Organic Evolution

Organic, or biological, evolution refers to the gradual modification of living organisms as they descend through successive generations from common ancestors. This process is distinct from other forms of evolution, such as chemical evolution, cultural evolution, or the emergence of life from nonliving matter. Organic evolution encompasses two primary processes: 

Anagenesis - It is the gradual evolution of a species that continues to exist as an interbreeding population.

Cladogenesis - This occurs when there is branching or splitting, leading to two or more lineages and resulting in separate species. 

1.0Introduction to Organic Evolution

Organic evolution is the theory that contemporary plant and animal species have evolved from pre-existing forms, with observable differences between ancestors and descendants arising from modifications across successive generations. While Charles Darwin (1809–1882) is often associated with this theory, he did not originate the idea. Earlier thinkers, such as the French zoologist Jean Baptiste de Lamarck (1744–1829), a pioneer in invertebrate palaeontology, and Erasmus Darwin (1744–1802), Charles's grandfather, also considered it. However, the concept of organic evolution did not gain widespread acceptance before Charles Darwin's time due to a lack of crucial data and the absence of the Huttonian idea of geologic time, both essential for supporting evolutionary theory. Charles Darwin's significant contribution was the introduction of a mechanism—natural selection—that explained how evolutionary changes could occur.

2.0Organic Evolution Definition

This theory suggests that environmental conditions in nature are constantly changing. Organisms possess an inherent ability to adjust to these changing conditions, a trait known as adaptability or adaptation. These adaptive changes in organisms drive the evolution and the emergence of new species. As these changes result from adaptations, the newly formed species are typically better adapted and more organized than their ancestors.

3.0Theories of Organic Evolution

Lamarckism/theory of inheritance of acquired characters 

A French naturalist proposed the first logical theory of evolution, Jean Baptiste de Lamarck (1744-1829) 

Book: Philosophie Zoologique (1809)

Basic concepts of Lamarckism- 

(i) Internal vital forces: Due to some internal vital forces, all organisms tend to increase the size of their organs or entire body. 

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(ii) Influence of the Environment and Emerging Needs: The environment impacts all living organisms. Changes in the environment lead to the emergence of new needs or desires. These new needs or desires prompt the development of new structures (known as the doctrine of desire/appetency) and alter the organism's habits.

(iii) Use and Disuse of Organs: When an organ is consistently used over multiple generations, it becomes more developed. Conversely, if an organ is not used, it will gradually degenerate and may become vestigial."

(iv) Inheritance of acquired characters: During an organism's lifetime, new characters develop due to internal vital forces, the effect of the environment, new needs, or the use and disuse of organs. All these acquired characters are inherited from one generation to another. By continuous inheritance through several generations, the variations accumulate to such an extent that they can give rise to new species. 

Examples: 

1. The long neck and forelimbs of giraffes: Lamarck proposed that giraffes, in their efforts to reach leaves high in the trees, adapted by stretching their necks. This acquired trait of elongated necks was then passed down to subsequent generations, resulting in giraffes gradually developing long necks over time.

2. Aquatic birds stretched their toes and developed webs. 

3. Snakes lost their legs. Snakes evolved from lizard-like ancestors, which had two pairs of limbs. Continuous creeping through holes and crevices made the snake's body elongated, and the continuous disuse of limbs, which hinders them while creeping in burrows, resulted in the loss of their limbs.

Criticism of Lamarckism

• Weismann's Theory of Continuity of Germplasm: According to this theory, two types of protoplasms are present in an organism: germplasm and somatoplasm. There is continuity of germplasm, and the variations influencing the germ cells are only inherited. However, the somatoplasm is not transmitted to the next generation; hence, it does not carry variations to the next generation.
• The next generations never inherit the boring of ear pinna and nose in Indian women. 
• Chinese women used to wear iron shoes to have small feet, but they still have normal feet.

Neo-Lamarckism

• According to Neo Lamarckism, "Changing environment may create some physical and chemical changes in somatoplasm of organisms, which may affect their germplasm also and such acquired characters can inherit." 

