Final answer:
Alfred Russel Wallace independently conceived the idea of natural selection concurrently with Charles Darwin, leading to a joint presentation of their findings in 1858 before the same theory was popularized by Darwin's 'On the Origin of Species' in 1859.
Step-by-step explanation:
The scientist who came up with the idea of natural selection at the same time as Charles Darwin was Alfred Russel Wallace. Both Wallace and Darwin made significant contributions to the scientific community with their ideas on evolution and the mechanism of natural selection. While Darwin's theory was influenced by his observations in the Galápagos Islands, Wallace developed his theory through his studies in the Malay Archipelago. In 1858, they jointly presented papers to the Linnean Society in London detailing their separate discoveries. A year later, Darwin's influential work, On the Origin of Species, was published, bringing the theory of evolution by natural selection to a wider audience.