Heredity And Evolution

Variation refers to any difference between groups of organisms or among individuals of any species that is caused either genetically (genotypic variation) or by the effect of environmental factors on the expression of the individual (phenotypic variation).

Heredity refers to the passing or transmission of characters or genetic factors from parents to offspring (or from one generation to the next). Heredity is the means by which an offspring acquires the qualities of its parents. Hereditary information is present in fertilized egg or zygote.

Mendel performed experiments on Pisumsativum(garden pea) and studied the inheritance of some contrasting characters like height, color of flower, color of seeds, etc. He gave the main rules of inheritance.

When Mendel crossed tall plant (with TT traits) with dwarf plant (with tt traits), all tall plants were obtained (F1 generation). But when F1 plants were self-fertilized, some tall (75%) and some dwarf plants were obtained (25%) in F2 generation. This means that both the characters (tallness and dwarfness) were inherited in F1 generation from the parents, but only the character for tallness was expressed.

In human beings, sex is determined by the genes, located on the chromosomes which are inherited from parents to offspring. The genetic constitution of the zygote formed after fertilization of an egg and sperm determines the sex of the child.

All human beings are a single species, called Homosapiens. The genetic footprints of human beings can be traced back to the African roots. We originated from Africa. Thousands of years ago, some of our ancestors left Africa and migrated to other parts of the world and some stayed back.

Homologous organs are those organs which have the same internal structure but perform different functions. These organs share a common ancestry in multiple organisms. They possess a similar structure of origin.

Modification of forelimb in different organisms

A fossil refers to any preserved remains, impressions or trace of any once-living organism from a past age. Due to some environmental conditions, the bodies of dead plants and animals do not decompose completely, and such body parts become fossil and are available on digging the earth. The position of fossil in the layer of earth helps to predict the time of extinction of the organism. Deeper the fossil in the earth layers, the older it is.

The process by which new species develops from the pre-existing species is called speciation. If DNA undergoes significant changes (mutation) in germ cells, it may result into formation of a new kind of species. A change in number of chromosomes will give rise to a new genetic constitution of an organism, leading to speciation. The lack of mating and no change in genetic material does not result in speciation.

The organs that have different internal structure but same appearance and also perform similar function to survive the prevailing environment are known as analogous organs. They do not share common ancestors and have different origin. They evolve independently of each other.

•Sex Determination is defined as the biological process for identifying gender and the development of sexual characteristics specific to an organism.

•Organisms reproducing through sexual reproduction have two sexes:- Male and Female respectively in some special cases there are Hermaphrodites(Bisexual species).

•Sex is mainly determined either by Genetic Factors or Environmental Factors

Different organisms use different mechanism of sex determination. In some species it is genetic and in a few cases, sex is determined by environmental factors, such as temperature. The temperature at which fertilized eggs are kept determines the sex of the organism developing inside it. In snails, sex is not determined genetically. Snails practice sex change. Adults start out as male, and then they become female. However, in human beings, birds and some reptiles, sex is determined genetically. The genetic constitution of the zygote formed after fertilization decides the sex of the offspring.

Wild cabbage (Brassicaoleracea) is the ancestor of broccoli, kohlrabi, cauliflower and kale. These varieties evolved from wild cabbage by the process of evolution. Swollen parts of wild cabbage developed into kohlrabi. Large leaves of wild cabbage came to be known as kale and flowers of wild cabbage developed into broccoli.

KALE

KOHLRABI

BROCCOLI

Five organisms which are now extinct:

Ammonite (marine mollusk animals)

Trilobite (marine arthropods)

Dinosaur (reptiles)

Sabre-toothed cat (predatory mammals)

Archaeopteryx (bird-like dinosaur)

Five varieties of vegetables produces from wild cabbage by artificial selection are:

i. Cabbage

ii. Broccoli

iii. Cauliflower

iv. Kohlrabi

v. Kale

WILD CABBAGE

cabbage, which is the common ancestor of broccoli, cauliflower and cabbage.

Fossil refers to any preserved remains, impressions or trace of dead plants and animals that once lived in the past. The dead bodies of plants and animals are usually decomposed by micro-organisms, but in some cases they do not decompose completely and are preserved. Fossils include bones, shells, impressions on rock, etc.

For example, if a dead leaf is trapped in mud, then mud forms an impression by setting around it as a mould. It then hardens and a fossil is formed.

Homologous organs are the organs that have the same internal structure but perform different functions to survive in the prevailing environment. They give strong evidence for evolution. They represent a case of divergent evolution.

For example, the forelimbs of human, frog and lizard possess the same internal structure but perform different functions. This indicates that they have evolved from a common ancestral animal and have the same origin.

Separation of species by physical barriers such as water forms, oceans, mountains etc. is defined as Geographic Isolation. Geographic Isolation won’t have any impact on the organism that reproduces asexually as they require only one parent. Therefore even if the organisms are separated from each other they can reproduce asexually. Asexual Reproduction leads to offspring which is an exact copy of the parent and there is no variation

The formation of new species from the pre-existing ones is called speciation. When population of same species is divided into two groups which get separated from each other geographically by some barriers (mountain, ocean), they are unable to reproduce with each other. It leads to reproductive isolation. As a result, there is no exchange of genetic material between the two separated groups. Variations arise due to natural selection. Thus, geographical isolation gives rise to a new species.

