Evolution—History   of   life.

Evolution is the process by which new species are formed from pre-existing one over a long period of time. It is believed that life exists only on earth of all planets.

Specific objectives

The learner should be able to

• Explain the theories related to the origin of life

Theories for origin of the earth

1. Stead – state. Cosmologist maintain that the earth and universe never had origin, has always been able to support life, has changed remarkably little if at all, and that the has 5,000 x10⁶ years based on radioactive decay rates.

2. Other hypotheses suggest that the universe may have begun as a ball of neutrons exploded in a ‘big bang’ emerged from one of several black holes

3. It was the design of a creator.

Theories for origin of life on earth.

1. Special creation; life was created by a supernatural being at a particular time. Genesis 1; 1- 26

2. Spontaneous generation; life arose from non- living matter on numerous occasions.

3.Steady-state. Life has no origin

4. Cosmozoan; life arrived on this planet from elsewhere.

5Biochemical evolution; life arose according to chemical and physical laws.

Modern view.

Modern geologists believe the earth is over a billion years old. They speculate that the mountains deserts and oceans of today formed from slow, gradual but continuous process of erosion and uplifting. The about 3.5 billion years ago, life began From simple unicellular organism, new life forms arose and changed in response to environmental pressures producing the past and present biodiversity [evolution]

Theories of evolution.

Evolution is an overall gradual development which is both ordered and sequential. In terms living organism it may be defined as; the development of differentiated organism from pre-existing less differentiated organism over the course of life.

Lamarckian theory.

The French biologist Lamarckian proposed, in 1805 a hypothesis to account for the mechanism of evolution based on two conditions; The use and disuse of parts and inheritance of acquired characteristics. Changes in environment may lead to changed pattern of behavior which necessitate new increased use or disuse of certain organism /or structure. Extensive use would lead to increased size and or efficiency whilst disuse would lead to degeneracy and a trophy. These traits acquire during the lifetime of individual were believed to be heritable and thus transmitted to offsprings.

According to Lamarckism, as the theory came to be to known, the long neck and legs of the modern giraffe were the results of the generation of short-necked and legged giraffe ancestors feeding on leaves at progressively higher levels trees. The slightly longer necks and legs produced in each generation were passed on the subsequent generation until the size of the present-day giraffe was reached.

Darwin, Wallace and the origin of species by natural selection

Darwin and Wallace proposed that natural selection is the mechanism by which new species arise from pre-existing species. This hypothesis/theory is based on three observations and two deductions which may be summarized as follows:

Observation 1; Individuals within a population have a great reproduction potential. eg American oyster produces 10 million eggs per season.

Observation 2; The number of individual in a population remain approximately constant

Deduction 1; Many individuals fail to survive or reproduce. There is a ‘struggle for existence’ with the population

Observation 3. Variations exists within all populations.

Deduction 2:  In the ‘struggle for existence’ those individuals showing variation best adapted to their environment have a ‘reproductive advantage’ and produce more offspring than less adapted organism.

Natural Selection

This is a natural mechanism by which those organisms which appear physically, physiologically and behaviorally better adapted to the environment survive and reproduce. Those organisms not so well adapted fail to reproduce or die. The former organisms pass on their successful characteristics to the next generation.

Variation.

The term variation describes the differences in characteristics shown by organisms belonging to the same natural population or species.

Types of variation

1. Continuous variation.

Variation is said to be continuous when there is a gradual change of character from one individual to another; e.g. skin color, length of leaves, the height of individuals. i.e. it is quantitative.

A graph showing continuous variation

2. Discontinuous variation

In the discontinuous variation, there is a clear – cut difference between the characteristics e.g. blood group tongue- rolling, sex, etc.

A graph showing discontinuous variation

Variation and selection

Selection is the process by which those organisms which appear physically, physiologically, and behaviorally well adapted to the environment survive and reproduce. They pass on their successful characteristic to the next generation while the less adapted die.

Therefore, selection can be seen to operate through the process of differential mortality and differential reproduction potential. The selection has an adaptive significance in perpetuating those organisms most likely to ensure the survival of the species and depends upon the existence of phenotypic variation with the population.

