Evolution - Biology Form 4
Previously we learnt how variation occurs due to sexual reproduction, mutations and interaction of organisms with the environment. Some of these variations are passed from the parents to the offspring and over a long period of time this results in production of individuals with totally different characteristics from those of the original forms. For instance man is believed to have descended from earlier forms that were of relatively different characteristics.
By the end of this topic, you should be able to;
Explain the meaning of evolution and the current concepts of evolution
Describe the straggle for existence and survival for the fittest.
Describe the evidences for organic evolution
Explain resistance to antibiotics, fungicides and pesticides.
Current organisms are believed to have risen from previous forms. In this topic of evolution we will learn how changes that led to the current diversity of modern organisms are believed to have taken place.This is the study of evolution.
Play the animation to see the evolution of man.
The origin of life
There are two theories that try to explain the origin of life. These theories are Special Creation and Chemical Evolution.
This theory postulates that the whole universe and all its constituents was created by a special being. This theory is taught in many religions across the world. The special creation theory is illustrated in the animation below. Click on the play button to view the animation.
This theory postulates that billions of years ago life started by it from simple inorganic chemicals found on the Earth. These chemicals combined through chemical reactions to form complex organic molecules. Specifically the atmosphere contained a mixture of carbon IV oxide, nitrogen, ammonia and methane. These substances combined due to energy from lightning and volcanic activity to form simple organic compounds such as amino acids, nucleic acids, nitrogenous bases and sugars. This led to the formation of nucleic acids which have the potential to replicate.
The animation shows an illustration of chemical reactions in which inorganic molecules progressively lead to the formation of complex organic molecules.
Evidence for organic Evolution
The organic molecules formed in chemical evolution are believed to have led to the formation of cells from which other organisms arose, becoming complex with time. This is organic evolution. The evidence of organic evolution include: -
1) Geographical distribution of organisms
2) Comparative embryology
3) Comparative anatomy
4) Cell biology
Geographical distribution of organisms
Continental Drift Theory
The current continents have complimentary shapes which can fit together like jigsaw puzzle. This suggests that at one time they were joined together as massive continent known as Pangea. At this time organisms on the continent mixed freely and had a lot in common. At one time Pangea broke up into continents. The various continents drifted into their present positions. As they drifted apart, they separated closely related organisms and isolated them from one another. The isolated organisms evolved differently with time into the present forms through the influence of their respective environments. Each population developed special features to survive in its respective environment i.e. adaptive radiations.
Play the animation to see continental drift
Fossils are remains of plants and animals which lived millions of years ago and have been preserved in rocks. The soft parts of the organisms decayed but the hard parts such as bones teeth and shells are preserved in rocks. Fossil records traces the way organisms have gradually changed through geological time. The study of fossils is called paleontology.
Play the animation to see the process of fish fossil formation.
From fossil records it is possible to trace the progressive change in organisms and their structures such as the human skull from ancestral forms to the current form.
Play the animation to see the progressive change in human skull from ancestral forms to current form
Comparative anatomy is the study of structural similarities and differences between organisms.
Homologous structures and Divergent evolution
A close examination of certain structure in different organisms reveal that they are similar in basic structure suggesting that they have common ancestral origin. An example can be seen in pentadactyl limb of vertebrates. The limb has same pattern of bone arrangement but perform different functions in different species.
The animation shows the pentadactyl limb and its adaptation in different vertebrates. Click on the play button to view the animation.
The fact that structure is similar in all vertebrates suggests that these organisms have developed through modification of a simpler basic design in a common ancestor. This common basic design is modified through a process called adaptive radiation which adapts animals to different habitats. Such evolution leads to formation of homologous structures.
Play the video to see adaptive radiation
Analogous structures and convergent evolution
Analogous structures have different embryonic origin but have evolved to perform similar functions due to exploitation of common ecological environment or niche. This leads to convergent evolution e.g. wings of birds and wings of insects; human eye and the eye of octopus and squid.
The illustration shows the bird and insect wing.
The video clip shows insects and a birds flying to illustrate common function but different anatomical origin of the structure. Click on the play button to view the video.
Comparative embryology is the study of morphological similarities and differences between embryonic stages of different vertebrates. Different vertebrates have a number of similarities. The most significant similarity is seen in the embryonic stages during early development. Very similar embryos indicate a recent common ancestry i.e. the evolutionary relationship is closer. For instance the embryos of all vertebrates have similar features. These include gill slits, a tail, and a two-chambered heart.
Play the animation to view comparative embryonic stages of different vertebrates.
Organisms sharing the same chemical characteristics are considered to be more closely related than those lacking such affinities e.g. most plants contain chlorophyll, cellulose and starch, all of which are absent from tissues of animals. Similarly chemicals such as nucleic acids, ATP and Cytochromes, and organelles like mitochondria, endoplasmic reticulum and ribosomes appear to be of universal occurrence in all organisms.
This strongly supports the view that all organisms have a common ancestry. Vertebrates possess adrenalin and thyroxin, neither of which is found in other groups. Animals have blood pigments which enhance transport of oxygen in the body. Such pigments include haemoglobin, chlorocruorin and haemocyannin. Each of these pigments is found in related organisms.
