Morphological Evidences (Homology, Analogy, Vestigial organs) - PowerPoint Presentation

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Morphological Evidences(Homology, Analogy, Vestigial organs)

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Comparative study of morphological and anatomical structures of animals and plants also indicate that they are constructed on the same basic plan and they shows certain similarities.

Such similarities are observed in:

1. Body

organizations

2. Gradual Modifications

3

. Homologous organs

4

. Analogous organs

5

.

Vestigeal

Organs

6

. Connection Links

7

. Atavism

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1. Body organizationsThe basic unit of all living being is cell.

In all

multicellular

organisms similar or different types of cell

aggregate to form

tissues

which gather to form

organs

and different organs interacted to form

organ system

.

This similarity in

body organization

indicates a

fundamental unity among all

animals.

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Examples In

sponges, the cells function more less independently and not organized themselves into tissues, this gives the sponges a

cellular level of organization

.

In

coelentrates

,

the cell are aggregated into tissues

but not form organs, this produce a

tissue level of organization

.

In

platyhelminthes

onward, the body has

organ-system level of organization

, consisting of cells, tissues, organs and organ-systems

.

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The organization of different organ systems like nervous system, blood vascular system, respiratory system, excretory system, etc are similar in many groups of organisms. Example

the

respiratory system

of

terrestrial vertebrates

has

two lungs

,

a trachea

,

a larynx

,

nasal chambers

and

nostrils

.

Likewise, the

blood vascular system

of all vertebrates contains

a heart

,

arteries

,

veins

and

lymph vessels

.

This change from simple to complex body organization (from sponges to mammals) and the presence of similar organ systems indicate a

common ancestry

.

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2. Gradual ModificationsThe organs like

Heart,

Brain

,

Kidney

etc. in certain vertebrates during the course of organic evolution gradually modified according to the environmental requirement to perform best function.

This modification was the actual mechanism of organic evolution.

The

heart

in case of

fishes

is two chambered .

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But in case of Amphibians it become three chambered (02 auricles and 01 ventricle).

In most of the

Reptiles

it is incompletely four chambered (02 auricles and partly divided 01 ventricle).

But in few

Reptiles

, all

Birds

and

Mammals

it is completely four chambered (02 auricles and 02 ventricles)

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3. Homology & Homologous organsHomology is the structural similarities between organs of different animals based on common ancestry or common embryonic origin.

The homology is seen in every organ system from fishes to mammals.

The homologous organs are those organs which have

the same fundamental structure but are different in

functions.

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These organs follow the same basic plan of organization during their development.

But

in the adult condition, these organs are modified to perform different functions as an adaptation to different environments.

The

homologous structures are

the

result of

divergent evolution

. Homology indicates common ancestry

.

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Examples:(a) Structural Homology in Fore limbs of vertebrates

The fore-limbs of seal/whale (flippers), bat (wings), mole, horse/cheetah and man look different, having different shapes and performs different functions but have the same basic structural plan.

That is the fore-limb in all the cases consists of the same set of bones i.e. single

humerus

, jointed radio-ulna, carpals in wrist, metacarpals in palms and phalanges in digits.

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Thus the skeletal parts of the fore-limbs of all these vertebrates are similar in structure and arrangement. But they look different in shapes and perform different functions.

In seal/whale it is used for swimming, in bat for flying, in mole for digging, in horse/cheetah for running and in man they are used for grasping(1. 7.19).

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(b) Structural Homology in BrainStarting from fishes to mammals, the brain consists of

same series of parts

such as:

a.olfactory

lobes

,

b.cerebral

hemispheres

,

c.optic

lobes

,

d.cerebellum

,

e. medulla oblongata

etc.

During the course of evolution some lobes shows enlargement e.g. the

cerebral hemispheres

in case of

fishes

are smaller than the

optic lobes

but in mammals they are much enlarged and hide the

olfactory lobes

in front and the

optic lobes

behind.

