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  • 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.
  1. PRIMARY FORCES: 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:
  • Mutation
  • Gene recombination
  • Natural section
NB: Natural selection must be present with either one of the two forces (mutation and or gene recombination)

2. SECONDARY FORCES: 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
  • Breeding
  • 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:
  1. Change in the environment creates new needs
  2. Use and disuse of organs (organs are elastic)
  3. 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
  1. Perpetuating those organisms’ with better traits and thus ensure survival of these organisms.
  2. 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.
  1. Natural selection
  2. 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
  • Inbreeding
  • 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).
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.



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