Old populations have more neutral alleles than new populations when N e is equal. Thus the center of gene diversity for a species is most often also the center of origin for a species. Plants and pathogens have coevolved for the longest time at the center of coevolution, leading to selection for a diversity of resistance alleles in the plant population. This is why plant breeders seek resistant germplasm at centers of diversity. If the pathogen coevolved with its plant host at the center of origin, we predict that the pathogen population also will exhibit maximum diversity at the center of origin.
Mutation plays an important role in evolution. The ultimate source of all genetic variation is mutation. Mutation is important as the first step of evolution because it creates a new DNA sequence for a particular gene, creating a new allele.
Recombination also can create a new DNA sequence a new allele for a specific gene through intragenic recombination. Mutation acting as an evolutionary force by itself has the potential to cause significant changes in allele frequencies over very long periods of time.
But if mutation were the only force acting on pathogen populations, then evolution would occur at a rate that we could not observe. In plant pathology, we are most often concerned with mutations that affect pathogen virulence or sensitivity to fungicides or antibiotics. In pathogens that show a gene-for-gene interaction with plants, we are especially interested in the mutation from avirulence to virulence because this is the mutation that leads to a loss of genetic resistance in both agroecosystems and natural ecosystems.
But mutations from fungicide sensitivity to fungicide resistance also are important in agroecosytems, as are any mutations that affect fitness. To demonstrate how mutation can lead to changes in allele frequencies, let's consider a simple model of mutation. Assume that we have two alleles at a single locus, call them A 1 and A 2 , where A 1 can mutate to become A 2 , and A 2 can undergo the reverse mutation to become A 1.
Let A 1 mutate to A 2 at a frequency of u per generation. We will call u the forward mutation rate. Let A 2 mutate to A 1 at a frequency of v per generation.
We will call v the backward mutation rate. Let the frequency of allele A 1 be p t at time t in the population, and let the frequency of allele A 2 be q t at time t. In every generation, a proportion of the A 1 alleles will mutate to A 2 alleles. This proportion will be the forward mutation rate u times the frequency of allele A 1 p , up.
This can be a bad or a good thing. Mutations can occur during DNA replication if errors are made and not corrected in time. Mutations can also occur as the result of exposure to environmental factors such as smoking, sunlight and radiation. Often cells can recognise any potentially mutation-causing damage and repair it before it becomes a fixed mutation.
Mutations contribute to genetic variation within species. Mutations can also be inherited, particularly if they have a positive effect. For example, the disorder sickle cell anaemia is caused by a mutation in the gene that instructs the building of a protein called haemoglobin. This causes the red blood cells to become an abnormal, rigid, sickle shape. However, in African populations, having this mutation also protects against Plasmodium.
In moving from one location to another they can cause mutations. If the element landed in the middle of the coding sequence of a gene, it most likely would lead to a frameshift mutation or introduce a stop codon , and knock out the function of that gene. Gene duplications can occur by unequal crossing over where gene families exist on the chromosome, homologous chromosomes may misalign and cross over recombine.
The daughter chromosomes include one with an extra copy and one with one fewer copies. Chromosome rearrangements can also be viewed as mutations. Classic cases: inversions were a section of the chromosome is inverted with respect to the "normal" chromosome.
Drosophila polytene chromosomes show characteristic banding patterns and allow for easy recognition of inversions. A paradigm of natural selection more later. An inversion does not prevent crossing over per se but the recombination products that result from a crossover within the inversion either have two centromeres and are pulled apart in division, or lack a centromere and are not transmitted.
Only the unrecombined parental chromosomes are transmitted. How will the frequency of an inverted chromosome in a population affect it role as a suppressor of recombination? The more frequent the inverted type gets, it will be present in a "homokaryotypic" state and recombination will not be suppressed. When entire chromosome arms are translocated or fused this can lead to changes in chromosome number.
Can also lead to genetic incompatibilities that may lead to reproductive isolation more in lectures on speciation. Learn more. The information on this site should not be used as a substitute for professional medical care or advice.
Contact a health care provider if you have questions about your health. How can gene variants affect health and development? From Genetics Home Reference. Topics in the Variants and Health chapter What is a gene variant and how do variants occur? Do all gene variants affect health and development?
0コメント