Heredity, Variation and Evolution of Organisms | Term Paper | Biology

Here is a term paper on the heredity, variation and evolution of the organisms.

Term Paper # 1. Heredity of Living Organisms:

Heredity includes those traits or characters that are transmitted from generation to generation and are therefore fixed for a particular individual called as heritable characters. Contrastingly, large number of ‘things’ like experiences, practice, mental ability, courage cannot be transferred, instead are acquired by the individual with time. These traits are NOT transferred called as non- heritable characters.

In nature, the process of heredity has great importance. We know all organisms have a finite lifetime. In their lifetime, they reproduce and pass on their characters to their progeny. This transfer is not 100% exact copy, but changes in the traits or characters occur.

If same characters had been passed on by all the existing organisms, say 1 billion years ago, we as humans might not ever come in existence. This world as we know it would not be here. Our fantasies about humongous dinosaurs would have no meaning.

This means the process of heredity is a glitch, a minor malfunction. These ‘glitches’ are manifested in the appearance of new characters and hence new organisms over a period. Thus, these glitches or variations are collectively shaping the concept of evolution. Study of Heredity and Variation is Genetics. The term genetics was for the first time used by W. Bateson in 1905.

An Austrian monk, namely Gregor Johann Mendel, was the first person to study genetics. He is referred as ‘Father of Genetics’.

Classical Genetics: Mendel’s Work:

Mendel was the first to demonstrate and study the mechanism of transmission of characters from one generation to the other. He carried out his work on garden pea, Pisum sativum because, the plant has several characters with two traits each. He selected seven pairs of contrasting traits of garden peas and found that for each character, there are only two possible traits.

The characters and possible traits are listed below:

Laws of Inheritance:

Definitions of the three important principles/laws deduced from Mendel’s work are given below.

A. Law of Dominance:

When two homozygous individuals with one or more sets of contrasting character are crossed, the characters that appear in the F1 hybrids are dominant characters and those which do not appear in F1 are recessive characters, however the genes are carried in generations.

B. Law of Segregation:

The law of segregation states that when a pair of contrasting factors or genes or alleles are brought together in a heterozygous condition, the two remain together without being contaminated only when gametes are formed from them, to show each character.

C. Principle of Independent Assortment:

If the inheritance of more than one pair of characters is studied simultaneously, the factors or genes for each pair of the characters are assorted and passed on to the progeny independent of each other.

Mendel’s Experiments:

Monohybrid Cross:

In 1857, Mendel began a series of experiments on the pea plant (Pisum satvium) to study the pattern of inheritance of various characters. He crossed a variety of pea plants, carrying particular traits (e.g. tallness, with another variety having contrasting traits e.g. dwarfness) of the same character.

These two types were called the Parental Types (P or P₁). The generation that was produced out of these two was called the First Filial Generation (F₁). When these were self -pollinated, the second generation that was produced was called the Second Filial Generation (F₂). This type of cross is called a Monohybrid Cross.

Notations in a Monohybrid Cross:

The dominant trait is written with a capital letter. For example, tallness is represented as T and dwarfness is represented with the corresponding small letters t. If, tallness is due to both the dominant alleles, it is written as TT. If, tallness is due to only one dominant trait, then it is written as Tt.

If both the alleles are recessive, making the organism dwarf, then it is written as tt. A homozygous condition is one in which both the alleles are of the same nature, for example, TT or tt. In Heterozygous condition, where the two alleles are of different nature, traits are written as Tt.

Observations from a Monohybrid Cross:

He observed that in F₁ generation, no dwarf plant occurred, or all plants were tall. He concluded that due to some reason, tallness trait is dominating or suppressing the dwarfness trait and thus named tallness as the dominant trait and dwarfness as the recessive trait. This observation is the proof of the Law of dominance.

In the F₂ generation, the phenotypic ratio was 3(tall):1 (dwarf) i.e. 75% tall and 25% dwarf. This led Mendel to conclude that the alleles representing ‘dwarfness’ were intact and were neither lost nor contaminated.

This led to the base of the Law of segregation. This means that although the alleles of a character remain invisible for generations, they are still present and separated in subsequent generations.

Di-Hybrid Cross:

In a di-hybrid cross, Mendel crossed genetically pure yellow round seeded (YYRR) pea plant with green wrinkled (yyrr) pea plant. All the plants of F1 were all yellow and round seeded (YyRr).

In F2 generation, four types of plants appeared as:

a. Yellow rounded – 9

b. Yellow wrinkled – 3

c. Green rounded – 3

d. Green wrinkled – 1

So here phenotypic ratio is 9: 3: 3: 1.

Notations in a Dihybrid Cross:

In a dihybrid cross, two characters are taken into account. Hence, the notation for the homozygous dominant would be AABB, and for the homozygous recessive it would be aabb. Thus, the law of segregation is valid for every case. All possible combinations are to be written before making a cross.

Observations of a Dihybrid Cross:

This shows that the chances for the pea seeds to be round or wrinkled have no effect on their chances to be yellow or green. In other words, each pair of alleles is independently separated of the other pair. This is the principle of independent assortment.

