THEORIES OF EVOLUTION
Organic evolution is the sum total of adaptive changes from pre–existing or old forms that has taken place over a long time resulting in diversity of forms, structures and functions among organism. The basis of evolution is that all organisms have pre–existing ancestors.
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Evidences of evolution
- Fossil record: A fossil is an impression of a plant or an animal that lived a very long time ago. The age of fossil is determined using radioisotope dating. Fossils are normally preserved in sedimentary rocks. Depending on the source, fossil records can be referred to as geological or paleontological or archaeological or historical record.
- Geographical distribution: Based on the effect of climate on all living things, variations in their forms, structures and functions can occur. After several years of isolation, organisms of one climate tend to differ slightly from organisms of another climate.
- Comparative anatomy: Evolution is obvious in anatomical comparison of vertebrates. The Pisces or fishes have simple heart with one auricle and one ventricle. The amphibians have two auricles and one ventricle. Reptiles have two auricles and a partially divided ventricle. Aves and mammals have two auricles and two ventricles.
- Embryological evidence: The embryo of man in the womb at different stages of development resembles the embryo of fish, amphibians and reptiles.
- Evidence of vestigial organ: Vestigial organs are minute and incomplete organs that have no special function. Evolutionarily, these organs are believed to be once functioning e.g. appendix and rudimentary tail in man.
- Evidence from domesticated animals: These include cats, dogs, hensetc which live with humans for many years.
THEORIES OF EVOLUTION
There are three prominent theories
- Jean Lamarck’s theories of use and disuse which states that
- Changes in the environment lead to changes in the species of organism.
- The changes cause the organisms to form new structures or habits to adapt to environmental changes.
- The organisms then develop specialized characters by use and disuse of organs.
- Frequently used organs become well developed and the unused ones degenerate or become useless.
- The well developed or dominantly acquired characters are inheritable.
Lamarck’s theory is unacceptable to modern scientists who have proved that only characters represented on genes are inheritable not physical ones got through use and disuse.
- Charles Darwin’s theory of natural selection (survival of the fittest) which states that
- Species of organisms can produce large number of offspring to the environment with limited resources.
- This then results in competition among the offspring.
- The survivors must have inherited the useful traits which are passed on to the offspring at reproduction.
- Those that could not survive the competition die off. As the population gradually becomes better adapted to the environment, new species emerge.
This theory is widely acceptable to many scientists till date.
NOTE: Both Lamarck and Darwin recognized the importance of environment.
Modern theories of evolution:
Based on the combination of natural selection (Darwin’s) and genetic origin of variation. This theory states that:
- There exist variations in the species population.
- Some of the variations have special survival advantages.
- Individuals with favourable variations are more adaptive to their environment than others.
- The individuals have to struggle for existence inthe environment.
- The fittest contribute more offspring to the next generation than the unfit ones.
- The main causes of variations are mutation and recombination of genes.
Roles of mutation in evolution
Mutation is a sudden change in DNA structure leading to a change in the phenotype of the species concerned. When mutation occurs in the gene of gametes, it leads to production of new species.