Organic evolution is a gradual change in the genetic composition of organisms in a population during successive generations leading to formation of new species from pre-existing species.



Are the ones which must be there for new species to be formed from pre-existing species. Thus, if absent no organic evolution takes place.

These forces include:


Gene recombination

Natural section


Natural selection must be present with either one of the two forces (mutation and or gene recombination)


These are forces that modify rate of organic evolution i.e. if present, rate of evolution of the species are faster, but it absent the rate is slower.

These include:

Gene flow

Genetic drift


Adaptive radiation/geographic isolation


1. Special creation

Species are immutable (not changing)
Life was created by a supernatural power at a particular time i.e. God made living things once upon a time and whatever is existing today is a result of reproduction. If God wants a new species he just

says BE and there it is.

Strength of the theory:

Faith i.e. believing and accepting this with no evidence.

Weakness of the theory:

Cannot be proved or disapproved by science as science goes with experiments and observation.

2. Spontaneous Generation

This theory was common in ancient Chinese, Babylonian and Egyptians
Nature passes from lifeless to animals in an unbroken sequence.
Particles of matter contained an active principle which could produce a living organism under favourable conditions. This active principle was present in fertilized eggs, seed, sunlight, wheat, decaying meat, natural forces and decaying earth, mud.
Van Helmont (15th century) did an experiment which gave rise to mice in 3 weeks; the raw materials were a dirty shirt, wheat grains and a dark cupboard. The active principle was thought to be human sweat.

Strength of the Theory

Life arises from non-living matter on a number of separate occasions.

Weakness of the theory

In the experiments, there were no controls, in which each variable could systematically be eliminated.

3. Cosmozoan theory (Panspermia theory)

There has been an extra-terrestrial organ i.e. life arose on this planet from somewhere else. life could have arisen once or several times at different times and on several parts of the universe.

Strength of the theory

The sightings of UFO’s, care drawings of rocket like objects and ‘spacemen’ and reports of encounters with aliens provide the background for this.

Weakness of the theory

The theory does not talk about the origin of life but perpetuation of it.



His account for mechanism of evolution is based on the following:

Change in the environment creates new needs
Use and disuse of organs (organs are elastic)
Inheritance of acquired characteristics.


Selection is a process by which organisms, which are better adapted to surrounding, survive and breeds, while those less adapted fail to do so.

Importance of selection:

It has adaptive significance in

Perpetuating those organisms’ with better traits and thus ensure survival of these organisms.
Selection determines which genes (alleles) pass to the next generation by virtue of the differential advantage; they exhibit as expressed as phenotypes therefore Selection pressure increases or decreases the spread of alleles, within the gene pool and the changes can be evolutionary.


Natural selection
Artificial selection (selective breeding)


Is a selective force such as natural disasters (drought, floods, earthquakes, diseases, fire), competition for food, water, mates, breeding areas operate on the variations are selected out by natural selection.
A greater proportion of the offspring in the next generation will be products of the suitable variants. This leads to changes in the gene frequency of the population, which can lead to evolutionary change.


Man selects animals and plants with traits he wants for mating, propagation, and pollination.
The individuals lacking desired qualities are prevented from mating by sterilization, segregation.
Members of the species selected are those that show favourable variations such as increased yield of eggs, milk, fruits, wool, early maturity such as cereal crops, resistance to diseases and hardness.
Man exerts a directional selection pressure which leads to changes in alleles and genotype frequency within the population.

Importance: Continued selective breeding by human has produced varieties, new breeds, strains, races, sub-species of animals and plants of agricultural importance.

Types of Artificial Selection

Out breeding


Inbreeding is a process involving selective reproduction between closely related organisms in order to retain and propagate the desired traits e.g. between offsprings produced by the same parent.
It is practiced in breeding show animals such as cats and dogs. It is used by livestock breeders to produce cattle, pigs, poultry and sheep with high yield of milk, meat, eggs, wool resp.
Prolonged inbreeding can lead to reduced fertility especially when breeding livestock.
It can lead to reduced variability of the genome (sum of alleles of an individual).


Resort to out breeding after several generations of inbreeding.

Forms/mechanisms of inbreeding:

Self fertilization


Involves crossing individuals from genetically distinct organisms i.e. between different varieties or strains and sometimes closely related species.
The offspring’s of such a cross are hybrids. It is applied mostly in plant breeding and commercial production of meat, wool and eggs.
The hybrids show hybrid vigour (phenotypes show traits which are superior to either of the parental stocks. Increased vigour results from increased heterozygosity in gene mixing.

Limitations of out breeding:

Interbreeding F1 for a long time decreases hybrid vigour (heterozygosity) due to increase in homozygosity.


Speciation is a process by which new species are formed from pre-existing species.

