Heredity & Evolution

Facts that Matter

Heredity is the transmission of characters or traits from the parents to their offspring.

Variation is the difference in the characters or traits among the individuals of a species.

Accumulation of Variations

    Sexual reproduction of organisms produces variations. The variations produced in organisms during successive generations get accumulated in the organisms. The significance of a variation shows up only if it continues to be inherited by the offspring for several generations.
    In sexually reproducing organisms there is a vast similarity. Variations if any are not genetic but due to environment and are non-inheritable.
      Variation increases the chances of survival of a species in a changing environment.
Selection of variants by environmental factors forms the basis for evolution process.

        Heredity: The rules of heredity determines the process by which traits are inherited.
        Inherited traits are those characters which are transferred from one generation to
another e.g., height, skin colour, blood group etc.
        Rules for Inheritance of traits (Mendel’s contributions):
   Inheritance is the transmission of genetically controlled characteristics from one generation to the next.
      Gregor Mendel choose pea plants for studying inheritance. Pea plants have a number of clear cut differences e.g., some plants were tall, some short, they are easy to tell apart and also they have a short life cycle.

Mendel’s Monohybrid Cross

1. Mendel first crossed pure bred tall pea plants with pure bred dwarf pea plant and found that only tall pea plants were produced in the first generation (F1).

2. Mendel then crossed the tall pea plants of the Ist generation (F1) and found that both tall plants and dwarf plants were obtained in the second generation (F2) in the ratio 3:1 i.e., 3/4 tall and 1/4 dwarf plant (monohybrid ratio).

Mendel’s Dihybrid Cross

        A breeding experiment dealing with two characters at the same time is called a dihybrid cross. Mendel considered shape as well as colour of seeds simultaneously.
         Mendel’s Conclusion: Based on the findings of monohybrid and dihybrid crosses
Mendel concluded—
        (i) In a monohybrid cross, only one of two contrasting characters (traits) appeared
             in F1 generation. However, in F2 generation, both the parental traits appeared in
             certain proportion.
  (ii) In a dihybrid cross, when two contrasting pairs of traits were considered simultaneously, only a parental combination appeared in F1 generation. However, in F2 generation, raised by self-pollination, other combinations of traits appeared. These included two parental type traits and two new combinations in approximately same proportion.

Mendel’s Interpretation: On the basis of monohybrid and dihybrid crosses, Mendel
postulated:
     (i) There are a pair of unit factors controlling each character in pea plant, one  inherited from each parent. Mendel considered these factors as the carriers of hereditary
information from one generation to another, i.e., from parents to the offsprings. At the time of reproduction, when gametes are formed, these factors segregate so that each gamete receives only one factor of each character. This is called law of segregation. Fertilisation brings these two factors again together in the offspring.
       (ii) In F1 generation, only one character was expressed. Mendel called it as dominant
character. The character which was not expressed was termed recessive character. The phenomenon of appearance of only of two contrasting traits in F1 generation, is termed as dominance.
       (iii) The characters are not lost even when they are not expressed.
     (iv) When F1 offsprings were allowed to be self pollinated, both the parental traits were expressed in definite proportion in F2 generation.
        (v) From the F1 generation of a dihybrid cross, Mendel postulated that inheritance of
factors controlling a particular trait in an organism are independent of the other. This is called law of independent assortment. Hence, at the time of reproduction, two pairs of factors of each of the two traits in a dihybrid cross segregated independently during gamete formation and randomly formed combinations.

 

How do these Traits get expressed?

        Traits of parents are transmitted to their offsprings (progeny) through genes present on their chromosomes during the process of sexual reproduction.
         In sexually reproducing organisms sex cells are produced by meiosis. Each gamete (sex cell) has a single set of chromosomes. Chromosomes are thread like structure present in the nucleus of a cell which contain hereditary information of the cell. Chromosomes are made up of DNA and proteins. The most important component of chromosome is DNA. Each species has a fixed number of chromosomes in each of its cells. Chromosomes occur in pairs.

One chromosomes comes from the father and the other from the mother. DNA carries the code used by the cell while making proteins. Each chromosome contains one molecule of DNA and genes are the segments of DNA. A section of DNA that provides information for one protein is the gene for that protein e.g., the amount of plant hormone made will depend on the efficiency of the process for making it. If protein works more efficiently, a lot of hormone will be produced, and the plant will be tall. 

During fertilisation when the sperm fuses with a female gamete the number of chromosomes of that species is restored. The progeny inherits 2 genes for each trait from its parents but the trait shown by the progeny is the dominant one e.g., in case of traits tallness and dwarfness, tallness is dominant over the gene for dwarfness.

Sex Determination

  The chromosomes that is associated with sex-determination is known as sex chromosome. The other chromosomes are called Autosomes

   Sex chromosomes was observed as an unpaired chromosomes in the male grasshoppers during spermatogenesis. This was larger and given the name X-chromosomes. Female carry 2 copies of this chromosome.
     In Drosophila and humans presence of a specific Y-chromosome was observed among males, in addition to single X-chromosomes. Female have two X-chromosomes.
     Females produce gametes both with X-chromosomes. Male have one X and one Y chromosome so half of the sperms carry X-chromosomes and the other half Y-chromosome. All gametes in the females carry one X-chromosome.
        When a sperm carrying X-chromosome fertilizes an egg, the zygote develops into a female (XX). When sperm with Y-chromosome fertilizes an egg, the zygote develops into a male (XY).

