Synthetic theory of evolution

a) Migration and change in gene frequency

Immigration when local population is large:

Immigration to a local population when it is large in size does not produce a marked change in the gene frequency of local such a condition the migrant or straggler genes coming from another population has one of the two facts.

-> the migrant or straggler gene is swamped and eliminated by local genes because local genes complexes are balanced to fit their own local environment, where as many of the straggler genes would be adapted to the environment where they came from.the hybrid between local and straggler would not be favored as it is very unlikely to be properly adjusted.

-> the straggler gene can be introduced in the local population by introgression i.e. the hybrids that it produce resemble one of the two parents those like the adjusted local population may breed successfully enough to introduce into it a few new useful genes and super genes.thus causing a change in gene frequency.

Immigration when local population is small:

It may or may not produce a marked change in the gene frequency of local population depending upon stability of the environment.

->when environment is stable

under such condition the local population though small is perfectly adjusted to the local environment and hybrids between them and stragglers would be at disadvantage and would not result in change in gene frequency.

->when environment is unstable

this is the condition when immigration is most effective because local population is not perfectly adjusted to the environment under such conditions hybrids between local population and stragglers may find the environment suitable enough to survive.if stragglers possess gene which are of survival value in the new environment selection would operate to incorporate these in the gene pool of next generation.

To conclude immigration do not produce a marked effect on local population when the latter is large or small in stable environment . it produces change in gene frequency when local population is small in unstable environment

b) Mutation and change in gene frequency.

-> Forward and reverse mutation pressure

mutation pressure is observed to be of low order of is estimated that mutation of one kind or another are present in from 1 to 10% of the germ cells produced in every generation.individual genes however vary greatly in frequency of mutation.some kind of genes may be so stable that only one in a billion will mutate.accordingly there must be great variation in the efficacy of mutation pressure in disturbing genetic equilibrium. some genes may mutate so frequently that the constitution of the gene pool is considerably altered from the equilibrium in a short span of time.

The mutation pressure is opposed by what is known as reverse mutation.this would also occur at a rather constant rate, although, judging by evidence available,at a lower rate. thus there are two opposed mutation rates: 1. the rate at which wild gene changes into mutant gene, 2. the opposite rate.the combined action of the two rates is to change the gene frequency until a point is reached at which the bot forward and reverse mutation pressure balance each this point an equilibrium is established.thus while mutation pressure may alter genetic equilibrium. their ultimate net effect is to establish equilibrium even though it is a different equilibrium from that which would otherwise prevail.

Measuring mutation rates in humans.

Microorganism and fruit fly are fast breeders, so forward and reverse mutation can be used to measure their rates but humans are much slower in breeding and also diploid. In recent times mutation rate is calculated by comparing electrophoretic rate of proteins of parent and offspring.although rate of mutation per gene is low, rate of mutation per generation is high. a moderate guess work has it that half of all sperms and eggs carry on altered gene not present in parent thus each of us has three in four chance of carrying a new mutation in a functional gene.thus there are 3 billion new mutation in the human population of present generation not present in previous generation.

c)Genetic Drift

Sewall Wright effect, in which certain genes without being advantageous to the population increase in frequency and may be fixed.this generally happens in small, isolated population.if population is large it will be illogical to suppose disappearance of majority of persons.if a population is not isolated some advantageous allele of the gene can be introduced in the population by migration which will hamper spread of disadvantageous allele.a population experiencing drift has to be small and isolated.


An isolated population of 100 individuals has 25 healthy male in reproductive age group and all of them vanish in a storm.the population will continue with residual males and their genes,howsoever deleterious,will circulate in the population.

Six-fingered dwarfism in old order Amish.

Spread of ITD genes in US.

Drift in dunker community, pennsylvania.


All the individuals may not be reproducing.

it may be that some of the individuals may not reproduce at all. because population is small each individual may be in possession of genes not available to other members of the population. with the death of the individual the gene is lost for ever to the population causing a shift in the gene frequency.

Individuals may not be living large no. of offspring.

the population must be heterozygotes for many genes.there is thus 50% probability of one member of a allelic pair to enter a gamete and thus if one child is born, there is 50% chances of one allele to be handed down to next generation.if individuals do not leave large number of offspring, there are chances that some of the alleles may not be inherited by the offspring causing a change in the gene frequency.


Bottleneck effect

when a natural calamity removes a good proportion of population so that population again grows with residual population.ex-glycogen storage related diseases in SE Asian islands.

Founder effect

When a population either migrates to a barren land or migrate to populated land but practices endogamy. eg-six fingered dwarfism in old order Amish.

Role of genetic drift in evolution.

a)formation of different gene combination

b)success of disadvantageous genes.


