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The Living World Class 11 Biology Notes

Ernst Mayr (1904-2004)

Ernst Mayr was a highly influential evolutionary biologist at Harvard University, often called "The Darwin of the $20^{th}$ century." His work covered many areas, including ornithology (the study of birds), taxonomy, evolution, and the history and philosophy of biology.

He was a key figure in making the origin of species diversity a central question in evolutionary biology.
He developed the modern, widely accepted definition of a biological species.
In recognition of his immense contributions, he received the "triple crown of biology": the Balzan Prize (1983), the International Prize for Biology (1994), and the Crafoord Prize (1999).

What is Living?

The world around us is filled with an incredible variety of life, from galloping horses and migrating birds to microscopic organisms inside a single cell. This vast diversity makes us wonder: what exactly is "life"?

This question can be broken down into two parts:

A technical question: What makes something living, as opposed to non-living?
A philosophical question: What is the purpose of life?

As scientists, we focus on the first question, trying to understand the defining characteristics of living organisms.

Diversity in the Living World

If you look around, you'll see a huge variety of living things. The more you explore—from your backyard to a dense forest—the more diversity you'll find.

Biodiversity is the term used to describe the number and different types of organisms present on Earth.
Currently, the number of known and described species is between 1.7 to 1.8 million.
As we explore new and even old areas, new organisms are constantly being identified.

Nomenclature and Identification

With millions of organisms in the world, we need a clear and consistent way to name them. Local names can be confusing because they change from place to place. For example, a single bird species might have dozens of different local names.

To solve this problem, scientists developed a standardized naming system.

Nomenclature is the process of providing a standardized scientific name to an organism, ensuring it is known by the same name worldwide.
Identification is the process of correctly describing an organism to know precisely which one the name is attached to.

To ensure consistency, international codes have been established:

International Code for Botanical Nomenclature (ICBN) provides the agreed-upon rules for naming plants.
International Code of Zoological Nomenclature (ICZN) provides the rules for naming animals.

Binomial Nomenclature

The system used by biologists all over the world is Binomial Nomenclature, which was developed by Carolus Linnaeus. This system gives each organism a name with two parts, which is convenient and descriptive.

Example

The scientific name for a mango is Mangifera indica.

Mangifera is the genus name.
indica is the specific epithet (or species name).

Universal Rules of Binomial Nomenclature

Language: Biological names are generally in Latin and are written in italics. They are Latinised even if they come from other languages.
Components: The first word represents the genus, and the second word represents the specific epithet.
Formatting: When handwritten, both words are separately underlined. When printed, they are in italics to indicate their Latin origin.
Capitalization: The genus name starts with a capital letter, while the specific epithet starts with a small letter. For example, Mangifera indica.
Author Citation: The name of the scientist who first described the species is often written in an abbreviated form after the specific epithet. For example, Mangifera indica Linn. indicates that Linnaeus first described the mango species.

Classification, Taxa, and Taxonomy

It is impossible to study every living organism individually. Therefore, we need to group them in a way that makes sense.

Classification is the process of grouping organisms into convenient categories based on easily observable characteristics. For example, when we say "dogs," we picture animals with certain features, distinguishing them from "cats."
Taxa (singular: taxon) is the scientific term for these categories. Dogs, cats, mammals, wheat, and plants are all examples of taxa, but they represent categories at different levels. A dog is in the 'dog' taxon, which is part of the 'mammal' taxon, which is part of the 'animal' taxon.
Taxonomy is the scientific study of the principles and procedures of classification. It involves four basic processes:
1. Characterisation: Understanding the characteristics of an organism.
2. Identification: Correctly identifying the organism.
3. Classification: Grouping the organism with others that are similar.
4. Nomenclature: Giving the organism a scientific name.

Modern taxonomy is based on a wide range of information, including external and internal structure, cell structure, developmental processes, and ecological information.

Systematics

Humans have long been interested not just in identifying organisms, but also in understanding the relationships between them.

Systematics is the branch of biology that deals with the diversity of organisms and their evolutionary relationships.
The term comes from the Latin word 'systema,' meaning a systematic arrangement of organisms. Linnaeus used Systema Naturae as the title of his famous publication.
Systematics includes identification, nomenclature, and classification, but it also places a strong emphasis on the evolutionary history that connects different species.

Taxonomic Categories

Classification isn't a single step. It involves a sequence of steps, where each step represents a rank or category. This sequence is called the taxonomic hierarchy. All the categories together make up this hierarchy. Each category is also referred to as a taxon.

The main taxonomic categories, in ascending order, are: Species → Genus → Family → Order → Class → Phylum / Division → Kingdom

Note

As we move up the hierarchy from species to kingdom, the number of common characteristics among organisms decreases. Organisms in the same species are very similar, while organisms in the same kingdom (like all animals) are very diverse and share only a few fundamental features.

Species

A species is the basic unit of classification. It is a group of individual organisms with fundamental similarities. We can distinguish one species from another based on clear morphological (structural) differences.

In the name Mangifera indica (mango), indica is the species.
In Solanum tuberosum (potato), tuberosum is the species.
In Panthera leo (lion), leo is the species.

Genus

A genus (plural: genera) is a group of closely related species that share more characters in common than species from other genera.

The genus Panthera includes the species leo (lion), pardus (leopard), and tigris (tiger).
The genus Solanum includes the species tuberosum (potato), nigrum, and melongena (brinjal).
The genus Felis includes cats and is different from the genus Panthera.

Family

A family is a group of related genera. It has fewer similarities compared to the genus and species levels.

In plants, families are characterized by both vegetative (stems, leaves) and reproductive (flowers, fruits) features. The genera Solanum, Petunia, and Datura are all placed in the family Solanaceae.
In animals, the genus Panthera (lions, tigers) and the genus Felis (cats) are put into the family Felidae. The dog family, Canidae, is separate.

Order

An order is an assemblage of related families that share a few similar characters. The similarities are even less than at the family level.

The plant families Convolvulaceae and Solanaceae are included in the order Polymoniales, mainly based on their floral characters.
The animal families Felidae (cats) and Canidae (dogs) are included in the order Carnivora.

Class

A class is a category that includes related orders.

The order Primata (monkeys, gorillas) and the order Carnivora (tigers, cats, dogs) are both placed in the class Mammalia. This class is defined by features like external ears and body hair.

Phylum / Division

This category is made up of related classes.

In animals, this rank is called a Phylum. The phylum Chordata includes classes like Fishes, Amphibians, Reptiles, Birds, and Mammals because they all share common features like a notochord and a dorsal hollow neural system at some stage of their life.
In plants, the equivalent rank is called a Division. For example, flowering plants are placed in the division Angiospermae.

Kingdom

The Kingdom is the highest taxonomic category.

Kingdom Animalia includes all animals, from insects to mammals.
Kingdom Plantae includes all plants, from mosses to flowering trees.