The other Darwin: Alfred Russel Wallace, co-discoverer of natural selection

Alfred Russel Wallace, co-discoverer of natural selection as the driving force of evolution
On the 150th anniversary (November, 24 2009) of Charles Darwin’s publication of On the Origin of Species in 1859, it is worth remembering that Alfred Russel Wallace made the discovery independently from Darwin. His essay, On the Tendency of Varieties to Depart Indefinitely from the Original Type, which he mailed to Darwin and other notable scientists from an island in southeast Asia, was read to the Linnean Society of London on July 1, 1858. It was read together with an extract of an unpublished 1844 essay by Darwin, whom Wallace considered something of a mentor, titled On the Perpetuation of Varieties and Species by Natural Means of Selection; and an abstract of a private October 1857 letter from Darwin to Professor Asa Gray, of Boston, U.S. These were published in the Linnaean Society’s journal on August 20 as one paper by Darwin and Wallace, but in fact they were separately authored and read separately at the July 1 meeting. The reading of these two papers prompted Darwin to rush to complete his book.
Wallace (January 8, 1823 – November 7, 1913) was born in Llanbadoc, Wales. A prolific author, he wrote on both scientific and social issues; the account of adventures and observations during his explorations in Indonesia and Malaysia, The Malay Archipelago, was one of the most popular and influential journals of scientific exploration published during the 19th century.
Although from a respectable, middle-class family, a deterioration of the family's finances forced young Alfred to withdraw from grammar school at age 13. He apprenticed as a surveyor with his brother William and worked at various surveying jobs, culminating in a position at the Collegiate School in Leicester to teach drawing, mapmaking, and surveying.
It was already more than a century since Carolus Linnaeus had published his Systema naturae in 1735, demonstrating the relatedness of species living close to one another, the relationships becoming more distant with increasing geographic distance. Scientists were already aware of the vast periods “geologic time” during which the various kinds of rock had been created. They knew that sedimentary layers had been build up over uncounted eons, occasionally interrupted with igneous layers indicating ancient lava flows. They knew that metamorphic rock was older, created from sedimentary or igneous layers by immense pressure and time. They could see that some layers were made of sediments eroded from mountains and volcanoes no longer there, and that some of these, containing shells of extinct sea animals, were found on mountain tops too far from any sea to be explained by cataclysm. They also knew, from the discoveries of fossils in different layers, that species of animals and plants evolved or “transmuted” over time, ancestral species giving way to similar, modern ones. Extinct fossil species most similar to extant ones were in the most recent layers at the top, while deeper, older layers held species that were increasingly different, indicating some process of gradual change. What they didn’t know was how.
In the Leicester library, Wallace, then about 21, met Henry Bates, at 19 already an accomplished zoologist. The two friends took beetle-collecting field trips together and discussed and corresponded about the important ideas of the day regarding human population and evolution. These included An Essay on the Principle of Population by Thomas Malthus (1798), the anonymous evolutionary treatise Vestiges of the Natural History of Creation (1844), Charles Darwin's Journal (1839), and Charles Lyell's three-volume Principles of Geology (1830–1833).
Because of these insights, and the thirst for knowledge of foreign places that accompanied the colonial period, museums throughout Europe had an unquenchable thirst for exotic animal specimens. The public was also hungry for travel books. Supplying the specimens and writing the books became profitable businesses for adventurers and ways to subsidise expeditions for explorers. In 1848, Wallace and Bates left for Brazil to collect insects and other animal specimens in the Amazon rainforest and sell them to collectors back in the United Kingdom. They also hoped to gather further evidence of the transmutation of species.
After about a year collecting together in the Amazon jungle, Wallace and Bates seem to have had some sort of falling out. Neither of their journals explains why, but anyone who has ever taken a long trip with a friend knows how the exigencies of travel can strain relationships. They separated and continued collecting separately for three more years, occasionally meeting to exchange notes and news. Then Wallace packed his collections and meticulous notes and illustrations and went home, while Bates stayed for seven more years. When Bates finally arrived home in 1859, he had sent back over 14,000 species (mostly of insects) of which 8,000 were new to science.
Meanwhile, en route home in 1852, Wallace's ship caught fire and the crew abandoned ship. All of his specimens, the vast majority of those he had collected during his entire trip, were lost. He could only save part of his diary and a few sketches. Although this was obviously a financial and scientific loss and a personal blow, the insurance payment for his lost collection plus the sale of a few specimens that he had previously shipped back to Britain allowed Wallace to spend eighteen months living in London. During this period, he wrote six academic papers (for example, On the Monkeys of the Amazon) and two books; Palm Trees of the Amazon and Their Uses and Travels on the Amazon. He also met and corresponded with a number of other British naturalists — most significantly, Charles Darwin.
The city could not hold his attention long, however. In 1854 at age 31 he embarked on an eight-year expedition through the Malay Archipelago to collect specimens for sale and to study nature. Among other discoveries, he noticed a sharp zoological divide across the strait between Java and the Seychelles. Unlike the islands of Sumatra, Java and Borneo, and the southeast Asian mainland to the northwest, which have many closely-related species, the Seychelles, Philippines and other islands to the south and east have mainly unrelated taxa, many of which are endemic. But neither are they related closely to species in Australia to the south, or to those on Papua New Guinea to the east. It has since been discovered that shallow ocean shelves connecting Java, Sumatra and Borneo with the mainland were occasionally exposed when glacial maxima in the late Pliocene and Pleistocene lowered sea levels, allowing species to move among these islands; however, the islands to the south and east had arrived there by plate tectonic movements and were always separated by deep water, so that no land connections were ever possible. Their species therefore evolved independently.
This biological realm is known today as Wallacia, and the line encompassing it, the Wallace Line. Wallace collected more than 125,000 specimens in the Malay Archipelago (more than 80,000 beetles alone). More than a thousand were new to science.
Because he was collecting for sale, Wallace collected many specimens of each species, allowing him to study the variations within each species throughout its range, and among related species. This allowed him to see how varieties with seemingly minor differences could prosper in different environments. His insight, in parallel with Darwin’s, was that even minor differences in an individual that conferred a survival or reproductive advantage would be passed on to its progeny in a higher proportion than from individuals without the advantage. In time, the differences in a variety from it parent type could become so great as to constitute a new species.
Neither Darwin nor Wallace discovered evolution or created a theory of it. Evolution is a fact that had been known long before. What they discovered was the theory of how it worked, a theory that has stood the test of time, has been strengthened and elaborated, and has become one of the foundations of biology.