evolution

1. EVIDENCES OF EVOLUTION
FROM
PATTERNS OF DEVELOPMENT

2. Evolution and Evidences of evolution:
 Evolution is a biological theory that postulates that all organisms on earth- plants,
and animals have a common origin and their differences today are the result of
modifications that occurred in successive generations.
 How Do We Know That Evolution Has Occurred?
The evidence for evolution has primarily come from four sources:
1. The fossil record of change in earlier species.
2.PATTERNS OF DEVELOPMENT:-morphological , embryological and anatomical
similarities of related life forms.
3.the geographic distribution of related species.
4. the recorded genetic changes in living organisms over many generations.
Evolution, which started out as a hypothesis, is now supported by evidence from
many fields of science.

3. 1.FOSSILS OF PLANTS
• Plant fossils are relatively much lesser as compared to animals. This is
because animals possess hard structures that can be easily preserved
which are not the case with plants.
• The fossil records of some protist and plant groups are shown in Fig.
The width of the shaded space is an indicator of the number of species.

4. FOSSIL of HORSE
• The earliest ancestor -Eohippus or
Hyracotherium
-living in the plains of North America in the
Eocene period
- had five toes
- size of the present day dog.
• replaced by the Mesohippus and Miohippus
- lived in the Oligocene period.
• Merychippus
-size of the pony
-had three toes in the fore and three in the hind
leg respectively.
-The median toe was longer and bore the entire
body weight.
• In Pliocene, the Merychippus was replaced
by Pliohippus .
• In the Pleistocene by the Equus.
• In all these forms, only the median toe
touched the ground and
bore a prominent hoof.
• Equus resembled the modern horse.

5. 2.Homology and Homologous Organs:
• Richard Owen introduced the term ‘homologous’.
• to those organs that have a common origin and built on the same fundamental pattern but
perform different functions .
• are morphologically different from each other.
• It is also known as divergent evolution.
• Homology does not prove that evolution has occurred; the existence of homology within a
group of organism is interpreted as evidence of descent from a common ancestor and
indicates a close phylogenetic relationship.
• Examples:
-the thorn of Bougainvillaea and the tendril of Passiflora
and Cucurbita .
-Leaves undergo modifications.
-The ovary wall in flowering plants is modified after
fertilization.
-Insect mouth parts modified differently.
-Limb Structures of Vertebrates.

6. • The thorns and tendrils are homologous.
- In plants, the thorn of Bougainvillaea and the tendril of Passiflora and Cucurbita
have different functions but both are axillary in position.
• Leaves undergo modifications of various kinds in different plants.
-In Cactus (Opuntia) the leaf is modified into a spine,
-in Lathyrus (sweet pea) it is modified into a tendril.
-be sessile (e.g., Zinnia) or petiolate (e.g., Pipal)
-simple (e.g., Mango) or compound (e.g., Cassia)
- reduced to scales (e.g.. Asparagus) modified into spines (e.g., Barberry) for
protection.

7. • The ovary wall in flowering plants is modified after fertilization in a
variety of ways to aid in seed dispersal.

8. • Insect mouth parts
-are made of the same parts – labrum, labium, mandibles, maxillae and hypopharynx.
- But in different insects they are modified differently – the biting and chewing mouth parts in
cockroach, the piercing and sucking type in mosquito, the sponging type in house fly.
• Limb Structures of Vertebrates
– The forelimb of a frog, wings of birds, wings of bat and seal, flippers of whale and the arm of man are
constructed on the same general plan, but perform different functions.
-The bones of the forelimb are made of the humerus in the upper arm, radius and ulna in the forearm,
carpals in the wrist, metacarpals in the palm and the phalanges in the digits.
- These bones become different from each other and include shortening or lengthening, decrease in the
number of bones or fusion of bones according to function.