Selection pressures may include food availability in animal and light in plants. This produces competition for resources between members of the population. Those organisms exhibiting characteristics which give them a competitive advantage will obtain the resources, survive and reproduce while organisms without those characteristics are at a disadvantage and may die before reproducing.

Both environmental limiting factors and population size operate together to produce a selective pressure which can vary in intensity and from time to time and from place to place.

Importance of natural selection

– organisms that are best adapted to a particular environment are allowed to survive and reproduce

– population size of a given environment is regulated to supportable limit.

– undesirable genes are eliminated from a population

– leads to the constant improvement of the population to better species

Types of selection.

Three types of natural selection

1.directional selection

2.stabilising selection

3.disruptive selection

1. Directional selection occurs when an extreme phenotype is favored. Such a shift occurs when a population is adapting to changing environment. E.g.,

(i) The gradual increase in size of a modern horse with change in environment from forest-like [which required penetration of thicket] condition to grassland [ which required escape from the predator] conditions.

(ii) Industrial melanism; the gradual increase in darkened peppered moth with increasing pollution that accompanied the industrial revolution in UK. Which enabled darkened form to hide easily on blacked walls and tree trunks.

Or

Industrial melanism is an evolutionary effect prominent in several arthropods, where dark pigmentation (melanism) has evolved in an environment affected by industrial pollution, including sulphur dioxide gas and dark soot deposits.

Or

Industrial melanism is the term used to describe increase in the frequencies of pale and melanic morphs in a variety of insect species, primarily cryptic moths, that have been noted since the advent of industrialization in many parts of the world

(iii) Emergence of bacteria and insects resistant to chemicals following prolonged indiscriminate use of antibiotics and pesticides

(iv) Increase in proportion of mammals with long fur when the average environment temperature decreases to increase the insulation of the animal or increase in the proportion of mammal with short fur when the average environmental temperature increases.

2. Stabilizing selection occurs when an intermediate phenotype is favored. It can improve the adaptation of a population for those aspects of the environment that remain constant. With stabilizing selection extreme phenotypes are selected against and individuals near the average are favored e.g. birth weight in human infants. Babies heavier or lighter than 3.6 kg in London are at a selective disadvantage and have a slightly increased rate of mortality.

3. Disruptive selection; Here two or more extreme phenotypes are favored over any intermediate phenotype. Fluctuating conditions in an environment, say associated with season and climate may favor the presence of more than one phenotype within a population. Selection pressure acting from within the population as a result of increased competition may push the phenotype away from the population mean towards the extremes of the population.

Effects of disruptive selection

• It split the population into two subpopulations, each which may give rise to a new species
• It can lead to the appearance of different phenotypes within the population, i.e. polymorphism.

Polymorphism

This is the existence or more forms of the same population and can be applied to biochemical morphological and behavioral characteristics e.g., land snail Cepaea nemoralis, the shell species may be yellow, brown or various shades

Types of polymorphism.

1. Transient polymorphism

This arises when different forms or morphs, exist in a population undergoing a strong selection pressure. The frequency of the phenotypic appearance of each form is determined by the intensity of selection pressure, e.g., melanic and non-melanic form of the peppered moth. Transient polymorphism usually applies in situations where one form is gradually replaced by another

2. Balanced polymorphism.

This occurs when different forms co-exist in the same population in a stable environment. E.g.

• Existence of the two sexes in animals and plants
• Existence of A, B, AB, and O blood group in humans.

Causes of variation

Variations are caused by both genetic and environmental differences. From the evolution point of view, genetic variations are more important because they can be transmitted from parents to offsprings.

The causes of genetic variations

1. Gene reshuffling

a.  Independent assortment of genes at meiosis I allow gene reshuffling in two ways

–   orientation on the equator in metaphase I

During metaphase, I of the first meiotic division homologous chromosomes came together in pairs and subsequently segregate into a daughter cells independently of each other. The result of this independent assortment is the production of a wide variety of different gametes depending on which particular chromosome ends up with one another in each cell.

– Crossing over

In prophase of the first meiotic division, homologous chromosomes came together and make intimate contact with each other. Chromatid of homologous chromosome may break and rejoin at any place called chiasmata.

b  Fertilization

Union of gametes at fertilization results in alleles present in one gamete being united with alleles in another. If a population consist of a large number of outbreeding individual, the amount of variation that may result from this is again virtually infinite.