Theories of Organic Evolution-Darwinism/Theory of Natural Selection

• Charles Robert Darwin was born on 12 Feb. 1809 in England. 
• Darwin travelled by H.M.S. Beagle, which left on 27 Dec. 1831 and returned on 02 Oct. 1836 through South America, South Africa, Australia, and the Galapagos Islands. 
• Two books influenced Darwin- 

(i) "Principles of the population" of Malthus 

(ii) "Principles of Geology" by Charles Lyell 

• Darwin later elaborated his theory in his book 'On the Origin of Species by Means of Natural Selection' (1859). The two fundamental concepts of Darwinism are Branching Descent and Natural Selection. Natural selection is based on several factual observations:

(i) Overproduction

• All organisms have the potential to produce a large number of offspring (multiplying in a geometric ratio). Theoretically, if all individuals reproduced to their maximum potential, population sizes would grow exponentially, as seen in bacterial populations. However, in reality, population sizes remain relatively constant.

(ii) Struggle for existence

• Natural resources are finite, and populations remain relatively stable, aside from seasonal variations. This stability has led to competition for resources, where only some individuals have survived and thrived at the expense of others. This phenomenon is known as the struggle for existence.
• It occurs in three forms:

(a) Intraspecific struggle: This is the competition among individuals of the same species for identical necessities such as food, shelter, and breeding opportunities. It is the most intense type of struggle.

(b) Interspecific struggle: This involves competition between individuals of different species for resources like food and shelter. It is the most significant driver of organic evolution.

(c) Environmental struggle: This struggle is between the organisms and their environment. All organisms struggle with cold, heat, wind, rain, drought, flood, etc. 

(iii) Variations and heredity

• Members of a population exhibit a range of characteristics, with no two individuals being exactly alike despite their superficial similarities. This means that there is inherent variation within the population. Characteristics that improve an individual's chances of survival under natural conditions (such as climate, food availability, and physical factors) are known as adaptive or beneficial variations. Conversely, characteristics that do not confer such advantages are referred to as nonadaptive or harmful variations. 
• Darwin's groundbreaking insight recognised that heritable variations, which enhance an individual's ability to utilize resources and better adapt to their habitat, would allow them to reproduce more successfully and leave more offspring.

(iv) Natural selection/ Survival of the fittest

• Individuals with more adaptive variations are "better fit" than those with less adaptive variations. Hence, those who fit better in an environment are naturally selected and leave more progeny than others. Darwin called this natural selection and implied it as a mechanism of evolution. 
• Fitness results from the ability to adapt and get selected by nature. 
• According to Darwin, fitness ultimately refers only to reproductive fitness. It is observed that all adult individuals of a population don't have equal chances of mating; females prefer some males with better phenotypes. This is called sexual selection. 

(v) Origin of New species

•  As a result of heritable variations and natural selection, there would be a change in population characteristics, and hence, new forms appear to arise.

Criticism of Darwinism 

• The main drawback of this theory is that Darwin didn't know genetics, and he had no satisfactory explanation for the cause, origin, and inheritance of variations. 
• This theory only explained the survival of the fittest but was unable to explain the arrival of the fittest. 
• Darwin was unable to explain why only a few individuals develop useful variations in a population and others have harmful variations. 
• Criticism of Darwinism was based on sexual selection. Why do only females have the right to be selected for mating? 
• Darwin couldn't explain the existence of vestigial organs. 
• Darwin was unable to differentiate the somatic and germinal variations. 

Neo-Darwinism/Mechanism of Organic Evolution

• Neo-Darwinism is a modified form of Darwinism, along with recent research by Weismann, De Vries, Stebbins, Dobzhansky, Sewall Wright, Mayr, etc. 
• According to this theory, the following factors are responsible for the formation of new species- 
• Rapid multiplication 
• Limited food and space 
• Struggle for existence 
• Genetic variations 

(a) Gene recombination—New combinations of genes are usually caused by the crossing over during gametogenesis. It is a continuous and common source of variation in a sexually reproducing population. 

