EVOLUTION, ITS EVIDENCES & PATTERNS

· Father/founder of the concept of evolution - Empedocles (493-435 BC)
· Father of modern applied ideas of evolution - Buffon (1707-1788 AD)
· Father of evolution/Term evolution was coined by Herbert Spencer (1820-1903 AD)
· Father of organic evolution- Charles Darwin (1809-1882 AD)
· Derived from 2 Latin words- (e- from, and volvere - to unroll)
· Charles Darwin in 1859 AD defined evolution as ‘Descent with modification', Evolution is a process by which related populations diverge from one another, giving rise to new species.

· Anthropoids gave rise to old world monkeys, apes and man. (Hominoid line- line remaining after the separation of old world monkeys).

· Changes in species over time/ Slow but continuous (Gradual) process/ branch of biology that deals with cumulative changes in the characteristic of organisms, over time in response to changes in an environment.
· History and development of race with variations
· Organic evolution is the process by which early forms of life, once originated, underwent gradual modification, with the changing environmental condition of the earth, to evolve into present-day diverse forms of life in millions of years.
· Three stages of evolution were given by Haeckel [BPKIHS 2002]. 

Evidence of Evolution

A) HOMOLOGOUS ORGANS
· Structure and origin– same, appearance and functioning- different [MOE 055, 062]

Examples of Homologous Organs

a) Structure and arrangement of bones of limbs of different vertebrates  

b) Basic plan of mouthparts of insects is the same but modified according to the mode of feeding. The mouthparts of cockroach, honey bee, mosquito and butterfly comprise labrum, a pair of mandibles and two pairs of maxillae but they perform different functions.

For example, Butterfly - siphoning, Housefly -sponging, Cockroach - biting and chewing, Honeybee - chewing and lapping, Mosquito - piercing and sucking.

c) Seven cervical vertebrae in mammals - All mammals have seven cervical vertebrae irrespective of their lengths of necks.

d) Thorns of Bougainvillea and tendrils of cucurbits (arise in auxiliary positions and are modified branches.)

e) Legs of different insects: Legs of insects are composed of five parts: coxa, trochanter, femur, tibia and tarsus. Except for tarsus, others are of a single segment.
Legs in mole cricket (forelegs), grasshopper (hindlegs), honey bee, mantids and water beetles are specialized for digging, jumping, collecting pollen, grasping prey and swimming respectively but in all these cases, the legs are formed of similar 5 podomeres.

f) Teeth of man

g) Penis and clitoris [BPKIHS 2002]

h) Testes and ovaries

i) Fin of fish and wing of bird [MOE 2003]

· Proves common ancestry- A group of quite similar organisms might have shared different habitats.
· Show adaptive radiation (Development of different functional structures from a common ancestral form). 

· The concept was given by H.F. Osborn in 1898.

· For example, modification in limbs of mammals from their common terrestrial and with the pentadactyl arrangement and divergent evolution. @ HOD= (Homologous- Origin same- Divergent evolution)

B) ANALOGOUS ORGANS

· Different structure and origin but same appearance and functioning

Examples of Analogous Organs
a) The wing of an insect or pterodactyl or bird or patagium of bat

· The wing of an insect is formed of a thin flap of chitin and stiffened by a series of 'veins'. It is operated by muscles attached to its base.
· In pterodactyl, the wing is an enormous fold of skin supported by an enormously enlarged fourth finger of the forelimb.
· In birds, the flight surface is formed by feathers attached to the bones of the forelimb.
· In bat, the wing is formed of a fold of integument (patagium), supported by the elongated and outspread phalanges of the last four digits (second, third, fourth and fifth digits.)
b) Eyes of Fish & Squids (Mollusca) - Function in the same way but their origin and structure are different.
c) Trachea of Insects / Vertebrates - [Used for respiratory purpose but the mode of development is different].
d) Stings of Honey bee / Scorpion - [Used for stinging but in the honey bee, the sting is modified ovipositor (the structure that helps in egg-laying) but in Scorpion, modified last abdominal segment]
e) Stem of Ruscus (modified into leaf-like) and a leaf of any typical plant- both are photosynthetic regions.
f) Gill of Fish / Prawn (respiration)
g) Fins of Fish / Flippers of whales are analogous organs. Fins of fishes are not pentadactyl but flippers of whales are pentadactyl.
· Supports convergent evolution as a group of quite dissimilar organisms might have shared a common habitat, also called homoplastic organs.
· No evolutionary significance
· Sir Richard Owen formulated the concept of Homology and Analogy.
· Study of functional anatomy is called Tectology.


