Rate of Evolutionary Change
Rates of evolution change vary widely over time, among characteristics, and among species. Evolutionary change can be estimated by examining fossils and species that are related to each other. The rate of change is governed by the life span of the species under examination, short-lived species are capable of changing more quickly than those that have a longer life span and reproduce less often. Yet, even short-lived species such as bacteria, which have generation times measured in minutes, do not manifest noticeable evolutionary changes in a humans lifetime.
One technique that has been used to examine the rate of evolutionary change is DNA analysis. This technology involves identifying the percentage of similarity between samples of DNA from two related organisms under study. The greater the similarity, the more recently the organisms are considered to have diverged from a common ancestor. The information that is obtained in this manner is compared to information obtained from other sources such as the fossil records and studies in comparative anatomy.
There are two competing hypotheses designed to explain the rate of evolutionary change. One is called the punctuated equilibrium hypotheses. This hypothesis states that there are periods of time in which the rate of evolutionary change is slow. These periods are interspersed with periods of rapid change. Rapid change usually prevails with the early establishment of a species, during which time the organism has not yet development environmental adaptation. For example, if a food source eventually becomes unavailable to a particular species, the rate of evolutionary change will increase rapidly. In this way, new characteristics will evolve so that the species can utilize a different food source or the species will become extinct. Once this trait is common within the species, the rate of evolutionary change will slow down. An example of punctuate equilibrium occurred millions of years ago when aquatics species inhabited the emerging land spaces. These species rapidly developed characteristics permitting life as land dwellers. Once these characteristics were developed and common among these species, the rate of evolutionary change slowed.
Another hypothesis is the gradual change hypothesis. This explanation of the evolutionary rate of change states that species evolve slowly over time. In this hypothesis, the rate of change is slow and species that do not change quickly enough to develop traits enabling them to survive will die. Although some species such as the sequoia (redwoods) or crocodiles have maintained distinct and similar characteristics over millions of years, some species such as the cichlids in the African rift lakes have rapidly change in appearance over thousands of years. These examples of slow changing species involve organisms that have an arrested rate of evolutionary change, but at one time, most likely endured a period of rapid evolutionary change resulting in tolerance to environmental changes.
There are several limiting factors that might control the evolutionary rate of change. Physical or biological changes influencing the environment can play a major role in the rate of evolutionary change. Another example is the mutation rate. The mutation rate in various species at different times can also limit the rate of evolutionary change such that a higher mutation rate correlates to a higher rate of change, assuming the environmental changes remain unchanged. Mutations do not seem to have major effects on limiting evolution because diversity in morphological evolution (evolution of physical characteristics) does not correlate well with DNA mutation rates. However, in some cases, evolution rates can depend on mutation rates. A good example is antibiotic resistance. Bacterial mutation rates can induce changes in the ability to become resistance to antibiotics.
If certain characteristic are more efficiently selected against in one species compared with a different species, then the rates of evolutionary change will vary between them. Selective pressures are greater in larger populations. Therefore, small populations might not be able to evolve rapidly enough in a rapidly changing environment. In this scenario, inefficient natural selection will be a limiting factor in the rate of evolution. Finally, constraints that occur when a mutation in a gene produces a beneficial characteristic for the species but impedes the function of other gene products can be a limiting factor on the rate of evolution. These architectural constraints can be bypassed if gene or whole genomes are duplicated. When this occurs, the extra genes can compensate for the negative effects on gene function imposed by the beneficial new function from the gene that is mutated. In a sense, these extra genes will speed up the rate of evolutionary change.
Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.
Milligan, B.G. Estimating Evolutionary Rates for Discrete Characters. Clarendon Press; Oxford, England, 1994.
Ridley, Mark. Evolution. Cambridge, MA: Blackwell Scientific Publications, 1993.