Evolutionary Change, Rate of
Evolutionary Change, Rate of
Rates of evolution change vary widely, among characteristics, and among species. Evolutionary rate of change can be estimated by examining fossils and species that are related to each other. The rate of change is governed partly by the life span of the species under examination; species whose individuals have short life spans are generally 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, may 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 describe the rate of evolutionary change. One is called the punctuated equilibrium hypothesis. This states that there are periods of time in which evolutionary change is slow—periods of stasis or equilibrium. These are interrupted or punctuated by periods of rapid change. Rapid change may occur, for example, when a small population is isolated from a larger parent population. Most small populations die out, but those that survive may be able to evolve more quickly because new genes can spread more quickly in a small population. If the species recolonizes a wider range, its rate of evolutionary change will slows down— and the species appears to make an abrupt appearance in the fossil record, which is unlikely to preserve any given individual and so is weighted toward common species.
The other primary hypothesis is that of gradual change. This 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. On this view, the sudden appearance of most species in the fossil record is due to the extreme rarity of fossilization events. The fossil record is thus like a movie film with 99% of the frames cut out and only a random selection of moments retained.
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 mere thousands of years. Most evolutionary biologists today acknowledge that there is evidence that both gradual and sudden evolutionary change occur. The question is not which occurs, but which occurs more often, and why either dominates in any given case.
Several factors can influence evolutionary rate. For example, there is the mutation rate, the rate at which random changes in appear in a species’s DNA. Higher mutation rates enable faster evolutionary change, in principle. However, in the field mutation rates 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. Yet in some special cases, especially in microorganisms, evolution rates do depend on mutation rates. A good example is the rapid evolution of resistance to antiviral drugs by the human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS). HIV has a very high mutation rate and allowing any virus to survive a course of medications can allow survivors of them medication to evolve resistance. This is why it is important for persons with AIDs to take their medicine consistently.
Selective pressure can also influence evolutionary rate. Selective pressure can be imagined as the importance of a given feature in a given environment. If a certain species of bird is accustomed to using fine, narrow beaks to extract seeds from a certain bush for food, but a drought kills off many of those bushes while leaving another kind of bush with large, heavy-shelled seeds relatively intact, then there would be selective pressure for beaks to become heavier and shorter in this group. That is, birds who randomly happened to have bills better suited to eating the available food would have a better chance of eating well and reproducing, so each new generation of birds would be descended from these heavier-beaked individuals. This form of natural selective pressure has been recorded on a painstaking, bird-by-bird basis over years among Darwin’s finches in the Galápagos islands.
Scientists have discovered that some species of bacteria and yeast, including the primary bacterium of the human digestive system, Escherichia coli, can change their mutation rate in response to environmental stress. More stressful environments trigger mechanisms allowing more mutations, which enables faster evolution and therefore adaptation of the species to the changing environments. In effect, certain species turn up the speed control on their own evolution in response to environmental factors.
Milligan, B.G. Estimating Evolutionary Rates for Discrete Characters. Clarendon Press; Oxford, England, 1994.
Dronamraju, Krishna R. and Bruce J. MacFadden. “Fossil Horses and Rate of Evolution.” Science. 308 (2005): 1258.
Pagel, Mark, et al. “Large Punctuational Contribution of Speciation to Evolutionary Divergence at the Molecular Level.” Science. 314 (2006): 119-121.
Pawar, Samraat S. “Geographical Variation in the Rate of Evolution: Effect of Available Energy or Fluctuating Environment?” Evolution. 59 (2005): 234-237.
Rosenberg, Susan M. and P.J. Hastings. “Modulating Mutation Rates in the Wild.” Science. 300 (2003) 1382-1383.
Bryan Cobb, PhD
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