Biological Evolution
Biological Evolution Biological evolution is the change in the allele frequency of a gene in a population over time. That is to say some genetic change has happened in the population between generations. Only populations can evolve, not individuals. Individuals can not change their genetic makeup. Only between generations, is there the possibility for genetic changes due to the forces of evolution. These forces are natural selection, mutation, gene flow, nonrandom mating, and genetic drift. Evolution is a measure of a population, not of an individual. Genetic variation, genetic differences between individuals, must exist for evolution to occur.
Charles Darwin defined evolution as descent with modification. However, Darwin did not understand the genetic basis to evolution. Not until Gregor Mendel’s work was rediscovered in 1900 could modification with descent be understood in terms of maintaining genetic variation. The mathematical proofs of Godfrey Harold Hardy and Wilhelm Weinberg, known as the Hardy-Weinberg theorem, started the field of population genetics, the integration of Darwinian selection and Mendelian genetics. Their proof showed how variation can be maintained because each individual had two alleles for each gene. This is in contrast to Darwin, who specified a kind of blending inheritance in which offspring were intermediate to the parents. Just as importantly, their work specified the forces (causes) of evolution. Population genetics is the foundation for modern evolutionary biology. Other population geneticists, such as Ronald A. Fisher, John B. S. Haldane, and Sewall Wright, contributed to the foundations of the theory of population genetics from the 1920s to the 1940s.
All animals are the descendants of a single common ancestor. Biological evolution has created the diversity of organisms we see today, as well as extinct animals such as dinosaurs for which we have the fossil record. The diversifying action of evolution to create new species is called speciation. Speciation is the splitting of one former species into two species that are reproductively isolated from each other such that they no longer successfully reproduce and exchange genes. Speciation is the result of a combination of biogeography, natural selection, adaptations, and the other evolutionary forces.
There are two main modes of speciation: allopatric and sympatric. Allopatric speciation is the division of one population into two populations because of some geographical barrier. While separated, each population evolves differently from the other population. When contact is restored between the two populations, they cannot reproduce, and so are unable to exchange genes because of the differences they acquired while separated. Sympatric speciation is when one population splits into two without any geographical barrier. While this mode of speciation was doubted for years, in the 1960s Guy Bush conducted experiments on fruit flies that supported this mode of speciation. In the early 1980s, Bill Rice conducted laboratory experiments in which he was able to cause sympatric speciation. Even though sympatric speciation is possible, it is not as common as allopatric speciation.
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Causes of Biological Evolution
There are five forces that cause evolution: natural selection, mutation, gene flow, nonrandom mating, and genetic drift. All five depend on the existence of genetic variation, which is necessary for any evolutionary change. Natural selection is the differences in the survival and reproduction rates of individuals with different phenotypes. When phenotypes can be genetically inherited, natural selection produces adaptations as the population evolves. Natural selection can remove variation from a population if it is stabilizing selection. Diversifying selection can increase the amount of variation in a population. Directional selection changes the average trait in the population.
Genetic mutations occur when errors are made in replicating (copying) and dividing DNA. Mutation is the ultimate source of genetic variation. Most of the time, mutations either have no effect on the phenotype, and therefore are neutral, or have a harmful effect. Rarely, a mutation will create a phenotype that is better, and so natural selection will favor this beneficial mutation. Mutations happen naturally at low levels of frequency. These levels can be much higher under some conditions. For example, exposure to radiation and to some toxic chemicals produces higher mutation rates.
Gene flow is the exchange of genes among populations or species. The exchange of genes between species is called hybridization. Introducing new genes into a population changes the gene frequency and causes evolution. Gene flow can be positive or negative for a population. Lots of gene flow can prevent local adaptation because any evolution produced by natural selection is swamped by the invading genes. On the other hand, gene flow can introduce a new beneficial gene into a population. Natural selection can favor this new adaptation, and it can spread through the population.
