Polymorphisms
  • happens when the heterozygotes have higher adaptive significance that the homozygote – a conditions c/a heterozygote advantage or heterotic balanced selection
  • Balancing selection is similar to disruptive selection where individuals of extreme trait values are favored against those with average trait values
  • Example
    • Sickle Cell Trait (HbA Hbs) – A balance between selection against homozygous sufferers & selection against homozygous normal. The gene for sickle cell anaemic has continued to exist in many African populations b/c the condition of sickle cell trait provides resistance against malaria(as dominant HbA HbAgets malaria). The advantage is responsible for maintaining a balanced polymorphism at sickle cell locus.
    • Thalassanemia
      • Due to defect in beta-chain hemohlobin 
      • Homozygous – Thalassemia major where both beta chain produced defectively & got disease. Where as Thalassemia minor are as carries were resistant to malaria in African tribes
      • Thus, population with different polymorphism traits can explore for an adaptive success. 

Conclusion – Polymorphism thus provides with alternative set of a traits of a character which is important for adaptive success in changing environment. Hence GP is a potential solution for future environment. GP is a predisposition of the population to adaptation.

Hardy Weinberg Equilibrium

Introduction – Population Genetics is primarily interested to know how genes behave in a population since the population is the unit of evolution.  So, to study the dynamics of the alleles in changing population, a reference model was provided by Godfrey Hardy, a mathematician & Wilhelm Weinberg, an experimental biologist (1908). The model represents a static genetic population, the one in which genes undergo no change & genetic frequencies are in a state of equilibrium called Hardy Weinberg Equilibrium

Hardy Weinberg lawstates that allele & genotype frequencies in a panmictic, randomly mating, population will remain constant from generation to generation in the absence of other evolutionary influences.  But if this equilibrium is disturbed in some manner, the genotype frequencies reach a new equilibrium value in one or more generations, depending on the nature of the disrupting influence. They gave an equation, in order to explain the situation

p2+2pq+q2=1

Where, p = [A] i.e., the frequency of dominant allele

q = [a] i.e., the frequency of recessive allele

‘A’ and ‘a’ are the allelic forms of one gene

  • This equation represents a hypothetical situation of no change when certain condition are met
  • This static model of H/W is in fact the allelic occurrence seen in the classic Mendelian inheritance of phenotypic ratio of 3:1

Organic Evolution & Gene Frequency (Race Formation) -Organic Evolution is a gradual change. Biological evolution can be defined as a change in the gene frequency of population.

  • Gene frequency means proportion of an allele to another allele of gene in a population.
  • If evolution → f changing ; otherwise not ‘; situation called Genetic Equilibrium

Genetic Equilibrium : A population is said to be in genetic equilibrium it the  frequencies of its alleles remain constant.

  • Certain Conditions must be met for the population to be in Genetic Equilibrium
    • All mating should be Random Mating
    • Large population size so that change don’t take place by chance.
    • Lack of Evolutionary Forces i.e Mutation or natural selection is absent
    • All mating should be equally Fertile i.e. they must produce same no. of viable offspring. 
  • As not  population can satisfy all of them simultaneously, thus an ideal population where the allelic frequencies are static hardly exists, it provides the fundamental theorem against which the actual framework of gene frequencies can be measured

Derivation of Hardy Weinberg Law –

By considering only one gene which occur in 2 alleles forms A-a, ; each with frequency of 50% i.e 1/2 frequency of A i.e [A] = 1/2 & [a] = 1/2 & they must add up to 1

[A] + [a] = 1/2 + 1/2 = 1

if [A] =p & [a] =q then their genotype will be

(p+q)2 = p2 + 2pq + q2 = 1

This is known as Hardy Weinberg Equilibrium

Properties of Hardy -Weinberg law

  • A population practising random mating in the absence of migration mutation & selection will be under Hardy Weinberg Equilibrium.
  • The genotypic frequencies of the progeny depends only on the gene frequencies of the parents & not on the genotypic frequencies.
  • One generation of random mating will produce the population under H.M equilibrium
  • The frequency of heterozygotes in the population will not be greater than 0.5
  • When the gene frequency of an allele is low, the rare allele occurs predominantly into the heterozygotes & less in homozygotes

Significance of Genetic Equilibrium for Evolution

Laws of chance or probability operate up on gene distribution in ways tending to preserve the status quo to maintain an unchanging equilibrium as generation pass.

if the equilibrium is upset there is a tendency to establish a new equilibrium. Evidently this tendency to equilibrium forms a sort of inertia which must be overcome if evolutionary change is to occur.

