Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift. Show Darwin’s grand idea of evolution by natural selection is relatively simple but often misunderstood. To see how it works, imagine a population of beetles:
If you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome. It is as simple as that.
Natural selection at work
Charles Darwin and Alfred Russel Wallace articulated the theory of evolution by natural selection without a modern understanding of genetics. But we have the advantage today of being able to discuss evolution with knowledge of genetics, so we’ll start with a brief primer on genetics. These basic genetic concepts are good to review to make sure we’re all on the same page:
Evolution by natural selection occurs when certain genotypes produce more offspring than other genotypes in response to the environment. It is a non-random change in allele frequencies from one generation to the next. In On the Origin of Species by Natural Selection (1859), Charles Darwin described four requirements for evolution by natural selection:
We can apply these postulates to consider whether evolution by NS is occurring in a specific case. Watch this TED Ed video of the evolution of antibiotic resistance in bacteria:
For bacterial evolution in the videos above, here’s how Darwin’s postulates apply:
This brings us to the idea of Darwinian fitness, that the organisms that best match their environment will have relatively greater survival and reproduction than those that match less well. Fitness is quantified relative to the average individual in the population; individuals that produce more viable progeny (progeny that can live and reproduce themselves) than average have greater fitness. A trait that is heritable and increases the survival and reproduction odds for those that carry that trait is called an adaptation. If a trait confers 1% greater reproductive advantage, it confers a fitness of 1.01. A trait that confers 10% greater reproductive advantage has a fitness of 1.1. Of all the mechanisms of evolution we’ll discuss in this course, only natural selection results in adaptations. Remember, the measure of fitness is production of viable progeny – adaptive traits promote survival of individuals to reproductive age and/or promote reproductive success. Evolution by natural selection results in individuals that are a better fit to their environmentEvolution by natural selection occurs when the environment exerts a pressure on a population so that only some phenotypes survive and reproduce successfully. The stronger the selective pressure or the selection event the fewer individuals make it through the sieve of natural selection. Those phenotypes that survive a strong selection event, such as a drought, are a better fit for an environment that suffers drought. Another way to say this is that they have higher Darwinian fitness. In the finch example above, the average phenotype has shifted so most individuals have larger beaks, which is a genetically controlled-trait in the finches. The larger beak size is an adaptation to the seed sizes available during drought conditions. A result of this shift is that small beak phenotypes have become rare or disappeared, so there is reduced phenotypic and therefore reduced genetic diversity in the finch population after selection. Stabilizing selection results in a narrowing of the normal distribution, because individuals who had the ‘average’ phenotype, or the phenotype closest to the mean, tend to leave more offspring than those with phenotypes at either extreme. Directional selection results in a shift toward one end of the normal distribution, because individuals who had one extreme of the phenotype tend to leave more offspring than those with the other extreme. Disruptive or diversifying selection results in separation of the normal distribution into two distributions with elimination of the middle of the peak, because individuals with either extreme phenotype tend to have more offspring than those with the intermediate phenotype. Balancing selection occurs when multiple phenotypes (or alleles) are actively maintained in the population (i.e., no single phenotype has a consistent selective advantage over any other). The two most common types of balancing selection are frequency-dependent selection, where fitness depends on how common the phenotype (or allele) is, and heterozygote advantage, where the heterozygote (with the combined phenotype of both alleles) has higher fitness than either homozygote. Does evolution of bigger, sexually reproducing organisms happen on time scales faster than geologic time?Yes! There are lots of great examples of evolution, even in sexually reproducing species, that happen pretty quickly, on the order of years or decades. In fact, the relevant time unit is generations. Rock Pocket mice in the desert southwest are a long-studied example. These small tan mice are hunted by owls, visual predators who spot the mice by their contrasting color against the sand. Most mice are exactly the same color as the sand. This short video explains what happens to a pocket mice population that migrates onto black volcanic rock, with mutation rates and the number of generations until the population shifts from all tan to all black coat color. Examples of how evolution matters to ordinary people
Evolution occurs in and all around us, because life evolves: Additional optional readingsThe Escape of the Pathogens: an evolutionary arms race Antibiotic resistance: delaying the inevitable |