Alternative ways of defining a species:

Biological aspect
A group of organisms capable of inter breeding and producing fertile offspring
A group of organism sharing the same ecological niche; no two species can share the same ecological niche
A group of organic showing close similarity in genetic karyotype.
A group of organic sharing a unique collection of structural and functional characteristics from certain ancestors.


Geographical isolation

A population which normally interbreeds freely may be separated by physical barriers/geographical barriers such as mountain ranges, deserts, oceans, rivers and canals etc. Under such circumstances,

the separate groups (demes) of the same species can no longer interbreed i.e. prevented from meeting hence barrier to gene flow.

Each side of a barrier has different environment conditions and thus new selective forces begin to operate.
The population will eventually be so different that they can no longer interbreed even if the physical barrier were to be removed therefore new species have been formed hence evolution.
This type of isolation may also be referred to as allopatric speciation.

Reproductive isolation/Physiological isolation

Groups live side by side but fail to produce fertile offsprings and to interbreed. This is due to the fact that the groups have accumulated sufficient structural, functional and behavioural characteristics difference that when mixed inter breeding does not occur. This is realised through

(a) Mechanical isolation

In case of animals, genitalia of 2 groups are incompatible i.e. failure of male penis to enter the female vagina and no successful copulation.
In case of plants related species of flower are pollinated by different animals.
Gametes are prevented from meeting

(b) Seasonal isolation

(c) Behavioural isolation

Occurs where animals exhibit courtship patters.
Mating only results if the courtship display by one sex results in acceptance by the other sex e.g. in some fish and birds.

NB: In some cases, fusion of the gametes may not occur sperms reach the ovum, pollen grains reach the embryo sac but the gametes may be incompatible and might not fuse.

Post zygotic mechanisms (Barriers affecting hybrids)

Hybrid inviability: Hybrids are produced but fail to develop to maturity; for example hybrids formed between northern and southern races of the leopard frog (rana pipiens) in North America.

Hybrid sterility: Hybrids fail to produce functional gametes, for e.g. the mule (2n=63) results from the cross between the horse (equals equus, 2n = 60) and the ass (Equals hemionus, 2n = 66).

Hybrid breakdown: F1 hybrids are fertile, but the F2 generation and backcrosses between F1 hybrids and parental stocks fail to develop or are infertile for e.g. hybrids formed between species of cotton (genus Gossypium).

Types of speciation

(a) Allopatric Speciation:

Characterized by the occurrence at some stage of spatial separation. Geographical barriers may produce a barrier to gene flow because of spatial separation. This inability of organisms or their gametes to meet leads to reproductive isolation drift in small populations lead to changes in allele and genotype frequencies. Prolonged separation of populations may result in them becoming genetically isolated even if brought together and thus arising of new species.

Example, the variety and distribution of the finch species belonging to the family Geospizidae on the Galapagos Islands. It is suggested that an original stock of finches reach the Galapagos Islands from the mainland of S. America and, in the absence of competition from endemic species. (Representing relaxed selection pressure), adaptive radiation occurred to produce a variety of species adapted to the particular ecological niche. The various species are believed to have evolved in geographical isolation to the point that when dispersal brought them together on certain islands they were able to co-exist as separate species.

Involves evolution/formation of species as a result of population being separated by the geographical barrier.

(b) Sympatric speciation

Genetic differences may accumulate allopathically in populations which have been geographically isolated for a much shorter period of time. If these populations are brought together, hybrids may form where these overlap. E.g. both the carrion crow and the hooded crow are found in the British Isles.
The carrion crow is common in England and Southern Scotland. The hooked crow is found in the north of Scotland. Hybrids formed from the mating of the two occupy a narrow region extending across central flow between the populations of the 2 crows.
In time selection against cross-breeding may occur leading to speciation in the same geographically area, hence sympatric speciation.
This speciation does not involve geographical separation of populations at the time at which genetic isolation occurs. It requires the development of some form of reproductive isolating mechanism which has arisen by selection within a geographically confined area. This may be structural, physiological, behavioural or genetic.
Involves formation of new species not involving any geographical isolation, but formed by other isolations e.g. physical.

Genetic recombination brings about variation to crossing over between non sister chromatids prophase I & meiosis, random assortment of the homologous x – somes and their movement to different cds. Also random fertilization.

These variations are inherited, some being useful to the organisms gives the organism a slightly better chance of survival and some are less useful. Incase of environmental change over a long time
In a course of time the surviving forms, may be sufficiently different from the original species that they can be called new species.

Mutation is sudden/spontaneous/abrupt change in the amount or chemical structure of DNA molecule. The changes create genetic variation among members of the same species. If the mutation confers/gives an advantage to the organism that under selective environmental selective pressure, it will be selected for against the less fitted mutants.

