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evolution
A change in the gene pool of a population over time. Evolution is not always discrete with clearly defined boundaries that pinpoint the origin of a new species, nor is it a steady continuum. Evolution requires genetic variation which results from changes within a gene pool, the genetic make-up of a specific population. A gene pool is the combination of all the alleles – alternative forms of a genetic locus – for all traits that population may exhibit. Changes in a gene pool can result from mutation – variation within a particular gene – or from changes in gene frequency – the proportion of an allele in a given population.
How genetic variation occurs
Every organism possesses a genome that contains all of the biological information needed to construct and maintain a living example of that organism. The biological information contained in a genome is encoded in the nucleotide sequence of its DNA or RNA molecules and is divided into discrete units called genes. The information stored in a gene is read by proteins, which attach to the genome and initiate a series of reactions called gene expression.
Every time a cell divides, it must make a complete copy of its genome, a process called DNA replication. DNA replication must be extremely accurate to avoid introducing mutations, or changes in the nucleotide sequence of a short region of the genome. Inevitably, some mutations do occur, usually in one of two ways; either from errors in DNA replication or from damaging effects of chemical agents or radiation that react with DNA and change the structure of individual nucleotides. Many of these mutations result in a change that has no effect on the functioning of the genome, referred to as silent mutations. Silent mutations include virtually all changes that happen in the non-coding components of genes and gene-related sequences.
Mutations in the coding regions of genes are much more important. Here we must consider the importance of the same mutation in a somatic cell compared with a germ line cell. A somatic cell is any cell of an organism other than a reproductive cell, such as a sperm or egg cell. A germ cell line is any line of cells that gives rise to gametes and is continuous through the generations. Because a somatic cell does not pass on copies of its genome to the next generation, a somatic cell mutation is important only for the organism in which it occurs and has no potential evolutionary impact. In fact, most somatic mutations have no significant effect because there are many other identical cells in the same tissue.
On the other hand, mutations in germ cells can be transmitted to the next generation and will then be present in all of the cells of an individual who inherits that mutation. Even still, mutations within germ line cells may not change the phenotype of the organism in any significant way. Those mutations that do have an evolutionary effect can be divided into two categories, loss-of-function mutations and gain-of-function mutations. A loss-of-function mutation results in reduced or abolished protein function. Gain-of-function mutations, which are much less common, confer an abnormal activity on a protein.
Drivers of evolution: selection, drift, and founder effects
We just discussed that new alleles appear in a population because of mutations that occur in the reproductive cells of an organism. This means that many genes are polymorphic, that is, two or more alleles for that gene are present in a population. Each of these alleles has its own allele or gene frequency, a measure of how common an allele is in a population. Allele frequencies vary over time because of two conditions, natural selection and random drift. Another important cause of genetic drift is the founder effect, the difference between the gene pool of a population as a whole and that of a newly isolated population of the same species.
Related category
• ORIGIN AND EVOLUTION OF LIFE
Source: National Center for Biotechnology Information
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