7 Determining Evolutionary Relationship

Dr. Ajeet Jaiswal

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Contents of this unit

 

1. Introduction: what is evolution?

 

2. How do we know that evolution has occurred?

 

3. Evidences from morphology and comparative anatomy

 

3.1 Homology and homologous organs

 

3.1.1 Homology in limb structure of vertebrates

 

3.1.2 Homology in brain structure

 

3.2  Vestigial organs

 

3.3 Evidences from atavism or reversion

 

4. Evidences from embryology

 

5. Evidences from palaeontology

 

5.1 Limitation of palaeontologist

 

6. Evidences from taxonomy

 

6.1 Evidences from connecting links

 

7. Evidences from geographic distribution

 

7.1 Discontinuous distribution

 

7.1.1 Diversity in fauna and flora

 

7.1.2 Discontinuous distribution of closely related species

 

8. Evidences from biochemistry and physiology

 

8.1  Protoplasm

 

8.2 Chromosomes

 

8.3 Genetic code

 

8.4 Haemoglobin

 

8.5 Cytochromec

 

8.6 Insulin

 

8.7 Enzymes and hormones

 

8.8 Comparative serology

 

9. Evidences from genetics  

 

10. Summary

 

 

Learning objectives:

  • This module focuses on the basic concepts of evolution.
  • The study of this module will enable the students to understand how evolution has occurred and what are the evidences of evolution.
  • This module also discusses about the important concepts of embryology, palaeontology, taxonomy, physiology and genetics.
  1. Introduction: What is Evolution?

   Biological evolution is the genetic change in a population from one generation to another. The speed and direction of change is variable with different species and at different times. Continuous evolution over many generations can result in the development of new varieties and species. Likewise, failure to evolve in response to environmental changes can, and often does, lead to extinction.

 

When scientists speak of evolution as a theory they do not mean that it is a mere speculation. It is a theory in the same sense as the propositions that the earth is round rather than flat or that our bodies are made of atoms are theories. Most people would consider such fundamental theories to be sufficiently tested by empirical evidence to conclude that they are indeed facts. As a result of the massive amount of evidence for evolution accumulated over the last two centuries, we can safely conclude that evolution has occurred and continues to occur. All life forms, including people, evolved from earlier species. Furthermore, all living species continue to evolve today. We now understand that there are a number of different natural processes that can cause evolution to occur. These are presented in later modules of this series.

 

For those who have difficulty in accepting evolution because of what they perceive as contradictions with their fundamental religious beliefs, it may be useful to distinguish the ultimate origin of life from its later evolution. Many, if not most, biological scientists accept that primordial life on earth began as a result of chance of natural occurrences 3.5-4 billion years ago. However, it is not necessary to believe in that view in order to accept that living creatures evolved by natural means after the origin of the first life.

 

Charles Darwin modified his religious belief, as did many others, as a result of the discovery of convincing proof of evolution. Darwin’s religious faith was also severely challenged by the death of his 10 year old daughter Annie in 1851. Apparently, he came to believe that his god created the order of the universe including the rules of nature that resulted in biological evolution. His famous book, on the origin of species, was not a denial of his god’s existence. However, he did reject a literal interpretation of the judeo-christian bible. His religious beliefs were probably very similar to those who advocate “theistic evolution” today.

 

2. How do we know that evolution has occurred?

The evidence for evolution has primarily come from four main sources:

  1. The fossil records of change in earlier species
  1. The chemical and anatomical similarities of related life forms
  1. The geographic distribution of related species
  1. The recorded genetic changes in living organisms over many generations

   The theory of organic evolution appears to be the most plausible explanation for the occurrence of varied forms of plants and animals on this earth. But an absolute proof in this connection is absent. The circumstantial proof is the only type of evidence available. The convincing evidence for the occurrence of descent with modification comes from:

  1. Morphology and comparative anatomy
  1. Embryology
  1. Palaeontology
  1. Taxonomy
  1. Geographical distribution
  1. Biochemistry and comparative physiology
  1. Genetics

 

3. Evidences from morphology and comparative anatomy

Morphological studies of various organ systems of vertebrates indicate that these are constructed on the same basic plan. The minor differences seen in some forms are the adaptive modifications to the diverse mode of living. This similarity is known as ‘homology’.

 

3.1 Homology and homologous organs:

Homology is the similarity among organs of different animals based on common origin. However, the structure might look different and the function would vary from one organism to the other. Homology is seen in every organ system from fish to man.

 

3.1.1 Homology in limb structure of vertebrates

The modifications include shortening or lengthening of bone, variation in shape, and reduction in the number of bones or fusion of bones in accordance with the function.