The selected ones will have a reproductive advantage and will reproduce more offspring. This process may result into formation of a new species different from its predecessor e.g. Biston, introduction of antibiotics in 1940 a strong selective pressure for strains of bacteria that have genetic capability of resistant to antibiotics as a result of mutation.

Natural Selection

The environmental changes are the main mechanism for effecting natural selection.

Mechanisms for natural selection

Individuals in a given population which are less fit are gradually eliminated while those with adaptive features are being favoured.

Organisms with variations (variation could have arisen from genetic recombination and or mutation) best adapted to the environment have a reproductive advantage thus producing more offsprings than the one of the favoured individuals increase while that of the unfavoured individuals decreases.

If the trend is maintained over a long time, a new species may arise e.g. evolution of Biston Bitullaria Carbonica is due to industrial revolution in UK which produced a lot of SO2. The SO2 made the leechens black and so camouflaging the black moth while the white one Biston Bitullaria Typical was preyed upon the birds. The black gene arose by mutation.

Geographic isolation

Two or more populations of the same species, occupying the same habitat become separated by a physical barrier like mountains, rivers, valleys. In such a situation, the gene pools of each population do not mix with one another i.e. no gene flow. Each population try to adapt itself to the changing environment.

The less fit individuals are eliminated and the more fit keep on increasing in number. As the process of adaptation continues for a long time in the different geographical areas, the population becomes distinctly different from each other. Hence new species have arisen e.g. Galapology, finches, Ilamas and camels.

How changes in the environment create needs?

Animals and plants respond to the environment by becoming better adapted to them.
As environmental conditions change it may lead to changes in behavioural pattern which can require increased use/disuse of certain organs.
Those organs which are frequently used will become stronger and develop and vice versa.
Developed structures are inheritable thus acquired characteristics e.g. webbed feet in ducks, long necks in giraffes, limbless, snakes.


The long neck and legs of modern giraffes were the results of generations of short necked and short legged ancestors feeding on leaves at progressively higher levels of trees.
The slightly long necks and legs produced in each generation were passed onto the next generation.

In a course of time, the whole population contained long necked and legged giraffes which we see today.

Hence they are evolved.

The case of webbed feet ducks also applies.

Strength of theory

He was correct by saying that change in environment creates new needs e.g. webbed toes.
The theory was useful in bringing up the idea of evolution i.e. created the foundation for Darwin and Wallace.
This theory applied explained the existence of vestigial organs i.e. when not in use; the organs will disappear or become non-functional.
He emphasized on the rate of the environment in producing the change in the individual.

Weakness of the theory

Inheritance of acquired characteristics was not true e.g. the son of a boxer would not necessarily be a boxer, because the change in the environment led to the change in behavioural pattern which did not

affect the gametes which were means of passing the straits to the next generation.


Was based on natural selection and survival of the fittest

Essential features:

Over production of offsprings
Constancy in number of population
Struggle for existence due to competition
There is variation among organisms
Survival of the fittest
Formation of new species


Individuals within a population have a great reproductive rate.
The number of individuals in a population remains approximately constant due to environmental factors such as food, space, light, water, breeding sites, etc.
Variations exist within individuals in a population.
Those surviving individuals have a reproductive advantage and will give rise to next generation which are better adapted to the environment.


From 1 and 2, many individuals fail to survive or reproduce therefore there is “struggle for existence” due to competition.
From 3 and 4, in the struggle for existence, individual showing variation has adapted to the environment and will be favoured than others, who are not adapted.

Strength of the theory

In absence of environmental factor every species tends to over produce remains fairly constant
High mortality rate especially in young occur and so only a few reach the reproductive age.
Competition occurs between organisms for the available resources and therefore there is struggle for existence.
There are variations among individuals.
In struggle for survival individuals with variation will reach reproductive maturity.
Favorable traits of these individuals will be passed on to the next generation and thus the survival of fittest.

Weakness of the theory:

The theory does not explain the (origin of the specie)
The theory does not explain the (existence of vestigial) structure.
According to the theory, the cause of imp (variations) complex structures among individuals which result information of new species are not explained e.g. imitation, polyploidy and meiosis)
The theory is silent on how (variations are passed) to the offsprings from one generation to the next.
The theory does not explain on the (survival of the unfit individuals) in nature that reaches reproductive age.
The theory does not explain (how natural selection occurs)
Does not explain how variations arise.
Does not explain the difference between environmental and genetic variation.

According to Darwin:

There are many populations of giraffe which could progressively reproduce.
However, their number remains constant
The giraffes were of many variations; some short, long and middle necked so in the struggle for existence, the short and middle necked would not survive and die out therefore the survival of fittest.
Long necks would reach for higher trees and therefore reproductive age and the next generation were of many giraffes with long necks, where as the short necked giraffes perished and in course of

time, due to natural selection, we have the long necked giraffes at present.

