6 Models of Natural Selection

Ms. Shalini Singh and Prof. GK Kshatriya

epgp books

 

Learning Objectives:

 

I. It aims to understand natural selection a blind process as one of the micro evolutionary force.

 

II. It aims to understand various components, modes and types of natural selection.

 

III. It aims to understand various case studies of natural selection in human population.

 

TABLE OF CONTENTS

 

1.  Introduction

 

2. History of natural Selection

 

3. Fitness

 

4. Measures of fitness

 

5. General mode of Natural Selection

 

5a. Directional Selection

 

5b. Stabilizing Selection

 

5c. Disruptive Selection

 

6. A general model of Natural selection

 

7.  Types of Natural Selection

 

7a. Selection against Recessive Homozygotes

 

7b. Selection against Dominant Alleles

 

7c. Selection against Co dominant Alleles

 

7d. Selection against Heterozygote’s

 

7e. Selection for the Heterozygote’s

 

8. Case studies in Human Population 8a. Haemoglobin S and Malaria

 

8b.Duffy blood group and Malaria

 

8c. Lactase Persistence and Evolution of Human Diet

 

8d. Genetic Adaptation to High Altitude Population

 

8e. Skin colour in Humans.

 

Introduction

 

Evolution in a larger aspect can be defined as encompassing changes over time. The various micro evolutionary forces contribute to the process of evolution. Natural selection being leads to the divergence of the individuals from their common ancestors. In 1995, philosopher Daniel Dennet in his book “Darwin’s Dangerous Idea” called Natural selection a blind process as the real one of the micro evolutionary force among mutation, genetic drift & gene flow and has a unique role in adaptation and evolution. As written by Charles Darwin in one of his masterpiece Origin of Species (1959), “the importance of selection lies in the power of selection accounting for unique & appreciable differences accumulated till the end resulting in fit and compatible offspring. These unique and appreciable features wonderfully quip them for survival and reproduction and it also design of the existing world and is not affected by supernatural & natural designer. Natural selection led to the evolution of adaptation a characteristic feature which enhances the functional role in the life history of an organism maintained with greater adaptive potential, an organism becomes best suited to some of their environment through change in their survival and reproduction. The thing which mattered in natural selection was the overall advantage of a trait or physical feature over number of traits varying and dependent on different environmental conditions. Like for example, small bodied organisms were best suited in some environmental conditions whereas large bodied organisms were suited in some other. Very often we are mistaken in considering natural selection with evolution, but that’s not the case. Evolution is an outcome of processes i.e. evolutionary process can occur only by genetic drift, without any evolutionary change.

 

Humans placed in the higher order among the organisation of life, face basically the same adaptive challenges as faced by all organisms but among the humans most of the adaptations is culturally transmitted, because of this complex form of cultural adaptations human’s have adapted themselves to the varied Earth’s ecological habitats.

 

History of Natural Selection

 

The theory of natural selection was proposed by Charles Darwin and Alfred Wallace on their voyage expedition. Both of them carried out an extensive study on natural world and added new observations which were further helpful in formulating the developing theory. Darwin more over emphasised on the competition taking place within the population whereas Wallace emphasized on the demands acquired by various species when subjected to different environment. Natural selection works with a bunch of different phenomenon and it was among the five vital theories given by Charles Darwin. The other theories were Evolution, Common Descent, Reproduction and Gradualism. All these theories supported one Idea or notion which was accounting the fact Origin of Species. Ernest Mayr (1991) in his book One Long Argument provided a convenient & simple way of understanding the process of natural selection. He gave five facts namely Super fecundity, steady populations, limited resources, unique individuals and heritability.

 

The first inference or conclusion is competition among the individuals for example goose which may hatch more than a half dozen goslings in a season with an excellent ability to reproduce can hatch more than a dozen of goslings in a season but due to limited resources such as food water or places and harsh environment conditions only few goslings survive after the stiff competition. Other two conclusions were based on the uniqueness and heritability differences among the individuals in a population. This uniqueness among the individuals will be due to the variation gives a boost to survival. The fitness among the individuals helped them to survive and reproduce offspring was termed as Darwinian fitness or relative fitness. The extent to which one trait relative to another becomes represented in the next generation through reproductive success and differential survival conferred as a consequence of that trait. Darwinian fitness is applied where a given trait is not recognized through its design to fulfil a particular function but as residual demographic effect of its performance of that particular trait. This variation among the individuals is inherited by their parents and is further passed to their offspring. The thing which mattered in Natural selection was the overall advantage of a given trait varying by environment like for example small body size or large body size can serve as a not advantage in various environment. It is also the machine that drives evolution and let those organisms that are abnormal to survive an environmental change and making them the new normal. Endler in 1986 defined as the consistent difference in fitness among phenotypically different classes of biological entities.

