Classification and Evolution: Our planet is home to millions of living organisms. We find that each organism is different from others to a lesser or greater extent. In order to study the characteristics of different organisms in an easier and fruitful way, several taxonomists put their effort into categorising organisms into different or similar groups based on their unique and shared characteristics.

As we all know that all living beings have arisen from a common ancestor however show variations in a few or more characteristics due to gradual changes. Hence these organisms are classified into different groups. Can we state that Classification and Evolution are interrelated with each other? If yes, then what is the evidence to prove the relationship between classification and evolution? Let’s answer these questionnaires by thoroughly reading the article.

What is Classification and Evolution?

Classification can be defined as the method of systematically arranging the different species of living organisms into closely related groups based on similarities and differences.

The sequence of gradual changes in the properties of a population of organisms or groups over the course of generations is called biological evolution. The evolution leads to the development of a complex life form, the simple life form. Human beings are the most recent and highly evolved organisms that exist at present.

Relation Between Evolution & Classification

There is a hierarchy of characteristics that help classify organisms into certain groups. Closer the groups, maximum the similar characteristics and vice-versa. Evolution refers to gradual changes, and a set of different characteristics leads to the classification of organisms.  Some of the important relationships which act as evidence of proving the relation between classification and evolution can be discussed as follows:

1. The classification started as an artificial system based on one or two characteristics that later developed into a natural system based on natural affinities and actual kinship found in the organisms. It is therefore concluded that:
i. Resemblances in animals are because they have arisen from a common ancestor, and
ii. Differences in them are chiefly due to adaptations to different types of environments for better survival. These adaptations lead to the gradual changes that result in the evolution of a new organism with variations in features.
2. The animals, when viewed together, seem to have evolutionary relationships with one another and are arranged in a series of increasing complexity. The Porifera are the simplest and placed at the bottom, and chordates (vertebrates) are complex and placed at the top in a series.
3. The biologists have summarised their study in the form of a family tree called a cladogram. In which the phyla represent the major branches of the tree of life. These are further divided into classes which are divided into orders.
This taxonomic tree with its branching system represents an interrelationship among groups of organisms.
4. The fact that animals could be graded in order of increasing complexity is evidence of evolution. The natural system of classification is based on similarities, and such similarities could be only due to their origin from common ancestors.
Conclusion of the interrelationship of classification and evolution:
The interrelationship between classification and evolution can be concluded with the following two facts:
i. Some groups of organisms have ancient body designs that have not changed very much, and these organisms are called primitive organisms.
ii. There are other groups of organisms that have evolved their body design very recently and exhibit variations from their ancestry, and are called advanced or higher organisms.
Charles Robert Darwin clearly and convincingly set forth the concept of natural selection as the mechanism of evolution. Darwin gave the biological world a master key that unlocked the previous intricacies about evolution in his book The origin of species. His theory of natural selection stated that a variety of existing species have originated from a common species as a result of adaptations to their surrounding environment.

Evolutionary Evidence Supporting Biological Classification

The ancient belief of the origin of life was entirely based on the supernatural phenomenon and the act of the almighty. Afterwards, Darwin proposed the theory of the Origin of species by natural selection justified that different species arise from a common ancestor and show the variations due to adaptations.

The following evidence is given in favour of the evolution of new species and their classification into different groups:

1. Comparative Anatomical and Morphological Evidence: Comparative study of the morphology and anatomy of living beings state that certain structural features are basically similar. These features include body organisation, gradual modification, homologous organs, analogous organs, connecting links, vestigial organs, and atavism.
I. Body organisation: Body organisation serves as one of the important bases for classification. The body organisation from the simplest to the complex organism show the following level of organisation:

The organisms are classified as sponges, Platyhelminthes, nematodes, annelids, arthropods, molluscs, echinoderms, and chordates based on body complexity. Vertebrates are highly evolved organisms with complex body organisation.

II. Gradual modifications: Studies of gradual modification of the heart of vertebrates and the enlargement of the cerebrum of the brain in vertebrates leads to their classification into different classes. The primitive vertebrates are fishes that have one atrium and one ventricle. Further, the complexity increases with the increase in the number of heart chambers.

