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The causes and importance of variation and diversity in organisms
Genetic diversity is the number of different alleles there are for a certain gene in a population at a certain time.
One cause of genetic variation in a population is through mutations. Mutations are changes in the base sequences of DNA that can be because of faulty DNA replication during the s phase of interphase, or due to exposure to mutagenic agents. These are substances that increase the chance of a mutation occurring, for example, exposure to x rays, ultraviolet light, certain chemicals, called analogs. Example of mutations can be inversions, translocations, substitutions, additions, and deletions. When a mutation occurs in the DNA base sequence, if it has a non-silent effect, where the mutation does change the mRNA sequence during transcription of the gene and affect the primary structure of the polypeptide. for example, during addition mutations, an additional base is added which causes a frame shift and affects the subsequent triplet of bases in the DNA base sequence. This can change the tertiary structure of a protein, such as an enzyme, or a transcription factor. This can have catastrophic effects for an organism, such as leading to a non-functional enzyme, like lactase. This can explain lactose intolerance. Not all mutations have a negative effect, some can lead to changes that result in the formation of new allele of a gene. Which under certain environmental situations, may provide advantageous survival and reproductive success to the organism. For example, if a mutation causes an allele for resistance against a certain disease, the individuals with this allele will more likely survive and reproduce. So, they will pass on their advantageous allele to their offspring. This is important, as mutations can be the driving force of natural selection and important for species becoming more well adapted to their environmental settings, helping to maintain greater biodiversity.
As well as mutations, genetic variation can be caused by processes in meiosis called independent segregation. Each homologous pair of chromosomes in your cells is made of one of your paternal chromosomes and one of your maternal chromosomes. The arrangement of the paternal and maternal homologous pairs of chromosomes during metaphase I have a random chance of being segregated into either daughter cell. Therefore, when the daughter cells divide, there will be a higher combination of different chromosomes in the gametes, after meiosis II. This leads to greater genetic variation in the offspring. By increasing the variation of different alleles in each gamete, when random fusion of gametes during sexual reproduction occurs, this means the zygote will have an even larger variety of chromosome combinations of different alleles. This is important as it helps us to understand mendelian inheritance and inheritance of certain diseases from our parents due to specific alleles and certain phenotypes of our parents. It is also important to organisms because it enables a greater variety of alleles to be passed on which increases the size of the gene pool. This increases a species ability to resist extinction from disease and environmental changes because it is likely that one member of the population will have the allele that is resistance to being affected by the disease or able to survive in that specific selective pressure. Wider variety of different alleles enables species to recognise members of the opposite sex and same species, this helps in successful courtship interactions and formation of a pair bond, and can also help to prevent inbreeding, which could further reduce the gene pool, and lead to increased risk of heritable diseases such as heart defects.
Like mitosis, genetic diversity can also be increased by environmental changes. For example, if populations become separated by a physical barrier, such as a lake, or a volcano. This can lead to species becoming geographically isolated, and so prevent any interbreeding or gene flow. This is called allopatric speciation. When this happens, different environmental selective pressures favour different allele which are advantageous. For example, if one leopard is separated and lives in a colder region those with alleles for thicker fur, are more likely to survive. So, these advantageous alleles are passed on to offspring. Over many generations, the allelic frequencies between the two populations becomes so genetically different, that if made to reproduce again they would not produce viable or fertile offspring. So, this process leads to formation of new species, and this increases biodiversity and species richness of habitats. This is important to living organisms because often intraspecific competition for abiotic and biotic resources is an issue for same species members. So, by increasing the number of different species it enables more different species to live together as they are not necessarily competing for the same resources, like food, mate, space, and shelter. This is how communities such as the amazon rainforest exist, as a variety of different species can all live in one area at the same time due to having unique niches and different needs.
As well as environmental factors, heritable changes in the expression of genes, but not the sequence can cause changes to the phenotypic variation and biodiversity of living organisms. These heritable changes are known as epigenetics. An example of an epigenetic mark is hyper methylation of genes. The entire range of epigenetic marks in a person cell is called the epigenome. Depending on what epigenetics a person has, will precis how their genes are expressed or not. For example, hypermethylation for genes in the hippocampus in rats was found to be linked to development of epilepsy. Similarly, hypermethylation of beta cells produced in the islets of Langerhans have led to non-functional beta cells leading to type one diabetes. When a gene is hypermethylated, this causes it to not be transcribed as transcriptional factors cannot bind to the promotor to activate RNA polymerase. Although epigenetics can have negative effects for living organisms. Epigenetic may also help to increase genetic diversity as the difference in gene expression led to different alleles being expressed in the phenotype, and this can lead to differential reproductive and survival success and leading to changes in allelic frequency and leading even wider gene pools. This is also important as it can help for medical treatment to be devised to reduce the negative effects of epigenetic marks on a living organism, and so epigenetics have helped in genetic screening for certain heritable diseases like cancer and this can be beneficial to genetic counselling.
So genetic diversity is a particularly important process to ensure that biodiversity is maintained and help to maintain imperative interactions between different species.