Reasons to Choose Pea Plant:

1. Pea plants are self-pollinating.

2. They are easy to cultivate. Mendel could observe inheritance patterns in up to two generations a year, because peas are cold season annual crops that can grow quickly.

3. They have sharply defined characters.

4. They are predominantly self-pollinated, which means different true breeding lines are easy to develop.

5. Easily grown in large numbers, occupy small space.

Concept of DNA, Genes and Chromosomes:

We are continuously saying that a character has different traits. These traits are expressed in some and not expressed in others depending on the arrangement of the ‘factors’ or the genes. But, what is really happening? Take an example of the character of height in the pea plant which has two possible alternates, either the plant is dwarf or, it is tall.

The combination on ‘factors’ responsible for a tall plant is either ‘TT’ or ‘Tt’. The presence of a ‘T’ is mandatory to have a tall plant. This T should be something that is imparting the height to the plant.

The plant can reach good height if it has a high amount of growth hormones released in its body. Subsequently, this T must be that part of the genetic material which in turn will be responsible for higher hormone production.

How is this possible that the traits or characters are transferred to other generations? It cannot be blood as fathers characters will not be transferred in that case. Then what is medium of transfer? A large research revealed it was DNA and not proteins.

The genetic material that is inherited by next generation is in the form of DNA (deoxyribo nucleic acid). This DNA is not present as a single thread, but it is arranged in highly folded structures called chromosomes, such that the entire DNA content is divided among many chromosomes. E.g. In humans, there are 23 pairs of chromosomes constituting a complete genome of human species. The genome is genetic material including coding and non-coding genes.

A part of DNA (or chromosome) which is responsible for taller growth hormone production, will be designated as T. This functional region of DNA which is responsible for a trait is called a gene. This gene has codes responsible for proteins, enzymes, hormones that control full life.

Illustration:

If a trait A exists in 10 percent of population of an asexually reproducing species and a trait B exists in 60 percent of the same population. Which trait is likely to have arisen earlier?

Sol: Trait B

---

Term Paper # 2. Variation of Living Organisms:

Any difference between individual organisms or groups of organisms of any species caused either by genetic difference or by the effect of environmental factors is called variation. Variation is observed in physical appearance, metabolism, behavior, learning and mental ability, and other visible characters of an individual.

‘Variation is concerned with the difference between the parents and off-springs of same species and among the offspring of the same parents.’

Types of Variation:

On the basis of heritability, variation is of two types:

a. Somatic variation:

Somatic variation affects the somatic cells of an organism and is unique for an individual. It is neither inherited from parents nor transmitted to the next generation. It is acquired by an individual during its life and is lost with its death. It is therefore also called an acquired variation, e.g. Tanning of the skin due to exposure to the sun, ability to swim, ability to solve problems, etc.

b. Germinal variation:

This variation affects the germ cells or reproductive cells of an organism and is consequently inheritable. It is received by the individual from the parents and is transmitted to the next generation. It occurs either by mutation or recombination of genes from both the parents, e.g. different eye colours, hair colour, the shape of the fingers, etc.

Significance of Variation:

Variation enables the organisms to adapt themselves to the changing environment.

i. It forms a raw material for evolution.

ii. It enables the organisms to face the struggle for existence in a better way.

iii. It helps men in improving the races of useful animals and plants.

iv. It is the basis of heredity.

v. It also leads to the existence of new traits.

---

Term Paper # 3. Evolution of Living Organisms:

It is the process by which different kinds of living organisms are developed and diversified from earlier forms during the history of the earth. It stands for change in species, generation after generation in accordance with environmental requirements to turn into better organized and more complex new species. It is due to opposing forces that constantly introduce variation in traits and processes that make particular variants more common or rare.

Evidences of Evolution:

(i) Fossils:

The remains (or impressions) of dead animals or plants that lived in the past are known as fossils. For example- Archaeopteryx, it looks like a bird, but also possesses many characteristics of reptiles. This is because Archaeopteryx has feathered wings like birds but its mouth and tail resemble those of the present reptiles.

Archaeopteryx is, therefore, a connecting link between birds and reptiles. Thus, fossils provide the evidence that the present animals (and plants) have originated from the previously existing ones through the process of evolution.

(ii) Homologous organs:

The organs that have a common origin but evolved to perform different functions are homologous. The presence of homologous forelimbs in a frog, a bird, and a man indicate that all these forelimbs have evolved from a common ancestral animal that had a ‘basic design’ limb. This hypothetical organism is the common ancestor for frogs, birds, humans, etc.

(iii) Analogous organs:

The organs that have different basic structure or design but have a similar appearance and perform similar functions are called analogous organs. For example, the wings of an insect and bird have different structures. The insects have a fold of membranes as wings that are associated with a few muscles.

In birds, a skeleton, flesh, and feathers support bird’s wings. The presence of analogous organs indicates that even the organisms having organs with different structures can adapt and perform similar functions for their survival. Analogous organs perpetuate that evolution process is diverse, and similar functions/adaptations can be acquired by more than one way.