This equilibrium tendency is conservative. It tends to conserve gains which have been made in the past & to prevent too rapid change.

It help to insure that a species will not gamble in undergoing evolutionary change. “Radical change, may lead to progress; it may also hustle a species down a blind valley to speedy extinction”

A further conservative function of the gene equilibrium is to maintain a good production of recessive genes in the population ; They may or may not be useful in the present day env but may assume significance in a future environment.

Example – Phenylthiocarbamide (PTC) taste in human population

  • In human population persons with gene TT find weak solution of PTC to be biter in taste whereas homozygous tt persons find this solution tasteless.
  • Moreover persons are unaware of their reaction to PTC & nobody selects their mate on the basis whether he or she can/can’t taste this substance. 
  • m’age take place at random & population is panmictic wrt to his trait.
  • Suppose, in an island or town the no. Of homozygous TT & homozygous tt (non taster) is equal, → so genotype frequencies in 1st generation will be TT = 25% ; Tt = 50% ; tt = 25%
  • Since it is an autosomal trait the population will be 75% tasters & 25% non- tasters. The same results are obtained if we consider the union of gamers at the time of fertilisation
  • Again, allele frequency of TT, Tt, tt add up to 1.

Significance of Hardy Weinberg Law

  • Hardy Weinberg model presents a hypothetical situation of no change. It is a fact that population does change if certain conditions are not met.
  • We can measure & analyze change in population or in other words this law suggests whether a population is evolving or not.
  • This model can be used to testify hypothesis about gene pool & evolutionary forces that work on them.
  • It can be used to calculate the frequency of specific allele & specific genotype such as deleterious recessive allele in population.
    • Thus with the help of this law various genetic disorder can be prevented e.g. Thalassemia : incidence has been checked in U.K. using this law.
  • This law describes how Mendelian inheritance preserves genetic variations from one generation to the next in population that are not evolving.

Application

  • To Know the genetic structure of a population e.g – We wish to know the genetic str. Of a given population wrt the ability to taste PTC (dominant genetic trait)
    • Let the f of the taster (dominant) allele be p & of the non-taster (recessive) allele be q. If we know the percentile of non tatters in the population (N), then using HQE, we can calculate as follows

q² = N

⇒ q = √Ñ

( ~ → — ; q can’t be negative) 

Since p + q = 1

p = 1 – q

⇒ p = 1 – √N

Thus, the genetic structure becomes known by Calculating the frequencies

  • To find the direction of evolutionary change in a population – This is done by two ways
    • Studying the population twice over a time period. The difference in the allele frequencies represents the direction of evolutionary change
    • Studying a population by dividing it into age groups. The regular change in allele frequencies from old to young represents the direction of evolutionary change.
  • To Find out the nature of a population –
    • suppose there are 3 alleles – A, B & O whereby A & B are dominant , & O is recessive. Let p,q, r be the respective gene frequencies of A, B, O
    • When, using Hardy Weinberg equation, the expected values of phenotype will be as follows

A’ = (p² + 2pr)n 

(n – observed population size)

B’ = (q² + 2 qr) n

O’ = (r² ) n

A’B’ = (2pq)n

The chi- squared test is then used to find the deviation of observed values of phenotype from the expected values. It is mathematically denoted as follows

𝛘² = (Observed – Expected Values) ÷  (Expected Values)

⇒ ∑ 𝛘 ² = ∑ (Observed – Expected Values)²  ÷  (Expected Values)

If ∑ 𝛘 ² is less than 3.84 (df =1, p < 0.05), then the population is a Mendelian Population

  • To test the validity of a sample drawn from a previously known Mendelian Population – if ∑ 𝛘2 is less than 3.84 , then the population is confirmed Mendelian population.

Conclusion – Thus Evolution has been made quantifiable by Hardy Weinberg law. So, evolution can be measured & can be predicted whether a population is evolving or not. Hence, the Hardly Weinberg law has tremendous impact on population genetics.