Despite the tremendous amount of variation these three processes may generate, they play only a limited role in evolution. The reason is that although they may establish a new combination of alleles in one generation, they do not generate long-lasting variations of a novel kind.

2. Mutation

This is a change in the amount or structure of the DNA of an organism. This produces a change in the genotype which may be inherited by cells derived by mitosis or meiosis. Individuals showing the new characteristics are referred to as mutants. Mutants arise spontaneously and in no sense ‘directed’ by the environment, although the environment greatly influences the mutation rate.

Factors or agents that speed up or lead to mutations are called mutagens include; Gamma rays, ultraviolet and a number of chemicals e.g., mustard gas, colchicine in plants

Effect of mutation

1. Mutations are persistent; they tend to be transmitted through many generations without further change.

2. The vast majority of mutation confers disadvantages on an organism that inherit them.

Causes of mutation

Ultraviolet light

Chemicals

Freezing of cells

Types of mutation.

1. Chromosomal mutation

This may be the result of changes in the number or structure of chromosomes

(a) Change in number of chromosomes.

This is usually due to errors occurring during meiosis and mitosis. The changes may involve the loss or gain of a single chromosome a condition called Aneuploidy or the increase in entire haploid set of chromosomes a condition called euploidy [polyploidy]

 Aneuploidy

In this condition where half of the daughter cell produced have an extra chromosome [n + 1], [2n + 1], and so on, while the other half have a chromosome missing [n- 1], [2n- 1] and so on.

An aneuploidy can arise from the failure of a pair or pairs of homologous chromosomes to separate during anaphase 1 of meiosis. If this occurs both sets of chromosomes pass to the same pole of the cell and separation of homologous chromosomes during anaphase II may lead to information of gametes containing either one or more chromosomes too many or too few. This is known as non-disjunction.

One of the commonest form of chromosomal mutation in human resulting from non-disjunction of the G 2-chromosome is Dawn’s syndrome [2n +1] characterized by

• mental retardation,
• reduced resistance to disease,
• congenital heart abnormalities a short stocky body
• and thick neck and the characteristic fold of skin over the inner corner of the eyes.

Dawn’s syndrome and other related chromosomal abnormalities occur more frequently in children born to older women.

Klinefelter’s syndrome is due to non-disjunction is sex chromosomes, which may result in individuals who have genetic constitutions, XXY, XXXY, or XXXXY.

These individuals are phenotypically male but have small testes and no sperm in the ejaculate. There may be abnormal breast development and the body proportions are generally female. Y is the cause of maleness.

Turner syndrome [2n- 1]: An individual inherits a single X-chromosome (XO). Individuals with this condition often do not survive pregnancy and are aborted. Those that do are phenotypically female but small in stature and sexually immature. Despite having a single X chromosome, like males, they are female.

Polyploidy

This occurs when there is an increase in the entire haploid sets of chromosomes; i.e., 3n triploid], 4n [tetraploid]

Polyploidy is rare in animals but common in plants. Polyploidy is often associated with advantageous characteristics such as

• increased sized and greater,
• the hardness of seeds, though such advantages are sometimes offset by reduced fertility.

Polyploidy is sometimes induced by colchicine an alkaloid substance extracted from the crocus colchicum

(b) Structural change in chromosomes

These include loss, multiplication or changes in the sequence of bases on a chromosome

• Deletion; This involved loss of a piece of a chromosome together with its genes.
• Inversion; Here a chromosome breaks in two places and the middle piece then turns around and joins up again so that the normal sequence of genes is reversed.
• Translocation; a section of one chromosome breaks off and becomes attached to another chromosome.
• Duplication; a section of a chromosome replicates so that a set of genes is repeated.

2. Gene mutation

Gene mutation arises as a result of a chemical change in an individual gene and it ‘s thought to be very important in generating evolutionary change. An alternation in a sequence of nucleotides in a gene may change the order of amino acids making proteins. This may affect the fitness of the organism. It includes.

• substitution; here one base is substituted with another e.g., sickle cell anemia
• insertion; here an extra-base nucleotide is inserted into the genetic code.
• deletion; here a base is lost from the genetic stand
• There may be a change in the sequence of nucleotides in the gene

3. Somatic mutation

Here mutation in non-reproductive cells of an organism. The resulting genetic change will present in all descended cells from the original mutant cell and may have a profound effect on individuals.