C) CONNECTING LINKS
· Living animals with characters of two different groups of animals are called connecting links.
· Extinct animals with characters of two different groups of animals are called missing links.

 

CONNECTING LINK BETWEEN

Reptiles: Teeth, jaw, claw, tail, non-pneumatic bones	Birds: beak, feathers, wings, 4 digits on each foot for perching, sternum with keel (distinct median ridge-like structure along the ventral part of avian sternum).	Archaeopteryx lithographica [KUMET 2000] [Aves]
Annelida: Worm-like body, non-chitinous cuticle, nephridia	Arthropoda: Compound eyes, antennae, haemocoel, semi-jointed paired legs, trachea, and tubular heart with Ostia.	Peripatus [Arthropoda] [MOE 2002]
Annelida: Gills, nephridia, trochophore like stage	Mollusca: Shell, Mantle, Muscular foot.	Neopilina [Mollusca]
Fish: Fin, scale, gill, lateral line system.	Amphibia: Internal nares, lungs, 3 chambered heart	Dipnoi [Lung fishes] (Proptopterus, Neoceratodus, Lepidosiren)
Non–chordates	Chordates	Balanoglossus [Chordata]
Cartilaginous fishes	Bony fishes	Chimaera (Rabbitfish)
Plant: Chloroplast, photoautotrophic nutrition	Animal: Flagella, contractile vacuole, binary fission	Euglena [Protozoa] [IOM 200]
Aquatic plants	Land plants	Fritschiella
Reptiles: Oviparous, variable body temperature (Poikilothermic), cloaca, some skeletal similarities	Mammals: Hair, mammary gland, diaphragm	Ornithorhynchus (Duck-billed platypus), Spiny anteater Echidna, Lycaenops (Extinct reptile).
Living	Non-Living	Viruses
Fish	Land vertebrates	Coelocanthus
Ape	Man	Australopithecus
Protozoa	Porifera	Proterospongia [Protozoa]
Amphibia	Reptilia	Seymoria [IOM 2007]

· Confirms the evolution of Arthropoda and Mollusca from annelids, amphibians from fishes and birds and mammals from reptiles.

D) LIVING FOSSILS

Animals that underwent very little change during long geological periods.
· Peripatus, Limulus - (Arthropoda)
· Neopilina, Nautilus - (Mollusca )
· Lingula - (Branchiopoda, Crustacean)
· Latimeria - (Coelacanth fish)
· Sphenodon - (Reptilia)
· Didelphis - (Opossum- tree-dwelling mammal)
· Lamprey – (Chordata- Cyclostomata)


E) VESTIGIAL ORGANS
· Rudimentary and functionless organs, organs of no use to the possessor.
· Weidershiem listed over 100 vestigial organs in man.

Examples of Vestigial organs in man are:
· Coccyx (Tail bone) [KUMET 2004]
· Wisdom (Third molar) teeth
· Nictitating membrane (Plica semi-lunaris or third eyelid)
· Body hair
· Ear pinna muscles or Auricular muscles [Indian Embassy 2002, 2006]
· Nipples in male
· Segmental muscles on the abdomen
· Vermiform appendix (Extension of caecum) [BPKIHS 2003; KUMET 2000]
· Melanocyte Stimulating Hormone

In other animals:
· Rudiments of pelvic girdles in Python
· Wings in flightless birds (Ostrich, Kiwi)
· Eyes of Proteus (Cave salamander )
· Reduced second and fourth metacarpals (splint bone in Horse )
· Pelvic girdle and Ear pinna of Whale
· Well developed vestigial organs in some animals and rudimentary vestigial organs found in other animals indicate their common ancestry.


