7.8 - Continuing Evolution

Evidence That Evolution Is Ongoing

1. Genomic Changes Over Time:
• Observation: New mutations show up in every generation, so a population's DNA keeps changing. Scientists can track these changes by sequencing genomes from different generations.
• Example: Since 1988, Richard Lenski's lab has grown populations of E. coli bacteria for tens of thousands of generations, freezing samples along the way. Their genomes have kept changing, and one population evolved the ability to use citrate (a nutrient in their growth medium) as food when oxygen is present, which E. coli normally can't do.

2. Continuous Change in the Fossil Record:
• Observation: The fossil record shows species changing over time, with transitional forms that link older groups to newer ones.
• Example: Whale fossils show a series from four-legged land mammals (like Pakicetus, about 50 million years ago) to fully aquatic whales. Over time, the hind limbs got smaller and the nostrils moved toward the top of the head.

3. Resistance to Antibiotics, Pesticides, Herbicides, and Chemotherapy Drugs:
• How It Happens: A population has variation, and a few individuals happen to carry alleles that let them survive the chemical. The chemical kills the others, the survivors reproduce, and the resistance allele becomes more common in each generation. The chemical doesn't cause the resistance mutation. It selects for individuals that already had it.
• Antibiotics: Bacteria like MRSA (methicillin-resistant Staphylococcus aureus) now survive drugs that used to kill them. Overusing antibiotics speeds this up.
• Pesticides and Herbicides: Insects resistant to insecticides like DDT, and weeds resistant to the herbicide glyphosate, became common after years of heavy spraying.
• Chemotherapy Drugs: The cells in a tumor vary genetically. If a few cancer cells have mutations that let them survive a chemotherapy drug, they keep dividing, and the tumor can come back resistant to that drug.

4. Pathogens and Emergent Diseases:
• Observation: Pathogens, especially viruses and bacteria, evolve quickly because they reproduce fast and in huge numbers. New strains can infect new hosts or get past immune defenses and vaccines, causing emergent diseases (new diseases, or old ones that suddenly spread more).
• Examples: HIV evolved from a related virus (SIV) that infects chimpanzees. Flu viruses change every year, which is why the flu vaccine is updated every year. SARS-CoV-2, the virus that causes COVID-19, kept evolving into new variants.