1. Variation: Individuals within a population must have differences in their traits. This variation can be in physical characteristics, behaviors, or even at the molecular level.
2. Heritability: These differences must be passed down from parents to offspring, meaning they have a genetic basis.
3. Differential survival and reproduction: Some individuals with certain traits are better able to survive and reproduce in their environment than others. This means they have a higher "fitness".
In simpler terms:
Natural selection is always happening in populations, as long as there is variation in traits, those traits are passed down, and some individuals are more successful at surviving and reproducing. This can occur in any environment, and it's a continuous process.
Examples of when natural selection occurs:
* A population of bacteria is exposed to an antibiotic. Some bacteria have a gene that makes them resistant to the antibiotic. These bacteria survive and reproduce, while the others die off.
* A group of birds is competing for food. Birds with longer beaks can reach food that other birds cannot. These birds are more likely to survive and reproduce, passing on their long beak genes.
* A population of rabbits is living in a snowy environment. Rabbits with white fur blend in better with the snow, making them less visible to predators. These rabbits are more likely to survive and reproduce, passing on their white fur genes.
It's important to note:
* Natural selection is a gradual process, happening over many generations.
* It doesn't have a goal or purpose. It simply favors traits that help organisms survive and reproduce in their current environment.
* The environment can change, leading to changes in which traits are favored by natural selection.