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(c) Structural Homology in HeartThe heart in

fishes

is two chambered where the auricle

receives

the blood and ventricle

pumps

it to the gills.

In

amphibians and reptiles

the heart is three chambered, 02 auricles and 01 ventricle. The

left auricle

received the oxygenated

blood from the lungs whereas the

right auricle

received the deoxygenated

blood from rest of the body.

Thus oxygenated and deoxygenated blood is stored separately but

gets mixed in the ventricle

while being pumped to the body organs.

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However, in higher reptiles, birds and

mammals the heart is 04 chambered where the

oxygenated and deoxygenated blood remain completely separated

during receiving from the body and pumping to the body

.

This shows a gradual modification in structure of heart during the course of organic evolution however the fundamental structure remain same.

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(d) Serial HomologySerial homology have been observed among invertebrates also especially in arthropods.

In

arthropods

the body is segmented and the

exoskeleton

in

chitinous

.

The exoskeleton is constructed on the same basic pattern in all classes of arthropod e.g. in crustaceans (prawn) all the segment of the body bears a pair of jointed appendages.

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All the appendages in prawn though appear drastically different in appearance but have same parts i.e. protopodites,

exopodite and

endopodite

.

These portions of the appendages in different segments modified to perform different functions

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Likewise, the mouth parts in cockroach, honey bee, mosquito and butterfly have the same fundamental plan

.

In

each of these insects the mouth parts comprise

labrum

, a pair of

mandibles

and two pairs of

maxillae

.

But in different groups they modify according to feeding habit like in cockroaches they are adapted for biting and chewing.

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In honey-bee for chewing and lapping, in mosquito for

piercing and sucking, in house-fly for

sponging

and in butterfly for

siphoning

.

Thus the similarity in the fundamental structure of the apparently different appendages or mouth parts indicate that they have evolved from the same ancestor over a period of time.

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Honey Bee

Beetle

Mosquito

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(e) Molecular HomologyHomology is also seen amongst the molecules

.

For

example, the proteins found in the blood of man and ape are similar.

The

phylogeny of an organism can be traced by using the base sequence in nucleic acids and amino acid sequence of proteins in related organisms.

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4. Analogy and Analogous organsThe analogous organs are those organs which are

similar in appearance and perform

same functions

but are different in their

structural details

and

embryonic origin

These organs are evolved because of

convergent evolution or parallel evolution

.

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Examples:

(a) The wings of an insect are analogous to wings

of

Pterodactyle

(extinct flying reptile) or

a

bird

or a

bat

because all of them are used for flying but their basic

structure

is different.

The

wings of

an insect in an extension of the integument which is stiffed by a number of veins while the wings of

Pterodactyle

was formed by enormous fold of skin which is supported by enlarged fourth finger of the forelimb. However, their function is similar.

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Similarly the wing of bird is made up of bones covered with flesh, skin and feather.The superficial similarity of these organs are due to

adaptation

to fly rather than to

inheritance

from a common ancestor.

Each has evolved from

separate ancestral

population

as a means of more efficient mode of locomotion.

The

similarity

that developed in

distantly related

groups

as an

adaptation

for the same function is called

analogy or convergent evolution

.

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(b) Pectoral Fins of sharks (fish) and flippers

of Dolphins

(aquatic mammals) are analogous organs because they are used for the same purpose, i.e. for swimming.

However, structurally they are totally different. The Pectoral fins of sharks are not

pentadactyle

, whereas The flippers of dolphins are the modified

pentadactyle

limb

.

Thus basic structure of pectoral fins of sharks and flippers of dolphins are different but both perform same function and are useful in swimming.

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(c) Stings of honey bee

and scorpion

are analogous structures because both perform the same function.

However, the sting of honey bee is a modification of its ovipositor (structure that helps in egg laying) while that of scorpion is modified last abdominal segment.

Stings of both arthropods perform similar function.

These functional similarities between analogous organs support occurrence of organic evolution.