However, as the genetic change is only in a non-reproductive cells, it cannot be transmitted to feature generation.

Gene pool

This is the total variety of genes and alleles present in a sexually reproducing population, and in any given population. The composition of gene pool may be constantly changing from generation to generation as a result of natural selection. Populations undergoing evolutionary change have continuously changing gene pools.

Gene pool

A gene pool is the stock of different genes in an interbreeding population

Species with small gene pools may be easier to wipe out due to a natural event that favors one trait over the other. A large gene pool means a variety of genes which prevents this

The composition of the gene pool may be constantly changing from generation to generation as a result of natural selection or maybe static in a stable environment.

Allele frequency

This the fraction of organisms in a population carrying a particular allele.

Genotype frequency

This is the fraction of organisms in a population carrying a particular genotype. The frequency of dominants and recessive alleles in a population will remain constant from generation to generation provided the following conditions exist.

1. The population is large
2. Mating is random
3. No mutation occurs
4. All genotype is equally fertile so that, no selection occurs
5. There is no emigration or immigration from and into the population, that is, there is no gene flow between population.

Any change in allele or genotype frequencies must, therefore, result from the alteration of one or more of the conditions above. These are factors that significant in producing evolutionary a

Factors producing a change of genotype are allele in a population.

1. non- random breeding

Mating in most natural populations is nonrandom. Sexual selection occurs whenever the presence of one or more inherited characteristics that increase the likelihood of bringing about successful fertilization of gametes of some organisms and not in others. There are many structural and behavioral mechanism in both plants and animals which prevent mating from being random, e.g. flowers possessing increased size of petals and amount of nectar are likely to attract more insects and increase the likelihood of pollination. Thus, sexual selection, as a mechanism of non- random mating ensures that certain individuals within the population have an increased reproductive potential so that their alleles are more likely to be passed to the next generation.

2. Genetic drift.

This refers to the fact that variation in gene frequencies with populations can occur by chance rather than by natural selection. E.g., chance events such as premature accidental death prior to mating of an organism in a small population which is the sole possessor of a particular allele would result in the elimination of that allele from the population.

Genetic load

This is the existence within a population of disadvantageous alleles in heterozygous genotype e.g. sickle cell trait in the region where malaria is endemic.

Gene flow.

It’s the movement of alleles from one population to another as a result of interbreeding between members of the two populations.

Heterozygotes as a reservoir of genetic variation (the Hardy-Weinberg principle)

For a particular character in a population, the dominant form expresses itself more often than a recessive form, for example, normal skin color is more common than albino.  In a large population, the proportion of dominant alleles and recessive alleles of a particular gene remain constant. It is not altered by interbreeding. This constancy is known as the Hardy-Weinberg principle is expressed by a mathematical law

P² +2pq+ q² =1

Where p = frequency of allele for dominant character

            q = frequency of allele for recessive character

The formula can be used to calculate the frequency of any allele in the population. For example, imagine that a particular metal defect is the result of a recessive allele. If the number of babies born with the defect is 1 in 20000, the frequency of the allele can be calculated as follow:

The defect will only express itself in individuals who are homozygous recessive. Therefore, the frequency of these individuals (q²) = 1/20000 = 0.00005

The frequency of the allele q = √0.00005 = 0.007

Since P + q = 1

The frequency p of the dominant allele = 1- 0.007 = 0. 997

From the Hardy-Weinberg formula, the frequency of heterozygotes is 2pq

i.e., 2 x 0.997 x 0.007 = 0.014

in other words, 14 in 1000 or 280 in 20000 are carriers (heterozygotes) of the allele.

This means that in a population of 20000 individuals, one individual will suffer the defect and about 280 will carry the allele. The heterozygotes are acting as a reservoir of the allele, maintain it in the gene pool.

As these heterozygotes are normal, they are not specifically selected against, and so the allele remains. Even if the defective individuals are selectively removed, the frequency of the allele will hardly be affected.