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Evolution

        Evolution is a change in the genetic composition of a population. Variations occur due to sexual reproduction or errors in DNA copying.

Illustration
        A group of 12 red beetles lives in a bush with green leaves. They are preyed upon by
crows. Many situations can develop in this population.
          1. Colour variations during reproduction. One beetle develops a green body colour. 

              Crows cannot locate green beetles in the green bush hence the number of                              green beetles increases as compared to red beetles. In this case Natural selection                 is exerted by crows and individuals more suited to the environment survive.
        2. One beetle develops a blue colour. Blue and red both are detected by crows and                   are eaten. If calamity strikes e.g., an elephant stamps on the bush and by chance                 only blue beetles survive then their number increase. This random change in                       the gene frequency occurring by chance irrespective of its being beneficial or                      harmful is called genetic drift.
        3. In this situation when bushes develops disease, food for beetles reduces. Due to
            scarcity of food beetles are poorly nourished. The average weight of beetles                           decreases. Then when scarcity is over the average weight of beetles are again                       increased.

Acquired and Inherited Traits

There are 2 kinds of traits in every organism.
(i) Inherited traits. These traits are controlled by specific genes and are passed on from one generation to another. Any alteration in the DNA will be passed on, through germ cells, to progeny resulting in variations in them.
(ii) Acquired traits. Certain traits are acquired by organisms in their life time. For instance, decrease in the body weight of beetles due to starvation is an acquired trait by the beetles during their life time. It involves changes in the non-reproductive tissues caused by environmental factors. It will not bring any change in the DNA. Therefore, even if some of the generations of beetle are low in weight because of starvation, this trait can not be inherited by the progeny over generations.

Speciation

           Origin of a new species from the existing one is called speciation.
      The important factors which could lead to the rise (or formation) of a new species are the following:
(i) 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.
(ii) Genetic drift caused by drastic changes in the frequencies of particular genes is by chance alone.
(iii) Variations caused in individuals due to natural selection.

      In most of the cases, new species are formed when the population of some species splits into two separate groups which then get isolated from each other geographically by the barriers such as mountain ranges, rivers or the sea. The geographical isolation of the two groups of population leads to their reproductive isolation due to which no genes are exchanged between them. However, breeding continues within the isolated populations producing more and more generations. Over the generations, the processes of genetic drift (random change in gene frequency), and natural selection operate in different ways in the two isolated groups of population and make them more and more different from each other. After thousand of years, the individuals of these isolated groups of population become so different that they will be incapable of reproducing with each other even if they happen to meet again. We say that new species have been formed.

Evolution and Classification

       Characteristics are details of behaviour or appearance. The more characteristics two
species will have in common, the more closely they are related. They will also have a common ancestor. Classification of species is a reflection of evolutionary relationship.

Evidences of Evolution

Homologous organs: Organs of different organism that have structural similarity, develop in the same way but perform different functions e.g., forelimb of a frog, a bird, and a human. Similarities indicate that all these vertebrates had a common ancestor.
Analogous organs: Organs of different organisms that are structurally and developmentally different but perform same function. e.g., wings of birds and insects.
Fossils: Remains or impressions of the hard parts of past organisms found in the strata of the earth.

AGE OF FOSSILS

1. Relative: Fossils found closer to the surface are more recent than fossils found in
the deeper layers.
2. Radioactive dating: The age of a fossil can be calculated based on the property of
radioactive uranium which changes to lead. The amount of lead in a rock can help to
calculate, the age of the fossil.

Evolution by Stages

Evolution by an organism or its organs from simple to complex has taken place in stages:
(a) Feathers were first developed in dinosaurs but they could not fly. They protected them from the cold. Later birds used feathers for flight. This indicates that birds are related to dinosaurs also that a character developed for one function is later on used for an entirely different function.
(b) Eyes developed first in planaria as photosensitive eye spots. Simple and compound eyes developed in insects and crustaceans. The structure of eye in each is different enough for them to have separate evolutionary origins.
(c) Artificial selection is the process by which man selects traits useful to him for improving the qualities of domesticated plants and animals e.g., different vegetables developed from the wild cabbage-cauliflower, broccoli, red cabbage, kale. Molecular phylogeny is the branch of science which is used to trace the change in DNA (introduced during cell division and accumulated over generations) backwards in time to find out where each change diverged from the other. Thus, closely related organisms accumulated lesser number of differences in their DNA as compared to distantly related organisms which accumulate greater number of differences in their DNA. Such studies help in teaching evolutionary relationships among organisms.

Evolution should not be Equated with Progress 

   Evolution is the generation of diversity and the shaping of the diversity by environmental selection. The only progressive trend in evolution is that more complex designs have emerged over time. But simple designs are also efficient e.g., bacteria, protozoan have survived. New species evolved from previous one. Natural selection and genetic drift lead to the formation of a population which cannot reproduce with the original one. New species may have better adaptive advantage to environmental conditions.

Human Evolution

 Human evolution has been studied by using various tools for tracing evolutionary relationships like excavating, carbon dating, studying fossils and determining DNA sequences. All human beings are single species called Homo sapiens. Earliest members of this species came from Africa.
     Human beings and chimpanzees are closely related species. Both these had a common
ancestor a long time ago. This ancestor diverged into several forms and each form evolved in its own separate way to give rise to present forms of human beings, chimpanzees and other great apes.

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