Heredity and Evolution

1 HEREDITY AND EVOLUTION HEREDITY The transmission of characters from parents to offspring’s from one generation to another is called heredity. Heredity explains that organisms resemble each other because they arise from common ancestor. Evolution is the orderly changes of various forms through a slow but continuous process. It gives rise to more complex body designs even while the simpler body designs continue to flourish. GENERAL TERM • Phenotype : An external appearance or body character of an organism irrespective of its genetic make-up is said to be phenotype. • Genotype : The genetic constitution of an individual is said to be genotype. • Monohybrid cross is a cross in which only one pair of characters is taken into consideration. • Dihybrid cross is a cross in which two pairs of characters are taken into consideration. • Trihybrid cross is a cross in which three pairs of characters are taken into consideration. • Gene : Gene is a segment of DNA which is responsible for the inheritance of a character from one generation to another. Variation. It refers to the differences in the characters or traits among the individuals of a species and also by the offsprings of the same parents are referred to as variations TYPES OF VARIATIONS

Continuous Variations

Discontinuous Variations

Variations in which characteristic concerned is not discrete, i.e. it can have any value inside a given range.

Variation in which characteristic concerned is discrete, i.e. one of two or more types.

There are intermediate forms which have blending of different alternative forms.

There is no intermediate forms.

These may be determined genetically or due to environmental and genetic factors working together e.g., height, weight, skin colour, hair colour, no. of leaves in plants.

These are determined genetically, e.g. fixed or free earlobes, rhesus blood group, genetic sex.

Variatation are produced both in sexual as well as in asexual reproduction. Although the amount of variations produced in asexual reproduction are subtle as compared to sexual reproduction.

Accumulation of Variation Heredity involves Inheritance of common basic body design, and minute changes in it, from one generation to the next generation i.e. from parents to offspring when the individual of this new generation reproduces. The second generation will have the basic body design, the differences that they inherit from the first generation, as well as newly created differences

2 Figure shows how diversity is created over succeeding generations in an organism that reproduces asexually. ncert p- 142 fig 9.1

• If one bacterium divides, then resultant two bacteria divide again, the four individual bacteria generated would be simi lar with minor differences due to small inaccuracies in DNA copying. • Depending on the nature of variations, different individuals would have different advantages like bacteria, which can withstand heat will survive heat wave better. • Selection of variants by environmental factors form the basis of evolutionary processes Importance of Variations, → They enable the organisms to adapt themselves in changing environment. → Variations form the basis of heredity. → They form raw materials for evolution and development of new species. Heredity and Variation in Asexual Reproduction. • In asexual reproduction, organisms raised are the exact copies of their parents. • They tend to preserve the similarities among all individuals belonging to a given line of descent. • They exhibit very little variations due to some environmental factors or mutations which are sudden changes in genes. Out of these Two factors, only mutations are heritable.

Heredity and . Variation in Sexual Reproduction. • • •

In sexual reproduction, two parents are involved and there is formation and fusion of gametes. The offsprings show variations from their parents due to crossing over and exchange of gene segments. They are not carbon copies of their parents, due to recombination of parental genes. So, variations which occur are heritable.

Inherited Traits - It is the transmission of particular characteristics from parents to their offsprings, generation to generation, which bear all basic features with great deal of variation. Inherited trait is a particular genetically determined characteristic that distinguishes a person. Gregor Johann Mendel was an Austrian Geneticist and regarded as the 'Father of Genetics'. • His experiments with garden peas (Pisum sativum) and the inferences together with his interpretations constitute the foundation of modern genetics. •

Mendel conducted his hybridization experiments in the garden behind the monastery.

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Mendel blended his knowledge of science and mathematics and was the first one to keep count of individuals exhibiting a particular trait in each generation.

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Mendel's experiments give us the mechanism for the inheritance of traits from one generation to the next.