Forces of Change in Gene Frequency 

introduction – The proportion of Gene in population is known as Gene frequency. Unlike assumption in Mendelian population with  HWE conditions, the natural population changes due to changes in gene frequency.

Acc to Falconer, broadly – there are two process through which changes in gene frequency occurs.

  1. Systemic Process (Directional Changes)
  2. Dispersive Forces : (Random Changes)
ParameterSystematic Forces / Directional Change   Dispersive Forces / Random Chnage
Explanationthe forces, which tends to change allelic frequency in a manner that is predictable both in amt & directionthe forces, which tends to change allelic frequency in a manner that is predictable  in amt but not in direction
Examples / IncludesGene Flow / Migration Mutation SelectionGenetic Drift Inbreeding
Act onboth small & large populationonly on small population
Leads / tends toTends to either reduce or increase the allelic frequencyit leads to ↑ in Homozygosity with corresponding ↑ in heterozygosity, b/c the dispersive forces a gene frequency from intermediate value towards extreme value 

Various factors responsible for gene frequency are as follows

Mutations

  • Mutation refers to sudden heritable change. It is ultimate source of the variation in the genetic composition of population.
    • These changes in genes are inherited in accordance with the Mendelian principles.
    • Hugo De Vries, first emphasised the importance of mutations in evolution & proposed mutation.
  • Features
    • Mutation are recessive,deleterious & some of them lethal
    • They may be somatic or germinal. Germinal are inheritable.
    • Can occur in any kind of cell (somatic / germ cells)
    • Mutations can be forward (common) & reverser which occurs at a constant rate till to the point an equilibrium is established.
  • Cause
    • Physical mutagens such as β rays, γ rays, chemical mustard gas etc.
  • Types
    • Chromosomal Aberrations
    • Point Mutation
  • Role in Organic Evolution – Mutation are regarded as the raw material for evolution which are acted upon by other factors of evolution such as Natural selection, isolation etc. Consequently Beneficial mutations cause change in gene frequency thus causing sub speciation & speciation.
  • Mutation Rate – In Recent time – mutation rate is calculated by comparing electrophoresis rate of proteins of parent & offspring
    • The largest survey was conducted by Japan where 3 new mutations were detected in half a million genes.
    • Although rate of mutation per gene is low, rate of mutation per generation is high

Genetic Recombination

  • It refers to rearrangement of genes through crossing over, during meiosis in sexually reproducing organism, resulting in new combination of parent gene in offspring.

Hybridization / Gene Flow

  • It is intermingling of genes of two population  of a species or b/w individuals of two species which are otherwise separated & it alter their gene frequency as a consequent i.e mating outside of mendelian population
  • It can either occur by migration or by artificial cross breeding.
  • It is effective in small populations.
  • It may result in genetic combination which are entirely new. I.e increase heterozygosity & thus increase genotypic variability
  • Cline – The Kind of Gene  Flow where a gene is spreading from a local population results in a gradually decreasing frequency along the line, which indicates the direction & magnitude of Gene Flow. This frequency gradient is called as a Cline.
    • An example of this gradient is the epicanthic fold which flow from Himalayas to Orissa.
    • Usually well separated populations are connected by numerous intermediate groups thr which gene can flow & give rise to geographic gradient of gene frequency b/w two polar populations.
    • Such gradients in gene frequency are k/n as Geno Cline & gradient of trait is called as a Pheno Cline.
  • Hybridization may also mean admixture of two or more genetically dissimilar population to form hybrids.
  • Hybridization or Gene Flow is basic evolutionary mechanism which brings about a change in the gene frequencies of a population by disturbing the Haedy weinberg Equilibrium.
  • It breaks the isolation. Isolation resulating from the absence of Gene Flow may permit genetic drift to casue a divergence. Gene Flow slows the process of population differentiation as the genetic drift speeds it up. Gene flow hence can also be referred to as a stabilizing mechanism. 

