BIOLOGY A LEVEL(FORM SIX) NOTES – EVOLUTION

EVOLUTION

• 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.

• Mutation
• Gene recombination
• Natural section

• Gene flow
• Genetic drift
• Breeding
• Adaptive radiation/geographic isolation

• 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

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

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

• 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.

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

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

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

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

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.

• 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.

• 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.

• 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,

• 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.

• 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

• 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.

• 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.

• 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 comm
  on 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.

• 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.

• 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.

• Animals and plants respond to the environm
  ent 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.

• The case of webbed feet ducks also applies.

• 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.

1. Over production of offsprings
2. Constancy in number of population
3. Struggle for existence due to competition
4. There is variation among organisms
5. Survival of the fittest
6. Formation of new species

1. Individuals within a population have a great reproductive rate.
2. The number of individuals in a population remains approximately constant due to environmental factors such as food, space, light, water, breeding sites, etc.
3. Variations exist within individuals in a population.
4. 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.

• 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.

• 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.

• 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

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

• 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

• 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

• 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.

• 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

3. BASIC BIOCHEMISTRY

4. BIO – GEOGRAPHY

• 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.

• 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.