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Heredity and Evolution

Chapter: Heridity and Evolution

 

Introduction:

Reproductive processes give rise to individuals that are similar, but not exactly identical, they show a slight difference. This difference is called variation and the numbers of successful variations are maximized by the process of sexual reproduction. Let us learn about the mechanism by which variations are created and inherited.

Accumulation of Variation During Reproduction:

Inheritance from the previous generations provides both a common basic body design along with some changes. The second generation will inherit differences from the first generation as well will have newly created differences. In asexual reproduction, let us look at bacteria for example. The bacteria first divides into two which will further divide into four and eight respectively. All these bacteria generated would be similar with minor differences between them because of small inaccuracies in DNA copying. In case of sexual reproduction we can observe much more variations in the second generation because of the involvement of two individuals in the process of reproduction.

Heridity:

Heredity is passing certain characteristics from parents to offspring's. The rules of heredity determine the process by which traits and characteristics are reliably inherited. Let us take a look at rules of heredity.

1. Inherited Traits:

A trait or a character that is inherited from one generation to the next generation is called as inherited trait. Some examples of inherited traits are: attached or detached ear lobes, dimples, tongue rolling etc.

2. Rules for the Inheritance of Traits – Mendel’s Contributions:

The rules of inheritance depend on the fact that both father and mother contribute equal amount of genetic material to the child and hence the child will express both maternal and paternal DNA. Thus, for each trait there will be two versions in the child. Mendel worked on inheritance and formulated certain rules which will explain what will be the trait of a child when he/she will express equal amount of genetic material from the parent. Let us look at the experiments carried out by Mendel.

Experiments of Gregor Johann Mendel:

  • Mendel used garden pea plant for his experiments. He used a number of contrasting visible characteristics of peas like: height (tall vs. short), seed coat (smooth vs. wrinkle), seed color (yellow vs.  Green).
  • He performed his first experiment by producing a progeny from a tall pea plant and a short pea plant and then he calculated the percentage of tall and short progeny.
  • He observed that the first generation plants were all tall and there were no medium height plants. He called the first generation as the F1 generation.
  • Mendel then self pollinated the F1 generation, the result was F2 generation or the second generation in which one quarter of them were short and the rest were tall this indicated that the first generation progeny, i.e. the F1 generation plants inherited both tall and short characters but only the tallness trait was expressed. Thus, two copies of the trait were inherited from each sexually reproducing organism.
  • Let us understand this by making a cross:

 

Tall(TT) × short(Tt)                all tall off springs(Tt)

                                                                  [F1 generation]     

      

                         F1 × F1                       Tall(Tt), Tall(Tt), Tall(Tt), short(tt)

                                                             (one quarter of the progeny is short)

 

From the above explanation it is understood that TT and Tt are representing tall plants and tt are representing short plants. Capital letters indicate dominant trait and small letters indicate a recessive trait. In the example given above tallness is the dominant trait and hence it is represented as T, whereas shortness is a recessive trait and is represented as t.

When pea plants with two different characters are bred with each other the F1 progeny will still have both the dominant traits. For example: the progeny of tall pea plant with round seeds and short pea plants with wrinkled seeds will be tall plants with round seeds in the F1 generation indicating that tallness and round shape of the seeds are dominating traits and when the F1 progeny was self pollinated it produced F2 progeny where some were tall plants with round seeds and short plants with wrinkled seeds. But some of the F2 progeny were tall with wrinkled seeds and some were short with round seeds. Thus, indicating that tall/short trait and round seed/ and wrinkled seed trait are independently inherited.

3. How do these Traits get expressed ?

DNA present in the cell is the information source for making proteins in the cell. A portion of DNA that makes a protein is called gene for that protein. Proteins control the characteristics. Let us take an example and see how proteins control the characters. Tallness of the pea plant depends on the amount of a particular plant hormone that controls the height of the plant. The amount of the plant hormone made will depend on the efficiency of the process for making it. Enzymes are required for making of hormone. If the enzyme works efficiently, a lot of hormone will be made. And the plant will be tall. If the gene responsible for making that enzyme is altered it makes the enzyme less efficient, the amount of hormone will be less and the plant will be short. Thus, genes control characteristics or trait. According to Mendelian experiments both parents must contribute equally to the DNA of the progeny during sexual reproduction. If both parents can help determine the trait in the progeny, both parents must contributing a copy of the same gene. This means each organism must have two sets of genes, one inherited from each parent. Therefore each germ cell must have only one gene set. Each gene set is not present as a long single thread of DNA, but as separate pieces called chromosomes. Therefore, each cell will have two copies of each chromosome, one from the male parent and the other from the female parent. Every germ cell will take one chromosome from each pair and these may be of either maternal or paternal origin. When two germ cells combine, they will restore the normal number of chromosomes in the progeny and ensure the stability of the DNA of the species. Such a mechanism of inheritance is used by all the sexually reproducing organisms and it explains the results of the Mendel experiments..

4. Sex Determination:

It is a biological system that determines the development of sexual characteristics in an organism. Different species use different strategies for the determination of sex. In some organisms sex is determined by environmental variables such as temperatures. Therefore, in such animals the temperature at which egg is fertilized determines whether the animal developing in the egg will be a male or a female.  In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined. But in most animals like human beings sex of the individual is genetically determined. All human chromosomes are not paired. Most human chromosomes have a maternal copy and a paternal copy, we have 22 such pairs. But, we have a pair of chromosomes called the sex chromosomes which will determine the sex of the foetus.  XX is the pair of sex chromosomes present in females. In case of men it is different they have one normal sized X and one short sized Y. So women are XX and men are XY. All the children will inherit X chromosome from their mother. If the child inherits Y from their father the child will be a boy and if the child inherits X from the father she will be a girl.