(iv) Vestigial organs:

Vestigial means useless. Those organs in an organism that possess no active function are called vestigial. Vestigial structures are actually the functionless remnants of the earlier functioning ones. For example, the vermiform appendix of the large intestine and the nictitating membrane of the eye are vestigial organs in human beings.

The appendix is a vestigial organ in humans but is functional in ruminant animals. This indicates that human beings may have evolved from such mammals that had a functional appendix.

Similarly, the nictitating membrane in humans (which is present as a small fold of skin in the corner of the eye) is a vestigial organ but the nictitating membrane is functioning in birds and provides protection to their eyes. This infers that human beings may have evolved from those ancestors who had a working nictitating membrane in their eyes.

(v) Embryology:

The study of the development of the embryo of an animal is called embryology. Developmental stages of the embryos of different vertebrate animal show similarities in their structure. In fact, the embryos of the different vertebrate animals are so similar in their early stages of development that it is difficult to distinguish one from the other.

The early embryos of all vertebrate animals like fish, salamander, tortoise, chick, calf and human look alike indicates that all these animals have evolved from a common ancestor.

Illustration 1:

Why does the tadpole of frog resemble a fish?

Sol:

Amphibians have evolved from fishes. Frog’s tadpole briefly recapitulates the fish – like ancestral stage during its development. It is an example of the biogenetic law.

Darwin and Evolution:

Charles Robert Darwin, an English scientist, sailed for five years on the survey ship, H.M.S. Beagle in 1831 as a young naturalist, when he was only 22 years old. Based on the data he collected, in 1839, Darwin published his book “The Origin of Species”.

The theory proposed by Darwin is popularly known as the “Theory of natural selection” or Darwinism. The main features are over production, limited food and space, the struggle for existence, variations, natural selection, and inheritance of useful variations.

There are two main points in Darwin’s Theory:

A. There is a continuous competition between the member of same species and different species for food, water, space, etc. Such that the organism that adapts best to this competition is selected by nature (natural selection) and others are gradually eliminated (survival of the fittest). This process goes on continuous, leading to the extinction of old species and appearance of new ones at the same time.

Example:

Evolution of Giraffe:

A giraffe has a characteristic long neck. Suppose, at some point of time a giraffe used to look like just a horse like an ancestor, with a normal neck. Now, this ancestor used to feed on grass and some leaves off the branches of trees that grow a bit low. After some time, the population of these ancestors increased and the competition among the individuals started increasing.

All the grass was eaten up, and the only food source left was leafy trees. We are aware that in the same population, individuals of different heights exist. For example, in humans, normal height ranges from 4.5-6.5 ft and also the son of a tall father tends to be tall, means this trait is also heritable.

Similarly, in these ancestors, there used to be the ones with long necks (not as long as a giraffe, but longer than the general height in the population). These ‘long-necked’ individuals could reach up to the height of feeding on trees where other individuals could not. This feature/trait gave them a competitive edge over the others, so their progeny that is also long- necked were to be able to feed and hence selected while ‘short-necked’ individuals were eliminated.

Over time, this process was repeated, such that individuals with much longer necks were selected. After thousands of years, this selection led to an emergence of completely new species of a giraffe from a primitive ancestor.

B. From the illustration above, one more fact can be deduced. The variations (in this case longer neck) are randomly existent in the population already. It is only because of environmental pressure; it is decided that the variation will be selected or not. When these variations are accumulated over time, new species are formed.

Illustration 2:

Give an example of characteristics being used to determine how close two species are in evolutionary terms.

Sol:

A thorn of Bougainvillea plant (glory – of – garden) and a tendril of Passiflora plant (passion flower) are homologous organs. Both these perform different functions but have a similar basic structural design, i.e., both are modified branches. During the course of evolution, these two species evolved from common ancestor having a fundamental structural design that developed subsequently into a thorn in one species and tendril in other species to perform different functions.

Speciation:

The origin of new species from existing ones due to reproductive isolation of a part of its population is called speciation.

The important factors that could lead to rise (or formation) of a new species are:

a. The geographical isolation of a population caused by various types of barriers (such as mountain, ranges, rivers and sea). The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of population.

b. Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.

c. Variations caused in individuals due to natural selection.

Human Evolution:

The sequence of evolution from human ancestors to humans present is a tree.

The major series are:

1. Australopithecus:

Earliest human like hominids that had a small brain and projecting jaw, straight walked with 2 feet, around 3.7 million years ago.

2. Homo habitus:

(Or handyman) earliest species of homo found in Africa woodlands with the brain size of 650-800 ml, made simple tool.

3. Homo erectus:

First to leave Africa and move to Europe and Asia. It had the slopping forehead, flattish face and the brain size of 850 – 1100ml, exploiting habitats.

4. Homo netherlands:

The first humans to have adopted to life in cold climates of Europe and Asia with strong physique and brain. Made clothes, range of tools and invented fire.

5. Homo sapiens:

A modern human with a large brain, intelligent, had the ability to use language, used surrounding for better use.