 

The given figure demonstrates the various components of selection in sexually reproducing organism &  can be extended to two levels. First level is the zygotic selection whereas the second level is gametic selection. In zygotic selection, the viability or probability of survival of the genotype to adulthood is different. It also accounts for the survival of the individual organism and further its mating success which is whether the number of males affects the individual’s number of progeny. Mating success will contribute to the sexual selection which will depend on the genotypes of both the mates. The second level operates between the parents and its gamete is gametic selection where the pressure is on the allele. An allele may affect the parent gamete’s ability to fertilize an ovum

 

Fitness

 

Fitness in simple term it refers to the probability of survival & reproductive success and it is equal to the average contribution to the gene pool of the next generation. The various components of reproductive success & fitness are

  • Probability of survival in various reproductive ages
  • Average number of offspring laid down by females and males.

 

The process of natural selection depends on the relationship between phenotypes, genotypes and fitness which further adds to evolutionary change. The fitness of a genotype is the average lifetime contribution made by an individual’s genotype of that population over one generation to another. Survival and female fecundity can be assigned as the components of fertility. The complexity of fitness depends on repeatedly sexually reproducing species during its lifetime. The term Darwinian fitness is often used to make distinction from physical fitness. A change in the allelic frequency over the generations affect the Darwinian fitness, the alleles with higher fitness become more common & favourable.

 

Measures of fitness

 

Relative and absolute fitness can be taken as various measures of fitness. In a co- evolutionary system relative fitness is used as a measure for dynamic assessment of the individual. To understand how Natural Selection operates in nature, it can be modelled through mathematical models. We start the model by considering a locus with two alleles, A and a with genotypes AA, Aa and aa. For a total of 2000 individuals, 500 individuals are of genotype AA, 500 for genotype aa and 1000 individuals for genotype Aa during birth. At the time of adulthood all organisms won’t be able to survive due to harsh environmental conditions or may be due to less physical fitness. Thus out of 2000 individuals at birth only 1500 individuals survived at adulthood.

We observe some mortality among the individuals which led to decrease in the genotype. Taking a look here we can define the absolute fitness, which accounts for the proportional change in each genotype by taking the ratio of the number of individuals at birth and surviving till adulthood which gives

Some individuals from each genotype did not survive. Proportionally, far more individuals died that had genotype aa than the genotypes AA and Aa. After we are done with calculating the absolute fitness, now we can calculate relative fitness which is denoted by “w” with subscripts used to denote various genotypes. Thus ,

 

wAA= used to refer to the relative fitness of genotype AA.

 

wAa= used to refer to the relative fitness of genotype Aa.

 

waa= used to refer to the relative fitness of genotype aa.

 

In relative terms, these numbers mean that for every 100 individuals with genotype AA or Aa that survive only 40 with genotype aa survive. This type of variation in fitness is typical for recessive allele aa and is harmful only when both the copies of this allele is inherited not when only one copy is inherited. If the absolute fitness of all the genotypes would have been same, their relative fitness would also have been same.

 

General mode of natural selection

 

Natural selection can alter the frequency of heretiable traits in three ways:

  1. Directional selection
  2. Stabilizing Selection
  3. Disruptive selection

 

Figure 1 demonstrating the three modes of natural selection. The shaded areas shows the groups being selected against. The top graph shows the original distribution of the individual whereas the final distributions of the individual appear in the graph after selection.

 

Directional selection: individuals at one end of the frequency distribution do well.

 

Features:

  • It occurs due to change in the environment in a particular direction
  • It favours the phenotype which has an extreme or non- average character.
  • It alters the mean value of the trait in the population in one direction.
  • It eliminates the normal or average individual.