For example, Darwin’s finches came into existence as a result of geographical isolation.

Heredity is the transmission of characters from parents to offspring. When the gametes of both parents fuse and form zygote, the characters are passed to the next generation. Organisms inherit the characters from their parents through genes. It is through inheritance that each species maintains the characteristics from generation to generation. Genes are the unit of inheritance. Sometimes, not all the characters inherited are expressed, but they are still present in the genetic makeup of the organism.

Heredity means continuity of features from one generation to the next. It is the tendency of every individual to express the inherited characters of its parent. Hereditary information is present in the zygote which develops into an organism of a particular type only.

Due to heredity, an egg laid by a sparrow always hatch into a sparrow. Similarly, a tigress gives birth to cubs only.

(a) •Homologous organs are those organs which have the same internal structure but perform different functions. These organs share a common ancestry in multiple organisms. They possess a similar structure of origin.

•The arm of a human, the wing of a bird or a bat, the leg of a dog and the flipper of a dolphin or whale are homologous structures.

HOMOLOGOUS ORGANS

•The organs that have different internal structure but same appearance and also perform similar function to survive the prevailing environment are known as analogous organs. They do not share common ancestors and have different origin. They evolve independently of each other.

•The wings in bats, birds, and insects and fins of penguins and fish are examples of analogous structures.

(b) •The organs with different basic design but similar appearance and similar function in different organisms are called analogous organs.

•The wings of insects and the wings of birds have different structure but they perform similar function.

•The presence of analogous organs in different animals gives the evidence that they have not evolved from a common ancestor but they perform similar function to survive in prevailing environment. Analogous structures represent a case of convergent evolution.

(a) •The process by which new species develop from the pre-existing ones is known as speciation.

•A number of ways leads to speciation:

1. Geographical separation of a population, leading to reproductive isolation and thus, speciation.

2. Lack of gene flow between two groups of a population results in speciation.

3. Occurrence of variations in organisms due to natural selection.

4. Due to random change n gene frequency (genetic drift), the organisms are unable to reproduce with each other, resulting in speciation.

•There can be more ways like DNA changes, the change in the number of chromosomes, the inability of the germ cells of two isolated groups to fuse with each other, etc. which can lead to speciation.

Thus, new species come into existence.

(b) •Geographical isolation signifies the separation of two populations of a species.

•A self-pollinating plant possesses both the male and female parts of a flower, required for pollination.

•Pollination occurs between the reproductive organs of the same flower, so geographical isolation is not at all significant in the speciation of a self-pollinating plant.

•Moreover, new gene variants will not be introduced in the population due to geographic isolation.

•On the other hand, accumulation of variations occurs in cross-pollinating plants due to geographical isolation.

•Mendel performed experiments on Pisumsativum (garden pea) for explaining the transmission of characteristics (traits) from parents to their offspring.

•He proposed the laws of inheritance.

•He studied the inheritance of some contrasting characters like height of plant, color of seeds, etc.

•He crossed a tall plant (with TT traits) with a dwarf plant (with tt trait), and obtained all tall plants in F1 generation. Thus, only one of the two parental characters was expressed in F₁ generation.

•When the plants of F₁ generation were crossed, 25% dwarf plants and 75% tall plants were obtained in F2 generation.

•Thus, in F₁ generation both the characters of tallness and dwarfness were inherited from the parents but only the character of tallness was expressed.

•In other experiment, Mendel selected two different traits in a plant. He crossed a plant having yellow and round seed with a plant having green and wrinkled seeds.

•In F₁ generation, all the plants were with yellow and round seeds indicating that yellow and round seed traits are expressed.

•When the plants of F₁ generation were self-pollinated, the plants along with new combinations (yellow and wrinkled seeds, green and round seeds) along with parental combinations (yellow and round seeds, green and wrinkled seeds) were obtained in F₂ generation.

•This indicates that yellow seeds/green seeds traits and round seeds/wrinkled seeds traits are independently inherited.

•In humans, the sex is determined by the genes located on chromosomes.

•Genes are inherited from parents to offspring.

•There are 23 pairs of chromosomes in humans. Out of 23 pairs, 22 pairs are autosomes which are similar in male and female.

•In female, the 23rd pair has two similar chromosomes called X-chromosome.

•In male, one chromosome in 23rd pair is X chromosome and the other is Y chromosome which is smaller in size.

•All eggs (ova) of female are similar. Each egg contains 22 autosomes and one X chromosome.

•Whereas in males, 50% sperms have X chromosome and 50% have the Y chromosome (along with 22 autosomes).

•If a sperm carrying X chromosome fertilizes an egg, the zygote develops into a female (XX along with autosomes).

•If a sperm carrying Y chromosome fertilizes an egg, the zygote develops into a male (XY along with autosomes).