Selection (Charles Darwin)

  • Selection is choosing the parents for the next generation
  • Selections are variety of mechanism that modify the gene pool of reproductive success of a genotype where biological traits either more or less common in population
  • Selection helps to increase desirable gene frequency over undesirable genes in gene pool. I.e who are better equipped are selected by nature & thereby they survive i.e “survival of fittest“
  • Fitness – the capacity of a given genotype to survive & reproduce under a specific environment is referred to as its fitness.(darawin fitness or adaptive/selective value. It has to component
    • Viability– the ability of newly formed zygote of a specified genotype to survive to the reproductive stage
    • Reproductive Ability – the ability of an individual of a given genotype to produce offspring during the reproductive period.
  • When selection operates on a particular gene its frequency is changed in successive generation because parents of different genotypes pass on their genes unequally to the next generation. Consequently Gene & genotype frequencies are changed by selection.
    • This change is brought about by fertility, viability or through Eugenics, euphonic & euthenic measure
  • Therefor selection may be natural or Artificial. The net result of selection is to make the population better adapted to its prevailing environment.
  • Natural selection produce systematic heritable changes in a population from generation to generation. 
  • Selection occurs/ operates through
    • Non-random mating (Selective Mating)
      • Are consanguineous mating / inbreeding & Assortative Mating i.e certain phenotypes are preferred Ex – white – white Mating
    • Differential Reproduction –
      • In it org best suited to env will produce more offspring than those which are less adaptive.
  • Selection may be induced by ; physical, chemical & biological agents ex – Food, light, presence of predators etc
  • Selection value – (W) of a genotypes – the ratio of offspring of a genotype that are viable & fertile as compared to those of other genotypes
    • It ranges from 0 to 1 ; where 1 is most successful genotype & 0 is least successful
    • Future selection occurs on whole genotypes of individuals & not against individual alleles.
    • Therefor selective value depends on the inter relationships in the expression of all the genes In genotypes & not specifically on one or more allele.
  • Types of selection
    • Directional Selection
    • Stabilising Selection 
    • Balanced Selection

Isolation (Muller & Dobzhansky 1973)

  • It refers to segregation of a population of a species into smaller units or the segregation of individual of different species by certain mechanism so as to prevent interbreeding among them
  • It can be – Types
    • Allopatric Isolation or Geographical Isolation (Wagner)
      • The separation in space of two populations by Landmass, water, Mountains, Deserts etc
      • Leads to allopatric speciation
      • Example
        • Ecological isolation,
        • Mechanical isolation
        • Seasonal Isolation
      • Continental Drift caused allopatric speciation -ex Frog varieties – Mantellinae (Madagascar only), Rhacophorinae (India/ southeast Asia) , Other Indian / South Asian frogs
    • Sympatric Isolation (Same Geographical Aea) (E.B Poulton) or Reproductive Isolation
      • Defined as any genetically determined agency which prevents interbreeding of Mendelian populations
      • Types
        • Prezygotic / Premating Isolation (Hinders Fertilisation) – external reproductive isolation which prevents inter specific mating ; prevent wastage of gamete ; prevent mating itself ; ex
          • Habitat Isolation
          • Temporal isolation
          • Behavioural Isolation (interfering in courtship) 
          • Mechanical Isolation (Different body size – computation not possible)
        • Postzygotic Isolation (Inviable & Infertile Development of Zygote) – internal reproductive isolation which reduces the formation of viable zygote or hybrids ;
          • allows mating but prevent fertilisation & further development of zygote
      • Pre zygotic isolation are highly susceptible for improvement by natural selection, whereas post zygotic natural selection acts indirectly. 
  • Isolation prevents gene exchange & allow formation of unique genotypes resulting in speciation.

Migration

  • It refers to moving  in or out of a population,
    • In unstable environment emigration and
    • In favorable environment immigration happens
  • Migration introduces gene from outside to a breeding population.
  • Population undergo change in frequency by selection of new traits in order to survive in new environment.
  • The intra group genetic variability within a species tends to increase & intergroup variability tends to decrease as a result of migration.
  • this migration cause admixture ex. European migrated to America & European migrated to South Africa where admixture took place & new population emerged.

