Evolution by Natural Selection

Learning Objectives

  1. Define and recognize fitness, adaptation, and evolution by natural selection
  2. Explain predictions of and evidence for evolution by natural selection
  3. Identify, explain, and recognize the consequences of evolution by natural selection in terms of fitness, adaptation, average phenotype, and genetic diversity
  4. Differentiate between directional, stabilizing, disruptive, and balancing selection

Evolution by Natural Selection

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:

  1. the trait under selection must be variable in the population, so that the encoding gene has more than one variant, or allele.
  2. the trait under selection must be heritable, encoded by a gene or genes
  3. the struggle of existence, that many more offspring are born than can survive in the environment.
  4. individuals with different alleles have differential survival and reproduction that is governed by the fit of the organism to its environment

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:

And a followup video that shows live action bacteria evolving antibiotic resistance:

For bacterial evolution in the videos above, here’s how Darwin’s postulates apply:

  1. The population initially contains only antibiotic-sensitive alleles (meaning the antibiotic will kill the cells), but mutations generate antibiotic resistant alleles. Now the trait under selection (antibiotic resistance) is variable in the population, with at least two alleles.
  2. The antibiotic-resistant individuals have offspring that are also resistant because they have the same gene mutation for resistance, indicating that the trait is heritable.
  3. Both sensitive and resistant bacteria have lots of offspring inside the infected human host (or on the petri dish) and compete for resources inside the host.
  4. Because the human host is taking a course of antibiotics (or the petri dish contains increasing dosages of antibiotic), bacteria with the sensitive alleles die more-so than bacteria with the resistant allele. The resistant bacteria are a better fit to the antibiotic-rich environment.

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. The resulting trait that is heritable and increases the survival and reproduction odds for those that carry that trait is called an adaptation. Of all the mechanisms of evolution we’ll discuss in this course, only natural selection results in adaptations.

Evolution by natural selection results in individuals that are a better fit to their environment

Evolution 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.

The finches on the Galápagos islands have provided a robust study system for observing natural selection in action over the past decades (see the work of Peter and Rosemary Grant and their collaborators). The small finches on the island of Daphna Major have strong beaks to feed on seeds. Smaller beaked birds can only crack open the smallest seeds, while birds with larger beaks prefer larger seeds. In 1977, drought reduced the number of small seeds, so many small-beaked finches starved to death.

A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the finches’ average beak size between 1976 and 1978.

A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the finches’ average beak size between 1976 and 1978. (Source: OpenStax Biology)

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.

When a population displays a normal distribution for a particular trait, natural selection can drive change in populations in different directions depending on the type of 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 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.

The image below illustrates the different effects on a population due to stabilizing, directional, or disruptive (diversifying) selection:

Figure_19_03_01 - types of selection

Different types of natural selection can impact the distribution of phenotypes within a population. In (a) stabilizing selection, an average phenotype is favored. In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed. In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against. (Source: OpenStax Biology)


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

  • The example of bacteria evolving resistance to antibiotics is just one example of how evolution affects people’s lives. Here are some questions for you to consider in the light of evolution:
  • How is cancer an evolutionary disease? Cancer arises because individual cells acquire mutations that they pass on to their progeny via mitosis. These mutations allow these cells to escape growth inhibition and hog resources (by creating new blood vessels and ramping up metabolism).
  • When we use insecticides in hour homes, and farmers spray their fields, how will the targeted insect population evolve?
  • When fishing regulations limit the catch to larger fish, what consequences might that have?

Evolution occurs in and all around us, because life evolves:

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Additional optional readings