   From the study of forelimbs of various animals it becomes evident that these vertebrates must have had a common ancestor with prototype of forelimb.

 

3.1.2 Homology in brain structure

Ranging from fishes to mammals, the brain consists of similar series of parts like olfactory lobes, cerebral hemispheres, optic lobe, cerebellum, medulla oblongata, etc. As we progress through the series from fishes to mammals some lobes present gradual enlargement (cerebral hemispheres). In fishes, the cerebral hemispheres are even smaller than the optic lobes, but in mammals these are so much enlarged that they hide the olfactory lobes in front and the optic lobes behind.

 

3.2 Vestigial organs

The vestigial or rudimentary organs are the useless remnants of structures or organs which might have been large and functional in the ancestors. These are often undersized, degenerate and nonfunctional.

 

Man alone possesses nearly 100 such vestigial structures like vermiform appendix (remnant of the caecum), muscles of external ear, nictitating membrane or plica semilunaris, vestigial tail vertebrae, wisdom teeth, lobe of the ear.

 

 

3.3 Evidences from atavism or reversion

Atavism or reversion is the reappearance of those ancestral characteristics in an organism or in the organism of a group, which do not occur normally or which represent the reminiscent of normal structures possessed by the individuals of other groups. Such abnormal structures are known as atavistic characters of reversions or atavisms. In such cases abnormal characters of structures appear in the embryo or in the adult, which were not present either in the parent or grandparents but in some remote ancestors.

 

Atavism of course, not very common is well illustrated by numerous examples, like cervical fistula in man, tail, mammary glands, hair on the body and face in Irish dogman, appearance of phalanges on 2nd and 4th digits in horse, homodont dentition in piscivorous cetaceans.

 

1. Evidences from embryology

Haeckel was much impressed by observing a generalized pattern of development and the general resemblances between the embryos of different groups of animals. Hackle formulated the ‘recapitulation theory’ or ‘biogenetic law’. It says “ontogeny recapitulates phylogeny”. Ontogeny is the life history of the individual starting from ovum and phylogeny is the series of adult ancestors of the individual which must have incurred in the evolution of the group of this individuals.

 

Von Baer’s principles of embryonic differentiation constitute a better guide to embryological evidence for evolution. These principles are as follows:

  1. General characteristics appear in the development before specialized characters.
  2. From the more general, the less general and finally the specialized characters appear.
  3. An animal during development departs progressively from the form of other animals.
  4. Young stages of an animal do not resemble to the adults of different groups but with their embryos.

 

5. Evidences from palaeontology

Evidence in support of evolution discussed so far are of circumstantial nature, but the direct evidence comes from the study of fossils. The term fossil (latin: fossilum, something dug out) refers not only to the bones, teeth and other hard parts of animals or plant body, but to any impression or imprint left by some previous organism in the soft mud, which subsequently gets hardened or the moulds and casts of the entire organisms of the soft animals is preserved.

 

Mostly fossils are found in the sedimentary rocks which are formed by the deposition of sand or mud on the bottom of lakes or sea. Entire bodies or parts of dead or dead organism become covered by sand deposits. After their burial most animals rot away without leaving any sign of their existence. But sometimes these deposits provide anaerobic situation required for fossil formation. Even under these conditions soft parts decay leaving only the hard parts like bones, scales, shells, etc. Slowly, over the centuries, the material of hard parts is replaced molecule by molecule with mineral matter from the surrounding mud. The replacement is sometime so accurate that even the cellular details can be studied accurately. Within thousands and millions of years layers of mud shrink and harden into rocks.

 

5.1 Limitation of palaeontologist

  1. Rare chance of fossilization.
  1. Often it is frequently distorted by pressure or fracture.
  1. The evidence is restricted to the details of skeletal structures and teeth and one can only draw inference about the soft parts e.g. Convocational pattern of the brain is always based on the cost of the skull.
  1. Difficulties of interpretation even when rich and more or less complete fossil record is available. This happens when one has some evidence of the earlier representative of an evolutionary line which has not yet developed the distinguishing taxonomic characteristics. This will be clear by the consideration of a spastic situation.
  1. Difficulty of correctly dating the fossil material both in absolute and relative terms.
  1. Differences in rates of somatic evolution which must be taken into account while selecting characters for their economic relevance in the assessment of phylogenetic status of the fossil types.

6. Evidences from taxonomy

The science of naming, describing and classifying organisms is known as taxonomy. Classification started as an artificial system of cataloguing of innumerable living organisms as a librarian classifies and catalogues books. It developed into a ‘natural system’ based upon natural affinities and actual kinship (based on genetic relationship) found in the organisms. It was devised by Linnaeus. It is, therefore, concluded that resemblances in animals are because these have arisen from a common stock, and differences in them are chiefly due to adaptations to different types of environment.