 

Example:

 

Industrial Melanism

 

The phenomenon of Industrial melanism is used to describe the evolutionary process in which light coloured organism’s population becomes dark as a result of natural selection. It was studied in the peppered moth, Biston betularia. Before industrial melanism these species had light colour pattern, dark coloured or melanic moths were rare. But after industrial revolution light coloured became highly vulnerable to predators due to darkened tree trunks and killed off lichens. Thus due to sudden change in the environmental conditions dark coloured moth became abundant, the cause of this change was selective predation by birds, as they favoured camouflage coloration in the moth.

Figure 2 demonstrating the light and dark coloured moth https://encrypted tbn0.gstatic.com/images?q=tbn:ANd9GcRuVmYSjIkHouAB6ApYBwKPX08yKk0cYZnOHdGex4a5K4f30r83

 

Stabilizing selection: the extreme varities for a particular heritable character is eliminated from both the ends of frequency distribution.

 

Features

  • It operates in a constant and changing environment.
  • It keeps a population genetically constant.
  • It favours the normal or average phenotypes.
  • It introduces homozygosity in the population.

 

Example:

 

The human baby with 73 lbs weight has less mortality rate whereas the human baby beyond 53 lbs and above 101 lbs has high mortality rate.

 

Diversifying/ Disruptive Selection: it is a process that enhances the adaptedness of population that live in heterogenous environments.

 

Features:

  • Previously homologous population breaks into several different adaptive groups.
  • Extreme values have highest fitness and intermediate or mean values are relatively disadvantageous.
  • It occurs when a population previously adapted to a non- homologous environment is subjected to divergent selection pressure in different parts of its distributional area.

 

Examples:

 

Darwin finches

 

Darwin finches were a group of 14 species of small bird, out of which 13 species of birds occurred on the Galapagos Islands. The Galapagos Islands, a cluster of 29 islands served as the cradle of evolution. These islands were relatively young and were never connected with the adjacent mainland of South America. Galapagos being a group of small islands were home to large flock of birds and would adjust themselves to the local conditions through the process of natural selection. With advent of time, they were different progressively from the original populations. The descendents now occupy many different kinds of habitats. These habitats encompasses a variety of niches comparable to those occupied by several distinct birds inhabiting the mainland.

 

Figure 3 showing various species of Darwin’s finches showing different bill and feeding patternshttp://www.pbs.org/wgbh/evolution/library/01/6/images/l_016_02_l.gif

 

Among the 13 species of Darwin’s finches there are only three main groups of finches which are ground finches, tree finches and warbler finches. Six varieties of tree finches also primarily in bill size and shape which reflected their adaptations to their adaptation. Six species of ground finches fed on seeds and the size of their bill was due to the size of seeds they used to feed on. Warbler finches differed from both tree and ground finches, they play the same ecological role in the Galapagoes woods that warblers play on the mainland.

 

A GENERAL MODEL OF NATURAL SELECTION

 

The concept of relative fitness can be related to natural selection by means of simple mathematical calculations or stimulations

Where p and q indicate the frequency of alleles A1 and A2 in the present generation.

 

w   = is symbolised as mean fitness which is the sum of all the genotype after selection.

 

Thus, w  = p2 w11+2pq w12+ q2 w22

 

 

These equations are important because they are used to demonstrate how different types of selection influence the allele frequency. These calculations will use a locus with two allele A and a where, the initial allelic frequency is p and q and the relative fitness of genotype AA, Aa and aa is wAA, wAa and waa respectively. The pyramid shows the steps involved in stimulation of Natural Selection where we start with allele frequencies of p=0.5 and q=0.5 and relative fitness of wA= 1.0, wAa =1.0 and waa=0.5.

STEP 1:

 

Computing Genotypic Frequencies Under Hardy Weinberg Equilibrium: Before selection the expected genotype frequencies of the population will be

 

AA                        Aa                               aa

 

p2=0.25            2pq= 0.50           q2=0.25

 

STEP 2:

 

Computing genotypic frequency after Selection: Due to selection the alleles in the present and last generation had a varied genotypic frequencies. Every genotype has a probability of a particular phenotype to be associated with it after their differential fitness. Their fitness will contribute genetically to the next generation. Therefore the probability that a particular trait will be passed into next generation can be solved by AND rule, where the probabilities of expected genotype proportions are obtained after selection.