9. 3.Analogy and Analogous Structures:
• The analogous organs are those that are similar in appearance and perform
the same function but are developed on a totally different plan.
• The presence of analogous organs indicate a similar adaptation by unrelated
groups through modification or evolution of different parts.
• It is called convergent evolution.
• Examples:-
- root of sweet potato and stem of potato .
- The tendril in edible pea is a modified leaflet, in Passiflora, a stem tendril, in wild pea a
whole leaf tendril, while in Gloriosa the tip of the leaf is modified as a tendril .
- The wing of a bird and the wing of an insect.
- Sting of honeybee and scorpion – the sting of both these organisms are different
structurally but functionally they serve the same purpose.
- Similarly, the fins of fishes, flippers of whales and penguin have similar appearance and
function but their structural details are totally different.

10. • The tendril in edible pea is a modified leaflet,
-in Passiflora, a stem tendril,
-in wild pea a whole leaf tendril,
-while in Gloriosa the tip of the leaf is modified as a tendril .

11. • The wing of a bird and the wing of an insect are used for flying, but their
basic structure is different.
-The wing of an insect is formed of a thin flap of chitin and stiffened by veins,
-but in birds, the flight surface is formed by feathers that are attached to the
bones of the forelimb.

12. 4.Vestigial Organs:
• Vestigial structures are remains of a structure that was functional in some ancestor but
is no longer functional in the organism.
• They are degenerate and non-functional.
• They are found in plants and animals, including humans.
• Some prominent examples are as follows:
-In plants like Ruscus and Asparagus, the leaves are reduced to spines.
-Vestigial Organs in Man.
-In snakes, vestiges of the bone of the hind limb and pelvic girdle are embedded in the flesh
of the abdomen.
- In animals living permanently in deep caves, the eyes are rudimentary or under-
developed.
- In flightless birds, small non-functional wings are present. For E.g. Ostrich, Emu,
Cassowary

14. 5.Atavism
• It is the reappearance of certain ancestral characters which had either disappeared or were
reduced.
• Atavism in plants.
-In Citrus leaf the lamina is separated from wing petiole by means of a joint or constriction. Sometimes
the winged part of the petiole is enlarged to produce two lateral leaflets making the leaf
trifoliolate.
-It shows that Citrus leaf was once trifoliolate compound but during evolution two leaflets have
degenerated.
-In many plants (e.g., Rosa, Hibiscus, Oxalis, Poppy), some of the stamens and even carpels get changed
to petal-like structures indicating that stamens and carpels have evolved from leaf-like structures.
• There are present some examples of atavism in human beings, viz., the power of moving pinna in
some persons, greatly developed canine teeth, exceptionally long dense hairsand short tail in
some.

15. 6.Evidences from Plant Embryos:
• Many bryophytes pass through a filamentous protonema stage before attaining
adult form. The filamentous protonema suggests algal ancestry for bryophytes.
• Bryophytes and pteridophytes have ciliated male gametes or sperms. They require
an external source of water for swimming to the female sex organs.
• In gymnosperms the sperms are transported by pollen tubes. Even then sperms of
cycas and Ginkgo are ciliated.
• The cycads are an intermediate group between the non-vascular plant and the
flowering plant and this represents a phylogenetic continuum within the plant
kingdom.
• In Pinus the foliage leaves do not occur directly on the main stems but are borne in
clusters on the dwarf shoots. However, in the seedling state the foliage leaves
occur directly on the main stem indicating evolution of Pinus from ancestors that
possessed foliage leaves directly on main stems.
• Australian species of Acacia possess phyllodes or foliaceous petioles instead of
normal bipinnate leaves as in other species of Acacia. Australian species show all
the transitional steps between bipinnate leaves and phyllodes during the seedling
stage.

16. Evidences from Animal Embryos:
• The early stages of embryos in all the
vertebrates exhibit remarkable similarity
and it is not easy to differentiate a
human embryo from the embryo of a
bird, reptile, amphibian and a fish.
• Fig. depicts the great similarity in the
early embryos of the forms mentioned
above. As they progress they become
more and more different.