Nonrandom mating changes what combinations of genes are mixed together in sexual reproduction. Sexual reproduction creates new individuals, half of whose genetic information comes from the mother and half from the father. If individuals within a population who have a particular genotype pair off and mate at a rate different from the occurrence of that gentotype in the population, then nonrandom mating is occurring. Nonrandom mating can be caused by mating among close relatives, or inbreeding, which can result from population subdivision. Nonrandom mating also happens when individuals choose mates based on particular phenotypes. In some animal species, a few males get most of the matings because they have some highly desirable phenotype. Assortative mating also produces nonrandom mating, which is the mating of males and females of the same phenotype. For example, large male frogs mate with large female frogs and small males mate with small females.
Genetic drift causes evolution by random changes in the allele frequencies. One way for genetic drift to happen is for some of the alleles to be left after some kind of fluctuations in population size. For example, if disease wipes out most of a population, only some alleles will be left in the population. Also, only some combinations of alleles for different genes will be left. If natural selection is not acting on a gene, then random genetic drift can be a stronger force than if selection is present. The impact of drift depends on the population size. Drift is stronger in smaller populations; they are more susceptible to random changes in allele frequencies since there are not as many alleles present.
Limits to Biological Evolution
What can limit evolution? Three main factors restrict the amount of change evolution can make in a population: the degree of genetic variation is limited; natural selection produces adaptations that are a compromise in form and function; and most forces of evolution are not adaptive.
First, genetic variation is the ultimate barrier to evolution. If there is no genetic variation, no evolution can happen. Genetic variation is limited to the history of the organism. A bear will not suddenly gain wings in a few generations of evolution. No bear has ever had wings, and it is unlikely that any bear will evolve them. An organism contains only so much DNA and the amount of existing genetic variation, the raw material for evolution, is restricted by the past history of the species. A bear does not have the underlying genetic variation necessary for a mutation to produce wings from the existing variation.
Second, adaptations are usually compromises and therefore limit evolution. Natural selection works on a whole organism rather than just single traits, so it is the combination of traits that natural selection favors. A cheetah is a fast runner but a poor swimmer. Any cheetah with webbed feet would be a better swimmer but could certainly not run as fast. Adaptations are trade-offs.
Third, many forces of evolution are not adaptive. Natural selection is the force of evolution that produces adaptations but the other forces of evolution are not necessarily adaptive. Gene flow can introduce genes into a population that are better suited to another environment. Nonrandom mating can break up existing combinations of genes that work well together. Mutation is typically harmful. Random genetic drift is frequently not beneficial. Most forces of evolution are random and can be working counter to natural selection.
Rates of Evolution
Does evolution proceed at a fast pace or a slow pace? How much of evolution can we actually observe? In 1972 Niles Eldredge and Stephen J. Gould wrote an article that presented the idea of punctuated equilibrium. Some organisms for which there are good fossil records show long periods of no morphological evolution (evolution in the form and structure of organisms); the animals remain unchanged over thousands of years. But then there suddenly appears what looks like a morphologically similar new species. The theory of punctuated equilibrium is that long periods of no change are followed by short periods of rapid transition. This is in direct contrast to gradualism. Gradualism suggests slow but continuous change over geological time. How is one to know if the fossil record is incomplete, and that the seemingly rapid change is accounted for by missing intermediate stages?
This question has inspired research on the rate of evolutionary change. It is possible to calculate rates of morphological evolution from the fossil record. Evolutionary rates can be measured over several generations in natural and laboratory populations. It is also possible to measure the relative rate of change in molecules for which the gene sequence is known. The sequence of a gene is the order of nucleotides within it. Sexual reproduction can also increase the rate of evolution compared to asexual reproduction. This is due to increased genetic variation by recombination and independent assortment. Gene sequencing has made it possible to investigate how the rate of evolution changes with the degree of underlying genetic variation, also called genetic polymorphism. In 1991 the first important test of rates of molecular evolution and molecular polymorphism was conducted by J. H. McDonald and Martin Kreitman. As the entire genetic material (genomes) of more and more organisms are sequenced, we will understand more about the rate and mechanisms of evolution. SEE ALSO Adaptation; Genes; Genetics; Natural Selection.