Rules for the Inheritance of Traits – Mendel’s Contributions The rules for inheritance of such traits in human beings are related to the fact that both the father and the mother contribute practically equal amounts of genetic material to the child. This means that each trait can be influenced by both paternal and maternal DNA. Thus, for each trait there will be two versions in each child. Mendel's Experiments on Inheritance of Traits. Mendel used a number of contrasting visible characters of garden peas like round/ wrinkled seeds, tall/short plants, white/violet flowers, etc,

One visible contrasting character

3 • •

Mendel took pea plants with different characteristics as a tall plant and a short plant. The first generation or F1 progeny thus formed are all tall. • Mendel then allowed the F1 progeny plants for selfpollination. • The second generation or F2 progeny of the F1 tall plants are not all tall, some are short. This indicates that both tallness and shortness traits were inherited in F1 plants but only the tallness trait was expressed. • Thus, two copies of the traits are inherited in each sexually reproducing organism. • In the figure, both TT and Tt are tall plants, while only tt is a short plant. •

A single copy of T is enough to make plant tall while both copies have to be ‘t ‘for the plant to be short. Therefore, traits like 'T' are dominant traits while 't" recessive traits.

Two visible contrasting character •

Mendel took pea plants with two different characteristics such as a tall plant with round seeds and a short plant with wrinkled-seeds, F1 progeny are all tall with round seeds, thus, tallness and round seeds are dominant traits. F2 progeny are tall plants with round seeds and some short plants with wrinkled seeds, But some F2 progeny shows new mixtures like tall having wrinkled seeds and short having round seeds. Therefore, tall / short trait and round seed/ wrinkled seed are independently inherited.

• • • •

The characters which always appear in two opposing conditions are called contrasting characters. A breeding experiment with a single character is called a monohybrid cross. A breeding experiment with a two characters at the same time is called a dihybrid cross

Control of Traits • • •

A section of DNA that provides information for one protein in cell is called gene for that protein. If the proteins work efficiently, the traits get expressed in a better-way. For example, plant height depends on the amount of growth hormone released by protein. If the protein is efficient, more growth hormone will be released and the plant will be taller. But if the protein is inefficient, the plant will be shorter. Thus genes control the traits.

Mechanism of Inheritance

trait

4 • • •

• •

If both parents help to determine the trail in the progeny, both parents must be contributing a copy of the same gene, Thus, each pea plant must have two sets of all genes, one inherited from each parent, so each germ cell must have only one gene set. each gene set is present , not as a single long thread of DNA, but as separate independent pieces each called chromosome and each cell will have two copies of each chromosome one each from male and female parents. When two germ cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of DNA species. Therefore, such mechanism of inheritance explains the-results of Mendel's experiments" and is used by all sexually and asexually reproducing organisms.

Chromosomes are long thread like structures present in the nucleus of a cell which contain hereditary information of the cell. Gene is the unit of inheritance. It is therefore, a unit of specific biological function. Gene is the part of a chromosome which controls the appearance of a set of hereditary characteristics. Sex Determination. Different species use different strategies for sex determination. • In some animals, environmental factors, the temperature at which fertilised eggs are kept determines whether the animals developing in the eggs will be male or female. •

In other animals such as snails, individuals can change sex, indicating that sex is not genetically determined. In human beings, the sex of the individual is genetically, determined.

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Sex determination is the process by which sex of a new born individual can be determined.

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Human beings have 22 paired chromosomes and one pair called sex chromosome which is not a perfect pair.

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A male has one X chromosome and one Y chromosome, i.e., half of the male gametes or sperms will have X chromosome and the other half will have Y chromosome. • A female has two X chromosomes, i.e.,all the female gametes or ova will have only X chromosomes.

Q How is the sex of the child determined? Sex of a child depends on what happens at fertilisation. (i) If a sperm carrying X chromosome fertilises an ovum which carries X chromosome, then the child born will be a girl. (ii) If a sperm carrying Y chromosome fertilises an ovum which carries X chromosome, then the child born will be a boy. Thus, the sperm determines the sex of the child.

EVOLUTION Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years and new species arc produced. Since the evolution is of the living organisms, so it is called 'Organic Evolution'.

