There are many mechanisms that lead to evolutionary change. One of the most important mechanism in evolution is natural selection which is the differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. Natural selection occurs when a environment makes a individual adapt to that certain environment by variations that arise by mutation and genetic recombination. Also it favors certain traits in a individual than other traits so that these favored traits will be presented in the next generation.
Another mechanism of evolution is genetic drift. Genetic drift is a random change in a small gene pool due to sampling errors in propagation of alleles or chance. Genetic drift depends greatly on the size of the gene pool. If the gene pool is large, the better it will represent the gene pool of the previous generation. If it is small, its gene pool may not be accurately represented in the next generation due to sampling error. Genetic drift usually occurs in small populations that contain less than 100 individuals, but in large populations drift may have no ignificant effect on the population.
Another mechanism is gene flow which is when a population may gain or lose alleles by the migration of fertile individuals between populations. This may cause the allele frequencies in a gene pool to change and allow the organism to evolve. The most obvious mechanism would have to be mutation that arises in the gene pool of a population or individual. It is also the original source of the genetic variation that serves as raw material for natural selection.
Not only are there mechanisms of evolution, but there is also evidence o prove that these mechanisms are valid and have helped create the genetic variety of species that exists today. Antibiotic resistance in bacteria is one example of evolutionary evidence. In the 1950’s, Japanese physicians realized that a antibiotic given to patients who had a infection that caused severe diarrhea was not responding. Many years later, scientists found out that a certain strain of bacteria called Shigella contained the specific gene that conferred antibiotic resistance.
Some bacteria had genes that coded for enzymes hat specifically destroyed certain antibiotics such as ampicillin. From this incident, scientists were able to deduce that natural selection helped the bacteria to inherit the genes for antibiotic resistance. Scientists have also been able to use biochemistry as a source of evidence. The comparison of genes of two species is the most direct measure of common inheritance from shared ancestors. Using DNA-DNA hybridization, whole genomes can be compared by measuring the extent of hydrogen bonding between single-stranded DNA obtained from two sources.
The similarity of the two genes can be seen by how tightly the DNA of one specie bonds to the DNA of the other specie. Many taxonomic debates have been answered using this method such as whether flamingos are more closely related to storks or geese. This method compared the DNA of the flamingo to be more closely related to the DNA of the stork than the geese. The only disadvantage of this method is that it does not give precise information about the matchup in specific nucleotide sequences of the DNA which restriction mapping does.
This technique uses restriction enzymes that recognizes a specific sequence of a few nucleotides and cleaves DNA wherever such sequences are found in the genome. Then the DNA fragments are separated by electrophoresis and compared to the other DNA fragments of the other species. This technique has been used to compare mtDNA from people of several different ethnicity’s to find out that the human species originated from Africa. The most precise and powerful method for comparing DNA from two species is DNA sequencing which determines the nucleotide sequences of entire DNA egments that have been cloned by recombinant DNA techniques.
This type of comparison tells us exactly how much divergence there has been in the evolution of two genes derived from the same ancestral gene. In 1990, a team of researchers used PCR(polymerase chain reaction) a new technique to compare a short piece of ancient DNA to homologous DNA from a certain plant. Scientists have also compared the proteins between different species such as in bats and dolphins. The oldest type of evidence has been the fossil record which are the historical documents of biology.
They are preserved remnants found in sedimentary rocks and are preserved by a process called pretrification. To compare fossils the ages must be determined first by relative dating. Fossils are preserved in strata, rock forms in layers that have different periods of sedimentation which occurs in intervals when the sea level changes. Since each fossils has a different period of sedimentation it is possible to find the age of the fossil. Geologists have also established a time scale with a consistent sequence of geological periods. These periods are: the Precambrian, Paleozoic,
Mesozoic and the Cenozoic eras. With this time scale, geologists have been able to deduce which fossils belong in what time scale and determine if a certain specie evolved from another specie. Radioactive dating is the best method for determining the age of rocks and fossils on a scale of absolute time. All fossils contain isotopes of elements that accumulated in the organisms when they were alive. By determining an isotope’s half-life which is the number of years it takes for 50% of the original sample to decay, it is possible to determine the fossil’s age.