Genetic Drift (Sewell Right Effect) – aka founder’s effect or chance Factor

  • Is a phenomena in which – certain genes, without being advantage to population increase in frequency & may be fixed. These random changes in gene f occurring  by chance & are not under control of natural selection are called Genetic drift by Sewell Wright.
  • According to Hardy-Weinberg law in a large randomly mating population without selection, mutation, migration the gene frequency remains constant. But in small populations, the gene frequencies are found to fluctuate purely by chance and smaller are the population the greater are the fluctuations.
  • For genetic drift to be significant in evolution the Conditions
    • population should be small & isolated &
    • there should be no random mating.
  • Genetic drift & natural selection tend to co-operate with each other in small population. The selection tries to keep the deleterious genes at a low frequency by differential reproduction and genetic drift change in allele frequency is by chance.
  • Causes
    • Significant random fluctuations in allele’s frequency are possible by chance deviation.
    • Natural calamities & migration / isolation
    • Inbreeding
    • Sterile Individuals etc
    • All individual may not be mating – As the population is small & the individuals may be in possession of genes not available to others, death of those individuals cause change / shift in gene frequency due to loss of genes.
    • Individuals may not be leaving large no. of offspring – The population must be heterozygous for many genes so that there is 50% probability of one member of allelic pair to enter a gamete & to handed over to next generation. Thus if individual don’t leave large no. of offspring, there are chances of some alleles may not be inherited by offspring causing a change in gene frequency.
  • Example
    • Retinitis pigmentosa in Tristan Islanders (Tristan Da Cunha), Small island in St. Helena, Pacific region etc
    • The cause of drift here is migration (from Europe) & Inbreeding.
    • Six fingered dwarfism (rare genetic disorder) in old order Amish – Highly endogamous sect of Pennsylvania, USA ; migrate to Germany from Pennsylvania in the 18th century.
  • Effects of Drift
    • Bottleneck Effect – This effect of drift comes into play when a natural calamity removes a good proportion of population so that population again grows with increased speed.
      • Glycogen storage related disease of ITD gene in some population of US are example where small population with deleterious gene is growing fast
    • Founder Effect – This effect comes into play when a population either migrates to populated land but practices endogamy.
      • Ex. Six fingered dwarfism in old order Amish, retinitis pigmentosa in Tristan Island.
    • Drift & Increase in Population – if isolated groups develop unique gene constellations through drift, these unique gene constellations have potentiality of spreading far and wide with increase in population size.
      • Eg. Recent out of Africa model of human evolution & distribution small fraction of Africans left Africa & subsequently populated the whole world in 50-60 thousand years.
    • Genetic drift along with selection acts on gene pool of population to bring about changes in gene frequency eg. Distribution of ABO blood group world wide.
  • Role of Genetic Drift in Evolution
    1. Formulation of different combinations
    2. Success of disadvantageous gene : A gene which in a large population, had a net disadvantage may prove to be advantageous in different combination of genes thus get a foothold in the population in which it was denied in a large population.

Inbreeding (also c/a Consanguinity)

  • Inbreeding refers to mating of two closely related parents i.e mating within Mendelian population 
  • Individual mating at random in small population are closely related & hence results in Reducing heterozygosity & increasing homozygosity in the population
  • Inbreeding in itself is not detrimental it is hazardous only to the extent when undesirable recessive genes are present in the original stock (gene pool).
    • If lethal genes are present in the heterozygous state inbreeding can bring the two recessive genes in one individual and causing its death.
    • Increase the risk of genetic disease among the inbred children
    • Such cases pose health & survival problems of deleterious or debilitating diseases. This is health & socio-economic burden to the society & to the country. Increase of deleterious character as a result of inbreeding is referred as ‘Inbreeding Load’.
    • Inbreeding in small population lead to ↓ in genetic diversity, & it may lead to ↓ Darwin Fitness of an organism.
    • In case if these results to less capability to survive in changing env, or eco-niche, with inbreeding such characters will eventually allow the species to extinction. Such characters are supposed to have less Darwinian fitness. Such decline in reproductive performance & fitness with inbreeding in a population is referred as “Inbreeding depression’
    • Therefore, inbreeding can only be harmful when the survivors can’t compensate the loss caused due to the autonomous recessive lethal/ semi lethal homozygous genes. Since human beings, in general, have sufficient fertility, the issue of harmful effects of inbreeding is merely a theoretical one.
  • Medical research council in England (1961) after studying isolated population in Tristanda Cunha Island (pacific Is.) concluded isolation led to inbreeding due to small size and restricted number of potential mates.
  • The measure of inbreeding is by coefficient of inbreeding, is defined as the fraction of homozygosity that is increased over the initial homozygosity of the population due to inbreeding. (vary b/w 0 or 1% to 100%)
  • Example
    • Those who say that it is harmful cite the example of great Andamanese, who has seen a sharp decline in the past century. The decline is attributed to inbreeding.
  • The total effect of such homozygosity is weaker, or less successful, or less fit member of the species. so thus it lower the fitness or adaptive value of species . one method in nature, therefore, of maintaining sufficient genetic variability in a population is to ensure crossbreeding.
  • The offspring of cross bred individuals have been found empirically to be stronger or better products than those of inbred lines.
  • This fact was exploited by breader geneticist in producing unusually vigorous progeny from widely different parents.
  • The excess superiority or vigor, or increase in degrees of expression of several traits, has been named heterosis, or hybrid vigor & has been exploited commercially. 

