A species of tan lizards have migrated to a swampy marsh with all the organisms of the species being 100% tan.
Genetic Variation- Some of the species have mutated to adapt to the dark grass of the marsh due to predators easily spotting the tan lizards.
Overproduction & Struggle for Existence- Lizards lay up to 8 eggs at once. Only those best fit for their environment will be able to evade predators and live to reproduce.
Differential Survival & Reproduction- The lizards who've inherited the dark green traits will be the fittest and'll be the ones who reproduce and pass their traits to their offspring. Occasionally a lizard will be born tan because of the existing genes from previous generations.
Changes in Gene Pool- The lizards had been 100% tan but were rapidly being hunted by predators in their new marsh environment so over generations they changed colors
The lizard population started off as a tan color but over time they had started change colors to better adapt to their new environment. Only the lizards with the traits best fit to their environment will reproduce and pass their traits down. This led to a major decrease in the tan lizard population.