 

All the animal phyla when viewed together seem to have no relationship with one another but it is possible to arrange them in a series of increasing complexity on a ladder like diagram. It is easy to concede that protozoan come at the bottom and chordates at the top of the ladder.

 

Taxonomists have summarized their studies in the form of tree-like diagram in which phyla represent major branches of the tree of life. These are divided into several small branches, classes, which are divided into orders. This taxonomic tree with its branching system like a real tree represents an inter-relationship among groups of organisms descended from a common ancestor and modified along different lines. The living animals constitute the terminal twigs of the phylogenetic tree and do not exhibit any direct relationship.

 

The fundamental relationship lies in the main branches and the trunk and the connecting links either exist as remote ancestors persisting today or died out in the past may be represented by fossil.

 

The mere fact that animals could be graded in order of increasing complexity is an evidence of evolution. The natural system of classification is based upon similarity and such similarities of structures could be only due to an origin from common ancestors.

 

6.1 Evidences from connecting links

While classifying animals one comes across certain animals or small animal groups which exhibit characteristics of more than one group. Such animal groups are called connecting links between those two groups. For example:

  1. Viruses: connecting link between nonliving and living
  2. Archaeopteryx: intermediate between modern birds and reptiles.
  3. Prototheria: connecting link between reptiles and mammals.

7. Evidences from geographic distribution

Distribution of animals and plants on the earth’s surface constitutes another convincing evidence of evolution. The distribution of animals drew Darwin’s attention to the possibility of the “origin of species by evolution” or “descent with modification”. The most important of them is the discontinuous distribution.

 

7.1 Discontinuous distribution

7.1.1 Diversity in fauna and flora: In many cases countries which are very near to each other and having similar climatic conditions differ in the flora and fauna. For example:

  1. Madagascar is only 260 miles from east coast of Africa, but its inhabitants are markedly different.
  2. The climate of Australia, South Africa and western South America is very much the same, but the fauna and flora in each region are strikingly-different.
  3. The fauna of North Africa and South Europe, which are widely separated by Mediterranean Sea, is much more identical than in the above cases.

7.1.2 Discontinuous distribution of closely related species: In some cases closely related species exist in widely separated places with no representatives in the intermediate territory. For example:

  1. Tapirs-are found in tropical America and Malayan islands.
  2. The camels are farmed in Asia, while their nearest allies llamas are found in South America.

   Many species have a very wide range of distribution. If a wide ranging species becomes extinct in the intermediate region of its range, this will result in widely separated population. Fossil evidences indicate the occurrence of such a phenomenon. For example, evolution of horse and camel had occurred in North America through land connections existing in the Pleistocene period. Due to certain reasons the intermediate population of camel in North America disintegrated. These forms in Asia evolved in different line forming camels (being adapted to desert life and the forms in South America evolved independently into llamas). It means that animals showing discontinuous distribution were also present in the past. These migrated from their place of origin and became modified. The intermediate population modified, whereas the intermediate population with no change or with some changes became extinct in due course of time.

 

 This uneven distribution of animals can easily be explained on the basis of organic evolution. A particular species originated at a particular place, migrated to far off places. The area of distribution was thrown up into a few small and as a result changes occurred in the organization of individuals and marsupials only in Australia and New Zealand can be explained that these islands were once continuous with the mainland of Asia but got separated in crustaceous period before the carnivorous eutherians evolved. In the mainland of Asia the monochromes and marsupials were replaced by carnivorous aeutherians but in Australia they continued to flourish and evolved in new species. Similarly, presence of different fauna in Madagascar which was once a part of Africa explains the origin and evolution of different species due to varied climate conditions.

 

8. Evidences from biochemistry and physiology

The most convincing evidence of descent from common ancestor comes from the similarities in the biochemical composition and physiological activities of organisms.

 

8.1 Protoplasm: the chemical composition of all beings is reduced to four main chemical elements, namely carbon, hydrogen, oxygen and nitrogen. These are combined to form their classes of organic compounds, the carbohydrates, fats and proteins. These are found suspended in water along with other organic and certain inorganic compounds forming the protoplasm. The protoplasm is organized into structural units, the cells, which are building block, forming tissues of both animals and plants.

 

8.2 Chromosomes: the essential components of nucleus in every living cell are the chromosomes. The chromosomes have fairly constant chemical composition in the living animals being composed of DNA and proteins. The basic unit of DNA is nucleotide consisting of a molecule of phosphoric acid, one molecule of pentose sugar ‘deoxyribose’ and a purine or a pyrimidine as nitrogenous base.