In our population of 20000, there is one individual who has two recessive alleles and 280 with one recessive allele- a total of 282. The removal of the defective individual will reduce the number of alleles in the population by just 2, to 282. Even with the removal of the defective individual, it would take thousands of years just to halve the allele’s frequency.

Occasionally, like the sickle cell anemia the heterozygotes individual have a selective advantage. This is known as heterozygote superiority.

Conditions that allow the Hardy-Weinberg principle to be true

1. No mutation
2. The population is isolated i.e. there is no immigration or emigration
3. There is no natural selection or individuals are equally fertile.
4. The population is large and mating random

Speciation

Speciation means the development of different genetic traits in an isolated subpopulation leading to a species distinctly different from the original parent population. Or speciation is the process by which one or more species arise from previously existing species.

A species is a group of organisms that are potentially able to breed among themselves but not any other species.

A single specie may give rise to new species [intraspecific speciation] or as is common in many flowering plants two different species may give rise to a new species [(interspecific hybridization).

If intraspecific speciation occurs whilst the population is separated it is termed allopatric speciation. E.g. Galapagos island.

If the process occurs whilst the population are occupying the same geographical area it is called sympatric speciation

Intraspecific speciation will occur when gene flow with a population is interrupted and each subpopulation is genetically isolated. Then change in allele and genotype frequencies within each subpopulation as a result of natural selection on the range of phenotype produced by mutation and sex recombination lead to the formation of race and subspecies. If genetics isolation persists over a long period of time the subspecies may form new species

Speciation will only occur as a result of the formation of the barrier which leads to reproductive isolation between members of the population.

 Isolation mechanisms

An isolating mechanism is a means of producing and maintaining isolation within a population. This can be brought about by mechanisms acting before or after fertilization.

(a) Prezygotic mechanism [barrier to the formation of hybrids]

(i) Seasonal isolation; occurs when two species mate or flower at different times of the year. E.g., California Pinus radiata flowers in February whereas Pinus attenuata flowers in April

(ii) Ecological isolation; occurs where two species inhabit similar regions but have different habitat preference e.g., Viola arvensis grows on calcareous soil, unlike viola tricolor. 

(iii)  Behavioral isolation; occurs where animals exhibit courtship patterns, that attract one individual for sex but not another.

(iv)  Mechanical isolation; occurs in animals where the difference in genitalia prevents successful copulation and in-plant where related species of flowers are pollinated by different animals.

(b) Postzygotic mechanism [barrier affecting hybrids]

(i) Hybrid in-viability; Hybrid is produced but fails to develop to maturing. E.g. hybrid formed between the northern and southern race of the leopard frog [Rana pipens] in North America.

(ii) Hybrid sterility; hybrid fails to reproduce functional gametes; e.g., the male [2n= 63] results from the cross between the horse [Equus hernionus 2n = 66]

(iii) Hybrid breakdown; F1 hybrid is fertile but the F2 generation and backcrosses between F1 hybrid and parental stock fail to develop or and infertile i.e., a hybrid formed between species [genus Gossypium].

Allopatric speciation.

This is characterized by an occurrence at some stage of spatial separation. Geographical barriers such as mountain ranges, seas, rivers, or habitat preferences may produce a barrier to gene flow because of spatial separation. This inability of organisms or their gametes to meet leads to reproductive isolation.

Adaption to new conditions leads to a change in alleles and genotype frequencies. Prolonged separation of populations may result in them becoming genetically isolated even if brought together. In this way, new species arise.

Sympatric speciation

This is the speciation that occurs within the population in the same geographical areas where reproduction isolation may result from structural, physiological behavior of individuals within a population, e.g. polyploidy in plants.

Artificial selection: This when breeders’ animals and plant select individuals with the characters that are wanted and allow them to interbreed, individuals lacking the desirable qualities are prevented from breeding. By rigorous selection over many generations, special breeds or varieties may be developed for a particular purpose.

Animals that have been subjected to artificial selection include.

-cow for beef and milk

-Sheep for wool and meat

-horse for racing and holing

-pig for bacon and lard production

-dog for beauty

Among plants crops such as wheat, barley, and potatoes, have been bred for higher yield, greater resistance to disease and drought.

Inbreeding.

