Evolution - Alleles and Variation
A mutation is any change to the quantity or structure of an organism's DNA. A mutation that changes the sequence of bases in gene results in a different form of gene called and allele. If the mutation occurs in a gamete, the allele will be passed on to the offs[ring. The phenotypes of the individuals that inherit this allele may be changed, adding to the phenotyipc variation within the population. It the individuals that inherit the allele are more like to survive and reproduce that other individuals without the allele, then after several generations the frequency of that allele will increase within the population. Evolution involves a change in the allele frequency in a population.
Within a population the phenotypic variation that exists is the result of genetic factors (e.g. presence of certain alleles, which would have originally arisen in the population by mutation), environmental factors (e.g. food availability) or a combination of both. In certain environments, individuals that inherit a mutant allele may be at a selective advantage. This means they are more likely to survive, and produce offspring that also carry the mutant allele, than individuals without the mutant allele. This leads to an increase in the frequency of the mutant allele in the population - the population in evolving.
Evolution - Selection
Living things produce more offspring than their environment can support. When large numbers of the offspring are produced within a population, intraspecific competition occurs. Phenotypic variation within a population will mean that some of the offspring will die or will not reproduce. This may be because they are more susceptible to disease, less able to escape predators, less able to find a mate or unable to obtain sufficient food. As a result, the offspring best suited to their environment are most likely to survive, reproduce and pass their alleles to the next generation. this is the basis of natural selection, and the survivors are said to have a selective advantage.
Evolution - Selection
Selection and resistance to antibiotics in bacteria populations:
Many bacteria carry genes for resistance to certain antibiotics on plasmids. The bacteria vary in their susceptibility to a particular antibiotic. The most susceptible bacteria die first when the population is exposed to the antibiotic. Gradually, the proportion of more resistant bacteria in the population increases. The bacteria that can survive exposure to the antibiotic are able to reproduce and pass on their genes for resistance to their offspring. The antibiotic is exerting a selection pressure on the bacterial population and the frequency of the gene for resistance to it is increasing within the population. If a person completes their course of prescribed antibiotics, the chance of any bacteria surviving is small. Their immune system can destroy the remaining bacteria. However, if a person stops the antibiotic treatment early, a high proportion of the surviving bacteria will be resistant. Anyone who becomes infected with these more resistant bacteria will need stronger doses of that antibiotic, or another type of antibiotic, to combat the infection. If incomplete antibiotic treatment is repeated often enough, then after many generations, all of the bacteria in the population will have evolved complete antibiotic resistance (e.g. MRSA). In the absence of antibiotics, the proportion of resistance genes in the population tends to decrease to a low level. This is because within the population, the resistant bacteria tend to be at a disadvantage relative to those bacteria that are not resistant to antibiotics.
Evolution - Isolation and the Formation of New Spe
Speciation is the evolution of a new species from an existing on. New species may arise in two different ways:
- Individuals from two different species cross fertilise and produce a fertile hybrid.
- Groups within the population become isolated from one another and cannot interbreed. This is called reproductive isolation. Gene flow amongst the separate groups is prevented and the population's single gene pool is now split into several separate gene pools. Allele frequencies within each gene pool my change due to selection pressures acting on phenotypes of the organisms may be different.The separate gene pools may become so different from each other that successful interbreeding of the separate groups is no longer possible. Each group is now a different species with its own gene pool. This type of speciation has two main forms: allopatric speciation and sympatric speciation.
Evolution - Allopatric and Sympatric Speciation
Allopatric speciation occurs when populations are prevented from interbreeding because they become geographically isolated.
Sympatric speciation occurs when populations living together become reproductively isolated. This isolation may take place:
- before mating occurs, thus preventing the exchange of gametes (prezygotic mechanisms - geographical, ecological, temporal, behavioural, mechanical)
- after mating has taken place, thus preventing the development of the zygotes into offspring (postzygotic mechanisms - gametic, hybrid sterility, hybrid inviability, hybrid breakdown)
Human Activities and Selection
Human activity is having a major effect on most of the ecosystems found on Earth. Populations living within these ecosystems are exposed to altered selection pressures as a result of human activity and this leads to differing survival and reproduction within the populations. This in turn affects the frequency of alleles within the gene pools of these populations. Humans are therefore affecting the evolution of populations and species.
For example, research has shown that the great **** living in rural and city environments have differences in behaviour. The noise in the city can cover up bird songs. These songs are vital for communication and attracting mates. Great **** have responded to this by increasing the frequency and speed of their songs. This means their song can be heard over the low frequency noise of their environment.