 

Step 3:
Normalize genotype frequency

 

The third step is normalizing the genotype where we express the obtained genotype frequencies after selection which is relative to the mean fitness of the genotypes. Normalization can be done by dividing each genotypic frequencies by the mean fitness

 

It should be noted that the sum of all the normalized genotype frequency adds up to 1 i.e (0.2857+0.5714+0.1429=1).

 

STEP 4

 

COMPUTING NEW ALLELE FREQUENCY

 

After normalizing the genotypes we can now compute the new genotypes by using the equation

 

p= fAA+fAa/2 = 0.287+0.5714/2 = 0.5714

 

q= faa+fAa/2 =0.1429+0.5714/2 = 0.4286

 

The impact of natural selection can be felt in single generation. Here the frequency of allele q has decreased drastically from q= 0.5 to 0.4286 and frequency of allele p has increased from p= 0.5 to 0.5714.

 

These results can be further extended to next can generation by following the above stated steps.

 

Various types of natural selection:

 

1.      Selection against Recessive Homozygotes

 

2.        Selection against Dominant Alleles

 

3.      Selection against Co dominant Alleles

 

4.      Selection against Heterozygote’s

 

5.   Selection for the Heterozygote.

 

1. Selection against Recessive Homozygotes

 

This type of selection favours harmful allele, where two copies of the allele lowers down an individual fitness. In case of a lethal recessive the fitness is zero. The model of natural selection can be best understood by understanding the measure selection coefficient, an opposite of fitness which is the probability of individuals not surviving and not reproducing. The two measures are related as w as fitness and s as selection coefficient, such that we derive an equation w+s =1. As an example if w=0.8 and s=0.2, this means the probability of an individual to survive and reproduce is 80% and probability of an individual to not survive and reproduce is 20%. Thus we assign the fitness values for type of selection.

 

w11=1, w12=1 and w22=1-s

 

The fitness of one type of quantity is reduced over generations which is relative to the other two phenotypes. We now derive the mean fitness, w .

 

we can assume that higher the frequency of a allele higher will be the amount of selection against the a allele which will lead to a lowered mean fitness. The mean fitness will be the highest when the harmful allele is completely deleted from the equation, where q=0. We can now see the impact on the recessive allele by substituting the mean fitness values.

 

Let us assume q= 0.5 and s=0.5 and substitute in the equation

This value can be extended to the next generation, we can observe a decreased frequency of a allele it was 0.5 initially whereas is 0.4286 in the present generation.

 

Through these equations we can derive the allele frequency change per generation which is Δq.

Figure 4 (Relethford: 2012) Graph showing selection against the recessive

 

Here s is the selection coefficient and s=0 denotes the complete selection against against lethal recessive allele. Progeny possessing this lethal allele will not survive. Like for instance in Tay- Such disease, individuals receiving the lethal recessive allele are homozygous and usually die in their successive period. Whereas the selection coefficient for the heterozygote increases with generation as it carries only one lethal allele and passes it to the next generation. The allele frequency drops from q=0.5 to about q=0.2 after 20 generations. Under selection against recessive homozygote , no heterozygotes are selected because the fitness is equal to 1

 

2. Selection against Dominant Alleles

 

The fitness of both the dominant heterozygote (AA) and the heterozygote Aa will be reduced relative to the recessive homozygote(aa). We start by assigning the fitness value to the genotypes.

The Δq will be always negative and it will continue to decrease until equilibrium. Selection against the dominant allele is expressed in the heterozygote. The removal of a dominant allele is possible in a single generation only when it is a lethal, such that any individual with genotype AA or Aa will be eliminated in the first generation and only the recessive genotype aa survives.

 

Mathematically also if we equate coefficient of selection as 1, the frequency of the dominant allele p’ in the next generation will be

Figure5 (Relethford:2012) Graph showing Selection against Dominant Alleles

 

Here the initial allele frequency are p=0.9 for the dominant allele(A) and q=0.1 for the recessive allele

(a). The fitness values are wAA = 1-s wAa = 1-s and waa = 1. Allele frequency was derived for three different selection coefficient s=0.2, s=0.4 and s=0.6. an interesting feature of this type of selection is that the dominant allele can be removed in one single generation as anyone having either the AA of Aa genotype will be eliminated and those with aa genotype will survive in the first generation.

