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It is clear that there is an inherent tendency to variation in sexual reproduction. An error in DNA copying during sexual reproduction also causes changes as we shall see. The long-term consequences of the accumulation of variations in species, is the basis for evolution and diversity in life.
All around us we see different organisms belonging to different species. Within each species too, no two individuals are identical to each other. Variation may be defined as differences among individuals of a species. Asexual reproduction produces identical offspring called clones and identical twins are seen in sexual reproduction. However, clones and identical twins can also differ from each other if subjected to different environmental conditions.
Variations arise either due to environmental influence or genetic factors. The environmental factors like the food, temperature, etc. influence the phenotype of individuals. However, phenotypic changes are not heritable. Genetic changes are those that affect the genes. These changes are heritable and important from the evolutionary point of view. Variations can be classified based on the origin.
They are somatic and non-inheritable. They are caused by the influence of the environment upon the somatic cells.
For Example
- Strong sunlight causes skin to darken in humans
- Honeybee larvae fed on royal jelly become fertile and others that are not become sterile. The fertile larva becomes the queen and the others become workers.
- Continuous exercise makes the muscles of sportsmen strong and well developed.
- Japanese women wear tight shoes to prevent their feet from growing larger
In all the above cases, the changes are not heritable and are caused due to changes in the environment.
Changes that occur in the genes of an organism may be heritable. The changes may be due to assortment of genes during meiosis in sexual reproduction or due to some sudden change in the composition of genes or chromosome number. An example of change due to meiosis is the difference in height of a population. Down's syndrome where there is an extra 21st chromosome is an example of change in the number of chromosomes. Unfortunately, Down's syndrome causes mental retardation, a moon face and thick lips in an individual who generally does not live to the reproductive age. An example of change in the composition of genes is sickle-celled anaemia. In this, one of the genes undergoes a change in its composition. This results in the red blood cells being sickle shaped which is a fatal to the individual.
Variations may also be classified as continuous and discontinuous.
Continuous variations are minute and non-hereditary. They may be due to influence of the environment or genetic. Variation in height of a human population, number of arms in a starfish, etc. are some examples.
Discontinuous variations are large and prominent. They are due to mutations. For example, six digits in man, tailless kitten, hornless calf, etc.
Variations, the raw materials for evolutionary change, are controlled by genes. When there is an error in DNA copying during sexual reproduction it leads to variation.
Variations occur due to microevolution, that is, evolution on a small scale-within a species or a single population. It is defined as a change in gene frequency in a population. The changes occurring in microevolution can be directly observed because of the short timescale. This sort of evolutionary change, provide the species a way to survive and adapt to the changing environmental conditions. For example, with as a result of fragmented or degraded habitat individuals with traits best adapted to the present environment will be selected to reproduce to a greater extent than other members of a population. Because selection occurs, gene frequencies change in a population thus evolution occurs.
Micro-evolutionary changes happen in a four basic ways: mutation, migration, genetic drift, and natural selection. All these evolutionary mechanisms directly affect gene frequencies in a population.
For example, in a beetle population if one observes the increase in the frequency of brown colouration genes and a decrease in the frequency of green colouration genes, one wonders how this change has occurred. Any combination of the following mechanisms of microevolution might be responsible for this pattern.
Mutation
Some "green genes" randomly mutated to "brown genes". But this process alone can account for the change over the gene frequency in just one generation because mutation is rare and occurs under severe conditions only.
Migration (or gene flow)
Some beetles with brown genes could have immigrated from another population, or some beetles carrying green genes could have emigrated.
Genetic Drift
The colour variation arises during reproduction just by random luck. More brown genes end up in the offspring than green genes, providing diversity without any adaptations. Accidents in small populations can change the frequency of some genes. Such variations in a population occur, even if it gives no survival advantage like camouflage.
Natural Selection
Beetles with brown genes escaped predation as their colour became one with the environment and they remained as if hidden. In the process they survived to reproduce more frequently than beetles with green genes. This allowed more brown genes to go into the next generation. In another situation an increase in the population of crows will decrease in the number of brown beetles and increase in the number of green beetles. Thus, natural selection is directing evolution in the beetle population. It results in adaptations in the beetle population to fit their environment better.
In these situations, a rare variation becomes a common characteristic in the population. The frequency of an inherited trait changed over generations. As genes control traits, one can say that the frequency of certain genes in a population changes over generations to bring about evolution.
An interesting case of variation has occurred for this grass species Anthoxanthum odoratum seen in the image below.
Some of these plants live near mines where the soil has become contaminated with heavy metals. The plants around the mines have experienced natural selection for genotypes that are tolerant of heavy metals. Meanwhile, neighboring plants that do not live in polluted soil have not undergone selection for this trait. The two types of plants are close enough that tolerant and non-tolerant individuals can potentially fertilize each other that of a continuous population. However, the two types of plants have now evolved different flowering times. This change could be the first step in cutting off gene flow entirely between the two groups and this variation may evolve into a new species altogether (explained later in speciation)
Man has also artificially made use of variations. It is because of such variations caused that he is able to improve the animals and plants that are useful to him.
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