F) ATAVISM
· The reappearance of ancestral characters
· Confirms ancestral history
Examples of Atavism are:
· Long and dense hair in some men (Lions boy of Russia )
· Newly born baby with a small tail
· Multiple nipples
· Power of moving pinna in some person
· Cervical fistula in man

· In man, in the normal course, only one pharyngeal pouch perforates to form an opening from the pharynx to the exterior in the form of the external ear canal as Eustachian tube.

· But rarely, the neck may possess an Additional opening through which the throat or nasal cavity communicates with the exterior. This represents the opening of an Additional pouch to the exterior and is known as a cervical fistula.
· Homodont condition in many aquatic animals or Homodont dentition in piscivorous cetaceans

G) EMBRYOLOGY
· Study of the development of an individual from fertilized egg to adult
· Embryos of different vertebrates.
· Development starts from a single cell and contains cleavage, morula, blastula and gastrula.
· Heart of vertebrates supports biogenetic law.
· Rule of embryonic development was given by Von Baer (1828AD).
· Biogenetic law/Recapitulation theory/Ontogeny recapitulates Phylogeny-Ernst Haeckel (1866 AD).
· Presence of gill slits (clefts) in vertebrate embryos - supports recapitulation.


H) BIOGEOGRAPHY
· Study of the geographical distribution of animals and plants
· Alfred Russel Wallace divided the world into 6 major biogeographical regions called realms - Palaearctic, Nearctic, Ethiopian, Neotropical, Oriental, Australian.
· Palaearctic and oriental regions are separated by high mountain ranges.
· Galapagos Islands are called “a living laboratory of evolution”.
· Darwin studied twenty related varieties of the bird belonging to the family Geospizidae, differing in the shape and size of the beak. These birds are now called 'Darwin's finches.


I) PALEONTOLOGY
· Study of fossils [MOE 2063; IOM 1998]

· Fossils is the remains of ancient organisms

· Fossils are found in sedimentary rock.

· Father of palaeontology- Leonardo da Vinci
· Founder of modern palaeontology- Georges Cuvier

· Age of rock is calculated on the basis of the amount of lead present.
· Written documents or Footprints of evolution.
· Geologists traced the evolutionary history of horses, camels, dogs, man and elephants with the help of the study of fossils.

Types of fossils: Unaltered, petrified, impressions, compressions, casts or moulds and amber.

i. Unaltered: Whole bodies, woolly mammoths were found buried in ice in Siberia.
ii. Petrified: Replacement of organic parts by minerals (Carbonates, Sulphates, Silicates, and Phosphates) is petrification, the most usual form of fossils- Bones, shells, teeth and trunk of trees.
iii. Impressions: Body parts of organisms when coming in contact with clay leave impressions on the soil. It later hardens to form rock. E.g. Feathers of Archaeopteryx.
iv. Compressions: Buried parts of organisms become flat due to overlying pressure. The outline is only left behind. Plant fossils.
v. Casts or moulds: Organism when gets submerged in water with lime, sediments surround the body in the form of the hard coat. They retain true copies of external morphology. E. g. Skeleton of Gastropods.
vi. Amber (Resin): Resinous secretion of certain coniferous trees.

Determination of Age of Fossil

· Age of fossil is determined either by radioactive carbon dating technique or by the amount of lead in the rock.
(a) Radioactive Carbon-dating Technique

· It is used to determine the age of recently found fossils. It is applicable to those specimens only which have carbon.

· Radioactive carbon has a half-life of 5568 years.

(b) The amount of lead in a Rock

· Uranium changes into the lead. The rate of this change is independent of the conditions under which it takes place.

· It has been estimated that one million grams of Uranium can produce 17,600 grams of Lead per year.
(c) Potassium-Argon method

· This method has recently been used to determine the age of hominid fossils in East Africa.

· The half-life of potassium40 is 1.3 * 10^9 years.


J) GEOLOGICAL TIME SCALE
· History of evolution based on fossils.
· The prokaryotes were originated in Precambrian.
· Fishes originated in Ordovician, Amphibians- Devonian, Reptiles- carboniferous, dinosaurs- Triassic period.
· Mammals originated about 250 million years ago.