 

2. Selection with the Co- dominant allele

 

The next model of natural selection works when the two alleles are Co-dominant and there selection is favoured for one allele as the heterozygote Aa will show the effect of both alleles. This kind of natural selection is done by assigning highest fitness to individual to genotype AA and lowest fitness is assigned to genotype aa( no A allele) and an intermediate fitness is allotted to individual with genotype Aa. So the fitness values allotted in this case are:

 

wAA= 1

 

wAa=1-s/2

 

waa=1-s

 

The fitness of the heterozygote is the average of the fitness values of the homozygotes or has an intermediate fitness

 

This equation shows that for a decrease in the selection coefficient leads to an increase in mean fitness. The selection against the allele a will lead to decline in the co dominance. Thus the amount of allele change over generation Δp=p-p’

 

The negative sign in the numerator shows the frequency of the a allele decreases with each generation. The selection against the recessive homozygote will lead to a decrease in the allele frequency and will further lead to a value approaching 0. When the selection is for the recessive homozygote, there is no selection against the heterozygotes because fitness value is equal to 1, whereas some heterozygotes are selected under codominance as the fitness of one allele becomes (1-s/2).

Figure 6 (Relethford:2012) Graph showing selection against the Codominant Allele for different values of selection coefficient s.

 

Figure 6 shows selection against the co dominant allele a for three different values of selection coefficient. The frequency drops from q=0.5 to q=0.1 after 20 generations of selection against codominant allele. Heterozygote’s are selected against under co dominance. The fitness of heterozygote’s is 1-s/2.

 

1.   Selection against the heterozygote

 

In this model of natural selection we assign a lower fitness to the heterozygote has been assigned as compared to the other two homozygotes. Thus the assigned fitness values are as follows:

 

Figure 7 (Relethford:2012) Graph showing Selection against the heterozygote for different values of q.

 

The initial allele frequency is greater than 0.5, the effect of selection will increase the allele frequency until q=0.1. the graph shows when the allele frequency is than 0.5, selection acts to decrease the frequency of the allele has value q=0.5, but when the allele frequency is equal to 0.5 there is no change.

 

 

2. Selection For The Heterozygote

 

In the natural selection model for the Heterozygote’s Selection, leads to an equilibrium value lying in the range of 0 to 1. There is no favouring of a particular allele over the other allele. This kind of selection for the homozygote is called Balancing Selection. In this the heterozygote’s have a higher fitness when compared with the homozygotes. In this model we assign different values to the selection coefficient for the homozygotes “AA” and “aa”. The “s” is the selection coefficient for the homozygote genotype AA and t is the selection coefficient for the homozygote genotype aa.

Figure 8 (Relethford: 2012) Graph showing selection for the heterozygote.

 

The relationship between fitness and allele frequency is parabolic, but the shape depends on s and t. The given graph provides an example for this relationship for three different sets of values of s and t. The law is parabolic, but replaces with each value for q.

 

Case studies of Natural selection in human population.

 

Cultural evolution meant that we no longer evolve genetically in any significant way. The rapid change are not as a result of natural selection but are instead changes in environmental conditions i.e. a primarily shift in infectious disease and nutrition, an unfortunate truth is that many humans continue to live in impoverished environments without proper health care or diet, and do not share with the existence of different conditions enjoyed by those living in other parts of society or the world. Natural selection shapes the human population over time. Here are some case studies which gives an idea about how does it work and shows the evidence of evolutionary change in recent human evolution since the initial beginnings of agriculture.