Fig: Gradual Evolutionary Changes in the Heart

III. Homology: It is the relation among the organisms of different groups that show similarities in basic structure and development but have different functions. It is based on divergent evolution.
i. Thorn of Bougainvillea and tendril of Cucurbita are homologous, both arising in the axillary position but perform the function of protection and support, respectively.
ii. Vertebrate forelimbs contain the same sets of bones organised in a similar way despite their functions. The similar basic structure of forelimbs justifies that all the vertebrates have a common ancestor.

Fig: Homology of Vertebrate Limbs

IV. Analogy: In analogy, the organs are functionally similar but anatomically different. It is based on convergent evolution. The examples of animals exhibiting analogy are as follows:
i. Though used for swimming, fish and whales’ flippers have different basic structures and origins. Fins are body extensions supported by fin rays, while flippers are modified pentadactyl limbs.
ii. The eyes of cephalopods and vertebrates are photoreceptors. However, vertebrate eyes originate as an outgrowth of the embryonic brain, and the eyes of cephalopods are the modification of skin.

Fig: Analogy of Vertebrate Limbs

These structural differences lead to the classification of vertebrates and invertebrates as two main groups of animals. Further structural differences lead to the classification of invertebrates into ten different phyla and the classification of vertebrates in different classes.

V. Connecting links: The organisms that show the characteristics of two adjacent groups are called connecting links. Euglena, Peripatus, lungfishes, and egg-laying mammals are familiar instances of connecting links.
i. Euglena is a protozoan that serves as a connecting link between plants and animals.
ii. Peripatus is a connecting link between annelids and arthropods.
iii. Lungfish is a connecting link between fishes and amphibians.
iv. Egg-laying mammals (duck-billed platypus) link the mammals to reptiles.
The connecting links represent the interrelationship between the organisms of two different phyla or classes. Connecting links supports the evolution of organisms from a common ancestor as they share the characteristics of two adjacent or closely related groups.
i. Euglena is a unicellular prokaryote that exhibits locomotion like animals and alike plants. It contains chlorophyll for photosynthesis. Therefore it shares the most significant and characteristic features of plants and animals and represents a connection between plants and animals.
ii. Duckbill platypus is a mammal as it possesses hairs on the skin, mammary glands, and some other characteristic features of mammals. The oviparity connects it to the other classes of vertebrates. Hence it can be said that all the classes of vertebrates are interconnected with respect to their evolutionary features.

2. Comparative Embryological Evidence:
I. The similarity in early development: All animal life starts as a single cell zygote. The zygote undergoes changes and forms an embryo. A series of embryonic development similar in all vertebrates ensures that the basic structure of the embryo reflects the common ancestry of all vertebrates.
II. The resemblance in the basic features of vertebrate embryos exhibits the common ancestry of all the vertebrates. They have more or less the same form and structures like gill clefts, tails, etc. Hence are together placed in the subphylum Vertebrata.
The embryos of closely related vertebrates resemble more and for a longer period than the embryos of distant vertebrates. For instance, the embryo of fish and salamander resemble more and for a longer duration than the embryos of a fish and a salamander. Therefore, the degree and the duration of resemblance in the embryos indicate the evolutionary relationship of adult vertebrates.

Fig: Resemblance in the basic plan of vertebrate embryos

III. A Recapitulation theory or Biogenetic law proposed by Baer is strong evidence for the evolution of organisms from their ancestors. The theory states that generalised features appear earlier than special features. This statement can be briefly put into three words; Ontogeny recapitulates phylogeny. Ontogeny is the life history of an individual animal; phylogeny is the evolutionary history of the race of the animal. This means that the organism repeats its ancestral history during its development. For example, the frog has a tadpole larva during its developmental stage that resembles fish.
IV. The development of certain organs such as the heart, brain, and ear in the vertebrates indicate their common ancestry. The heart, during its development in birds and mammals, passes through two-chambered and three-chambered stages before becoming four-chambered. This indicates that both birds and mammals have originated from fish.

