English for

4.2 Reading Resource #1
: Charles Darwin and Natural Selection
In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russell Wallace, 
independently conceived of and described the actual mechanism of evolution. Importantly, both spent time in 
the tropics exploring the natural world. Darwin traveled around the world on the H.M.S. Beagle from 1831 to 
1836, visiting South America, Australia, and the southern tip of Africa. From 1848 to 1852, Wallace traveled 
to Brazil to collect insects in the Amazon rainforest, and from 1854 to 1862, he traveled to the Malay 
Archipelago. Darwin's journey, like Wallace's later voyages in the Malay Archipelago, included stops at 
several island chains, the most recent of which were the Galápagos Islands (west of Ecuador). 
Darwin observed species of organisms on these islands that were clearly similar but had distinct 
differences. For example, the Galápagos Islands' ground finches included several species, each with a distinct 
beak shape (Figure 56). He noticed that these finches resembled another finch species on South America's 
mainland, and that the group of species in the Galápagos formed a graded series of beak sizes and shapes, with 
very small differences between the most similar. Darwin imagined that the island species might be all species 
modified from one original mainland species. In 1860, he wrote, “Seeing this gradation and diversity of 
structure in one small, intimately related group of birds, one might really fancy that from an original paucity 
of birds in this archipelago, one species had been taken and modified for different ends.” 

---

95 
Figure 56.
As Darwin’s observation, beak shape varies among finch species. He postulated that the 
beak of an ancestral species had adapted over time to equip the finches to acquire different food sources. 
This illustration shows the beak shapes for four species of ground finch: 1. 
Geospiza magnirostris
(the large 
ground finch), 2. 
G. fortis
(the medium ground finch), 3. 
G. parvula
(the small tree finch), and 4. 
Certhidea 
olivacea
(the green-warbler finch). 
Wallace and Darwin both observed similar patterns in other organisms and independently conceived 
a mechanism to explain how and why such changes could take place. Darwin referred to this mechanism as 
natural selection. Darwin argued that natural selection was an unavoidable result of three natural laws. First, 
organisms' characteristics are inherited, or passed down from parent to offspring. Second, more offspring are 
produced than can survive; in other words, survival and reproduction resources are limited. All organisms' 
reproductive capacity exceeds the availability of resources to support their numbers. As a result, each 
generation competes for those resources. Both Darwin and Wallace derived their understanding of this 
principle from an essay by economist Thomas Malthus, who discussed it in relation to human populations. 
Third, offspring differ in terms of their characteristics, and those differences are inherited. Out of these three 
principles, Darwin and Wallace reasoned that offspring with inherited characteristics that allow them to best 
compete for limited resources will survive and have more offspring than those individuals with variations that 
are less able to compete. Because characteristics are inherited, they will be more prominent in the next 
generation. This will lead to change in populations over generations in a process that Darwin called “descent 
with modification.” 
Natural selection papers by Darwin and Wallace were read aloud before the Linnaean Society in 
London in 1858. The following year, Darwin's book, On the Origin of Species, was published, outlining his 
arguments for evolution by natural selection in great detail
35
.