Darwinism, Its Criticism and Neo-Darwinism, Difference between Darwinism and Lamarckism

You must have often wondered how life originated on earth. Were all the life forms that we see today, present even millions of years ago? The most accepted theory about the origin of life says that life originated spontaneously from inorganic matter present in the primitive earth environment. It originated as simple prokaryotic organisms and today after millions of years, earth has millions of species of living organisms on it. So how did the primitive organisms that were first borne on earth give rise to such diverse varieties of species?

You must have heard that apes are our ancestors, that lizards are the descendants of dinosaurs and that birds have evolved from reptiles. In fact, all throughout history there have been the extinction of old forms of life and evolution of new forms. But what drives evolution towards a specific direction? What are the factors that influence the process of evolution? There have been multiple theories to explain evolution but one of the most widely accepted theories is the one given by naturalists Charles Darwin and Alfred Wallace. Darwin’s theory was based on his observations of life forms and fossils during his world tour on the ship HMS Beagle whereas Wallace’s conclusions were based on observations made during his study in the Malay Archipelago.

Both these scientists believed that evolution occurred by natural selection due to which nature selects the individuals which have favourable variations that allow them to survive and outbreed others under changing environmental conditions. In this article we will discuss Darwinism, the salient features of his theory, the evidence that support it, its criticism and the laying down of Neo-Darwinism theories.

Table of contents

• Introduction to Darwinism
• Salient features of Darwin’s theory of Evolution
• Evidence in support of Darwin’s theory
• Criticism of Darwin’s theory
• Neo-Darwinism
• Difference between Darwinism and Lamarckism
• Practice problems
• FAQs

Introduction to Darwinism

Charles Darwin is known as the father of evolution due to his contribution to the establishment of the theory of evolution. Darwin’s evolutionary theory of natural selection gave a more specific explanation of the formation of new species. As per his theory, organic evolution occurs through natural selection and accumulation of small variations which provide structural and functional superiority to certain individuals over others in their survival and differential reproduction.

                                                          Fig: Human evolution

Historical background

Charles Robert Darwin went around the world as a naturalist on the survey ship HMS Beagle from 1831-1836. The journey and his observations have been described in his book ‘A naturalist's voyage round the world’. His observations were -

• Different areas and islands possess different flora and fauna despite their having a similar climate and topography.
• Plants and animals of nearby islands are related to each other but are otherwise different not only from one another but also from those found in the mainland.
• The same organism shows different variations in different areas. The flightless bird Rhea was found to have different varieties in different parts of South America.
• He observed that 13 different species of finches inhabited the Galapagos island and they differed from each other and from the ones in the mainland in their size, beak shape and food habits.
• Some of the fossils that he collected resembled modern day plants and animals while others were quite different.

Darwin came across a paper by Thomas Malthus ‘An essay on the Principle of Population’ in which he had explained the presence of limited resources, geometrical increase in population, struggle for existence, and correction of population size through wars, famines, earthquakes, etc. Darwin also studied domestication of plants and animals and observed that conscious or artificial selection helps breeders to develop high-yielding varieties of cattle and poultry. 

In June 1842, Darwin wrote a brief central theme of his theory of evolution and received a communication from Alfred Wallace whose conclusions on the theory of evolution were similar to that of Darwin’s. 

Darwin and Wallace presented a joint reading of their theory of evolution in front of the Linnean Society of London in July 1858 and next year Darwin published his work in the famous book - ‘On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life’, more popularly known as ‘On the Origin of Species’.

Salient features of Darwin’s theory of Evolution

Rapid multiplication or prodigality

According to Darwin, living beings have a high biotic potential or reproductive potential. Their number can grow in geometrical proportions. For example, 

• Paramoecium divides every 16 hours and within five years a single organism can produce offspring with a volume 1000 times the volume of earth.

                                   Fig: High reproductive potential of Paramoecium

• Bacteria divide every 20 minutes and within 24 hours a single bacteria can give rise to millions of offspring.
• A single plant of evening primrose produces around 25,200 to 118,500 seeds per season. If all of them were to successfully germinate, then within 5 generations, the population of evening primrose will be 14 x 1080.

Limited food and space

 The availability of food and space on earth is limited. Each ecosystem has a carrying capacity for different populations which is the maximum number of individuals in a population that it can sustainably support. This number depends on the number of producers it supports. 