5 Variation in Population : An Example. 12 red beetles live in a green leafy bush grows by sexual reproduction and generate variation. Situation -1 • Crows eat these beetles, leaving only fewer beetles available for reproduction. • Due to colour variation during reproduction, only one green beetle evolves and therefore, all its progeny beetles become green. • Crows cannot see green coloured beetles on green leaves and hence cannot eat them, resulting more green beetles than red ones in the beetle population. • This type of variation gives a survival advantage. In this case, natural selection is directing evolution in beetle population. The natural selection is exerted by the crows. The more crows there are, the more red beetles would be eaten, and the more the proportion of green beetles in the population would be. It results in adaptations in the beetle population to fit their environment better. Situation - II • Due to colour variation during reproduction, a blue colour beetle appears and all its progeny beetles become blue, • Crows can see both red and blue beetles and therefore, eat them. • Initially there are less number of blue beetles and more of red beetles. • Then an elephant stumps on the bushes and kills most of the beetles. By chance, few beetles that survived were mostly blue. • Thus, the blue beetle population slowly expands. • There is no survival advantage on this variation and provides diversity without adaptation. In this case, the colour change gave no survival advantage. It was simply a matter of accidental survival of beetles of one colour that changed the common characteristic of the resultant population. The elephant would not have caused such major havoc in the beetle population if the beetle population had been very large. This random change in the gene frequency occurring by chance irrespective of its being beneficial or harmful is called genetic drift which provides diversity without any adaptations. So, accidents in small populations can change the frequency of some genes in a population, even if they give no survival advantage. In both the above situations rare variation came to be a common characteristic in the population. The frequency of an inherited trait changed over generations. Since genes control traits, therefore the frequency of certain genes in a population changed over generations. This is the essence of the idea of evolution.

Situation - III • As the beetle population begins to expand, the bushes suffer from a disease and amount of leaf

available for beetles have reduced. • Thus, the beetles are poorly nourished and the average weight of an adult beetle has decreased. • After few years, the plant disease is eliminated and enough food is available for the beetles. Thus,

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the beetles have come back to its normal size and weight. This change is not inherited over generation.

Non-reproductive variations (Variations in somatic cells)

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Not passed from one generation to another because they do not change the DNA ot germ cells. Die with the death of the organism. Cannot direct evolution. Examples • Change in the weight of beetles. • Experience of an organism. • Boring of ear pinna in human females.

Variations Reproductive variations (Variations in germ cells)

Passed from one generation to another because they change the DNA of germ cells. Do not die with the death of organisms. Can direct evolution.

Examples • Rolling and non-rolling of tongue. • Free and attached ear lobes.

Types of variations

Acquired Trait. It is a particular characteristic that is developed during the lifetime of an individual by the use and disuse of organs or by the influence of the environment. Such characteristics are not genetically controlled and cannot be passed on to the next generation. → The environment and use and disuse of organs affect somatic cells only. → This means acquired characters are restricted to somatic cells alone and do not influence germ cells. So, these traits cannot be inherited. FOR EXAMPLE –  The weight of the beetle is reduced because of starvation that will not change the DNA of the germ cells. So, this change in weight is not an example of evolution. It is also not passed (inherited) over generations.  If we breed a group of mice, all their progeny will have tails, as expected. Now, if the tails of these mice are removed by surgery in each generation, these tailless mice will not have tailless progeny because removal of the tail cannot change the genes of the germ cells of the mice.  The strong muscles of a wrestler are not inherited by his children.

7 Acquired Traits

Inherited Traits

(i)

These are somatic variations.

(i)

These are genetic variations.

(ii)

Acquired traits develop due to the effects of environmental factors, use and disuse of organs and special (conscious) efforts.

(ii)

Inherited traits develop due to reshuffling of genetic material and mutations.

(iii)

These traits develop throughout the lifetime of an individual, and die with the death of that individual. Example—Learning of dance, music, etc. and muscular body of a wrestler.

(iii)

These traits are transferred (inherited) by the parents to their offspring. These do not die but are passed on to the next generation.