Result of Change in Gene Frequency

Matings

Introduction – Simplest types of mating is called Random Mating c/a Panmixis. In this type of mating, an individual of one sex has an equal probability of mating with any any individual of the opposite sex in the population. It forms the foundation of population genetics. However, random mating is prevented by certain factors like Mate preferences & Social factors. One such deviation from random mating is inbreeding

Non Random Mating – These mating causes deviations from the Hardy Weinberg expectations by affecting combination frequencies of genotypes.

  • Genetic recombination doesn’t by itself alter allele frequencies. however any consistent bias in mating patterns can the genotypic proportions.
  • It therefore sets the stage for the action of natural selection

Models of Non Random Mating

  • Positive Assortative Mating
  • Negative Assortative Mating
  • Inbreeding ( c/a Genetic Consanguinity)
    • The closest type of inbreeding in the bisexual org. or the hermaphrodites is Self Fertilization.
    • In the unisexual org. the closest type of inbreeding is Back cross.

Consanguineous Mating / Inbreeding

Introduction – Consanguity refers to mating b/w individuals related by blood, having common ancestors a few generations ago. The degree of consanguinity depends upon the distance of relationship.

Thus highly consanguineous marrages are; father & Daughter; mother & son; brother & sisters. It is becasue of large no, of genes they share between them.

Evidences

  • In ancient time the pharaohs of Egypt and the royal families in the Inca society (Peru, South America) marriages of brother and sisters were favored to maintain the purity of royal blood.
  • In some societies, due to the small no. of potential mates available,  inbreeding among fairly close relations such as cousins is actively encouraged or is unavoidable
    • Pitcain Island & Tahitian Wives (Four Families, 50 inhabitant as of 2010)
    • Founder population like Amish in Pennsylvania
  • There are some areas,where inbreeding is avoidable but it is still actively encouraged.
    • Japan – First Cousin Marriage – 10% of all marriages
    • Andhra Pradesh – Uncle Niece Marriage – 10% of all marriages
  • The cultural rule of endogamy prescribe marriages inside a small group within the population.

Incidence of Inbreeding in Indian Population

  • States : TN,AP,Kerla -almost 35% each.
  • Communities : Memon, Vohra, Parsi, Khoza & Muslim

Types of Consanguineous Marriage

  • Cross Cousin – cross cousins are the children of siblings of opposite sex
    • Father Sister’s children
    • Mother’s brother’s children
  • Parallel Cousin – Parallel cousin are the children of the siblings of same sex i.e one’s paralel cousin are the
    • Children of mother sister
    • Children of father brother
  • Uncle Niece

Coefficient of Inbreeding (F) – is defined as the fraction of homozygosity that is increased over the initial homozygosity of the population due to inbreeding. (vary b/w 0 or 1% to 100%)

Coefficient of Inbreeding (F) = E(1/12)N(by Sewell wright)

N = Number of Persons in the path of relationship / ancestry

F = 0 (No Consanguinity)

F = 1 (Consanguinity)

  • Example
    • F = 1/16 (2nd cousin Marriage)
    • F = 1/64 (uncle – Nice Marriage)

Effects of Consanguineous Marriage – Any departure from random mating naturally leads to complications in the relationships b/w allele frequencies & genotype frequencies

  • Inbreeding has imp. medical consequences in addition to its effect on genetic equilibrium. When relatives mate, their offspring have increased probability of inheriting an allele in homozygous dose
  • It was discovered by the geneticists that close inbreeding is detrimental to the offspring & cross breeding is beneficial.