Struggle for existence

Population of each species tends to increase due to high reproductive potential but the resources available to each population is limited due to limited carrying capacity of the environment. This results in competition for obtaining optimum resources necessary for survival and this is known as the struggle for existence. The struggle seen in an ecosystem can be of three types -

1. Intraspecific struggle between members of the same species which compete for food, shelter, mates, water, light, etc.

2. Interspecific struggle between members of different species which compete for similar requirements, such as plants of different species competing for light and water. In fact, prey-predator interaction is also an example of interspecific struggle.

3. Environmental struggle exists between organisms and the restraining factors of the environment such as carrying capacity, limited resources, famine, drought, floods, etc.

Variations

Variations are differences in the genetic makeup of individuals belonging to the same species. These variations are responsible for the difference in size, form, structure, behaviour, physiology, etc amongst different individuals of the same species. Depending upon their contribution to the struggle for the existence of an organism, variations can be useful, harmful or neutral. Useful variations have a selective advantage while harmful ones have a selective disadvantage.

Natural selection

Under changing environmental conditions, only those organisms which have useful variations, have a selective advantage which helps them adapt to the environment. Thus, they are selected by nature for survival while the others which are unable to adapt, die in the process. This is called natural selection by Darwin and survival of the fittest by Herbert Spencer. The fitness here refers to reproductive fitness which is the ability of an organism to reproduce and produce fertile offspring. Organisms with higher reproductive fitness will survive in the struggle for existence and leave more progeny than the others.

For example, during prolonged drought, only those plants which have adaptations to conserve water efficiently will survive.

Inheritance of  useful variations

Under changing environmental circumstances, the organisms which survive are the ones with maximum reproductive fitness and the highest number of useful variations. They interbreed and pass on these variations to the progeny. The progeny would again exhibit the struggle for existence, natural selection and the survival of the fittest, Thus, a continuous selection process operates in nature due to which each generation is an improved version of its predecessor.

Formation of new species

There is continuous selection and accumulation of useful variations. Useful variations accumulate over many generations and add up subsequently to eventually give rise to offspring who are considerably different from their ancestors. Thus they evolve into a new species. Therefore, natural selection operates through interaction between the environment and the variability that naturally exists in a population.

However, the environment does not remain stable and it may show unexpected changes, causing surviving individuals to perish and establishing a new set of variations. This would change the path of evolution and newer species of different kinds will be formed.

Evidence in support of Darwin’s theory

It is evident from nature that even though organisms have a high reproductive potential, the realised potential and actual number of individuals present in a population of that species is never as high as its actual reproductive potential. Although each Evening Primrose plant can give rise to 1,18,000 seeds, not all successfully germinate into plants. This is due to the availability of limited resources. 

We have also seen that no two individuals of the same species are exactly identical.

Struggle for existence is also seen in all spheres of life, for example, the relationship between a predator and a prey.

Some other evidences in support of Darwinism include -

Coevolution of floral nectaries and insect proboscis

Flowers are pollinated by insects and the flowers produce nectar to attract the insects and feed them in return for their service. The nectaries in a flower are seated at a particular depth and the proboscis of the insects which pollinate a particular species of flower are long enough to reach the nectaries. Such a correlation between the floral nectaries and insect proboscis is possible only if the two have coevolved gradually with suitable variations and their natural selection.

                         Fig: Coevolution of floral nectaries and insect proboscis

Mimicry

The term mimicry was coined by Bates (1862). It is the resemblance of an organism to another organism or its natural surroundings which gives it the benefit of deceiving its predator or prey, e.g, the praying Mantis, leaf insect, etc. Mimicry suggests that the mimic must have evolved along with its model and have developed variations which provided it the desired benefit.

                                        Fig: Mimicry in Praying mantis

Extinct forms

Study of fossils shows that new organisms have regularly evolved while older ones became extinct. This is possible only when newly evolved organisms are better than the older ones in the struggle for existence.

Artificial selection

Artificial selection parallels natural selection. It is seen when breeders artificially select plants and animals with desirable traits and breed them to obtain progeny having desirable characteristics of both parents. 

Adaptations

Variations present in organisms help them to adapt to changing environmental conditions. For example, peaches are deciduous in Europe but evergreen in India, insect pests rapidly develop resistance to new insecticides due to variations.