Mating of two closely related parents.Brothers and sisters,mating with first cousin is very close inbreeding.inbreeding tend to eliminate hybrid from the population to replace them by pure breeds of homozyguous type. cit does this by simple process of segregation according to Mendels first law of segregation.thus the proportion of homozygotes in the population is increased.inbreeding increases homozygosity but it does not alter the gene frequency, this can be shown by comparing crossers in two family.when two children of heterozygosity and two children of homozygosity crossed between each both the cases recessive is present in the same percentage.they are in existing in outbreeding too but simply under cover.

inbreeding in itself is not detrimental it is hazadrous only to the extent when undesirable recessive genes are present in the original stock.if lethal genes are present in the heterozygous state inbreeding can bring the two recessive gene in one individual and causing its death.

thus, if recessive gene is lethal, formation of recessive homozygotes will lead to its elimination. for a self fertilizing species, the recessive gene can be theoretically eliminated from population in 9th generation.


Selection is variety of mechanisms that modify the gene pool for reproductive success of a genotype.

Population Geneticist concept:

As they are statistician, they are more interested in many offspring a particular genotype left for the next generation?this is the question they ask while looking for an aspect of selection. they are naturally interested more in fertility and fecundity of the population than merely agents of natural selection and its benevolent holier than thou actions.

Gene frequency approach of selection

the gene frequency for any gene can be calculated .if there has occurred any change in the gene frequency then it is considered that the population is explain the changes in gene frequency various factors such as chance or genetic drift,migration etc are invoked.if such factors fails to explain the nature of gene frequency, it is supposed that selection has acted.

Selection not the sole agent.

it is one of the causative factors of evolution along with migration drift hybridization etc.

Basics on selection.

a result of natural selection is a change in allele frequency relative to specific environmental factors.if environment changes selection pressure changes as well.such a functional shift in allele frequency is what we mean by adaptation.selection acts on individuals,it is individual who reproduce or does not reproduce and who continually attempt to maximize their own reproductive success.

f)Isolation and speciation.

Separation of species by varous factors and do not able to mate and remain isolated over a much longer period of time result in new speciation.


Convergence refers to the development of similar characteristics or adaptations in animals that differ in direct ancestry.the hummingbird and the humming moth. for example-have converged in their flying habits as a result of their common search for nectors in flowers for food.convergence applies to one or a few characteristics rather than the whole makeup.


parallelism implies a similarity in biological makeup of the ancestral forms, whereas convergence does not. the term is usually applied to two species of organisms that were similar in origin, and that remained similar as they evolved like having some of the same changes occurfing in both of them even after they have separated and evolved into two different species.the old world and new world monkeys provide an example of parallelism.

Homology and analogy.

Homology means similarity in origin(implying a common ancestor). Homoplasy means similarity in appearance but not in origin, and Analogy means similarity in function but not in origin.The wings of different species of birds are homologous as wings, but there is no homology between the wings of a bird and a bat.wings of birds and bat are merely analogous.but both evolved from parts of arms,forearm,hand of four limbed, five fingure vertebrates, and as such are homologous as forelimbs.

these features are valuable in determining whether they have evolved in a parallel or convergent manner.

Serial homology

It is the similarity of structures between one of the animal to the other part of the same arm and leg of man illustrate an instance of serial homology.

Adaptive radiation

It refers to the way a particular species evolves into progressively dissimilar organisms.How various organisms end up adapting in different forms and circumstances with different characteristics and environments and some eventually became extinct. various orders and suborders of mammals have undergone further differentiation, branching or radiating into types adapted to different habitats.

Dollo’s law

Dollo published the theory that evolution is irreversible.A change of the natural environment in an exact step wise reverse order is exceedingly rare.hence an occasion for reversed natural selection involving a number of factors in a given order would be just as rare.furthermore, if an previous allele has been entirely eliminated from the gene pool, a reversal of evolution would also require precisely the correct reverse mutation just as the environmental condition also reverse in proper order.

Single step backward, returning to an ancestral condition, sometimes occur in reversed mutations, as has been well documented in bacteria.however, that such reversal will not recapitulate a whole sequence is evidence for Dollo principle.

Mosiac evolution-the evolution of parts and not wholes.

The evolution of the species tends to be inconstant and asymmetrical.that is it may be rapid at one time and slow at another,in a rare cases it may stop one time, evolution may affect the limbs,and at another it may affect the jaw.with a change in food supply and some other alteration in the environment,running or biting abilities, for example would become more important or less important in natural selection.

the evolutionary trends of different organs systems like locomoter system, nervous system, respiratory, respond separately to environmental conditions.

the complex Mosiac pattern of human evolution led first to the evolution of a manipulative apparatus of hand and eye coordination,then to bipedalisation to permit use of hands even during locomotion, but human enchephalization was late and rapid.

Cope’s rule.

Living organism have a tendency to increase in their size during the course of organic evolution. this tendency has been found in vertebrates invertebrates and plants.though a majority of the present species are probably the largest of their class,this principle is not universal and has exceptions in both plant and animal kingdoms.

Gause’s rule or principle of competitive exclusion.

In the normal course of evolution, if two organisms occupy the same trophic level in the ecosystem, these organisms try to reach equilibrium through different strategy.however in some cases it so happens that the very presence of a competitor results in the domination of other organism which tries to adapt very effectively ultimately resulting in the exclusion of the competitor.such adaptive strategies adopted by the organisms to reduce the other to theΒ  point of extinction, called competitive exclusion, and not seen in absence of a competitor.

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