The first scientific theory about how species evolve is known as "Natural Selection," a concept primarily credited to Charles Darwin. In his groundbreaking work "On the Origin of Species," published in 1859, Darwin proposed that individuals within a species exhibit variations, and those with traits better suited to their environment have a higher chance of surviving and reproducing. Over time, these advantageous traits are passed down, leading to gradual changes in species. This theory fundamentally challenged the prevailing notions of static species and laid the groundwork for modern evolutionary biology, emphasizing adaptation and survival in changing environments.