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5. Vestigial Organs:

The organs which are present in

reduced

form

and

do not perform any function

in the body but correspond to the

fully

developed functional organs

of related animals are called

vestigial organs

. They are

believed

to be remnants of organs which were complete and functional in their ancestors.

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Examples:

(a) Vestigial Organs in Human Body:

Human body has been described to possess about 90 vestigial organs.

Some

of these are

nictitating

membrane

(

plica

semilunaris

),

auricular muscles

, (muscles of pinna),

segmental muscles of abdomen

,

panniculus

camosis

(subcutaneous muscles),

vermiform

appendix

,

caudal vertebrae

(also called

coccyx

or tail bone),

third molars

(wisdom teeth),

hair on body

, and

nipples in

male

.

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(b) Vestigial Organs in Animals: Important examples are vestiges of

hindlimbs and pelvic girdles of pythons and

greenland

whales, (which show that snakes and whales originally evolved from four-footed ancestors), wings of flightless birds such as Ostrich; Emu, Cassowary, Kiwi, Rhea and Dodo (extinct), splint bones in feet of horse and brow spot in frog’s head (a vestige of 3rd eye).

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6. Connecting Links:

Those

organisms

which possess the

characters

of

two

different

groups

are called

connecting

links

.

Following

are some important examples of connecting links

.

(

a)

Euglena

, which is a

protist

, is considered as connecting link between the

animals and plants

.

The

animal characters

such as

mouth, contractile

vacuoles, binary fission

as well as the

plant

characters

like

chloroplast

and

photoautotrophic

mode of nutrition are present in one organism.

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(b) Proterospongia is a link between

Protozoa and Porifera

.

It is a colonial

protozoan

.

It consists of flagellated and collared individuals that resemble

choanocytes

(collar cells) of

sponges (

Porifera

)

.

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(c) Neopilina . It is a connecting link between

Annelida

and

Mollusca

.

It resembles

molluscs

as it possesses a

shell

, a

mantle

and a large

muscular foot

.

Its

annelid

characters are presence of

segmentally

arranged

gills

,

nephridia

and

muscles

and a

trochophore

-like larval stage.

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(d) Peripatus

(walking worm), an arthropod, is a connecting link between

annelids

and

arthropods

.

Its

annelid

characters are the

worm-like body

,

structure of the eyes

,

unjointed

legs

, presence of

segmental

nephridia

,

soft cuticle

and continuous

muscle layers

in the body wall.

The

arthropod

characters

exhibited are

haemocoel

,

tracheae

as respiratory organs and

tubular heart

with

ostia

.

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(f) The lung fishes, e.g.,

Protopterus (African lung fish),

Lepidosiren

(South American lung fish) and

Neoceratodus

(Australian lung fish) may be considered the connecting links between the

fishes and amphibians

.

The lung fishes have all the characters of a typical fish, but they are capable of respiring through lungs and possess a three chambered heart.

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(g) Latimeria (Coelacanth fish) is considered a connecting link between

fish and amphibians.

(h)

Chimaera

. It is a connecting link between

cartilaginous fishes and bony fishes

.

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Chimaera

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(i) Egg-laying mammals

(e.g.,

Ornithorhynchus

known as

Duck-billed platypus and

Tachyglossus

known as Echidna or Spiny ant eater) are connecting link between reptiles and mammals.

The mammalian characters are hair and mammary glands.

The reptilian characters are presence of

cloaca

. They lay eggs. They also shows some skeletal similarities.

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Short-beaked

Echidna

 (

Tachyglossus

)

Duck-billed

platypus

(

Ornithorhynchus

)

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(e) Balanoglossus is a connecting link between non-chordates and chordates.

Although it is a hemichordate (non-chordate).

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7.

Atavism:

It is the reappearance of certain ancestral characters which had either disappeared or were reduced.

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 hairs, short tail in some

babies.

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Atavism is also observed 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.

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