What is evolution?
Definitions of evolution
The word “evolution” may conjure up images of a fish crawling out of the ocean, or humans descending from apes. But the scientific definition of evolution both broad and simple.
Evolution means change. Here are three definitions from leading college-level textbooks on evolution:
Evolution has been defined as the change in the form and behavior of organisms between generations (1), and as a change over time of the proportions of individual organisms differing genetically in one or more traits (2). The first definition emphasizes changes at the organism level; the second describes changes in the proportions of organisms themselves. An even more simple definition is to describe evolution as changes in allele frequencies over time (3). An allele is a specific version of a gene. A population evolves as the proportion of any one allele increases or decreases, or as new alleles are introduced by spontaneous mutation. So here we have one phenomenon—evolution—described on three different levels of biological organization, from individuals to traits to genes. In fact, these subtle variations reflect how evolution is studied on different levels of biological organization.
What do these definitions have in common? Most importantly, evolution is changes in heritable characteristics within a population. For example, suppose a particular type of flower has either yellow or white petals, depending upon its genotype. Imagine a population of these flowers over two seasons: in season one, 50% of the individuals have yellow petals and 50% of the individuals have white petals; in season two, 60% have yellow petals and 40% have white petals. Between season one and season two, this population has evolved. In this example it is easy to see why evolution is most commonly described by population-level change. It doesn’t make sense to talk about the evolution of a single individual. If a single individual varies in some way over the course of its lifetime, it is not (generally speaking) due to alterations in genotype.
These fluctuations in the frequencies of petal color are an example of microevolution, evolution that takes place over relatively short periods of time. Of course, evolution occurs over very long periods of time as well. Scientists estimate that life on earth has been evolving for about 3.7 billion years. Evolution that occurs over relatively long periods of time is called macroevolution. The formation of new species is an example of a macroevolutionary process. If our yellow and white flowers eventually split into two different populations and cease to interbreed, speciation might occur. Speciation typically occurs on the order of tens of thousands to millions of years.
When speaking about macroevolutionary processes, such as speciation, confusing language can make it sound as though evolution of a single form has occurred. For example, a paleoanthropologist may describe an increase in skull size over time of an ancient hominid. Of course, this telescoping of terminology just means that the representatives from different populations through time show a trend of increasing skull size, indicating that average skull size got bigger as time went on. For most of us, anyway.
1. Ridley, Mark. Evolution, third edition. Blackwell Publishing: Malden, MA, 2004.
2. Futuyama, Douglas J. Evolutionary Biology, third edition. Sinauer Associates: Sunderland, MA, 1998.
3. Freeman, Scott and Jon C. Herron. Evolutionary Analysis, fourth edition. Pearson Prentice Hall: Upper Saddle River, NJ, 2007.
Is natural selection the same thing as evolution? No, but evolution can occur by natural selection.
The theory of evolution by natural selection, or “descent with modification,” was proposed by Charles Darwin and was a tremendous contribution to the discipline of biology. By Darwin’s time in the mid-19th century, many people had observed that organismal forms appeared to have changed over time; they had observed evidence of evolution. Several theories were put forth to explain these changes, and Darwin’s changed history.
The theory of natural selection is comprised of several logical steps, based on observation and inference:
- There is competition among individuals in a population.
- There is variation among individuals in a population.
- This variation is, at least in part, heritable.
- This variation contributes to fitness; fitter individuals will leave a larger contribution of offspring in the next generation.
- The succeeding generation will have an increased proportion of the traits that confer the higher fitness.
Is natural selection the only way evolution happens? No! It is often the most interesting way that organisms evolve, but evolution can also happen by other processes. Consider the yellow and white flower population in the preceding section. The petal color proportions could have changed between seasons because the yellow flowers were fitter, or simply because the white flowers were disproportionately killed by a snap frost, grazing animal, or overzealous gardener, irregardless of their color. Evolution by this type of random process is called genetic drift. Moreover, it is important to note that not all natural selection leads to evolution. Remember that evolution is defined as the change in gene frequencies over time. If new variants pop up in a population (due to mutation), but these variants are less fit, natural selection will select against them. Thus the existing gene frequencies will be stabilized (not changed) in the population. This is called stabilizing selection.
Extraordinarily complex adaptations, such as the vertebrate eye or a highly specialized orchid-pollinator morphology, are not credited to random processes. Quite the opposite: adaptations are by definition the result of natural selection, which is a specifically deterministic process. However, the underlying variation upon which natural selection acts is the result of spontaneous mutation, which does occur randomly. Because variants that are more fit leave more and fitter offspring, we can think of natural selection as a process that produces not a random assemblage of phenotypes, but a highly specified collection of the best-adapted organisms.
Yes, evolution is a fact. Evolution is simply a change in a population over time, and this is easily observed. We observe microevolution all the time: one important example is how pathogenic bacteria have evolved antibiotic resistance over the last several decades. Macroevolution cannot be observed in real time, but the fossil record demonstrates that organismal forms have changed over millions of years. Inferring exactly when and how evolution occurs is a difficult process, and scientists often disagree about the details. Thus precise phyletic relationships and the specifics of evolutionary events are continually debated, but scientists universally agree that all living things are descended from ancestors that inhabited the earth millions of years ago.
To scientists, the term “theory” does not imply speculation the way the word might in common usage. In science, a theory is an explanation. The scientific method tests theories through experimentation; by accumulation of facts, a theory becomes more robust. A theory never becomes a fact, only a stronger (or weaker) theory. The vast accumulation of facts and observations in evolutionary biology make the theory of natural selection a very robust theory.