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An overview of what evolution is and the core concept of change in populations over time.
Did you know that whales have tiny, useless hip bones buried deep inside their bodies? Why would a sea creature that never walks have parts meant for land?
In biology, evolution is not about an individual changing during its lifetime—like a Pokémon evolving. Instead, it is a change in the genetic makeup of an entire population over many generations. Think of a population as a 'gene pool.' Evolution occurs when the proportions of certain traits in that pool shift. For example, if a population of beetles goes from being green to green over fifty years, evolution has occurred. This process is driven by the fact that individuals with traits better suited to their environment are more likely to survive and pass those genes to the next generation.
Quick Check
True or False: An individual bird can evolve thicker feathers during a particularly cold winter.
Answer
False. Individuals can adapt or change behavior, but biological evolution only refers to genetic changes in a population over generations.
Scientists look at evolution on two scales. Microevolution refers to small-scale changes within a single population. A classic example is bacteria developing resistance to antibiotics. The species hasn't changed, but its traits have. Macroevolution refers to much larger changes that occur over long geological time scales, resulting in the formation of entirely new species (speciation) or the extinction of others. While they happen at different scales, the mechanisms are the same; macroevolution is essentially the result of many microevolutionary changes accumulating over millions of years.
1. A farmer sprays a field with pesticide. 2. of the insects die, but have a rare genetic mutation that lets them survive. 3. These survivors reproduce. 4. In the next generation, the frequency of the resistance gene increases from to . This is microevolution in action.
Quick Check
If a group of dinosaurs gradually evolves into the first birds over 50 million years, is this microevolution or macroevolution?
Answer
Macroevolution, because it involves the emergence of a new group of organisms (speciation) over a long period.
The concept of common ancestry suggests that all living things on Earth are related. If you trace the lineage of any two species back far enough, you will find a common ancestor—a species from which both descended. This is visualized through the Tree of Life. In this diagram, the 'trunk' represents the earliest life forms, and the 'branches' represent the splitting of lineages. The closer two species are on a branch, the more recently they shared an ancestor. We use evidence like DNA sequences and homologous structures (similar physical features) to map these relationships.
Scientists compare DNA sequences to determine how closely related species are. 1. Humans and Chimpanzees share approximately of their DNA. 2. Humans and Bananas share approximately of their DNA. 3. Because the percentage of shared DNA () is higher between humans and chimps, we conclude they shared a common ancestor much more recently than humans and bananas.
Let's revisit the whale's hip bones. 1. Whales are mammals that live in the sea. 2. Their DNA and bone structure show they are closely related to land-dwelling hippos. 3. The presence of 'vestigial' (useless) hip bones is evidence that whales evolved from four-legged land ancestors. 4. This transition from land to sea is a prime example of macroevolution driven by environmental changes.
Which of the following is the best definition of biological evolution?
If we observe a population of birds where the average beak size increases by mm over years, what are we witnessing?
According to the concept of common ancestry, humans and bacteria share a very distant common ancestor.
Review Tomorrow
In 24 hours, try to explain the difference between a 'population' and an 'individual' in the context of evolution to a friend or family member.
Practice Activity
Look up 'vestigial structures' online and find one example in the human body. How does it provide evidence for our evolutionary past?