Chapter 9: Heredity and evolution

Chapter 9

Heredity and Evolution

Heredity is defined as the transmission of characteristics from parents to offspring.
The differences in characters of parents and offspring are known as variations. 
There are two types of variations - somatic variations and genetic variation.

• Somatic variations occur in the somatic cell of the body. They are not inherited or transmitted in the next generation. So, they are also known as acquired traits.
• Gametic variations occur in the germ cells of the body. They are inherited in the next generation. So, they are known as inherited traits.

Importance of Variations

• It is the basis of heredity.
• It is the basis of evolution also.
• It increases the chances of the survival of the organism according to the changing environment.

 Causes of variation
The most common causes of variations are mutation, recombination and random mating. Recombination or crossing over is one of the important reasons for variation. It is an exchange of chromosome segments at the time of gamete formation.

Mendel and his contribution to Genetics
G.J. Mendel started his work on Pisum sativum (garden pea). He was known as the Father of genetics. He had chosen seven pairs of contrasting characters-

The reason for choosing garden peas for the experiment was-

• Short life cycle
• A large number of seeds produced
• Self-pollination
• Several contrasting characters can be found

Mendel Laws 

• Law of Dominance: If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
• Law of Segregation: The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.
• Law of Independent Assortment: Each pair of alleles segregates independently of other pairs of alleles during gamete formation

Monohybrid cross
When one pair of contrasting characters is taken to cross two pea plants, it is known as a monohybrid cross.

 The image depicts the monohybrid cross between the true-breeding yellow pod and the true-breeding green pod. All the pods obtained were green in colour. The offspring obtained are known as F1 progeny or First filial generation.
In the case of the heterozygous condition,

In this Image, the parents are heterozygous, so phenotypically 3 purple flowers and 1 white flower were produced. But genotypically, 1 homozygous dominant (BB), 2 heterozygous dominant (Bb) and one homozygous recessive (bb).

Dihybrid cross

When two pairs of contrasting characters are taken to cross two plants, it is known as a dihybrid cross. 

The phenotypic ratio was found to be 9:3:3:1
9 are round yellow
3 are round green
3 are wrinkled yellow
1 is wrinkled green
But the genotypic ratio was found to be 1:2:1: 2:4:2: 1:2:1.

Sex determination

Sex determination is used to define the sex of the offspring. Environment and genetic factors determine the sex of the offspring. Environmental factors, such as gender in turtles, are determined according to the temperature.

Types of Sex Determination

• Different types of sex determination are- XX-XY type (humans), XX-XO type (insects), ZW-ZZ type (chicken) and ZO-ZZ type (moths and butterflies).
• Genetic factors include the presence of sex chromosomes. For example, in humans, the presence of two X chromosomes leads to female offspring whereas the presence of one X and one Y chromosome forms male offspring.
• In human beings, there are 23 pairs of chromosomes. Out of these 22 pairs of chromosomes are known as autosomes whereas the 23rd pair of chromosomes are known as sex chromosomes or allosomes. The sex of the offspring is determined by the chromosome inherited from the father.

Evolution

 The sequence of gradual changes that take place in an organism over a million years and leads to the formation of new species is known as evolution.
J.B. Lamarck was the first scientist who gave the theory of evolution. He gave the theory of inheritance of acquired characters. Later on, Charles Darwin came and gave the theory of natural selection or Darwinism. According to his theory, evolution occurs through natural selection.

Theory of Lamarckism

This theory is also known as the “Theory of inheritance of acquired characters”. Lamarckism is based on the following postulates-

• Living organisms tend to increase in size.
• New needs lead to the formation of new organs.
• Continued use of a particular organ makes it more developed and disuse of an organ leads to its degeneration.
• New characters are acquired by individuals during their lifetime.

Theory of Darwin
Darwin's theory was also known as the “Theory of Natural selection”.
Postulates of Darwin Theory

• Speciation (formation of species) - Useful variations from generation to generation gives rise to the formation of new species.
• The struggle of existence due to the multiplication of organisms and limited food and space, there exists competition among the organisms.
• Survival of the fittest or Natural selection Nature selects those characteristics or organisms that are useful and are best adapted to the prevailing conditions. “For example, industrial melanism observed in peppered moths in Britain”

Speciation
The origin of new species from already existing species is known as speciation. Speciation can take place through-

• Gene flow can lead to speciation. It is a transfer of genetic variation from one population to another
• Random change in allele frequency known as genetic drift can also lead to speciation.
• Natural selection is another reason through which speciation can take place
• Geographical barriers such as mountains, rivers can also lead to speciation. This is known as geographical isolation. 

Evolution and classification

Evolution and classification are linked to each other. There is different evidence of evolution given-

• Homologous organs are the organs evolved from the same ancestors but they have different functions. For example, the forelimb of a horse and the wings of a bat. The Flippers of the whale, the human hand are other examples of homologous organs.

Examples of homologous organs

• Analogous organs are the organs that arise from different ancestors but have the same function. For example, wings of bats, wings of birds, wings of insects etc.

Examples of analogous organs 

• Paleontological (fossil) evidence was also given for evolution. The dead remains of the organisms are known as fossils. For example, Archaeopteryx possesses features of both reptiles and birds. This concludes that birds evolved from reptiles. There are two methods for finding the age of the fossils- one is carbon dating and the other is by digging. In the digging method, the deeper the fossil is, the older it is. Biogenetic law states that stages of development of an animal embryo are the same as adult animal stages. 
• Vestigial organs are rudimentary in nature. They have lost their function through evolution. For example, appendix in humans, muscles of ears, wisdom tooth etc.

Evolution by Stages
Evolution can take place in stages also. For example, the evolution of eyes. Flatworms have rudimentary eyes, whereas insects possess compound eyes. In the last humans have binocular vision.
The evolution of feathers is also an example of evolution by stages. For example, dinosaurs have feathers but are unable to fly. But later on, birds used feathers for flying.

Molecular Phylogeny
It is also evidence for evolution. According to this, changes in DNA during reproduction are the basic events of evolution. Organisms that are related to each other most distantly will have greater differences in their DNA.

Evolution by Artificial Selection
Artificial selection selects special phenotypic characters to produce organisms with enhanced characteristics. For example, plants that are disease or insect resistant. Artificial selection can be used to produce different cabbage varieties such as broccoli, cauliflower, red cabbage etc.

Human Evolution
Excavating fossils, time dating and determination of DNA sequences are used to study human evolutionary relationships. The study of human evolution indicates that all of us belong to a single species that evolved in Africa and then spread across the world in stages.

The stages of human development are as follows-