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

A Short Primer on Genetics

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:

  • All living things have DNA as their genetic material. DNA is composed of nucleotide bases, commonly abbreviated as A, T, C, and G.
  • DNA is organized into one or more chromosomes, which are linear or circular structures comprised of DNA and associated proteins
  • An organism’s genome is the complete set of genes or genetic material for that species.
  • gene is a hereditary factor that determines (or influences) a particular trait.  A gene is comprised of a specific DNA sequence and is located on a specific region of a specific chromosome.  Because of its specific location, a gene can also be called a genetic locus.
  • An allele is a particular variant of a gene, in the same way that chocolate and vanilla are particular variants of ice cream.
  • An organism’s genotype is the particular collection of alleles found in its DNA.  An organism with two of the same alleles for a particular gene is homozygous at that locus; an organism with two different alleles for a particular gene is heterozygous at that locus.
  • An organism’s phenotype is its observable traits, which can include physical features and behaviors. Some aspects of phenotype are influenced by genotype, and some are influenced by environment.
  • mutation is a change in the DNA sequence (A, T, C, or G). Some mutations are deleterious (‘bad’), some have no effect (‘neutral’), and some are beneficial (‘good’).  Mutations create new alleles, so without mutations, there would be no new genetic variation.

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. 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 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.

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 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.

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

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 our 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:

Questions about any of this? Let us know in the IKE before class.

Additional optional readings

The Escape of the Pathogens: an evolutionary arms race

Antibiotic resistance: delaying the inevitable

Good Health and Well-being

UN Sustainable Development Goal (SDG) 3: Good Health and Well-being – Understanding the principles of evolution and natural selection can help us combat antibiotic resistance and develop effective medical treatments. Strong foundations in evolution can aid the development and proper application of vaccines and limit the spread of new and emerging infectious diseases.