We often think of evolution as a process requiring many thousands, even millions, of years. First, random DNA mutations must happen in a gamete which is passed along to an offspring. If the DNA mutation affects the coding region of a gene, and the gene product (the protein) is altered, more often then not, the mutation has a negative effect on the viability of the offspring. However, occasionally a mutation will be beneficial, providing the offspring with an increased chance of surviving and having offspring of their own (passing along the new mutation). Again, mutations occur randomly and it takes time for mutations to spread through a population, something that only happens if the mutation is either beneficial, or at least is not harmful.
Sometimes evolution appears to happen quickly. This occurs when a genetically diverse population (one with several different alleles, or versions, of a given gene) undergoes selective environmental pressure. For example, imagine a new virus appears on the scene. The virus enters cells by binding to a specific cell surface receptor and then injecting its nucleic acid into the cell which can then be copied by the cell to make more virus. The human population has multiple different alleles of the cell surface receptor. This is important: each individual person in the population has cells with one version of the receptor (one allele), while there are several alleles of the receptor within the entire human population. The alleles for the receptor were present before the virus. Individuals with properly functioning receptors went about their daily life, growing up, having kids, working, working, working, and taking the occasional vacation.
Suddenly, with the arrival of the new virus, the environment changed. The virus represents a selective pressure. Because the virus binds to a particular cell surface receptor allele expressed by some (not all!) humans in the population, those humans are vulnerable to infection by the virus. Sadly, infection by the virus results in the death of anyone infected. Imagine what will happen to the human population. If there were five different cell surface receptor alleles before the virus, how many alleles would remain in the population after the virus? With only four remaining alleles, the population has evolved. But remember…so has the virus. Viruses accumulate mutations as well, often at a very fast rate, and given enough time, a mutated virus may be able to bind to one of the remaining four cell surface receptor alleles. Hopefully scientists will develop a vaccine before that happens! Note: this is a hypothetical example virus – not the current coronavirus!
Well that was intense! Let’s lighten the mood with another example of evolution in action: antibiotic resistance. After watching the Amoeba Sisters video below, complete the Natural Selection Video Recap assignment. To complete the assignment:
- Create a Google Doc titled “Week 32 – Your Name” (example: Week 32 – Pickles Swart).
- Share the document with firstname.lastname@example.org.
- Create a section in the document titled “Natural Selection Video Recap”
- Answer questions 1-9 from the worksheet in your doc.
Return to the Week 32 – Phylogenetic Trees post and continue our work for the week.