The causes and consequences of variation
The causes and consequences of variation
Variation is the differences that exist between members of the same species.
It can be caused by environmental or genetic factors. Genetic variation tends to be permanent and stays within the population gene pool (unless it is erased via evolution) and can effect both the phenotype and the genotype of an organism. The environmental differences within or between species cannot be inherited by offspring and are only able to influence the phenotype of a species.
Variation can either be continuous and discontinuous. Continuous variation is when there is a complete range of measurements between two extremes. An example of this type of variation can be seen in the height of humans, which ranges from the shortest to the tallest individual in a population. This is an example of polygenic inheritance and the pattern of variation reflects the fact that there are many different combinations of alleles possible for human height.
Discontinuous variation is variation in which individuals fall into distinct categories is between one type and another, this type of variation is caused by major genes and is unaffected by the environment. An example of this type of variation if the ABO blood groups, a persons blood group doesn't change just because they don't have the right diet for example or get a tattoo. Other examples include the ability to roll ones tongue, gender and eye colour.
Genetic variation in the same species arises from the inheritance of different genotypes from parents. Differences between gametes are caused by independent assortment and crossing-over during meiosis in cell division.
Independent assortment has the potential to produce slightly different gametes in enormous numbers. This process takes into account all the possible arrangements of bivalents at metaphase 1 in cells containing a number of chromosomes. In humans for example the number of chromosomes is 23, which means that more than 8 million (232) different haploid combinations of chromosomes are possible.
Crossing over occurs in late prophase 1, the process involves the allele exchange between homologous chromosomes. Chromosomes are able to take up different forms of the same gene or substitute alleles between each other. This also increases variation between gametes and thus lead to variation between individual of the same species.
Random fertilisation can also cause variation between a species. It is the indiscriminate fusion of the female and male gametes. This also gives rise to huge numbers of different possibilities of a gamete genotype.
The consequences of the 3 ways in which genotypes are combined through meiosis and sex is much greater ability of sexual species to respond to environmental changes. The advantages of sexual reproduction are that it allows beneficial mutations, which have arisen, in separate individuals to be brought together to form between adapted individuals. However, many mutations of this type are recessive to normal ...
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Random fertilisation can also cause variation between a species. It is the indiscriminate fusion of the female and male gametes. This also gives rise to huge numbers of different possibilities of a gamete genotype.
The consequences of the 3 ways in which genotypes are combined through meiosis and sex is much greater ability of sexual species to respond to environmental changes. The advantages of sexual reproduction are that it allows beneficial mutations, which have arisen, in separate individuals to be brought together to form between adapted individuals. However, many mutations of this type are recessive to normal alleles, a recessive mutant allele awaits replication in the gene pool over many generations before chance brings recessive alleles together, so they are expressed.
Gene mutations can also be caused by what's known as base pair substitution. Base-pair substitutions occur when one nucleotide and its partner from the other DNA strand are replaced with another pair. Since these substitutions only occur at one site on the DNA strand they are sometimes called point mutations. Point mutations often cause no effect at all but minor changes to proteins may occur (if one amino acid is altered but not in a way that changes the protein's function too much), and occasionally such mutations may be lethal (protein product absent or non-functional).
One example of a gene mutation caused by base pair substitution is that causing sickle cell anaemia. The DNA molecule, which codes for the beta amino acid chain in haemoglobin has a mutation whereby the base adenine, replaces thymine. This is then replicated; the mRNA produced has the triplet code GUA (for the amino acid valine) rather than GAA (for the amino acid glutamic acid). The haemoglobin molecule containing the abnormal beta chains forms abnormal long fibres when the oxygen level of the blood is low, S-haemoglobin is produced and it is this that causes the red blood cell to become sickle shaped.
This condition can be both beneficial and lethal in depending on the whether the patient is a carrier or has the disease. A person who only shows the sickle cell trait (is heterozygous for the disease) benefits from the mutation if they are living in an area where malaria is a problem, as it makes them immune to the malaria parasite. However, those with the disease generally suffer from pain in joints, kidney damage and eventually death. People infected by the disease tend to die young and are less likely to pass on their genes. This means that their genes will eventually be eliminated from the gene pool (in certain areas, for example Britain -where malaria is a very rare disease) however, in other areas that gene may be beneficial, as in this case, so its function in the gene pool is significant. This is an environmental cause of variation between a species (human) -the process of selection, described above.
