Monday, July 27, 2020

Evolution - Notes | Class 12 | Part 2: Evidences for Evolution


1. Paleontological evidences

Paleontology is the study of fossils.
Fossils are remnants of life forms found in rocks (earth crust). They are written documents of evolution.
Significance of fossils:

a.    To study phylogeny (evolutionary history or race history). E.g. Horse evolution.
b.    To study the connecting link between two groups of organisms. E.g. Archaeopteryx.
c.    To study about extinct animals. E.g. Dinosaurs.
d.    To study about geological period by analysing fossils in different sedimentary rock layers. The study showed that life forms varied over time and certain life forms are restricted to certain geological time spans.

2. Morphological & Anatomical evidences
Comparative anatomy and morphology shows that different forms of animals have some common structural features. This can be explained as follows:
a. Homologous organs

-    Homologous organs are the organs having fundamentally
similar structure and origin but different functions. This phenomenon is called Homology.
-    E.g. Human hand, Whale’s flippers, Bat’s wing & Cheetah’s foot. These forelimbs have different functions but similar anatomical structures such as bones (e.g. humerus, radius, ulna, carpals, metacarpals & phalanges).
-    Homology is also seen in heart, brain etc.
-    Homology in plants: E.g. Thorns of Bougainvillea and tendrils of Cucurbita.
-    The origin of homologous organs is due to Divergent evolution. It is the evolution by which related species become less similar to survive and adapt in different environmental condition.
-    Homology indicates common ancestry.

b. Analogous organs
These are the organs having similar function but different structure & origin. This phenomenon is called Analogy. E.g.
§ Wings of insects (formed of a thin flap of chitin) and wings of birds (modified forelimbs).
§ Eyes of Octopus (retina from skin) and mammals (retina from embryonic brain).
§ Flipper of Penguins and Dolphins.
§ Sweet potato (modified root) & Potato (modified stem).
§ Trachea of insects (from ectoderm) and lungs of vertebrates (from endoderm).
Origin of analogous organs is due to Convergent evolution. It is the evolution by which unrelated species become more similar to survive and adapt in similar environmental condition.
3. Adaptive radiation (Biogeographical evidences)
Adaptive radiation (evolution by adaptation) is the evolution of different species from an ancestor in a geographical area starting from a point. It is a type of divergent evolution. E.g. 
o  Darwin’s finches in Galapagos Islands.
o  Australian marsupials (Marsupial radiation).
o  Placental mammals in Australia.

When more than one adaptive radiation is appeared in an isolated geographical area, it results in convergent evolution.
E.g. Australian Marsupials and Placental mammals.
Placental mammals
Australian Marsupials
Marsupial mole
Ant eater
Numbat (Ant eater)
Marsupial mouse
Spotted cuscus
Flying squirrel
Flying phalanger
Tasmanian tiger cat
Tasmanian wolf

4. Biochemical evidences

-    Organisms show similarities in proteins, genes, other biomolecules & metabolism. It indicates common ancestry.

5.  Embryological evidences

-     Proposed by Ernst Haeckel.
-     He observed that all vertebrate embryos have some common features that are absent in adult.
-     E.g. all vertebrate embryos (including human) develop vestigial gill slits just behind the head.  But it is functional only in fish and not found in other adult vertebrates.
-     However, Karl Ernst von Baer rejected this proposal. He noted that embryos never pass through the adult stages of other animals.

6. Evidences for evolution by natural selection

Natural selection is the process in which organisms with better favourable & heritable variation are survived and reproduced.
Some evidences are given below:
§ Industrial melanism: In England, before industrialization (1850s), there were more white-winged moths (Biston betularia) on trees than dark winged or melanised moths (Biston carbonaria). After industrialization (1920), more dark-winged moths and less white winged moths were developed.
Before industrialization: There was white lichens covered the trees. In that background, white winged moths survived but dark winged moths were picked out by predators.
After industrialization: The tree trunks became dark due to industrial smoke and soot. No growth of lichens. So white winged moths did not survive because the predators identified them easily. Dark winged moth survived because of suitable dark background.
§ Development of resistant varieties in organisms against herbicides, pesticides, antibiotics or drugs etc.
These are the examples for natural selection by anthropogenic action (evolution due to human activities).

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