TOPIC 5: EVOLUTION ~ BIOLOGY FORM 6
ORGANIC EVOLUTION.
FORCES/PRESSURES LEADING TO ORGANIC EVOLUTION
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
THEORIES OF ORIGIN OF LIFE
1. Special creation
says BE and there it is.
Strength of the theory:
Weakness of the theory:
2. Spontaneous Generation
Strength of the Theory
Weakness of the theory
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
Weakness of the theory
The theory does not talk about the origin of life but perpetuation of it.
THEORIES AND MECHANISM OF ORGANIC EVOLUTION
LAMARCKIAN EVOLUTION
His account for mechanism of evolution is based on the following:
SELECTION
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
TYPES:
NATURAL SELECTION
ARTIFICIAL SELECTION
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:
Solution
Forms/mechanisms of inbreeding:
OUTBREEDING
Limitations of out breeding:
Interbreeding F1 for a long time decreases hybrid vigour (heterozygosity) due to increase in homozygosity.
SPECIATION
Speciation is a process by which new species are formed from pre-existing species.
Alternative ways of defining a species:
TYPES OF ISOLATION
Geographical isolation
the separate groups (demes) of the same species can no longer interbreed i.e. prevented from meeting hence barrier to gene flow.
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
(b) Seasonal isolation
(c) Behavioural isolation
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.
(b) Sympatric speciation
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.
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?
Example
In a course of time, the whole population contained long necked and legged giraffes which we see today.
Hence they are evolved.
Strength of theory
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.
DARWIN’S THEORY
Was based on natural selection and survival of the fittest
Essential features:
Observations
Deductions
Strength of the theory
Weakness of the theory:
According to Darwin:
time, due to natural selection, we have the long necked giraffes at present.
EVIDENCE FOR ORGANIC EVOLUTION
There is several evidence
1. PALAENTOLOGY
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.
years.
Evidence:
contain more varieties of fossils with increasing complex structure.
Archaeopteryx – reptilia and aves
Symoria – amphibia and reptilian
Synognathics – reptilia and mammals
been discovered yet.
2. COMPARATIVE ANATOMY/ MORPHOLOGY
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.
Steady State
At that moment he wasn’t listening to music, he was living an experience.