(b) Mutation - Discontinuous source of variations 

(c) Hybridization—This process involves the crossing of organisms that are genetically different in one or more traits.

(d) Gene Migration and Gene Flow—When a portion of a population relocates to a different area and integrates with another population, the gene frequencies in both the original and new populations are altered. The new population gains new genes/alleles, while the original population loses them. Repeated instances of this gene migration lead to gene flow.

(e) Genetic drift - If the change in gene frequency occurs by chance, it is called genetic drift.

• Natural Selection: Natural selection is a process in which heritable variations enabling better survival enable reproducing and leaving a greater number of progenies. A critical analysis suggests that variation results in a changed frequency of genes and alleles in future generations. Coupled with enhancing reproductive success, natural selection makes it look like a different population and leads to new species formation. 
• Isolation - It involves the separation of populations through various barriers that hinder interbreeding. Reproductive isolation arises between these populations due to factors like geographical barriers, ultimately resulting in the emergence of distinct species.

4.0Evidences and examples of Organic Evolution

1. Palaeontological Evidences

• The study of fossils is called palaeontology. 
• According to Charles Lyell, Fossils are impressions or remains of complex parts of life forms found in rocks. 
• Most fossils are found in sedimentary rocks. 
• Fossils found in rock sediments of varying ages likely represent the life forms that perished as those sediments were deposited. Analyzing fossils from different sedimentary layers can reveal the geological periods during which these organisms lived.
• Some of them represent extinct organisms (e.g., Dinosaurs). 
• The study shows that life forms vary over time, and certain life forms are restricted to specific geological periods. 
• New forms of life have arisen at different times in Earth's history, i.e., evolution has taken place. 
• Generally, fossils found in older rocks are simpler, and those found in newer rocks are both simple and complex.  
• Using fossils, we can study the evolutionary pedigree of animals like horses, elephants, and man. 
• The geological history of Earth closely correlates with its biological history.

ARCHAEOPTERYX: 

It is a missing link between reptiles and birds. The connecting links that are not found at present are called missing links. Andreas Wagner discovered its fossil in Bavaria, Germany. It was found in the rocks of the Jurassic period. 

Reptilian characters

• Long lizard-like tail with free caudal vertebrae 
• Non-pneumatic bones 
• Weak sternum 
• Teeth present in jaw

Avian Characters

• Feathers on body 
• Jaws modified into beak 
• Forelimbs modified into wings (reduced) 
• Hind limbs built-in avian plant

2. Evidences from Connecting Links 

Some organisms possess characters of two separate groups called connecting links, proving that higher group members have evolved from the lower group. 

• Examples
• Virus - Between living and nonliving 
• Euglena - Between plants and animals 
• Proterospongia - between Protozoa and Porifera 
• Neopilina - between Annelida and Mollusca 
• Peripatus - between Annelida and Arthropoda 
• Balanoglossus - between Non-Chordata and Chordata

3. Evidence from Comparative Morphology and Anatomy

Similarities and differences are found among organisms of today and those that existed years ago. Such similarities can be interpreted to understand whether common ancestors were shared or not. 

These similarities are of two types-

1. Homology 
2. Analogy

• Homology: Organs originating from the same embryonic source and sharing fundamental structures, despite performing varying functions, are termed Homologous organs, demonstrating the principle of Homology.
• Examples of homologous organs :- 

(i) The forelimbs of mammals such as Whales, bats, Cheetahs, and humans exhibit a common anatomical blueprint, comprising humerus, radius, ulna, carpals, metacarpals, and phalanges. Despite serving diverse functions across these mammals, their forelimbs showcase structural similarities owing to their shared evolutionary ancestry.

(ii) Thorn of Bougainvillea and Cucurbita tendril are both axillary bud modifications. 