Evolution:

The variations which occur during sexual reproduction because of the errors in DNA copying leads to evolution.

Acquired and Inherited Traits:

The germ cells of sexually reproducing populations are made in specialized reproductive tissue. Therefore, changes which occur in the non-reproductive tissues cannot be passed on to the DNA of germ cells. Thus,  the experiences of an individual during its lifetime cannot be passed on to its progeny, and cannot direct evolution. For example if we breed a group of mice, all their progeny will have tails. If we remove the tails of this mice in each generation still mice in further generations will have tails and that is because removal of tail cannot change the genes of the germ cells of the mice.

Speciation:

Micro evolution is the reason for changes in the common characteristics of a particular species. But this cannot explain how new species can come into existence.  Speciation is an evolutionary process by which new biological species arise.

Evolution and Classification:

Similarity between the organisms will allow us to group them and then study the groups. Characteristics are the details of appearance or behavior. For example we have two hands, it is a character, the plants can perform photosynthesis this is a character of plants. Some basic characters are shared by the organisms. Cell is the basic unit of life in all the organisms. A basic character of cell that differs among the organisms is whether the cell has nucleus or not. Bacterial cells do not have nucleus while most of the organisms do have the nucleus. Among the organisms with nucleated cells some of them are unicellular and some are multicellular. In multicellular organisms the cell divides to form special tissue and organs while unicellular organisms perform all functions with a single cell. Multicellular organisms can be further classified based on their ability to perform photosynthesis. The multicellular organisms which cannot perform photosynthesis can be classified based on whether the skeleton is inside the body or around the body. The more characteristics two species will have in common, the more closely they are related. And the more closely they are related, the more recently they will have had common ancestors. For example: A brother and a sister are closely related. They have common ancestors in the first generation before them, i.e, their parents. A girl and her first cousin are also related, but less than the girl and her brother. This is because cousins have common ancestors, their grandparents, in the second generation before them, not in the first one. We can now appreciate that classification of species is in fact a reflection of their evolutionary relationship. We can therefore build groups of species with common ancestors, then super-groups of these groups with more distant common ancestors, and so on.

Tracing Evolutionary Relationships:

The characteristics in different organisms would be similar because they are inherited from a common ancestor. Let us just look at an example: mammals have four limbs and so do birds, reptiles and amphibians. All of them have the basic structure of the limbs with slight modifications in functions they perform. Such a homologous characteristic helps to identify an evolutionary relationship between apparently different species. The similarities in the organ shape are not necessarily because of common ancestry. For example: birds and bats, both of them have wings but their ancestry is not common. Bats wings are skin folds stretched mainly between elongated fingers. But the wings of birds are a feathery covering all along the arm. The designs of the two wings, their structure and components, are very different. They look similar because they have a common use for flying. This makes them analogous characteristics, rather than homologous characteristics.

Fossils:

The preserved traces of living organisms are called fossils. Usually when an organism dies, their will be decomposed and lost. But sometimes some parts remain in the environment without decomposing for example a dead insect gets caught in hot mud it does not decompose quickly, eventually the mud becomes hard and retains the impression of the body parts of the insect, such impressions are nothing but fossils. We can find out the age of fossils by digging into earth. The fossils that are found closer to the surface are the more recent fossils than the fossils are found in the deep layers.

Evolution by Stages:

Complex organs like eye are created bit by bit over generations. The eye – like the wing – seems to be a very popular adaptation. Insects have them, so does an octopus, and so do vertebrates. And the structure of the eye in each of these organisms is different – the difference is enough for them to have separate evolutionary origins. a change that is useful for one property to start with can become useful later for quite a different function. For example: feathers can start out for the purpose of providing insulation in cold weather but later they become useful for flight. Dinosaurs also had feathers but they could not use them for flight. Birds later on adapted feathers for flight. This means that birds were closely related to reptiles, since dinosaurs were reptiles. It is true that analysis of the organ structure in fossils allows us to make an estimation of how far back evolutionary relationships go. Another way of tracing evolutionary relationships is comparing DNA of different species. The changes in DNA during reproduction are the basic events in evolution. If that is the case, then comparing the DNA of different species should give us a direct estimate of how much the DNA has changed during the formation of these species. This method is now extensively used to define evolutionary relationships.

Evolution Should not be Equated with ‘Progress’

We need to remember certain things while tracing the family tree of a species. Firstly, there are multiple branches possible at each and every stage of this process. A new species has emerged does not mean that old species will disappear.  Also, It is not that the newly generated species are better than the older ones. It is just that natural selection and genetic drift have together led to the formation of a population that cannot reproduce with the original one. It is not true that human beings have evolved from chimpanzees; it means that both have common ancestors. There is no real progress in the idea of evolution. Evolution is simply the generation of diversity and the shaping of the diversity by environmental selection. The only progressive trend in evolution is only that more and more complex body designs have evolved over a period of time. This does not mean that older deigns are inefficient. Human beings are not the pinnacle of evolution, but simply yet another species in the teeming spectrum of evolving life.

Human Evolution:

Excavating, time-dating and studying fossils, as well as determining DNA sequences – have been used for studying human evolution. There is a great diversity of human forms and features across the planet. Long time ago people used talk about human races. Skin color was the commonest way of identifying races. Some were called yellow, some black, white or brown. There is no biological basis to the notion of human races. All humans are a single species. Regardless of where we have lived for the past few thousand years, we all come from Africa. The earliest members of the human species, Homo sapiens, can be traced there. Our genetic footprints can be traced back to our African roots. A couple of hundred thousand years ago, some of our ancestors left Africa while others stayed.