Animals	Evolved or Originated	Dominated or Age
Fishes	Ordovician	Devonian
Amphibians	Devonian	Carboniferous
Reptiles	Carboniferous	Jurassic or Mesozoic Era [IOM 2008]
Birds	Jurassic	Cenozoic Era
Mammals	Triassic	Cenozoic Era (Pliocene Epoch)
Dinosaurs	Triassic (evolved), Jurassic (dominated), Cretaceous (Extinct)	-
Archaeopteryx	Jurassic	Cretaceous
Invertebrates	-	Ordovician
Protista	-	Archaeozoic era

· Dinosaurs became extinct probably due to the direct hitting of a comet or a meteorite having rich amounts of Iridium.

Outline evolution of the horse
· The evolution of the modern horse proves the process of evolution by revealing that the earlier simpler forms have themselves changed into better-organized complex forms.
· Horse originated in the Eocene epoch in North America.
· First horse-like animal from which the modern horse (Equus) evolved was Hyracotherium (old name Eohippus).

PATTERNS OF EVOLUTION

A. Divergent (Adaptive radiation) Evolution: Evolution of different groups of organisms from a common ancestor.

Examples:

· Australian marsupials adapted as herbivores, carnivores, burrowers and fliers,

· Darwin’s finches, varied limb structure of mammals etc.

B. Convergent Evolution: Organisms with different ancestors became more alike due to sharing of similar habitats.

Examples:

· Wings of birds & bats [KUMET 2000],

· Streamlined body of Fish & Whales,

· Wing of mosquito, sparrow & bat [MOE 2062],

· Fin of Fish & flipper of a Whale,

· Australian marsupials and placental mammals,

· Spiny and scaly anteaters etc.

C. Parallel Evolution: Two organisms acquire quite similar characteristics independently although they have stemmed from related ancestral lines.

Examples:

· New and old world monkeys,

· Asiatic and African elephant,

· Running in 2 toed Deer and 1 toed Horse. (Adaptive convergence of closely related species).

D. Progressive Evolution: Evolution of complex form of life from simple form, the evolution of multicellular organisms from unicellular ones. 

· Example: Fish → Amphibia → Reptilia → Birds and Mammalia.

E. Retrogressive Evolution: Evolution of simple form of life from complex form.

Example:

· Ascidian larva of Herdmania (retrogressive metamorphosis),

· Most monocotyledonous plants are herbaceous and simple in structure,

· Vestigial organs, 

· No digestive system in tapeworm & absorbs nutrients through the body surface.

F. Micro-Evolution: Evolution at the genetic level occurs below species level, i.e. number of series of invertebrate fossils.

G. Macro-Evolution: Evolution on the grand scale of geological time, above species level- adaptive radiation.

H. Mega-Evolution: New origin from predecessor, rare, origin of reptiles from amphibians.

I. Quantum Evolution: Rapid evolution of new organisms in a short time due to large scale environmental change.
Example:

· Development of land plants, wingless insects and scorpions.

J. Co-evolution: Evolution in 2 species of adaptations caused by selection pressure each imposes on the other. 

Examples: Most host/parasite, predator/ prey, cleaner /cleaned relationships are likely to involve co-evolution.

K. Allopatric Speciation: Speciation due to geographic isolation in a part of the population, i.e. Australian marsupials radiated to form new species, Darwin's finches of Galapagos Islands.

L. Sympatric speciation: Speciation due to reproductive isolation in a part of the population.

Polyploidy is the additional chromosome in animals that is lethal.
Tobacco is made by the Hybridization of 2 smaller species.
Wheat consists of 14, 28, 42 chromosomes.

Sources of Variation

· Variation is the differences between the closely related individuals of the same species and offspring of the same parents.

· It is caused either by the genetic differences or by the effects of the environmental factors.

· Variation may be shown in the physical appearance, metabolism, fertility, mode of reproduction, behaviour, learning and mental ability and other obvious or measurable characteristics.

· Variation enables a population to adapt to changing environmental conditions.

· The sources of variation are as follows.