(iii) Hearts of vertebrate 

(iv) Brains of vertebrate 

• Adaptive Divergence- It refers to the phenomenon where various species evolve within a specific geographic area, originating from a common point and then spreading out into different habitats. This process is driven by divergent evolution, resulting in homology among species.
• Examples: 

Darwin's finches - During his journey to the Galapagos Islands, Darwin observed a fantastic diversity of creatures. Of particular interest, small black birds, later called Darwin's Finches, amazed him. All the varieties, he conjectured, evolved on the island itself. Many other forms with altered beaks arose from the original seed-eating features, enabling them to become insectivorous and vegetarian finches.

4. Evidences from Comparative Morphology and Anatomy

Analogy

• Organs that have different origins and fundamental structures but perform similar functions are called Analogous organs, and this phenomenon is called analogy. When different structures evolve for the same function due to similar habitats or adaptations, this is called convergent evolution. The analogy doesn't indicate common ancestry. 
• Examples

(i) The Wings of butterflies and birds are not anatomically similar structures, though they perform identical functions, i.e., they are used for flying. 

(ii) Eye of the octopus and mammals 

(iii) Flippers of penguins and dolphins 

(iv) Sweet potato (root modification) and potato (stem modification) 

(v) Sting of bee and scorpion

Convergent evolution or Adaptive convergence

• When multiple instances of adaptive radiation unfold within a confined geographic locale, each inhabiting distinct habitats, it signifies a phenomenon known as convergent evolution. 
• In Australia, placental mammals showcase this adaptive radiation by diversifying into various forms, each bearing a striking resemblance to a corresponding marsupial counterpart. A notable example is the resemblance between placental wolves and marsupial Tasmanian wolves. 
• Parallel evolution, on the other hand, manifests when closely related species undergo adaptive convergence, evolving independently yet in sync towards similar traits. This occurs when two distinct but akin species progress in parallel trajectories, acquiring comparable characteristics autonomously.

5. Evidences from Embryology 

• This was proposed by Von Baer (father of embryology). He stated that "in embryonic stages, general characters appear first and specialized characters appear later".
• In 1866, Ernst Haeckel proposed 'Biogenetic law' which explained "Ontogeny recapitulates phylogeny".
• Ernst Haeckel's law was based on observing certain features during the embryonic stage that are common to all vertebrates and absent in adults. For example, the embryos of all vertebrates, including humans, develop a row of vestigial gill slits just behind the head. Still, it is a functional organ only in fish and not found in other adult vertebrates. 
• It means an organism shows its ancestral adult stages during its embryonic development. In other words, embryos of advanced species pass through stages represented by adult organisms of more primitive species. 
• It shows that all organisms have a common ancestry. 
• Interestingly, Von Baer (1828) had disproven the 'Biogenetic law' before Haeckel invented it. He observed that embryos never pass through the adult stages of other animals. 

Examples

(1) The tadpole (larva of amphibians) resembles fish. This indicates the origin of amphibians from fishes. 

(2) During the development of the heart in higher vertebrates like birds & mammals, it initially exhibits the 2–chambered state, the same as fishes. Later on, it develops into 3- chambered as in amphibians & reptiles and finally, in the last embryonic stages, it becomes 4– chambered as present in the adults. This proves that all vertebrates have evolved from common fish-like ancestors and also that both birds & mammals have evolved from reptiles.

5.0Sample Questions from Organic Evolution

Q.1 Explain adaptive convergence.

Ans. When more than one adaptive radiation appears to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution. 

Q.2 What is reproductive isolation?

Ans. It prevents interbreeding between the populations of two different or closely related species. It maintains the characteristics of the species but can lead to the origin of new species.

Q.3 How does speciation help in evolution?

Ans. The formation of one or more new species from an existing species is called speciation. So, speciation forms diversity in living organisms. Speciations are of two types- (1) Divergent speciation and (2) Transformation speciation. 

Q.4 Name different types of Natural selection.

Ans. There are three different types of Natural Selection

• Stabilizing selection
• Directional/ Progressive selection
• Disruptive selection

Q.5 Explain Genetic Drift.

Ans. Genetic drift is a random change of gene/allelic frequencies in a population merely by chance. It operates rapidly with a small population. This is due to habitat fragmentation, isolation, natural calamities, or epidemics.