 

The chemical composition of DNA is basically the same in all living beings except for differences in the sequence of nitrogenous bases. How can such diverse organisms have the same basic fundamental composition? It means all have gradually evolved from the same common ancestor.

 

The degree of similarity between the DNA of two species can be estimated by the pairing property of DNA strands. The DNA is melted into single strands. The single stranded DNA of species A is broken into small fragments. These fragments are brought into contact with the single stranded DNA of species B. The fragments of DNA of species A will pair with those fragments of species B with DNA having complementary base sequences.

 

Any fragments of DNA strand of species A that are not complementary to any region of species B remain unpaired. The greater the degree of pairing in the DNA strands of species AB, the closer are the similarities in the genetic messages contained in the DNA of two species.

 

8.3 Genetic code: It is surprising to note that the same genetic code having triplet code is found from viruses to man.

 

8.4 Hemoglobin: Hemoglobin is a conjugated protein. It is formed by two identical alpha chains and two identical beta chains. Each alpha chain has 141 amino acids and each beta chain has 146 amino acids. Beta chain of hemoglobin of humans and gorillas differ in one amino acid of humans and pig in ten amino acids and of humans and horses in 26 amino acids.

 

The above account makes it clear that gorilla is the nearest living being who is nearer to man and the two species have evolved from a common ancestor.

 

8.5 Cytochrome c – It is a respiratory pigment present in all eukaryotic cells. It forms a part of the electron transport system and all eukaryotes accepts electrons from h+ ions. It is formed of 10µ amino acids. In chimpanzees and humans cytochrome-c molecules are identical.

 

8.6 Insulin: Insulin analyzed from different animals (beef, sheep, pig) in one to three amino acid positions. Beef insulin is so similar to human insulin that it has been used for treatment of human diabetes.

 

8.7 Enzymes and hormones: In the same manner similar or identical enzymes and hormones are found in large number of animal groups. For example, trypsin-the protein splitting enzyme, is found in animals from protozoa to mammals and analyzed from sponges to man. Beef thyroid can be used in the treatment of human thyroid defeciencies.

 

8.8 Comparative serology: The body fluids and tissues also play an important role in providing evidence for the relationship among organisms. Blood relationship is the most popular method used in solving so many taxonomic problems. Dr. George H.F. Nuttal has widely used the precipitation method for finding out the interrelationship among different animals. This has brought human beings or man close to monkeys and apes.

 

9. Evidences from genetics

The final line of evidence from evolution is drawn from genetics, the science of heredity. It has been established now that genes (the hereditary determiner) are quite constant and are inherited unchanged generation after generation. But genes undergo change producing mutation and validations. These changed genes determine the character in a different direction than the original. The natural forces of isolation and natural selection operate on these mutations.

 

The selection and breeding of domesticated animals and cultivated plants for the past several thousand years provide models as to how some evolutionary forces operate in nature. Organisms change with the ever changing environment and many of these variations are heritable. Mutations always affect their genetic makeup leading to the development of new characteristics. Some of these modifications are beneficial to the organisms in question. By selecting and interbreeding these selected varieties new races have been established by man. The same selection and interbreeding might have occurred and may be occurring on large scale in nature and, thus would have established new species.

 

 

 

10. Summary

  • Biological evolution is genetic change in a population from one generation to another.
  • When scientists speak of evolution as a theory they do not mean that it is a mere speculation.
  • Charles Darwin modified his religious beliefs, as did many others, as a result of the discovery of convincing proof of evolution.
  • The theory of organic evolution appears to be the most plausible explanation for the occurrence of varied forms of plants and animals on this earth.
  • The convincing evidences for the occurrence of descent with modification came from: morphology and comparative anatomy, embryology, palaeontology, taxonomy, geographical distribution, biochemistry and comparative physiology and genetics.
  • Homology is seen in every organ system from fish to man.
  • The vestigial or rudimentary organs are the useless remnants of structures or organs which might have been large and functional in the ancestors.
  • Atavism or reversion is the reappearance of those ancestral characteristics in an organism.
  • Haeckel formulated the ‘recapitulation theory’ or ‘biogenetic law’.
  • Evidence in support of evolution discussed so far are of circumstantial nature, but the direct evidence comes from the study of fossils.
  • The science of naming, describing and classifying organisms is known as taxonomy.
  • While classifying animals one comes across certain animals or small animal groups which exhibit characteristics of more than one group. Such animal groups are called connecting links.
  • Distribution of animals and plants on earth’s surface constitutes another convincing evidence of evolution.
  • The most convincing evidence of descent from common ancestor comes from the similarities in the biochemical composition and physiological activities of organisms.
  • The final line of evidence from evolution is drawn from genetics, the science of heredity.

 

you can view video on Determining Evolutionary Relationship

 

 

References

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