Criticism of Darwin’s theory

Variations

Darwin’s theory of evolution was based on variations, however, he could not explain the origin of variations. He believed that new species developed due to accumulation of small variations. But small variations arise either due to random segregation and crossing over during meiosis or due to chance recombinations between homologous chromosomes during fertilisation. These are not new variations and rarely go beyond the limits of species.

Pangenesis

In order to explain the inheritance of variations from one generation to the next, Darwin came up with the theory of pangenesis. According to this theory, every organ produces minute hereditary particles called gemmules or pangenes and the pangenes of the body organs are released into the blood which carries these to the gametes, to be inherited by the offspring. This theory was disproved by Weismann’s theory of germplasm which states that variations in germ cells are only inherited.

New characters

Darwin could not explain the origin of completely new characters. His theory could only explain the mechanism of modification or improvisation of already existing characters.

Arrival of the fittest

Natural selection explains the survival of the fittest, however, it does not explain the arrival of the fittest. Any new character introduced in a population will have no functional advantage and should not be selected by nature.

Overspecialisation

Overspecialised organs are those that have grown beyond their maximum usefulness and have now become a burden, e.g, tusks of elephants, branched antlers of deer. If we consider Darwin’s theory of natural selection, then such organs should not be naturally selected beyond the level of their optimum utility.

                                    Fig: Antlers of deer

Vestigial organs

A number of vestigial organs which are no longer functional are found in the organisms of a species, e,g, vermiform appendix and fused tailbone in humans. If natural selection allows the selection of useful traits only, then how are the vestigial organs existing in organisms, generation after generation? Darwin’s theory of evolution could not explain this.

                                  Fig: Vestigial organs in humans

Unchanged forms

Some organisms such as Selaginella have remained unchanged for several million years. Darwin’s theory states that continuous natural selection allows accumulation of variations that help in the gradual evolution of a new species. Occurrence of such unchanged forms could not be explained by Darwin.

                                                Fig: Selaginella

New species

Darwin believed that the new species that evolve are better versions of the parent species. But this is not always true. Often the newly formed species are genetically very different from the parent species which makes them reproductively isolated from each other.

Neo-Darwinism

This is an altered explanation of Darwin’s theory to understand natural selection and Mendelian genetics better. The main force driving speciation is the gathering of genotypic variations in a gene pool. Sometimes this theory is also referred to as the Modern synthetic theory of natural selection.

According to this concept, variations in the gene pool of a population are introduced by -

• gene mutations : sudden changes in the composition of DNA. 
• genetic recombination : exchange of genetic material either between multiple chromosomes or between different regions of the same chromosome by breakage, rejoining, and copying of chromosomes or chromosome segments. 
• chromosomal aberrations : change in chromosomal structures 
• genomic mutations : change in number of chromosomes 
• hybridisation : crossing/breeding of genetically different organisms. 
• migration : entry or exit of individuals in and out of the population leading to addition of new alleles to the gene pool or loss of certain alleles. Thus the frequency of occurrence of specific alleles in the gene pool changes resulting in variations.
• genetic drift - sudden changes in the frequency of occurrence of certain genes in the gene pool of isolated populations (a population that gets separated from the parent population due factors such as geographical barriers) of a species due to chance factors.

Variations that occur only in the genome of germinal or reproductive cells of an organism can be passed onto the future generation. Different genetic variations interact to express a range of characters that allow better adaptability of an organism to its environment. Natural selection of genetic variations allows individuals with advantageous variations to survive, reach maturity and reproduce to contribute genes in the gene pool (differential reproduction).

Isolation or separation of a segment of a population from the rest helps to accumulate specific variations in a group of individuals of a population or in different populations of a species. The gene pool of an isolated population gradually changes and fails to interbreed with the parent population. Thus a new species is formed. This theory suggests that population, and not an individual organism, is the unit of evolution.

Difference Between Darwinism and neo - Darwinism

Difference Between Lamarckism and Darwinism

Practice problems

Q 1. The development of mimicry in organisms to evade predators is an example of

a. saltation 
b. natural selection
c. differential reproduction
d. coevolution

Answer: Mimicry is the resemblance of an organism (mimic) with another organism (model) or its surroundings which helps them deceive predators or prey. The resemblance can be with respect to appearance, sounds produced, etc. Every time the resemblance between the mimic and the model becomes obvious, the mimic has to evolve. This would require simultaneous evolution in both the organisms and thus mimicry is an example of coevolution. Thus, the correct option is d. 