Similar explanation for development of webbed feet in duck and development of limbless snakes.


There is several evidence


Palaeontology is the study of fossils

Definition: Fossils are any form of preserved remains thought to be derived from living organism over millions of years.

They are formed from preserved hard parts of an organism e.g. bones, teeth and shells. These are found in sedimentary rocks formed from deposition of mud, silt, or sand one thousands to millions of


Entire organism
Encased in tar
Frozen into ice during glaciations
“Mummies” found in asphalt and lakes of California.
Woolly mammoths in Siberian
Hard skeletal materials
Trapped by sedimentary sand and clay which form sedimentary rocks e.g. limestone, sand store
Bones, shells and teeth
Impressions of remains of organic in fine grained sediments on which they died.
Feathers of Archaeopteryx in upper Jurassic. Jelly fish in Cambrian in British carboniferous leaf impressions.
Footprints, traits, tracks and tunnels of various organisms made in mud are rapidly baked and filled with sand and covered by further sediments.
Dinosour foot prints and tail scrapings indicate size and posture of organism
Faecal pellets prevented from decomposing, later compressed in sedimentary rocks often contain evidence of food eaten e.g. teeth and scales.
Cenozoic mammalian remains.


Deeper layers usually contain simpler forms of organisms much older compared to upper layers which are complex but young i.e. most fossils occur in sedimentary simple structure fossils. Younger rocks

contain more varieties of fossils with increasing complex structure.

Fossils show gradual change from one form to another as we move from the deeper layer cupboards.
Therefore changing environment may have favored a mechanism for evolutionary change that account for a progressive change in the structure of the organic e.g. fossils we have today.

Archaeopteryx – reptilia and aves

Symoria – amphibia and reptilian

Synognathics – reptilia and mammals

Fossil record is not continuous i.e. missing links due to unfossilization of some soft bodied organisms on, dead organisms of decay rapidly or they may have been eaten by scavengers or they have not

been discovered yet.


Likenesses in anatomy among different kinds of organisms produce evidence that evolution has taken place as revealed by homologous analogous organs.

Homologous structures – Structures performing different functions but showing a common ancestral origin some position in the embryonic state and have the same microscopic origin of tissue.

Organs with different functions with similar basic form, microscopic structure, embryonic development but different functions due to changing environment. E.g. ear bones – jaw of fish, haters (hind pair of

wings in most insects have been modified in dipteral to form natters) to maintain balance during flight, pentadactyl limb, fertilized ovary wall, mouth parts of insects.

Analogous Structures:

Different ancestral origin and development but with same function e.g. eyes of vertebrates and cephalaploid molluscs, winds of birds and insects.

Vestigial Structures;

Structures which have ceased in function due to course of time and are passed non-function due to course of time and are passed non-functional from generation to generation. These are homologous to

structures that are functional in other organisms e.g. appendix in humans is vestigial but in herbivores, it is for digestion, the coccyx in humans and tails in crocodiles.

Snakes have no limbs but some function less limb bones can be found in the skeleton of some insects; other reptiles like lizards have limbs suggesting that ancestors of snakes had limbs.


Basic cellular structure – known to occur in large number of groups of organisms e.g. nucleic acid in all organisms e.g. Genetic code/DNA is the same in many organisms and there is very basic similarities in

chemical processes taking place in cells; chlorophyll in all photosynthetic plants, chtochronein all aerobic respiring organism and energy stored in ATP within cells. In vertebrate Hb is for transport of O2 and

haemocyanin in insects (common pigment) genetic code is the same, therefore protein synthesis is similar (but diff sequence of amino acid) ATP molecules in all Eukaryotic plants and animals.

Basic physiological processes: a great similarity in the physiological processes among different groups.

The above suggest a common ancestral origin from which these features are inherited.

There is no even distribution of species on the earth due to continental drift which separated the land mass organisms were isolated.
Different zones have their characteristic flora and fauna though they may have same climatic conditions e.g. elephants are found in Africa and India but not in South America which has similar habitat Britain (England Ireland) with similar climatic conditions have different flora/plants (veg) and fauna/animals (climate).
The above suggest that the species have originated from a particular area.
To avoid overcrowding the intra and inter specific competition, the organisms disperse/spread.
In the new area, the organisms undergo adaptive radiation to cope up with the new condition.
The climatic/topographical and other.

Steady State

Life has no origin
This theory assets that the Earth had no origin, has always been able to support life, has changed remarkably little, if at all and that species had no origin
This theory proposes that species never originated, they have always existed and that in the history of species the only alternatives are for its nos. To vary, or for it to become extinct.




Please enter your comment!
Please enter your name here