 

A. Haemoglobin S and Malaria( Balancing Selection)

 

The story of natural selection and haemoglobin molecule is a classic example in anthropology. Haemoglobin is a protein of the blood that transports oxygen to tissues throughout the body (different tissues). Normal haemoglobin consists of 4 protein chains i.e. 2 identical chains of alpha (α) and Beta (β). The normal form of β haemoglobin gene is known as the A allele and people with AA genotype have haemoglobin functions normally in transferring oxygen.SS genotype results in manifestation of disease Sickle cell anaemia where a single mutation replaces the sixth amino acid of the beta chain Glutamic Acid with amino acid Valine. The disease can cause the red blood cells to become distorted and change the levels of oxygen. The deformed blood cells do not carry oxygen effectively, causing serious problems throughout the body. With respect to natural selection, relative fitness is assigned to different genotypes (AA, AS and SS). The genotype AA is reported to have highest relative fitness, genotype AS slightly lower relative fitness whereas the genotype SS is expected to have the lowest fitness. A main focus is laid on A and S alleles, because S allele is harmful in the homozygous case and a selection against allele is done. Allele S is mutant and has the lowest frequency.

 

Balancing selection and Haemoglobin S: Balancing selection refers to a selective process by which multiple alleles get added and are maintained in the gene pool of a population. A moderate to higher frequency of S allele are found in the populations inhabiting the areas of West Africa, Middle East, South Africa and India. Populations having malarial histories tend to have a high frequency of S allele and can be best explained by balancing selection. Malaria an infectious disease is one of the harmful disease recorded in human history of which the parasite Plasmodium falciparum is the most fatal. Presence of an S allele renders the red cell inhospitable to the malaria parasite, thus protecting the individual from its effects. The fitness difference among the SS and AA genotype can be seen by an example provided by Bodmer and Cavalli-Sforza(1976) from the Yoruba of Nigeria inhabiting the malarial environment. The observed genotype numbers in adults were AA=9365,AS=2993 and SS=29) for a total of 12387 adults, where 21723 A alleles and 3,051 S alleles, for a total of 24774 alleles. Hardy Weinberg proportions can be easily computed and will give the genotypic and allelic frequencies.

 

Allele frequency   Allele a Allele S

 

21723/24774                        3051/24774

 

=0.877                                     =0.123

 

Genotypic frequencies

 

AA= p2(12,387)= (0.877)2(12387)

 

=9527.2

 

AS=2pq(0.877)(0.123)(12387)

 

=2672.4

 

SS=q2(12387)= (0.123) 2(12387)

 

=187.4

 

The absolute fitness for the various genotypes can be observed

 

AA=9365/9527= 0.983

 

AS= 2993/2672.4= 1.120

 

SS=29/187.4= 0.155

 

The absolute fitness can be further used to calculate relative fitness

 

AA= 0.983/1.120=0.878

 

AS=1.120/1.120=1

 

SS=0.155/1.120=0.138

 

The selection coefficients (obtained by subtracting the relative fitness from 1) for the homozygotes are s=1-wAA= 1-0.878=0.122

t=1-wSS=1-0.138= 0.862

 

These values shows that for every 100 people with genotype AS surviving to adulthood, whereas 88 and 14 with genotype AA and SS respectively.

 

b. Duffy Blood Group and Malaria

 

Duffy blood group is defined by the presence of antigens on the surface of red blood cells. The gene of duffy blood group is present on the chromosome 1 and consist of three codominant alleles: Fy0, (coding for the absence of any Duffy antigen); Fya ( coding for a antigen) and Fyb (coding for b antigen). A selection and fixation for the Duffy negative alleles will be there in populations experiencing vivax malaria. The highest frequencies of Duffy negative alleles is found highest frequencies in central and South Africa reaching a value of 1.0 (100%) in number of populations.

 

c. Lactase Persistence and Evolution of Human Diet

 

Many humans today have developed lactose intolerance, physical effect of lactose intolerance can vary and include flatulence, diarrhoea, bloating and cramps. The gene controlling lactase activity is present on the chromosome 2. The critical factor that explains global variation variation in lactose persistence is diet. Populations indulged in dairy farming tend to have higher frequency of lactose intolerance. The fact can very well be explained by natural selection where lactase persistence was selected for the populations engaged in dairy farming because of the nutritional advantage among individuals who are able to digest the milk. An European cattle study revealed that the geographic distribution of various protein genes was correlated with levels of lactase persistence in humans. The estimated age of lactose persistence fits the age of domestication of cattle. Cattle farming began in Africa and the Middle east between 7500 and 9000 years ago.