3. Comparative Palaeontological Evidence:
Palaeontology is the study of past life based on the fossil record. Fossils include not only the bones, teeth, and other hard parts of the animal or plant body but also any impression or imprint left by the past organisms in the soft soil, which got hardened over a period of time.
A comparative study of fossils and the present living organism shows that the basic characteristics of an existing organism resemble the features of an extinct organism that had been observed by studying the fossils of extinct species. Moreover, the differences and complexity in the body features of the present-day organism ensure that certain changes have been evolved according to their habit and habitat. There are the following aspects of fossil study that prove the organic evolution of organisms:
I. Number and nature of fossils in early rocks: The rocks of the early earth contain a smaller number of fossils than later eras, and only the fossils of simple marine invertebrates have been found. Hence life first originated in the sea in the simple form.
II. Distribution of fossils in successive strata: Fossils present in the lower strata are simple compared to the recent fossils found in the upper strata of rocks are more complex. This leads to classifying the organisms from primitive to complex into different phyla or divisions based on their palaeontological studies.
III. Missing links: The fossil record contains some missing transitional links, also called connecting links. For example, Archaeopteryx is a missing link between birds and reptiles. It was found in the rocks of the Jurassic period.
Archaeopteryx is a missing connecting link between reptiles and birds. The mouth is replaced with a beak, as in birds. However, teeth are present like in reptiles. Also, the presence of wings and a tail respectively represent the features of birds and reptiles.
Missing links between the two groups of species show the interrelationship between the two adjacent classes of living beings and therefore help in the hierarchical arrangement of different groups in classification.

Molecular Phylogeny

Molecular phylogeny supports the evolutionary relationship between the organisms by analysing the nucleotide sequences in DNA, amino acid sequences and structural features of proteins in the hereditary molecules (DNA and RNA).

I. Every organism possesses a genome (genetic material) that contains all the biological information required to construct and maintain a living organism.
II. The biological information stored in a genome is encoded in the genes.
III. The information stored in a gene is read and translated by proteins, which attach to the genome and initiate a gene expression.
IV. Mutation in chromosomes (genome) is one of the main reasons for genetic variations.
V. Several mutational changes that may be caused due to chromosomal aberrations and their inheritance show that the two species are interrelated or diverge from each other. These chromosomal aberrations can be of two types:
i. Change in the chromosome structure due to the duplication, deletion, inversion, and addition of a chromosomal segment.
ii. Change in the chromosome number (monoploid, triploid, tetraploid, pentaploid, hexaploid, etc.)

Advanced technologies are now being used to easily obtain the DNA sequence for molecular phylogeny. The polymerase chain reaction is a good example of advanced technology that is widely used in the molecular phylogeny of living organisms.

Summary

Classification deals with the arrangement of organisms into different groups based on a certain set of characteristics. Various studies by taxonomists and biologists put a light on the interrelationship between classification and evolution. It has been stated that all the organisms share some basic characteristics in common and therefore represent a common ancestry. The gradual increase in the complexity of the organisms is the result of adaptation that leads to a more highly evolved organism. As the year’s pass, organisms’ complexity gradually increases, and organisms are classified into a hierarchical series of groups. Comparative morphological, anatomical, embryological, and palaeontological studies describe that classification and evolution are the two sides of a coin.

Frequently Asked Questions (FAQs) on Classification and Evolution

Q.1. What is the evidence of evolution for classification?
Ans: Comparative morphology and anatomy, the fossil record, comparative embryology, molecular phylogeny are the evidence of evolution that support classification.

Q.2. What is classification and evolution in biology?
Ans: Classification is defined as the arrangement of organisms into different groups based on their similar and dissimilar characteristics. Evolution refers to the gradual changes in a group of organisms over the course of a generation.

Q.3. What are the three types of evolution?
Ans: The three types of evolution are convergent evolution, divergent evolution, and parallel evolution.

Q.4. What are homologous organs?
Ans: The organs that have similar basic structures but are adapted to perform different functions are called homologous organs. For example, forelimbs of vertebrates of different classes exhibit homology.

Q.5. How is evolution related to classification?
Ans: The phylogenetic system of classification is based on the evolutionary relationship between the organisms and therefore arranges the organisms into hierarchical series starting from the simplest organisms to the complex organisms.

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