Genetic Effect

  • Continuous inbreeding – results in development of Pure Line by establishing homozygosity & eliminating heterozygosity in a population.
  • consanguinity increase homozygosity
    • With every succeeding generation of self fertilization, the frequency of the heterozygotes drops off by 50%, reaching 0.008 by generation seven and 0.001 by generation ten. At this stage, the population is 99.9% homozygous
  • It result into expresstion of large no. of recessive autosomal genes in the offspring as chances of heterozygote carrier for a disease increase due to consanguineous marriages.
    • At least 600 human traits are so for known to show an autosomal recessive pattern of inheritance. Ahost of them are autosomal recessive diseases.
    • Around 0.1 % marriages in the USA, Tanzania is b/w 1st cousins, around 8% of albino children result from the consanguineous marriages.
    • Dwarfism is frequent among the older Amish people of Pennsylavnia. No. of Amish people is small & they practise consanguineous marriages in order to maintain cultural identity.
    • Recessive autosomal disease are quite frequent among communities practicing consanguineous marriage like Muslim Communities.
  • Reduces the genotypic variability of a population, thereby decreasing its chances of survival in a changed environment i.e Genetic Fitness
    • mutant genes though harmful can be proven to be useful. eg. Sickle cell anemia can control the incedence of Malaria in a malaria prone region ;mutant genes through harmful can be proven to be useful.
  • two alleles in an individual will identical by descent from a common ancestor
  • Has number of effect on inheritance of dominant & codominant genes since  the condtition is expressed as soon as mutation is casued.

Examples of Genetic Effects

  • inheritance of Albinism (Panama Area)
  • Inheritance of Six-fingered Dwarfism
  • Colour blindness, erythroblastosis foetalis etc
  • The frequencies of disease, physical defect & mental defects are more among offspring’s of first cousin m’age than among offspring of m’age b/w unrelated persons. 

Harmful or Not – Consanguineous marriage are not always considered to be harmful.

  • Long Term – It is found that people who have tradition for 100 years fo inbreeding, have lesser incidence of recessive alleles. Since, consanguineous marriages of much longer duration reduce the proportion of heterozygotes &  through natural selection the eliminates the apnoramal homozygotes & leaving an increase in the proportion of normal homozygotes. Thus, in the long run the net result of inbreeding is same to those of the population without such practice.

Cases studies in India

  • A study of new born children in Hyderabad by Murty & Jamil found significantly higher rates of genetic anomalies in offsprings of CM as compared in non-consanguineous marriage. 
  • In-depth studies on a few endogenous caste group in A.P → inbreeding associated with higher childhood & foetal losses.
  • As per Basu and Rizvi, consanguineous marriages are most prevalent in the southern states of Tamil Nadu, Andhra Pradesh and Kerala, with Tamil Nadu alone accounting for 37% of the total such marriages.
  • A 2014 study on the tribes of Kerala found that the reducing populations of tribes like Koda, Kochu Velan and Paliyan, is because of the increased frequency of recessive traits. One chief reason being attributed herein is the prevalence of consanguineous marriages.
  • Other studies include Afzal and Sinha’s (1984) study showing that consanguinity reduces Intelligence Quotient in progeny; and Rao’s (1976) study that indicates foetal development is slowed in inbred population.

Control of Inbreeding :

  • Most groups actively work very hard at maintaing exogamy. As a general rule most human populations do not inbreed if they can at all help it.
  • Factors
    • Endogamy rules
    • Incest taboo
    • Gradual infusion of education
    • Modernisation
    • Urbanisation
  • Inbreeding occurs less frequently than predicted under random mating, b/c all societies have some form of incest taboo, esp. nuclear family

Conclusion – thus, inbreeding is mainly harmful however with the passage of generation its impact can be naturalised. Though it is responsible for many genetic diseases, but it is conrolled by social norms.

Non Consanguineous Mating

Introduction – Mating among unrelated individuals.