(iv)

(iv) Example—Attached or free earlobe and curly hair

→ Acquired Traits : Acquired characters are those variations or changes which an individual develops during its lifetime due to some special efforts, use or disuse of organs and due to some environmental factors. They are not controlled by genes. → Inherited Traits : Inherited traits are those characteristics which are inherited from parents to their offspring generation after generation as they are controlled by genes. → Genetics : Genetics is a branch of biology concerned with the study of heredity and variation. → Genetic Drift : The random change in the frequency of alleles in a population over successive generations due to error during DNA copying in the gametes. There are two alleles of each gene in an organism. Charles Robert Darwin - theory of natural selection, explains idea of evolution is not able to explain mechanism of heredity. In this theory changes in the DNA of reproductive cells are not involved to explain evolution. Darwin's theory of evolution tells us how life evolved from simple to more complex forms. Origin of Life on Earth. J.B.S. Haldane, a British scientist suggested in 1929 that life must have developed from the simple inorganic molecules which were present on earth to more complex organic molecules. Thus, the first primitive organisms would arise from further chemical synthesis. What is a Species? A species can be defined as a population or group of populations of similar organisms which are able to interbreed and produce fertile offsprings.

8 What is Speciation? It is the evolution of reproductive isolation more species from an existing species.

among

once - interbreeding populations, i.e., the development of

Speciation is the development of new species. It happens when different populations of the same species evolve along different lines. Populations are groups of interbreeding individuals of species occupying a particular geographical region, e.g. all the oaks in a forest, all the elephants in a game reserve, water snakes in a pond. How are populations isolated? Populations are usually isolated by physical barriers such as stretches of water or mountain ranges. This is known as geographic isolation, which is the most common way of separating the populations reproductive!y. This can lead to formation of a new species.

For example: beetles In both the conditions genes of migrant beetle enter the new population. Thus we can conclude that: • When populations are partly separated there is a gene flow between them. • The level of gene flow decreases when populations are completely separated by geographic isolation. Over generations different changes accumulate due to natural selection and genetic drift. • Natural selection also operates differently in different locations, e.g. if one sub-population of crow is eliminated by eagles, it does not mean that other sub-populations where number of crows is more also gets eliminated. • This also holds true to the example of red beetles eaten by crows (i) The green variation is no selected in the first situation and the number of green beetles is less, (ii) In the second situation of I same example green variation gets strongly selected as the number of green beetles increases. • Also, the two sub-populations of beetles, i.e. blue and green are isolated by the process of genetic drift and natural selection. The members of these two groups do not interbreed, and two new species; formed. DIFFERENT WAYS BY WHICH NEW SPECIES ARE FORMED

→ Change in the chromosome number, i.e. changes in the DNA. → A new variation is introduced due to which there is no interbreeding, i.e. green beetles don't mate with red ones, leading to strong selection of green beetles. This leads to formation of a new species.

→ Micro-evolution. It is the evolution on a relatively small scale, involving the emergence of new species or of new groups below the species level, such as races and subspecies which are significant. They also change the common characteristics of a particular species. When a population of a species splits into two, it cannot reproduce with each other, and then a new species is generated, for example : • A huge population of beetles feed on bushes spread a wide mountain range, • Individual beetle however feed on nearby bushes. • There is sub-population of beetles in a neighbourhood and reproduction takes place within the sub-population. 

Occasionally a migrant beetle enter a different sub-population and reproduce with them, thus genes of the migrant beetle enter in a new population.

9    

This kind of gene flow is bound to happen between populations that are partly, but not completely separated. If, however, between two such sub-populations a large river comes into existence, the two populations will be further isolated. The levels of gene flow between them will decrease even further. Over generations, genetic drift will accumulate different changes in each sub-population. Also, natural selection may also operate differently in these different geographic locations. Thus, for example, in the territory of one sub-population, crows are eliminated by eagles. But this does not happen for the other sub-population, where crow numbers are very high. As a result, the green variation will not be selected at the first site, while it will be strongly selected at the second.

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Changes due to genetic drift and natural selection will result in isolation of two sub-population which become more and more different from each other.

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Ultimately these two groups will be incapable of reproducing with each other and two generation of beetles are being generated.

FOR EXAMPLE: If the DNA changes are severe enough, such as a change in the number of chromosomes, eventually the germ cells of the two groups cannot fuse with each other. Or a new variation emerges in which green females will not mate with red males, but only with green males. This allows very strong natural selection for greenness. Now, if such a green female beetle meets a red male from the other group, her behaviour will ensure that there is no reproduction between them. Effectively, new species of beetles are being generated.

