This unit dwells on the process of evolution


Evolution is the gradual process by which the present diversity of plants and animal life arose from the earliest and most primitive organism over avery   long  period of time due to change in the  environment.

Theories of the origin of life.

  1. Special creation

It suggests that all living things were created separately and simultaneously in 6 days by a super natural power and the organism have never been changed. It argues that the similarities that occur between organisms are merely circumstantial and net a case of evolution.


  1. This theory has no scientific proof; can’t be investigated using observable scientific phenomena hence it is difficult to disprove or prove it using scientific cateria. It is prominent among most religions in the world  and it is based on faith.
  2. Spontaneous generation.

It was a belief that life  began  by repeated appearance of living organisms  from  non living matter it followed observation that maggots came from rotting matter ,snakes from the bush ,earthworms  from mud , rats and flies from garbage. A lab experiment by a scientist ‘van  helmont’ suggested that  mice would be formed if  a few  grains of wheat and a dirty shirt were left in a  dark corner.

  1. Steady state

Suggest that the earth had no origin. It has always been able to support life. It has changed remarkably very little if at all but species have no origin. It challenges in the history of the species that the only alteration are for its members to vary or for it to become extinct.

Colonization of dry land.

Changes necessary fro an animal moving from water to dry land.

(i) Support

Since H20 is more dense that air, it allows greater support therefore a need to develop a stronger bony framework on land. A strong vertebral column is an additional means of support developed in reptiles and higher animals. Insects are adapted by developing a strong and rigid exoskeleton.

(ii) Locomotion

H20 is more viscous than air there provides a strong hold 4 locomotory organs. A move to land necessitated stronger locomotory organs hence development of limbs in amphibian, reptilian, mammals and insects. Wings in insects, birds and bats.

(iii) Gaseous exchange

They had to develop air breathing by development of lungs in vertebrates and tracheal systems in insects.

(iv) Instability of air in terms of temperature.

A mechanism of temperature regulation in both ectothermic and endothermic organisms had to be developed.

(v) Reproduction

There being no medium for the mobility of gametes, fertilization had to change from external to internal and eggs were laid externally and protected from desiccation hence the evolution of the amniotic egg. A few life amphibians still have to return to H20 for fertilization for they aren’t fully evolved for terrestrial conditions.

(vi) Excretion

Less toxic nitrogenous wastes like ured in amphibians and mammals, uric acid in insects, reptiles and birds. These require less water to remove than ammonia hence allows water conservation.

(vii) Sense organs

The new medium required the development of special senses e.g. detecting sound vibrations in air during heating and the many new responses required a larger brain.

Account of evidence to support evolution

(i) Cell biology

This is provided by comparative bio-chemistry or study of the organelles. Cells have structures like mitochondrian, ribosomes and chemicals like ATP, DNA, RNA whose structural and organization are basically the same. These findings point to phonogenetic links in plants and animals due to biochemical homology.

Here, we assume that the fewer the differences in the molecular or cellular structure, the fewer the mutations that have occurred thus the closer the links between the two organisms.

Among many plants, chlorophyll cellulose and starch are present but absent in animal cells. Vertebrates posses adrenaline and thyroxine which are absent in all other organisms.

Distribution of blood pigments is another case of biochemical homology. Hemoglobin, heomoergythin, hamocyanin, all contain iron in most vertebrates and in all invertebrates and in all invertebrates. These pigments occur in phonogenetically related gps.

The distribution of phosphagens is also evidence for biochemical homology. Phosphagens are molecules that provide phosphorus gps for synthesis of ATP. Chordates and Echnoderms posses phosphocreatine as the phosphagen in their muscles while phosphoarginine is the phospagen in the other phylum.

(i) Embryology



This is the study of development stages of the embryos of organisms. All vertebrates start their life histories from a single cell, the fertilized ovum and go through the same stages of development showing membrane in embryos at certain stages hence evidence of relationship among them in addition to similarities in their adult structures. Some embryos of all vertebrates have a notochord embryo of higher animals repeat structures of lower animals some of which are later lost during development. I.e. Karl Ernost’s recapituloaiton theory which states that ‘ontogeny recapitulates phylogeny’ it the development stages from zygote to adult repeats the history of the whole race in its evolution through the ages.

Ernast haeckel’s biogenetic law states that each organism tends in its ontogeny to recapitulate or repeat the stages which its assertors passed through during the phylogeny of the whole raw.