This is the crossing of closely related individuals. Inbreeding leads to a loss of fitness known as inbreeding depression. This is because an individual produced as a result of the crossing of the close relatives is more likely to have two copies of harmful or ever lethal recessive alleles.

Hybrid.

Is the result of a cross between individuals belonging to two different varieties [out breeding] such individuals show hybrid vigor. This is because hybrid tends to be heterozygous at many of their loci. Any harmful allele, therefore, has its effect masked by healthy ones.

The green revolution

This is the production of new varieties of the world’s major food crops such as rice, wheat, maize, and barley by agriculturalists in the recent past.

In general, the new varieties display some or all the following advantages over the older one.

1. There stems are shorter, resulting in dwarf varieties that are less likely to be flattened by wind and rain and can be more easily harvested.
2. They give a higher yield per unit area
3. They show a greater response to water & fertilizer.
4. They are relatively insensitive to day length and or imperative, with the result that two or even three crops may be grown per year.
5. They are more resistant to pests and diseases.

The disadvantage of the green revolution.

New varieties required a high level of fertilizers which are expensive and not always available in developing countries introduction of these species in the developing world concentrates wealth in the hands of the minorly of farms able to afford artificial fertilizer.

EVIDENCE OF EVOLUTION.

1. Geographical distribution
2. Comparative anatomy
3. Embryology
4. Taxonomy
5. Paleontology

Geographical distribution

The earth is believed to have been composed of one continent from which different day present continents involve apart. For instance, the following provides evidence that the eastern coast of South America was once in contact with the western coast of Africa.

1. The two coastlines of the two continents are complementary.

• Presence of fossil deposit of mosasaurs [fossil reptile] only on the eastern part of Africa with no evidence of migration.

The distribution of animals and plant on different continents provide evidence  for a possibility of evolution as described in the following;

Example

1. The Northern hemisphere where continents are close together and where evidence point that there was a continuous land bridge linking them in the geological past; mammals on these continents are very similar. It possible that mammals evolved to give rise to new species and generally some which moved from North America to Euroasia or vice versa. In other words, different mammals evolved in these two continents but the geographical closeness of the two regions kept their faunas together and prevented them from diverging greatly.
2. The southern hemisphere where the continents were widely separated from each other. there is a sharp contrast in mammals that inhabit Africa [lion, giraffe], Southern America [e.g., tapir, puma, and Australia [kangaroo]. The wide separation meant that only very rarely was there an exchange of mammals between them, so the mammals each evolved independently along with their own.
3. Oceanic Islands [islands that originated from volcanic eruptions and never had any contact with the main land]

• Species on these islands had close similarities to those on the mainland but plants and animals on the islands such as the giant tortoise were noticeably larger in most case probably because they lacked competition from larger and more dominant, Advanced species which were absent from the islands but which cohabited with smaller related species on the mainland.

• The competition for food, space, and mate between the abundant Iguana lizard on the Galapagos island probably lead to the evolution of the two species one on land and another in water. The aquatic form had adaption for locomotion in water such as a laterally- flattened tail and well-developed webs of skin between the toes of all for limbs and feeds on marine algae.

• The competition for food probably lead to the evolution of finches’ beaks shape to enable different finches to feed on different food e.g. Darwin found finches with beaks for crushing seed, eating insects, sucking nectar, catching insects in flight and others for eating insect larvae or Galapagos island

Comparative anatomy

A comparative study of the anatomy of groups of animals and plants[morphology] reveals that certain structural features are basically similar.

For example, the basic structure of all flower consists of sepals, petals, stamen, stigma, style, and ovary, yet the size number of parts and specific structures are different for each individual.

Similarly, the limb-bone pattern of all tetrapods from amphibians to mammal has the same structural plan; called the pentadactyl limb.

The common basic structures on organisms suggest common ancestry.

In each case, structures are modified for a particular function in a particular environment.

Some homologous structures that fail to develop full functions and/or structure are referred to as vestigial organs.

Homologous structure

These are organs that have a similar basic structure, similar topographic relationship as structures in other species the same histological appearance, similar embryonic development but showing adaptation to different environmental conditions and modes of life. E.g., wings of bird and arms of man, Halters of Diptera [housefly], and hind wing many flies.