The table shows pigments and organisms where they are found.
Mechanism of evolution
Several theories have been formulated on how evolution may have occurred. Some of the theories include those proposed by Larmack in 1809 and Darwin in 1859.
Larmarck proposed a theory to account for the mechanism of evolution based on two conditions.
- The use and disuse of parts of organisms
- Inheritance of acquired characteristics
He proposed that when the environment demanded the need of a particular structure in an organism, the organism developed in response to the demand during its lifetime. The traits acquired during the lifetime of the individual were believed to be inheritable and thus transmitted to the offspring. Lamarck cited as an example the long neck and legs of modern giraffes. These were thought to have resulted from short-necked and short legged ancestors believed to have stretched their necks to feed on leaves at progressively higher level of trees. At each stretch, the length of the necks increased and was passed over to the offspring of the particular giraffes.
Play the animation to see short necked and short legged giraffes trying to feed on tall trees.
Lamarck also cited the example of flightless birds such as ostrich, emu and kiwi. He proposed that the wings became reduced and functionless due to disuse in an environment not requiring flight. The video clip shows ostrich, emu and kiwi. Click on the play button to view the video.
Lamarck emphasis on the role of the environment in producing phenotypic changes in the individual is correct e.g. body building exercises will increase the size of muscles.
Play the video clip to see a weight lifter lifting weights.
However, his explanation that such acquired characteristics are inherited makes his theory unacceptable. It is important to note that acquired characteristics do not affect the genetic make up of an individual and therefore are not inherited. Lamarck's theory was therefore unscientific and is no longer acceptable.
Evolution by natural selection
The environment in which organisms live provides tests for survival. These include climate, soil type, predation, disease and competition for resources such as food, space and shelter. Natural selection occurs when the environment permits only organisms which are best adapted to survive. The survivors are the fittest and best suited for the particular conditions of the environment at that time. They are said to be selected naturally by the environment. The selected types survive and breed, hence pass on their genes for these adaptations. Those not selected die, hence their genes disappear from the population. For example millions of years ago the earth was cooler than it is now. Large animals called dinosaurs thrived.
The animation shows dinosaurs and other animals.
When the world climate became wormer, they had problems with regulating their body temperature due to their large size, hence died and are now extinct. On the other hand the relatively smaller animals survived and passed on their genes to succeeding generations. If the conditions of the environment change again new survivors appear since the previous survivors may no longer be the fittest. This process is continually repeated hence the population changes in characteristics.
The animation shows a mixture of different animals.
The population is said to have evolved. Any mutation which helps an individual organism to survive in a changed environment also gives it a better chance to breed and pass on the new gene. By breeding these new genes are spread in the population. The population changes in characteristics as compared to the original, making interbreeding between them difficult. Eventually different species develop.
Natural selection in action
Darwin was not able to demonstrate evolution taking place, showing transitional forms. However recent scientists have put forward cases to indicate that evolution is taking place even today. A good example of evolutionary change is the response of moth species to the atmospheric pollution which accompanied industrial revolution. The moth Biston betularia occurs in two forms. The common type is speckled white and black melanic form.
The illustration shows the two forms of the moth.
Before the industrial revolution the tree trunks had lichens growing on them. The speckled white moth camouflaged well with the tree trunks with similar pattern and colouration as the lichens on the trunks.
The illustration shows speckled white moth against tree trunks with similar pattern and colouration.
At this time the black melanic form did not camouflage well with the environment and therefore was prone to predation by birds.
Play the animation to view the black melanic form on white background.
During the industrial revolution around 1848, a lot of soot was released which killed the lichens on trees and darkened the tree trunks. Due to this the black melanic form blended well against the bark of trees, while the speckled was exposed.
The illustration shows dark tree trunks with the black melanic form of the moth.
The black forms were therefore better protected in the industrial area against predation by birds than the speckled white form. The animation shows dark background of tree trunks with both melanic forms of moths.
It is important to note that the occurrence of the melanic form was due to mutation which represents possible transition towards speciation.The agent of natural selection is selective predation on the unprotected forms of the moth.
Resistance to drugs and antibiotics
Drugs and antibiotics are chemicals used to kill disease causing microorganisms. Pesticides are substances used to kill pests such as aphids, mosquitoes and tsetse flies. Many disease causing organisms are becoming resistant to drugs and antibiotics.
The animation shows various bacterial forms and pests. Click on the play button to view the animation.
This resistance is due to the fact that within a population of these organisms, some individuals possess a gene for resistance or acquire it through mutations. This makes them to resist the chemicals or pesticide. Those that survive transmit these characteristics to their offspring hence leading to establishment of a new population of resistant forms.
Play the animation to see the development of resistant forms of bacteria.
For example penicillin, an antibiotic, at one time was used to kill many disease causing bacteria. However mutations occurred in the bacteria which resulted in resistant forms that are no longer killed by the drug. Diseases caused by these resistant forms would not be cured by penicillin. Similarly some mosquitoes possess a gene which makes them resistant to the pesticide DDT. Pesticides in which DDT is the active ingredient cannot kill such flies. The malaria causing plasmodium rapidly changes its surface coat, making it resistant to drugs.
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