Mutation and Evolution
Mutations and natural selection cause adaptations. Mutations can cause instant adaptations, while natural selection is the process by which adaptations occurs. Understand the connection between genetics and evolution; Understand how environmental changes and selective pressures impact the spread of mutations, . Mutation: A mutation is a change in DNA. Various agents Biology: What are the differences between genetic mutations, evolution and natural selection? 1, Views . What is the relationship between mutation and adaptation? Views.
According to their magnitude mutations can occur at different levelsthey can be divided into three different groups: Gene mutations, chromosome mutations and genome mutations.
The DNA is constantly subject to mutations, thus its sequence can be altered in several different ways. A gene mutation can be defined as any change in the sequence of nucleotides of the genetic material of an organism. A chromosome mutation is a change in the structure or arrangement of the chromosomes.
These mutations can involve duplications or deletions of chromosome segments, inversions of sections of DNA reversed positions and translocation. Genome mutations are alterations in the number of chromosomes in the genome. They can be classified into two groups: US Department of Energy Chromosome: A thread-like structure found in the nucleus of living cells that carries genetic information in the form of genes.
Chromosomes come in pairs. A unit of hereditary information consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic of an organism. Every person has two copies of each gene, one inherited from each parent. Different versions of the same gene are called alleles.
They determine how physical traits are expressed in a given individual. A long double-stranded nucleic acid molecule arranged as a double helix and joined by hydrogen bonds between the complementary bases: Adenine and Thymine or Cytosine and Guanine. It is the main constituent of chromosomes and it is the responsible for the transmission of hereditary characteristics from parents to offspring. The sequence of nucleotides determines individual hereditary characteristics.
The complete genetic material of an organism which is contained in the chromosomes, including the genes and DNA sequences. Viruses, despite not being considered living entities are also subject to evolutionary pressures, mutations and natural selection. The real problem when assigning this classification to viruses comes from the fact that they do not have a cellular structure and they are obligate intracellular parasites. Viruses do not possess a cell membrane in the sense of a phospholipid bilayer or metabolize on their own and they need to hijack the machinery of the cell to reproduce.
Viruses cannot reproduce outside a living cell. Their real position in the tree of life continues to generate controversy in the scientific community as they move in a thin boundary line between the living and the nonliving. Viruses have a simple genome that contains the genetic information encoded in either DNA or RNA and also high replication and mutation rates.
Mutation and Evolution
Influenza viruses, for example, are RNA viruses belonging to the family Orthomyxoviridae. There are three types of flu viruses: A, B, and C. Influenza type A viruses can eventually cause severe illness and are the unique responsible for Influenza human pandemics. They are subdivided into differente subtypes based on the two antigenic proteins on the surface of the virus: These viruses are formed by eight single-stranded RNA segments and have a very high rate of mutation the error rate of the viral RNA-polymerase is times higher than the error rate of the human DNA- polymerase.
Thus, replication of RNA viral genomes is error-prone and leads to several mutations on which natural selection can act. The high mutation rates, the existence of large population sizes and short generation times are the fundamental features that have contributed to the rapid evolution of viruses. Interesting Review on Viral Evolution. These minor changes are produced by the accumulation of spontaneous mutations in genes encoding these surface proteins. Drift is an ongoing process that conducts to the emergence of new virus strains.
When these minor changes occur, antibodies only provide a limited protection or partial immunity against the new strain, basically due to older exposures. This is the reason why people get flu more than on time throughout their lives, since the antibodies previously produced against the older strains no longer recognize the new virus strain and as a consequence, they are not effective anymore in fighting and controlling the infection. For this specific reason, flu vaccines have to be modified every year based on the strains that are supposed to circulate each season.
On the other hand, Antigenic shift is the process according to which at least two different strains of a flu virus are combined, giving rise to a new subtype with a new genetic material. In other words, it is a result of recombination events between different viral strains that infect the same cell and combine.
The surface viral proteins are replaced by significantly different HA and NA. These radical changes lead to the emergence of new Influenza A virus subtype that has never circulated among the population and thus, most individuals are not immune against it. If this new virus were able to cause severe illness and could spread easily from one person to another through various regions or continents, then a flu pandemic would occur.
While influenza viruses are changing all the time by the antigenic drift mechanism, antigenic shift only occurs occasionally. The introduction of new alleles increases variability within a population and allows for new combinations of traits.
Relationship Between Genetic Mutation and Evolution
Horizontal gene transfer HGT also known as lateral gene transfer LGTis a process in which an organism recipient acquires genetic material from another one donor by asexual means. It is already known that HGT has played a major role in the evolution of many organisms like bacteria. In plant populations, the great majority of cases linked to this mechanism have to do with the movement of DNA between mitochondrial genomes.
Horizontal gene transfer is a widespread phenomenon in prokaryotes, but the prevalence and implications of this mechanism in the evolution of multicellular eukaryotes is still unclear. Nevertheless, many investigations on HGT in plants have been carried out during the last years trying to reveal the underlying patterns, magnitude and importance of this mechanism in plant populations as well as its influence on agriculture and the ecosystem.
Plant populations can experience gene flow by spreading their pollen long distances away to other populations by means of wind or through birds or insects bees, for example and once there, this pollen is able to fertilize the plants where it ended up. Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes comparable to sperm cells.
Of course, pollination does not always lead to fertilization. Maintained gene flow also acts against speciation by recombining the gene pools of different populations and in such a way, repairing the developing differences in genetic variation. Thus, gene flow has the effect of minimizing the genetic differences between populations. Gene duplication in a diploid organism provides a second pair of genes so that one pair can be safely mutated and tested in various combinations while the essential functions of the parent pair are kept intact.
- Large Changes in Phenotype Can Come from Small Changes in Genotype
Over time, one of the duplicates can acquire a new function. This can provide the basis for adaptive evolution. But even while two paralogous genes are still similar in sequence and function, their existence provides redundancy "belt and suspenders".
This may be a major reason why knocking out genes in yeast, " knockout mice ", etc. The function of the knocked out gene can be taken over by a paralog. After gene duplication, random loss of these genes at a later time in one group of descendants different from the loss in another group could provide a barrier a "post-zygotic isolating mechanism" to their interbreeding.
Such a barrier could cause speciation: Genes in one species that have arisen by duplication of an ancestral gene. Duplication of the entire genome. Genome analysis of three ascomycetes show that early in the evolution of the budding yeast, Saccharomyces cerevisiaeits entire genome was duplicated. Each chromosome of the other ascomycetes contains stretches of genes whose orthologs are distributed over two Saccharomyces cerevisiae chromosomes.
There is also evidence that vertebrate evolution has involved at least two duplications of the entire genome. Mutations in Regulatory Regions Not all genes are expressed in all cells.
In which cells and when a given gene will be expressed is controlled by the interaction of: A mutation that would be lethal in the protein coding region of a gene need not be if it occurs in a control region e. In fact, there is increasing evidence that mutations in control regions have played an important part in evolution.
Humans have a gene LCT encoding lactase ; the enzyme that digests lactose e. In most of the world's people, LCT is active in young children but is turned off in adults.