This shows that we and other vertebrates continue to pass through many embryonic stages that our ancestors passed through because we have all inherited developmental stages from a common ancestor therefore the more closely related 2 vertebrates are the longer their embryonic development processes share common stages and vice versa.

(ii) Geographical (distribution) studies.

Areas of almost identical climate have different fauna and flora e.g. Africa had lions elements, hippos, rhinos, short tailed monkeys, South America has taps, panthers, long tailed monkeys, Australia has a few mice species, egg laying menotrop.

It is argued that these animals had common ancestry but because of migration this continous geographical distribution, they settled where they are now and they became extinct intervening regions. They hence developed into a variety of species. This explanation is developed from the theory of continental drift. E.g. it is believed that mammals first developed in Asia and the spread into Europe, Austria, Africa, and America from where they developed into different species by adaptive radiation.

(iv) Comparative anatomy

The anatomy of all vertebrates shows the following similarities;

  • Development of respiratory system from anterior part of the alimentary canal.
  • Development of two part and limbs which bear similar relationship to the main body axis except in degenerate types.
  • Possession of red bloods cells.
  • Metameric segmentation which paired segmented nerves and blood vessels which are more evident in embryos.
  • Distribution and position their internal organs e.g. brain, heart, liver, pancreas, spleen, kidney and the distribution of muscles, blood vessels and nerves.
  • Homologous structures among different vertebrates like the pentadactyl limb found in all terrestrial vertebrates.

Homologous structures are structures with the same basic structure but perform different functions e.g. leg of bird and leg of man. This general anatomy of vertebrates confirms evolution of many organisms from similar ancestors.

Analogous organisms are those with different basic structures but performs the same functions in the adult e.g. wings of birds and insects. These result from different ancestry but show that evaluation along different lines can serve same environment. This is convergent evaluation the results due to similarities in way of living sometimes due to adaptation to similar living conditions. Whales and dolphins are gill less, warm blooded mammals but similar to fishes in external morphology.

Vestigial organs are degenerate with no apparent function in the organism. They show that due to evolution, organs that were formerly of use in the old environment but useless in new environment degenerate due to loss of function e.g. the appendix and coccyx in man.

Therefore homologous structures support evolution because they show the organisms which posses them are related therefore have a common ancestor. Different in them show divergent evolution security them to different functions thus different habits but the similarities are evidence the evolution hasn’t occurred by sudden appearance of new forms but they slight modification of existing a new e.g. the pentadactyl limb has been modified for grasping in monkeys, running in houses, swimming in whales and flying in bats. This is an example of adaptive radiation i.e. when a gp differentiates into many other types to adapt 2 different modes of life while the internal organization has remained the same.

(v) Paleontology

This is the study of fossilized remains and animals found in the strate of the crust of the earth.

Fossils are the remains of organisms that inhabited the earth in old geological ages.

These provide the following evidence for evolution.

  • Fact of persistent change i.e. the floras and faunas which have covered the earth in the past have constantly altered. New types of plants and animals appeared, flourished became extinct and were succeeded by other types which had the same cycles. This succession is evidence of change by the former existing plants and animals, that species have come into existence and have gone out of existence.
  • Existence of precursors and transitional forms between types. In vertebrates, there are 5 classes i.e. Pisces, amphibian, reptilian, aves and mammalian but fossils found are in each care roughly intermediate between the classes thus show how evolutionary transition was affected.
  • The existence of complete fossil records of some organisms eg the hone from ancestor to the present day descendants. This reveals the actual course of evolution.

(vi) Taxonomy



The members of each gp of plants and animals during classification are classified in gps that are contained in larger gps i.e. species

Orders       families      phyla.

This is a natural classification reflecting relationships and reflects the course of evolution. Members of each gp of plants and animals are placed together because of common characters and the gps in turn are characterized by other resemblances. This is due to relationship of genetic affinity. The different between them is due to descent with divergence and modification from ancestors shared in common. All this is evidence for evolution.


Lamarck’s theory i.e. The inheritance of acquired characters.

Lanarck’s idea was that evolution was brought about the handing on of characters acquired by one generation to the next. An acquired character is one which was developed in the course of the life of an individual in the somatic cell, through some external change in the environment or through excess we or complete disuse of a part of the body.

