Heredity and Evolution | Class 10 CBSE | Web Notes | Part 4

EVOLUTION AND CLASSIFICATION

Evolutionary relationships of the species can be worked out by identifying hierarchies of their characteristics.
Characteristics are details of appearance or behavior; i.e., a particular form or function. For example, four limbs of animals, photosynthesis in plants, etc.
Most organisms share some basic characteristics. Based on this, a hierarchy of classification is given below:

If two species have more common characteristics, they are more closely related and will have a recent common ancestor. For example, a brother and a sister have immediate common ancestors (parents). But the common ancestors of first cousins are grandparents. Thus, the classification of species reflects their evolutionary relationship.
Going backwards, we reach a single species (single common ancestor) at the beginning of evolutionary time.

Evolutionary relationships between different species are traced by identifying common characteristics.
The organs having a similar basic structure but modified to perform different functions are called homologous organs. For example, mammals, birds, reptiles, and amphibians have four limbs. Their basic structure is similar but performs different functions.

The organs having the same function but different structure and origin are called analogous organs. For example, wings of birds and bats.
Squirrels and lizards have no wings. This does not mean birds and bats are more closely related.
Wings of bats are skin folds stretched between elongated fingers, but a bird’s wings are a feathery covering along the arm. So, the design, structure, and components are different.
However, the arms (forelimbs) of birds and bats can be considered homologous because they have nearly the same sets of bones. The forelimbs are modified into wings.

Fossils are preserved traces of organisms that lived in the past. They help us understand extinct species. For example:
A dead insect caught in hot mud will not decompose quickly, and the mud will eventually harden and retain the impression of its body parts.
The dinosaur skull fossil in the Narmada valley.
Age of fossils can be estimated in two ways:
Relative: Fossils closer to the surface are more recent than those in deeper layers.
Detecting the ratios of different isotopes of the same element in the fossil.

How do fossils form layer by layer?

Imagine some invertebrates on the seabed die 100 million years ago and are buried in the sand. As more sand accumulates, sandstone forms under pressure. Millions of years later, dinosaurs living in the area die and are buried in mud, which also becomes rock above the rock containing the earlier invertebrate fossils.
Millions of years later, the bodies of horse-like creatures die and are fossilized in rocks above the earlier rocks.

Evolution of complex organs, such as the eye, is the result of the selection of several intermediate stages. For example, an intermediate stage, such as a rudimentary eye (e.g., eye spots of Planaria), gives a fitness advantage.
Also, a change useful for one property can become useful for a different function. For example, feathers start out providing insulation in cold weather but later become useful for flight. Some dinosaurs (reptiles) had feathers but could not fly. Birds later adapted the feathers for flight. This means birds are very closely related to reptiles.
All dissimilar structures evolved from a common ancestral design. This can be analyzed using fossils.
Such evolutionary relationships can be understood with the help of wild cabbage.
Humans started cultivating wild cabbage more than 2000 years ago and generated different vegetables from it by selection. This is artificial selection rather than natural selection.

Artificial Selection	Produced Plant
Very short distances between leaves.	Cabbage
Arrested flower development.	Broccoli
Sterile flowers.	Cauliflower
Swollen parts.	Kohlrabi
Slightly larger leaves.	Kale (leafy vegetable)

All these structures are descended from the same ancestor.
Changes in DNA during reproduction are the basic events in evolution. Comparing the DNA of different species gives a direct estimate of how much the DNA has changed during speciation and helps identify where each change diverged from the other. This method is called molecular phylogeny. It is extensively used to define evolutionary relationships.

There are multiple branches possible at each stage of tracing the family trees of species.
If a new species emerges, it does not necessarily mean that the old species disappear. It depends on the environment. Also, the newly generated species may not be ‘better’ than the older one.
Human beings did not evolve from chimpanzees. Rather, both humans and chimpanzees have a common ancestor. The two resultant species from that common ancestor evolved in separate ways to give rise to humans and chimpanzees.
There is no real ‘progress’ in the idea of evolution.
Evolution is simply the generation of diversity and the shaping of diversity by environmental selection.
The only progressive trend in evolution is that more complex body designs have emerged over time. However, many older and simpler designs still survive. For example, bacteria inhabit habitats like hot springs, deep-sea thermal vents, and the ice in Antarctica. In other words, human beings are not the pinnacle of evolution but simply another species in the evolutionary process.

All humans are a single species.
The earliest members of the human species, Homo sapiens, evolved in Africa.
A couple of hundred thousand years ago, some human ancestors left Africa.
The migrants slowly spread from Africa to West Asia, then to Central Asia, Eurasia, South Asia, and East Asia. They traveled down the islands of Indonesia and the Philippines to Australia and crossed the Bering land bridge to the Americas. This separation and mixing led to the evolution of modern humans.

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