Q 2. According to Neo-Darwinism, variations which drive evolution occur

a. within an individual.
b. in the gene pool of a population.
c. in every member of a species.
d. in all species of an ecosystem.

Answer: Neo-Darwinism is an improvement of Darwin’s original theory of natural selection in order to remove its defects and incorporate features like Mendelian inheritance which are in tune with present day knowledge. 

Unlike Darwinism, which failed to explain the cause of variations, Neo-Darwinism believed that the raw materials for evolution are the genetic variations that occur in the gene pool (set of all genetic information of a population) of a population. 

Only those variations which occur in the genes of germ cells of organisms in a population can be passed onto the offspring during reproduction. Only a few individuals of a population carry the favourable genetic variations that provide them with better adaptability in a changing environment and increased reproductive fitness. 

Nature selects such organisms for survival and an isolated group of such individuals can gradually evolve into a new species as their gene pool undergoes multiple variations due to mutations, recombinations, etc.  Thus variations that become the raw materials for the evolution of a new species are not restricted to an individual and occur in selected individuals only.

Thus, the correct option is b.

Q 3. Explain the evolution of Giraffe neck with the help of Darwin's theory of evolution by natural selection

Answer: According to Darwin's theory of natural selection, the population of giraffes consisted of both short-neck and long-neck ones. As the ground vegetation was depleted, the long neck ones could reach the leaves at higher levels of tall trees and could survive by feeding, whereas the short-neck ones were deprived of food and died, eventually being eliminated from the population. The long neck ones were thus naturally selected and as they reproduced and passed on the long-neck variations to the offspring, gradually the long-neck species of giraffes evolved.

Q 4. Which of the following statements is not true?

a. Darwin’s theory of natural selection cannot explain the existence of vestigial organs.
b. Coevolution of floral nectaries and proboscis of pollinators supports Darwin's theory of evolution.
c. According to neo-Darwinism, the unit of evolution is the population.
d. Darwin’s theory of evolution explains the origin of variations.

Answer: Darwin, along with Alfred Russel Wallace, laid down his Theory of natural selection. He believed that minor inheritable changes or variations naturally occur in organisms and natural selection of useful variations is the driving force of evolution. 

Darwin’s theory of evolution was based on variations, however, he could not explain the origin of variations. He believed that new species developed due to accumulation of small variations. But small variations arise either due to random segregation and crossing over during meiosis or due to chance recombinations between homologous chromosomes during fertilisation. These are not new variations and rarely go beyond the limits of species.

Thus, the correct option is d.

FAQs

Q 1.What is the importance of  Darwin's theory?
Answer: Darwin’s evolutionary theory of natural selection gave a more rational explanation of the formation of new species. As per natural selection, various species originated from a single species as a result of adaptation to the changing environment.

Q  2. What are the three types of natural selection?
Answer: The three types of natural selection are - 

• Stabilising natural selection - When selective pressures work against two extremes of a trait, the intermediate trait is selected for. 
• Directional natural selection - In this case, selective pressures work in favour of one extreme of a trait and thus the graph representing distribution of traits will lean in a particular direction.
• Disruptive natural selection - In this type of natural selection, selective pressures work in favour of the two extremes of a train and against the intermediate trait.

Q 3. Are humans still evolving?
Answer: Recent studies have shown that humans are still evolving. One of the most common examples is the existence of lactose tolerant individuals. Most human adults are unable to digest lactose because the body switches off the lactase enzyme production after the weaning period. And yet, more than 70 per cent of European adults can easily digest milk. They carry a mutation in the DNA segment that controls the expression of the gene coding for lactase enzyme. This mutation allows the expression of the lactase gene to be switched on and lactase production to continue, even after weaning. This genetic change appears to have happened between 5,000 and 10,000 years ago, which is approximately when domestication of milk-producing farm animals began in Europe.

Q 4. Which species did humans evolve from?
Answer: Modern humans are believed to have evolved from their common ancestor Homo erectus and originated around 200,000 years ago in Africa.

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