He proposed that changes in conditions create needs which lead to new methods of behaviour involving fresh uses or disuses of existing organs of the body which leads to changes in their structures or method of functioning and that the resulting body changes are inherited and reappear in some of the individuals in succeeding generations.

It is based on the propositions ie.

  • Under the influence of the environment, changes can be included in the body of an animal and in many cases, they have adaptive significance eg when the muscles of a blacksmith become enlarged as a result of wielding the hammer therefore becomes better adapted to wield it.

The fact that animals produce offsrpoings like themselves not only in physical features but also in functional characteristics such as method of speaking. He concluded that parents who have been modified by the environment can produce offsprings showing the same modifications but this is genetically incorrect. He illustrated his theory with the evolution of the longneck of the giraffe suggesting that the ancestor had a short neck

  • but due to feeding on leaves of trees, it stretched its neck to reach higher leaves after finishing the lower ones. A resulted into longer necks which were inherited by offsprings.

Natural selection



Refers to the differential contribution of offsprings by the different members of a population to the succeeding generations due to their possession of such characteristics. Sites as may suit them better the environment condition.

Such individual are variants and their variations are genetical thus inherited. The contribution of the offsprings of one generation to the next is not entirely random but depends on this variability hence some variant individuals are consistently more successful in leaving offsprings than others. The successful variants are therefore selected by natural selection since they have more survival value in the face of hostile environment where all organisms live.

Importance of natural selection

  • Brigs about new species because it is organisms with the best characteristics which survive.
  • Leads to extinction of some species reducing competition.
  • Increase hybrid vigour.

Examples today.

  • Extinction of while rhinos
  • Resistance of pests to pesticides
  • Polymorphism

Observation and conclusion on which natural selection is based


The potential offsprings from individuals are numerous because organisms tend to have high reproductive potential. If their offspring

  • reproduced by the same rate, then in a few generations, they would be very numerous.
  • Despite the large potential, under natural conditions the numbers of a species or population tend to remain constant from year and the majority of the offsprings die before reproduce.


  • There should be stiff competition between individual for space, food etc such that a struggle for existence exists. There is death of many individuals due to loss of the struggle.


(iii)       An examination of a large number of individuals of a species

shows much variation.


  • In a struggle for existence, only the fittest survive i.e. those whose variations are most advantageous (favourable), in their suitability to the environment.
  • Thus all those qualities which enable the survivors to survive would tend to be passed on to their offspring and so in generation after generation would appear in larger proportions in the offsprings while those unfavourable would tend to disappear i.e. natural selection hence through it, a species would undergo evolution and would change by becoming adapted to its surroundings and finally a new species.


Hence, natural selection in conclusion depended on 2 factors i.e.

  • Occurrence of variations some of which increase the likelihood of survival more than others do and which tend to be inherited.
  • The selection of favourable variations by the survival of the fittest in the struggle for existence.


(i) Stabilizing Selection

Natural selection favours individuals near the mean in the distribution and selects against individuals at the extremes. This brings about constancy in the population and does not fevour evolution. It ensures that most individuals surviving are those adapted to the environment conditions.

Example is inheritance of sickle cells anaemia. Individuals at the extremes die of either sickle cells diseases or malaria while the majority heterozygote survive.

Birth mass in humans

1st generation                2nd generation           3rd generation  

(ii) Directional selection


Environmental change favours a new phenotype then the individuals of this phenotype are likely to become numerous in the population at the expense of those not favoured. Hence the man shifts to a new one (extreme) and the composition of the population changes accordingly. It leads to evolution of new forms by producing a selection pressure which favours the increase in frequency of new alleles within a population. Examples include industrial melanism, resistance to antibiotics and selective breeding. Hence the effect of directional selection is to move the distribution of the phenotype so that the made coincides with a new environment optimum.  

Disruptive selection



Natural selection favours those phenotypes towards the extremes of the range of phenotypic variation and selects against those near the mean. This is clearly the reverse of stability selection and may lead to the splitting of a single gene pool into two in which case to different species result.

This is not very common. E.g. grass hopper i.e. brown and green wings no intermediately leads to bimodal distribution.

Phenotypic characteristics


Darwin’s finches

Those with short strong books were able to feed on the nuts exclusively while those with the long standers beaks were able to feed on the insects exclusively. Those with an average unspecialized beak had competition with the other birds’ species and reproduced less successively. Can develop into a dence genetically isolated unit of population.



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