Notes for HEREDITY AND EVOLUTION (Class X)

HEREDITY AND EVOLUTION (NOTES)

SEX DETERMINATION

The process by which the sex of a person is determined is called sex determination. The chromosomes that determine the sex of a person are called sex chromosomes. They are of two types: X chromosome and Y chromosome.

·         A male has one X and one Y chromosome. This means that half of the sperms will have X chromosome and the other half will have Y chromosome.

·         A female has two X chromosomes but no Y chromosome. All the ova will receive one X chromosome.

Q: What are autosomes? What is the number of autosomes in a human cell?

A: The chromosomes in the human cells that do not take part in sex-determination are known as autosomes. The number of autosomes in a human cell is 22 pairs (44 chromosomes).

Q: Mention the other methods of sex-determination in which sex chromosomes are not involved.

A: In some animals, the incubation temperature of eggs is responsible for determining the sex of the offspring. Eg: In turtles, high incubation temperature results in female offsprings whereas in lizards, high incubation temperature results in male.

Evolution

Q: What is evolution?

A: Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are produced.

Q: Differentiate between Acquired traits and inherited traits.

ACQUIRED TRAITS	INHERITED TRAITS
1) A trait in an organism which is not inherited but develops in response to the environment.	A trait in an organism which is caused by a change in its genes (or DNA) in the reproductive cells of the parents.
2) These traits cannot be passed on to the offsprings	Can be passed on to the offsprings
3) Do not lead to evolution	Lead to evolution
4) Eg: low weight of beetle population due to scarcity of food, a mark on someone’s face due to an accident.	Eg: The change in color of beetle population due to mutation, inheritance of eye color, hair color, hair texture etc

Theory of Natural Selection:

Natural selection is the process of evolution of a species whereby the characteristics that help individual organisms to survive and reproduce are passed on to their offsprings and those characteristics which do not help are not passed on.

The theory of natural selection was given by Charles Darwin, hence is also known as Darwinism.

Salient features of the theory:

·      Within a population, there are variations occurring naturally. Some individuals have more favourable variations than the others.

·      All the species produce a large number of offsprings, but the population size remains fairly constant naturally. This is due to the struggle between members of the same species(intra-specific) and different species(inter-specific) for food, space and mate.

·      This struggle for survival within a population eliminates the unfit individuals. The fit individuals possessing favourable variations survive and reproduce. This is called Natural Selection. (Survival of the fittest).

·      The individuals having favourable variations pass these variations to their progeny from generation to generation, and when accumulated over a long period of time, lead to the origin of new species.

Q: What is speciation? How does it occur?

A: The process by which new species develop from the existing species is called speciation.

     New species are formed when the population of the same species splits into two separate groups, which then get isolated from each other geographically by the barrier such as mountain ranges, rivers or seas.

The gographical isolation of two groups of population leads to the reproductive isolation due to which no genes are exchanged between them. However, breeding continues within the isolated population producing more and more generations.. The process of Genetic Drift and Natural Selection operate in the isolated groups and make them more different from each other.

After thousands of years, the individuals of these isolated groups become so different from each other that they will be incapable to reproduce with each other even  if they happen to meet again.

Q: List the important factors which can lead to the rise of new species.

A: 1) Geographical isolation of the population

     2) Genetic Drift (Random change in the gene frequency occurring by chance alone).

     3) Natural selection

EVIDENCES OF EVOLUTION

Homologous Organs:

Those organs which have the same basic structure or same basic design (or same origin) but different functions are called homologous organs.

Eg: Forelimbs of a man, lizard(a reptile), a frog(an amphibian), and a bat (a mammal) are built from the same basic design of bones, but they perform different functions. The forelimbs of a man are used for grasping, that of a lizrd are used for running and climbing, forelimbs of a frog are used for jumping above the ground level whereas of a bat and bird are modified for flying.

The presence of Homologous organs indicates that all these forelimbs have evolved from a common ancestral animal that had a basic design- limb.

Analogous Organs:

Those organs that have different basic structure (or different designs) but perform similar functions are called analogous organs.

Eg: Wings of an insect and wings of a bird. The wings of an insect are membranous folds while the wings of a bat are formed of a structural framework of bones with flaps of skin in between, but both of them are used for flying.

Since the analogous organs have different basic designs, so they do not indicate a common ancestor for the organisms. It rather indicates that even the organisms having organisms having organs with different structures can adapt to perform similar functions for their survival under unfavourable environmental conditions.

Fossils:

These are the preserved traces of living organisms formed when the body of the living organism does not decompose completely due to some environmental factors. Eg: If a dead insect gets caught in hot mud, it will not decompose quickly. The mud will eventually harden and retain the impression of the body parts of the insect.

Q: What is archaeopteryx? Explain its significance in the field of evolution.

A: Archaeopteryx is a fossil bird. It looks like a bird but has many other features that are found in reptiles. This is because Archaeopteryx has feathered wings like a bird, but teeth and tail like those of reptiles. It is therefore a connecting link between the reptiles and birds and hence suggests that birds have evolved from the reptiles.

Thus, fossils provide evidence that the present day animals and plants have originated from the previously existing ones through the process of continuous evolution.

Q: What are the ways to determine the age of fossils?

A: The two methods used to determine the age of fossils are:

Relative Method: The fossils that are found in the layers closer to the surface of the earth are more recent. The ones found in the deeper layers are older, whereas the ones found in the deepest layers of the earth are the oldest ones.

Carbon dating Method: Fossils that are found today were once living objects. All the living beings contain some C-14 atoms that are radioactive. When a living being dies and forms a fossil, its C-14 radioactivity goes on decreasing gradually. In carbon dating method, the age of fossils is found by comparing the C-14 radioactivity left in the fossils with the C-14 radioactivity present in the living objects today.

Evolution by Stages:

EVOLUTION OF EYES

The complex body design of animals such as eyes have been created in stages over many generations.

The eye is a very important organ for the animals. It is a complicated organ which can’t be generated by a single DNA change.

First of all, the rudimentary eye (basic eye) like that of a flatworm (planaria) was formed. The eyes of a flatworm are very simple that are actually just ‘eyespots’ which can detect light. Even these rudimentary eyes provide a survival advantage to the flatworm. Starting from this basic design, more and more complex eyes were then evolved in various organisms. Most of the animals have eyes- insects, octopus and other invertebrates. But the structure of the eye in each of these is different, that suggests separate evolutionary origins.

EVOLUTION OF FEATHERS

Sometimes an evolutionary change produces in an organism for one purpose later on becomes useful for an entirely different function. Eg: Feathers have been evolved in Aves(birds) for providing insulation (warmth) to their bodies in cold weather but later on these feathers became more useful for flying.

Even some dinosaurs have feathers though they could not fly using them. The presence of feathers in reptiles tells us that the birds are closely related to reptiles.