Gene flow. - It is the exchange of genetic material by interbreeding between populations' of the same species or between individuals' within a population. Gene flow increases' variation in the genetic composition of a' population. Genetic drift - It is the random change in the frequency of alleles in a population over

successive generations due to sampling error in the gametes. • Each new generation differs from its parental generation with regard to allele frequencies simply because of random variation in the distribution of gametes. • This process is more rapid in smaller populations, hence genetic drift can cause loss of genetic diversity if there are no counteracting factors. • Over generations to come genetic drift will accumulate different changes in sub-population.

Natural Selection. It is the process, according to Darwin, which brings about the evolution of new species of animals and plants. • It was noted that the size of any population tends to remain constant despite the fact that more offsprings are produced than are needed to maintain, • Darwin found that variations existed between individuals of the population and concluded that disease, competition and other forces acting on the population eliminated those individuals less well adapted to their environment. • The surviving population would pass the hereditary advantageous characteristics to their offsprings.

• But with time this process would give rise to organisms different from the original population and new species are formed,

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Characteristics. - It is the detailed appearance or behaviour of a particular form or a

particular function. For example, the four limbs of human beings is a characteristic and that plants can perform photosynthesis is also a characteristic. Classification. - It is the arrangement of organisms into a series of groups based on

physiological, biochemical, anatomical or other relationships. Basic characteristics of organisms. • • • • •

Cell is the basic unit of life in all organisms. Cells of some organisms have nucleus and some do not. Amongst organisms with nucleated ceils some are unicellular and others are multicellular. Some multicellular organisms can perform photosynthesis and others cannot. Among multicellular organisms, some have skeleton inside the body and others around the body.

Evolutionary Relationship with Classification. • The more common characteristics two species have, the more closely they are related. • The more closer the species are, the more nearer they have a common ancestor. • For example, a brother and a sister are closely related and they have a common ancestor, their parents in the first generation. • A girl and her first cousin are closely related but less related than her brother. The cousins have a common ancestor, their grandparents in the second generation. Thus, evolutionary relationships are traced in the classification of organisms. Homologous organs

are those organs which have the same basic structural design and

developmental

origin but have different functions and appearance.

Example : The forelimb of a frog, a lizard, a bird and a man seem to be built from the same basic design of bones, but they perform different functions. Analogous organs

are those organs which have different basic structural design and

developmental origin but have similar appearance and perform similar functions.

Example : The wings of birds and bats look similar but have different design in their structure. Wings of bats are skin folds stretched between elongated fingers but wings of birds are covered by feathers all along the arm.

S.NO.

Features

Differences between homologous and analogous organs Homologous organs Analogous Organs

,

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Definition

Similar in origin and basic structure but may differ in function.

Dissimilar in origin and basic structure but may have similar functions.

2.

Idea of ancestry

Give idea of common ancestry

Do not give the idea of common ancestry.

3.

Examples

Hands of human beings and forelimbs of horse.

Wings of birds and insects.

Fossils - They are all the preserved traces or remains of living organisms of geological past. • When organisms die, their dead bodies decompose and get lost. • But some parts of the body may be in the environment that does not let it decompose. • Example, a dead insect will not decompose in hot mud, it will harden and retain the impression of the insect body parts and thus preserved as fossils. • Evolution can be worked out by the study of not only living species but also fossils. Kinds of Fossils. - Fossils are of various kinds depending upon their differences in appearances and degrees of detail and preservation. They are –  Fossil tree trunk,  Fossil invertebrate (Ammonite),  Fossil invertebrate (Trilobite)  Fossil fish (Knightia) and  Fossil dinosaur skull (Rajasaurus). Fossil Dating. The age of fossils can be estimated by two ways – (i) If we dig into the earth and start finding fossils, it can be assumed that the fossils closer to the surface are more recent to those found in deeper layers. (ii) By detecting the ratios of different isotopes of the same element in the fossil material. Formation of Fossils. - Fossils are formed layer by layer in the earth's crust. • Suppose 100 million years ago invertebrates that were dead on the sea bed were buried in the sand, and with time more sands accumulate and sandstone are formed.  After million years, the dinosaurs living in the area die and get buried in mud which are compressed into rock above the earlier invertebrate fossils.  Eventually again million years later, the bodies of horse-like creature dies and their fossils are found in rocks above the earlier rocks.  But much later, due to erosion, the water flow wears some of the rocks and the horse-like fossils are exposed. And as we dig into deeper layers the older fossils are found. Uses of Fossils. Fossils help to analyse – (i) racial history of plants and animals, (ii) past climatic conditions of the earth and

(iii)

to measure the geologic lime.