1. Mutation
· It is the sudden heritable changes.

· Mutation rates are slow but are sufficient to create considerable genetic variations.

· Mutations are of two types: chromosomal mutations and gene mutations.

A. Chromosomal mutations: These appear due to changes in structure and number of chromosomes.
i. Changes in chromosome number: These mutations are caused by changes in the number of chromosomes.

· They are of two types:
a. Aneuploidy: It is a mutation in which a numerical change occurs in the chromosome number of the genome like monosomy (2n-1), nullisomy (2n-2), trisomy (2n+1), tetrasomy (2n+2) etc.
b. Polyploidy: It is the increase in the number of chromosome sets.

· Example: triploidy (3n), tetraploidy (4n), pentaploid (5n), hexaploidy (6n).

· Increase in the number of the same genomes is called autopolyploidy (e.g. AAAA).

· Increase in the number of chromosome sets is called allopolyploidy or interspecific polyploidy.

b. Structural changes in chromosomes or chromosomal aberrations: It is the change in the morphology of chromosomes.

· It consists of 4 types: duplication (doubling of a segment), deficiency (deletion of a segment), translocation (passage of segment of a chromosome to a non-homologous chromosome) and inversion (reversal in the order of genes).

B. Gene mutations: It is the change in the structure and expression of genes due to addition, deletion, substitution or inversion of nucleotides.

· The rate of gene mutation is increased by the presence of radiations and certain chemicals called mutagens.

· Mutated genes add new alleles to the gene pool.

· Accumulation of many mutations may add up to large-scale changes which finally leads to speciation.


2. Genetic recombination
· It occurs due to the following reasons:

a. dual parentage

b. independent assortment of chromosomes

c. crossing over

d. random fusion of gametes

e. formation of new alleles.

Genetic recombination adds new alleles and a combination of alleles to the gene pool, so it causes variations.

3. Genetic drift (Sewall Wright Effect)
· The random changes in the gene frequencies in a population by chance rather than by natural selection are called genetic drift.

· Although genetic drift occurs in all populations, its effects are most marked in very small isolated populations.


4. Natural selection (Selection)
· It is the most widely accepted theory concerning the principal causal mechanism of evolutionary change profounded by Charles Darwin and Alfred Russel Wallace.

· It results from differential reproduction (some members of a population produce abundant offspring, some only a few and still others none), one phenotype as compared with other phenotypes in the same population.

· According to Darwinism, survival and fertility mechanisms that affect the reproductive success or promote differential reproduction are called selection.

· But according to modern view, the selection is the consistent differences in the contribution of various genotypes to the next generation.

Concept of Species

· Evolutionary taxonomists have defined a species as the basic unit of the classification.
1. Morphological species concept: Taxonomists define species as an assemblage of individuals with common morphological features and separable from each other such assemblage by correlated morphological discontinuities in the number of features.
2. Biological species concept: According to this, species is a sexually interbreeding or potentially interbreeding group of individuals separated from other species by reproductive isolation that is by the absence of genetic exchange.
· When the two species are morphologically almost identical but do not normally interbreed, they are called sibling species.

· For example, Drosophila pseudoobscura and Drosophila persimilis are the two species of the fruit fly which do not cross-fertilize.
3. Evolutionary species concept: It includes morphological, genetic, behavioural and ecological variations.


Important Characteristics of a Species
i. All the members found in different areas of the globe are derived from a common ancestor.
ii. The members resemble more closely than they resemble individuals of the other species.
iii. There is complete anatomical similarity.
iv. All the members of a species have similar karyotypes.
v. There is a broad similarity in morphological character.
vi. There is a similarity in proteins, enzymes, hormones and other biochemicals.
vii. The members can interbreed freely and produce fertile offspring.

· They are reproductively isolated i.e. they do not interbreed with the members of the other species.
· Mayr (1987AD) prefers to use the term para species while Ghiselin (1987AD) uses the term pseudo species for asexual groups.

Speciation

· Evolution depends upon the origin of new species.

· Speciation is the phenomenon of the formation of one or more new species from the existing one by evolutionary means.