Types – It is of two types

  • Positive Assortative Mating
    • When individuals of like phenotype mate more often than expected by random mating predictions.
    • As Like phenotype individuals are similar to some degree in genotypes. The positive assortative mating increases the amount of homozygosity in the population & reduces heterozygosity
    • Ex. Stature & IQ of Eye Color
  • Negative Assortative Mating
    • mating with an individual who is phenotypically dissimilar
    • Theoretically, if this occurs more than expected by random mating predictions, it should increase the amount of heterozygosity in the population while correspondingly reducing homozygosity.

Genetic polymorphism is not necessarily an unqualified boon to the population. The vast genetic diversity provides raw material for evolution & also equips genetic predisposition to adaptability in populations.

But, it also includes several alleles which don’t impart optimum fitness to the population & at times may be harmful to it. Such deleterious genes are a genetic liability which tends to decrease the fitness of the population & is known as genetic load

Genetic Load

Introduction – Genetic load is a quantity to measure the loss of fitness in a population. Coined originally as ‘Costs of Natural selection’ by J.B.S Haldane, it was renamed as genetic load in 1950 by Muller.Genetic load is the presence of harmful alleles in a population;

Definition – According to Crow “Genetic Load is the proportion by which the fitness of a average genotype in the population is reduced in comparison with the best genotype” I.e relative decrease in the avg fitness of the population to that of the maximum fitness shown by optimum genotype.

Genetic load = (ωmax –ώ) / ωmax

Where, ωmax = Maximum Fitness & ώ (bar) = average fitness

If ωmax = 1 (ideal condition) then –

Genetic Load = 1 – ώ

Source of Genetic Load – source of which is always mutation.

  • It can be increased by high rate of mutation & decreased by low mutation rate.
  • Natural selection can only increase / decrease the no. of such genes in the population, has no role in the formation of genetic load.

Types of Genetic Load – Three major types of genetic loads originate & maintain in a population.

  • Mutational Load
    • It involves the slicing away of genetic variability due to recurrent mutations,
    • along with deleterious genes, even beneficial genes may also be eliminated through mutation leading to genetic burden on the population
  • Segregationally Load
    • It refers to a process were inferior homozygotes are continuously segregated. & these represent a liability in the population
    • As they are carried in population without any immediate & obvious utility
    • Segregationally load can cause greater lowering of variability then the mutational load
  • Incompatibility Load
    • it is encountered when certain genotypes are unable to survive in the environment of specific parental genotypes
    • Ex. Erythroblastosis Foetalis

Effect of Genetic Load – Effect of genetic load may be +ve or negative depending upon the environmental conditions.

  • Extinction of species due to huge genetic load
  • Increase in incidence of disease
  • Due to induced sex-linked lethal sex ratio may alter in the long run
  • They may be expressed through death or genetic diseases or through sterility or inability to find a mate or by any other means that reduce reproductive ability of the optimum genotype.

Factor Affecting It

  • Mutation – Most mutations are harmful to the effect that they result in deleterious recessive traits. This causes decline in fitness, meaning ↑ing genetic load, & thus more genetic deaths.
  • Inbreeding & hybridisation
    • Inbreeding → homozygosity → ↑ possibility of recessive deleterious gene to show its phenotype expression. Thus reduces fitness & increase genetic load
    • Conversely hybridisation lowers genetic load.
  • Environment – genetic Load varies with time & place, even within a population. This is because the optimum genotype is not a stable construct. This means that a population having high genetic load in a particular env may have a lower load in a new env due to the deleterious genes becoming non-disadvantageous in the new environment. E.g Sickle Cell Trait (HbA/HbS) is a advantageous in malarial conditions.

Genetic Load & Changing Environment

  • It is usually conceived that a population with small genetic load will survive & population with large genetic load may become extinct.
  • But it is not always true. Population with small genetic load may become extinct in a rapidly changing environment. While a population with relatively large genetic load may be successful when subjected to new environment in which formerly dangerous alleles are able to survive.
  • The absence of genetic load may be more detrimental to a population than its presence.

Self Thinning – For survival of a species it must match with the caring capacity of the env. Self thinning is essential in population which may be done by genetic load.

Conclusion – Thus contrary to earlier beliefs of genetic load as a dreaded monster, genetic load may increase population fitness. In other words it may decrees the fitness of individuals but increase that of population.