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Evolution by Stages. The evolution in an organism or its organs from simple to complex forms has taken place in stages. It has taken bit by bit over generations. The evolution cannot take place by a single DNA change. Let us take few examples. (a) Feathers were firstly developed in dinosaurs, but they could not fly.  Probably, it was developed to provide insulation in cold weather.  Later on feathers developed in birds, where it was used for flight.  This indicates that birds are closely related to reptiles (dinosaurs were reptiles).  This also indicates that the character which developed for one function is later on used for entirely different function.

(b) Eyes for the first time developed in Planaria  Rudimentary eyes present in planaria are just photosensitive eye-spots.  Simple as well as compound eyes have developed in insects and crustaceans.  Eyes have also developed in Octopus ant vertebrates.  The structure of the eye in each of these organisms is different enough for them to have separate evolutionary origins. By this selection process, very dissimilar looking structures may evolve from a common ancestral design. One of the classical examples is wild cabbage plant . Humans have, even more than two thousand years, cultivated wild cabbage as a food plant, and generated different vegetables from it by selection. The various crop plants developed from wild cabbage plants are : • Cabbage - It is selected for its terminal buds, where there is a very short distance between leaves. • Kohlrabi - It is selected for its swollen stem position. • Kale It is selected for its slightly larger leaves. • Broccoli - It is selected for its flower (arrested flower development) and stem. • Cauliflower - It is selected for its flower clusters (sterile flowers). The other way of tracing evolutionary relationships depends on the changes in DNA during reproduction. If we compare the DNA of different species then we can directly estimate of how much the DNA has changed during the formation of these species.

Complex organs may have evolved because of the survival advantage of the intermediate stages.  Complex organs like eyes are created bid by bid over generation which is a very popular adaptation. The structure of eye in all organism is different enough to have separate evolutionary origins.  Organs or features may be adapted to new functions during the course of evolution. As for example, feathers are thought to have evolved for warmth and later adapted for flight.  Some dissimilar looking structures also evolved from common ancestor, Example - Wild cabbage plant, from which different vegetables are generated by artificial selection like broccoli, cabbage, cauliflower, etc.  The traces of evolutionary relationships depend on the original idea that 'changes in DNA during reproduction are its basic events' 

13 Adaptation. It is any change in the structure or functioning of an organism that makes it better suited to its environment, Phylogeny. It is the evolutionary history of an organism or group of related organisms. Molecular Phylogeny. It is the idea that organisms those are more distantly related will accumulate a greater number of differences in their DNA. Such studies traces the evolutionary relationships. Evolution versus Progress. Evolution cannot be said to 'progress' from lower forms to higher forms. It seems to have given rise to more complex body designs even while the simpler body designs continue to flourish. For example, human beings have not evolved from chimpanzees, but both have a common ancestor. 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 probably evolved in its own separate way to give rise to the present forms of human beings, chimpanzees and other great apes. Thus, evolution is simply the generation of diversity and shaping of diversity by environmental selection. Environmental selection. It is the selection within a population resulting from the influence exerted by the environment. It leads to a change in the composition of genes within a population. Human Evolution. The study of human evolution indicates that all of us belong to a single species that evolved in Africa. The earliest member of the human species, Homo sapiens can be traced there. • DNA sequences have been used for studying human evolution. • Due to the diversified human forms and features, skin colour is the common way for identifying the races. • Few thousand years ago some ancestors left Africa while others stayed back. • The residents spread across Africa and the migrants spread across the planet from Africa to West Asia, Central Asia, Eurasia, South Asia. East Asia, Indonesia, Philippines, Australia and America. • They went forwards and backwards with groups separating from each other, or sometime coming together. • Like all other species, they were also living their lives to the best of their ability.

What are the different tools used for studying human evolution? Different tools used for studying human evolution are: (a) excavating (b) time-dating (c) studying fossils (d) determining DNA sequences.

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