Evolution describes the changes in the inheritable features of a population of organisms over many generations due to various factors, such as natural selection or genetic drift.
Shown below is a timeline chart of the evolution of a chicken.
Using the chicken as an example, we can see that the modern chicken (Gallus gallus domesticus) is a domesticated form of the Red Jungle Fowl (Gallus gallus). These two forms of chicken come from a common ancestor, Basal Tryannoraptoran from the Triassic Period, which is shared with the dinosaur that we know today as the Tyrannosaurus Rex. Looking at the past forms of the chicken, we can see various physical features such as the length of feathers, the wingspan, the color, and even whether they have hair-like feathers or fur. The splitting of the two lines of evolution shows that there was genetic drift that occurred or natural selection, which caused one side of the lineage to become more bird-like with a bigger wing span with feathers, whereas the other lineage grew shorter arms with claws and lost their fur/feather-like feature.
Below is an animation of a modern chicken (Gallus gallus domesticus) reverting to the non-domesticated version, the Red Jungle Fowl (Gallus gallus), then reverting to the Basal Tyrannoraptoran from the Triassic period.
Resources:
- https://www.britannica.com/science/evolution-scientific-theory
- https://livestock.extension.wisc.edu/articles/origin-and-history-of-the-chicken/
- https://www.britannica.com/animal/Phasianidae