· Species is defined as an interbreeding population, which is reproductively isolated from other similar populations.

· Each species is an isolated pool of genes and each species fills an ecological niche not utilized by another species.

· Each species remains in the process of continually adjusting to its environment.

· Species is a collection of deme (sub-species) and deme is the smallest population unit and is the group of genetically similar individuals


Types of Speciation
1. Allopatric speciation:

· In this speciation, a part of the population becomes geographically isolated from the main population with the formation of a geographical barrier, such as a mountain, sea, river or land bridge (Isthmus of Panama) or ocean, across the species range.

· It prevents gene flow.

· Adaptations to new conditions or random genetic drift in small populations lead to changes in allele or genotype frequencies.

· Prolonged separation of populations may result in them becoming genetically isolated even if brought together. Hence new species arise.

· The geographical speciation is the most exclusive mode of speciation among animals.

Examples
a. Darwin’s finches that formed separate species in Galapagos Islands
b. Australian marsupials radiated to form new species
c. One-horned and two-horned rhino
d. Asiatic and African elephants

2. Sympatric speciation:

· It is the formation of new species within a single population without geographical isolation.

· It may result from polyploidy.

· The additional chromosomes in animals are mostly lethal.

· However, polyploidy occurs commonly in plants.

· Tobacco is a polyploid that evolved by the hybridization of two smaller species. 

· In wheat, there are species with 14, 28 and 42 chromosomes.

· Different varieties of wheat have been obtained by polyploidy.

· Sympatric speciation requires the development of some form of reproductive isolating mechanisms which has arisen by natural selection.

· It occurs within a geographically confined area.

· It may be structural, physiological, behavioural or genetic.

Isolation

· It is the segregation (separation) of the population of particular species into smaller units that prevents interbreeding between them.

· The separation may be due to physical (e.g. geographical, ecological) and biotic (e.g. physiological, behavioural, mechanical, genetic) barriers.

· These separated organisms develop different characteristics, so they become separate species.

· Hence, isolation is essential for speciation.

Types of Isolation
· Evolutionary biologists have recognized many types of isolations that cause speciation.

· Some of them are as follows.
1. Geographical isolation:

· It occurs when an original population is divided into two or more groups by geographical barriers such as rivers, deserts, mountains, glaciers, forests, bridges, gorges, snow glaciers, lakes or oceans; it prevents interbreeding between them, then in the course of time, different mutations may become incorporated in the gene pools of the different groups.

· Often these differences are of such a nature that the separated groups, when they come in contact again, do not interbreed.

· Due to this, species have been formed.

· One-horned and two-horned rhinos are geographically isolated living in Asia and Africa.

· Another example is Darwin finches. Out of 26 groups of finches found among the Galapagos Islands, only five of these groups resemble the mainland finches. Other groups (21 groups) were peculiar to the groups of the islands. The southern and northern elephant seals are even so similar to each other but their breeding populations are separated about 3000 miles of warm tropical seas, and, hence, are not capable of genetic exchange. The distribution of land snails in Hawaiian Island (Island of Oahu) is another example.

2. Reproductive isolation:
· It is the prevention of interbreeding between the populations of two different species; whether they are geographically isolated or living close together.

· Some genetic mechanisms prevent them from interbreeding and producing fertile offspring.

· Such mechanisms of reproductive isolation preserve the integrity of species by preventing hybridization.

Origin of species (Speciation)

· Speciation is the phenomenon of the formation of one or more species from the existing one by evolutionary means.
· The populations of a species are present in different environments and are separated by geographical and physiological barriers, accumulate variations (genetic differences) due to mutations (both point and chromosomal), recombination, hybridization, genetic drifts and natural selection. These populations, so, become different from each other morphologically and genetically and are reproductively isolated, forming new species.
· Besides this, those individuals with new traits which can adjust to the changing environment are selected by nature and new traits spread by differential reproduction.

· Accumulation of such useful traits in the population, alter the individuals in structure and function and develop into a new species.

Also, Read our other Notes Important for Entrance Exams:

• Life and Its Origin
• Evolution, Its Evidence & Patterns
• Theories of Evolutions
• Human Evolution