 

d. Genetic Adaptation to High Altitude Population

 

A change in the adaptability was observed among the humans, as our ancestors expanded out of Africa and spread across the world. One particular challenge occurred in adapting themselves to high altitude and overcome the physiological stress. When a low native enters a high altitude environment, the hypoxic conditions can be encountered by various individuals and an adaptive response is further generated like increase in the red blood cells and increased respiration, increased chest dimensions, greater lung volume relative to high altitude. There are some genetic influences on high altitude environment then some physiologic and biochemical traits as a result of natural selection as the genetic make-up of the respiratory components of the Tibetan and the Ethiopian populations are significantly different. These two populations differ in the level of oxygen in the blood, the level of oxygen saturation in the blood and levels of haemoglobin concentration.

 

e. The Evolution of Skin Colour

 

Human skin colour (pigmentation) is a quantitative trait that shows an immense amount of variation between human groups around the world, ranging from very dark to extremely light. The wide range of skin colour is affected by natural selection. Skin colour is affected by geographical distribution and tends to increase and decrease with distance from equator, both north and south as the amount of ultraviolet (UV) varies with distance. UV radiation is strongest at the equator and diminishes with increasing distance.

 

Summary

  • The theory of natural selection was proposed by Charles Darwin and Alfred Wallace on their voyage expedition. Both of them carried out an extensive study on natural world and added new observations which were further helpful in formulating the developing theory
  • Selection is primary of all the forces that cause differential survival and reproduction among the genetic variants. When the selective agencies are primarily done by human wishes and choice, the process is called artificial selection.
  • Natural selection is the machine that drives evolution. This mechanism leads to an evolution of new normal which can survive the harsh environment and bring a change in the life of that organism.
  • Natural selection operates in two levels one is zygotic selection whereas the other one is the gametic selection. The viability or probability of survival of the genotype to adulthood is different. It also accounts for the survival of the individual organism and further its mating success which is whether the number of males affects the individual’s number of progeny.
  • The process of natural selection depends on the relationship between phenotypes, genotypes and fitness which further adds to evolutionary change. The fitness of a genotype is the average lifetime contribution made by an individual’s genotype of that population over one generation to another.
  • There are three different modes of natural selection- directional, stabilizing and disruptive. Directional selection favours the variant at one extreme. The second mode removes extreme variants from the population and preserves the intermediates. The final mode favors both the variants.
  • Natural selection due to the difference in the genotypic viability is called hard selection as they fail to reproduce and results in their death. It is also termed as soft selection as all the individuals in the parental generation reproduce by differing amount.
  • There are five types of natural selection. One is the when the selection is against the recessive homozygote, the other is when the selection is against the dominant homozygote , the third one is the general dominance and the last two is heterozygote disadvantage and heterozygote disadvantage.
  • One of the classic examples of natural selection and balancing selection in human population is change in the allele frequency of haemoglobin S, which is selected for the malarial population. Here the highest fitness is that of heterozgote (AS) relative to AA genotype and SS genotype (presence of sickle cell anaemia).
  • Change in diet, change in skin colour all are the various aspects of selection in natural selection which has lead to adaptation of humans in harsh conditions. humans have moved to high altitude and have adapted genetically to various physiological and ecological stress.
  • Human evolution is not ceased but continues both culturally and genetically. Due to a rapid demographic and cultural changes has lead to a variant form and pattern of genetic diversity among humans.

 

you can view video on Models of Natural Selection

Suggested Readings.

  • Dobzhansky, T. (1977). Evolution. San Francisco: W.H.Freeman & Co.
  • Futuyma, D. J. (2009). Evolution. Sunderland: Sinauer Associates Inc.
  • Hamilton, M. B. (2009). Population Genetics. Chischester: Wiley- blackwell.
  • J.Rougharden. (1979). Theory of population genetics and evolutionary ecology: Introduction.New York: Macmillan Publishing Co.,Ltd.
  • Pierce, B. A. (2013). Genetics: A Conceptual Approach. W.H.Freeman.
  • Relethford, J. H. (2012). Human Population Genetics. new york: Willey Blackwell.
  • Russell, p. J. (1997). genetics. Benjamin-Cummings Publishing Company; 5 Sub edition.
  • Sforza, W. &. (1975). Genetics Evolution and Man. W H Freeman & Co .