Gene flow can also causes variation, it is the exchange of genes between populations of a species. For example, the various populations of magpies found in different locations around Australia do not usually interbreed, and they have observably different phenotypes in different areas. However, if some magpies migrate between locations, and interbreed with the magpies in the new location, their genes have 'flowed' out of the original population and into the second population.
This introduction of new genes into a population is often beneficial as it may supply the so-called hybrid vigour, which increases the survival potential of the next generation of individuals.
Environmental differences can be very important when considering variations within a species. For example, some twins have an identical genetic make-up and it would expect that these be exact copies of each other throughout their lives if they grew in similar conditions but big differences in diet could make one twin much taller or heavier than the other. Similarly, plants bred from identical seeds may grow in a stunted way in exposed, dry areas with few nutrients in the soil or may grow large and luxuriant in sheltered, well-watered environments, which are rich in nutrients.
Mutations may arise due to environmental factors. For example a mutation can be caused by the breakage of one or both strands of DNA molecules. This happens when additional energy (from X-ray, Light or radiation) is provided to the bonds holding the molecule together or when other molecules with similar shapes and reactivity (e.g. Nitrous acid, mustard gas) insert themselves into the DNA polymer or chemically alter bases. If this change is still present in the DNA molecule when it is copied by semi-conservative replication, then the alteration in the sequence is made permanent.
These chance mutations that occur can be beneficial for certain species. For example a chance mutation in some bacteria makes them resistance to anti-biotics. The selection pressure favouring resistance types causes the non-resistance types to die. Therefore only the resistance types survive and pass on their genes to their off spring by the process of natural selection.
The peppered moth is another example of this type of mutations. Before the industrial revolution the most common type of moth in Britain was the light coloured, as they could camouflage well on the barks of the trees. In heavy industrial areas the light coloured moths could no longer camouflage and so were eaten more often by predators. At this time a mutation occurred producing the black coloured moth. This black moth had a selective advantage in the industrialised areas. However, the light coloured moths did not completely die out because they thrived in the cleaner areas of the country where it was a benefit to be pale-coloured.
Gradually varying phenotypes and genotypes of a species over a range of geographic locations are called clines. A species that varies with geographic location in this way is said to show clinal variation.
One of the most obvious examples of clinal variation is in the population of the species Homo sapiens. If we consider the many groups of humans in the world that we call 'races' this idea of clinal variation is well illustrated. Although we all belong to the same species, different environmental pressures have, over time, selected for different phenotypes (and different genotypes) in different geographical locations.
Disruptive selection is an example of speciation. This type of selection favours the extremes in a range of variation at the expense of the mean. Organism who's phenotypes correspond to the mean in this range are at a disadvantage. These are selected against, while selection favours both of the extreme varieties. Over time this will result in the evolution of the two distinct forms.
Genetic engineering can also cause variation between species. It is an aspect of biotechnology, which involves altering the genetic make up of an organism. However, many organisms, which are genetically engineered, are not allowed to return to their natural environment again and thus are not able to 'mix' with the species in the case of animals especially.
However in the American Northwest this may not be the case, salmon are being genetically engineered to grow twice as fast, using growth hormone genes, so they can be sold sooner and are therefore more profitable. These gene-altered fish pose threats to wild fish, in rivers and oceans, and as well as to those who eat them. These threats are irreversible, once genetically engineered fish escape into open waters, they can never be recalled. This may cause variation in rare cases and even cause a new species to evolve.
Many crops and vegetables are also genetically modified; certain desirable characteristics are emphasised a lot more in the genotype than in the natural form of the organisms. This type of variation can be disastrous on a species ability to evolve and adapt to new environments via evolution.
To conclude variation has as many wide spread causes as it does consequences. Its importance on earth is immense and it is due to variation in species that the human has evolved. The true, literal source of variation has been shut up inside the genome of species for a long time. It consists of (in the case of the structural genes which code for proteins) an abundance of genetic material. Mutations damage those genes, which upsets balances and/or causes 'new' characteristics to appear. However, those characteristics were already contained within the organism. In other words, precision prevention and repair of mistake mutations are programmed into the genome, or designed if you will. As a result of the built-in designed, system of natural variation