Adaptive radiation

This is a term used to describe the differentiation of homologous structure to perform a variety of function e.g., mouthparts of insects consist of the same basic structure but maxilla in butterfly is modified for suckling and manipulating food in grasshopper;

Adaptive radiation is the relatively fast evolution of many species from a single common ancestor. Adaptive radiation generally occurs when an organism enters a new area and different traits affect its survival. An example of adaptive radiation is the development of mammals after the extinction of dinosaurs

Divergent evolution.

This is the process whereby groups from the same common ancestor evolve and accumulate differences, resulting in the formation of new species.

Divergent evolution may occur as a response to changes in abiotic factors, such as a change in environmental conditions, or when a new niche becomes available.

Analogous structure

These are the structure of organism bearing no close phylogenetic link but showing adaptation to perform the same function e.g.,

-wings of insects and bats

-the jointed legs of insects and vertebrates

Convergent evolution

This is the evolution where structures believed to have different ancestry origins are modified to perform the same basic function. E.g. wings of bird and insects

Vestigial structures.

Vestigial organs are non-functional organs in an organism that are functional in related animals and were functional in the ancestors.

 There are 90 vestigial organs in the human body and mainly include coccyx (tail bone); nictitating membrane (3rd eyelid); caecum and vermiform appendix; canines; wisdom teeth etc.

Pentadactyl limb.

It is so-called because it typically has five digits is formed in all four classes of terrestrial vertebrate [amphibian, reptile, birds, and mammals] some of the limbs bone can even be traced back the fins of certain fossil fishes from which the first amphibians are thought to have evolved.

Through divergent evolution, the pentadactyl limb has become adapted for different function in some species some or all the foes or finger have been lost eg

Seal                –  for swimming

Human         –  for manipulation / grasping

Horse              –  for running

Mole                – for digging

Bat or bird        – for flight

Molecular biology / Biochemistry / cell biology

The following examples of comparative molecular biology / Biochemistry provide evidence for evolution.

• The presence of similar biological molecules such as nucleotide, ATP, cytochrome, and certain organelles such ad the ribosome in all living organisms support the view that all living organisms had a common ancestry.
• The differing degree in an amino acid sequence of proteins from one species or another is also used to show evolution. The closure species in evolution the closure in the similarities in the aminoacid sequences in their proteins
• Like with proteins close evolutionary species show close similarities in their DNA base sequences, the comparison carried out by a technique of DNA hybridization.

Embryology

Similarities in the embryonic development stages are thought to have evolutionary significance. Species believed to have a close evolutionary past have been found to show similar embryonic stages.

For example, the presence of branchial groves and segmental myotomes in the human embryo is suggestive of a fish ancestry.

Paleontology

This is the study of animals and plants of the past as seen in the fossil records.

Fossils are any form of preserved remains thought to be derived from living organisms. They include entire organisms, hard skeletal structure, molds and casts, petrifactions, impression, imprints, and coprolites [fossilized fecal pellets]

• There is evidence from fossils that there has been a progressive increase of complexity of organism which denies the fixity of species. The oldest fossil-bearing rock contains very few types of fossilized organism and they all have a simple structure. Young rocks contain a greater variety of fossils with increasingly complex structures.

•  Throughout the fossil record many species that appear at an early stratigraphic level disappear at a later level. This is interpreted in evolutionary terms as indicating the time at which species originated and became extinct.

• Geographical evidence suggests that geographical regions and climatic conditions have varied throughout the earth’s history. Since organisms are adapted to a particular environment, the constantly changing condition may have favored a mechanism for evolutionary change that account 4 that progressive change in the structure of an organism as shown in the fossil record.

Ecological consideration also fit in with the fossil evidence. e.g. Plants appeared on land before animals, and insects appeared before insect-pollinated flowers

Taxonomy

This is the study of principals, rules, and methods of classification. The system of classification proposed by Linnaeus before the time of Darwin and Wallace, based on phylogenetic similarity and differences between organisms may suggest the existence of an evolutionary process. There are similarities and differences between organism may be explained by an organism with each taxonomic group to particular environmental conditions over time.

Numerical taxonomists, working mainly from comparative phenotypic characters have found it possible to construct a phenetic classification system which inconsistent, to the extent of present knowledge with the concept of evolution organisms that has close evolutionary relationship share very many similar characteristics.

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