Course: BIS 105 Environmental Biology

Unit 3 Learning Module

Darwin and His Voyage

Charles Darwin and Evolution

Charles Darwin’s Theory of Evolution represents a giant leap in human understanding. It explains and unifies all of biology – thousands of years of natural history from before Darwin’s time, as well as the 150 years of genetics, molecular biology, and even ecology since Darwin published the theory. It directs our responses to disease and our practice of agriculture. It enlightens conservation biology. It has the potential to guide our future decisions about biotechnology. Apart from science, the Theory of Evolution has dramatically changed how we think about ourselves and how we relate to the world. Because the theory has influenced so many aspects of human life, it is crucial that you understand it thoroughly.

The “Theory of Evolution” contains two major ideas:

The first is evolution itself.

- Present life has arisen gradually from past life forms. The millions of species of plants, animals, and microorganisms that live on Earth today are related by descent from common ancestors.

The second describes how evolution happens.

- Natural selection explains how the diversity of life has arisen through time.

The main goal of this lesson will be to clarify these ideas. The lesson will begin by exploring Darwin’s experiences. The ideas of others who influenced Darwin’s thinking will also be presented. Finally, the content and significance of the theory itself will be analyzed.

The Voyage of the Beagle

Captained by a 26-year-old Royal Navyman and carrying a 22-year-old “gentleman’s companion” who collected beetles competitively, His Majesty’s Ship Beagle set sail on one of the shortest days of the year 1831 to chart South American coastal waters. Alarmed by the suicides of his own uncle and the previous Beagle commander, Captain Robert FitzRoy had sought a social and educational equal to accompany him at dinner and in scientific endeavors throughout the anticipated two-year voyage. Charles Darwin, financed by his wealthy father, assumed the unpaid positions of the ship’s naturalist and captain’s friend.

Darwin resisted his family’s hopes that he become a doctor or clergyman. During the two years before he dropped out of medical studies, he was repulsed by the brutality of surgery but fascinated by natural history – field observations of plants, animals, rocks, and fossils. He observed marine mammals on the English coast, and learned taxidermy from a freed slave whose talk of rain forests ignited curiosity in Darwin. After his disappointed father switched him to a school of theology, Darwin again gravitated toward natural history, becoming a protégée of botanist John Steven Henslow in order to learn the popular pastime of competitive beetle collecting. He managed to pass his theology exams, but his interests continued to reflect his passion for natural history, including William Paley’s “argument for divine design in nature.” He had just postponed entry into the clergy in order to study geology – mapping rock layers in Wales – when he received the invitation to join FitzRoy on the Beagle.

Planned to last two years, the voyage stretched to five years. Darwin spent over 3 years of this time on land, carefully observing rock formations and collecting animals, plants, and fossils. Throughout the journey, he used his observations to develop a series of ideas which later became the foundation for his theory of evolution by natural selection. A few of his ideas, observations, and experiences follow.

Rock and Fossil Formations

During the voyage of the Beagle, Darwin made a number of geological observations that helped form his theory. Rock and fossil formations that he observed suggested that continents and oceans had changed dramatically over time.

- Darwin found rocks at a continental divide, 13,000 feet above sea level, which contained fossil seashells.

- A river in Argentina rose gradually through a series of plateaus, which Darwin and FitzRoy interpreted as ancient beaches.

- After experiencing a volcanic eruption and an earthquake in Chile, Darwin found a bed of newly dead mussels, which the quake had lifted nine feet above the sea.

- A petrified forest embedded in sandstone at 7,000 feet had been a sunken coastal woodland, buried in sand and then uplifted into mountains.

- Near Lima, Darwin recognized coral atolls as the result of sinking volcanoes, with coral adding layer after layer to keep the living reef close to the sunlit surface, as shown below.

Darwin explained coral atolls in terms of slowly sinking volcanoes. Evidence for slow geologic change contributed a great deal to his thinking about slow changes in life.

Tropical Rain Forests and Many New Plant, Animal, and Fossil Species

During the voyage of the Beagle, Darwin made a number of observations of plants, animals, and fossils that helped him form his theory. Observations of tropical rain forests and many new plant, animal, and fossil species encouraged Darwin to reconsider the source of the vast diversity of life.

- In Brazil, Darwin collected great numbers of insects – especially beetles!

- Inland from Montevideo, Darwin dug up the hippopotamus-like skull of an extinct giant capybara.

- After collecting his first marsupial in Australia, Darwin exclaimed that some people might think “'Surely two distinct Creators must have been [at] work."

Marine Iguanas (left) and Blue-footed Boobies (right) were among the tremendous variety of new and very different plants and animals Darwin identified during the voyage of the Beagle. He developed his ideas about evolution and natural selection to explain the remarkable similarities and differences he had observed.

Sedimentary Rocks Implied Gradual Changes

Darwin also made a number of observations that implied gradual changes in both the Earth and in living organisms, as opposed to catastrophic changes, including:

- Many inland sediments had clearly been deposited by quiet tides rather than catastrophic floods.

- Gauchos, cowboys of Argentina, helped Darwin find and excavate fossils of gigantic extinct mammals, including armadillos and one of the largest mammals of all time, the ground sloth Megatherium. Darwin recorded that these sediments bore no trace of a Biblical flood.

Darwin found two separate fossils of one of the largest mammals of all time, a giant ground sloth, Megatherium. He noted that they were found in sediments which had been deposited slowly over long periods of time, rather than suddenly as by a catastrophic flood.

Life on Island Chains

The distribution of life on island chains challenged the dogma of the unchangability of species. The Galapagos Islands are arguably where Darwin made his most influential observations. The Galapagos Islands are a group of 16 volcanic islands near the equator about 600 miles from the west coast of South America. Darwin was able to spend months on foot exploring the islands.

- Darwin noted that locals could distinguish each island’s variation of Galapagos tortoise, shown below. Surprisingly, he did not collect their shells, despite dining on the giant reptiles during the voyage.

- A series of birds now known as the Galapagos (or Darwin’s) finches were also specific to certain islands. Darwin failed to label the locations in which he had collected these rather drab-looking birds, but fortunately, FitzRoy and the ship’s surgeon were more careful with their collections.

- Darwin interpreted the different Galapagos mockingbirds as varieties, but wrote that if varieties were a step on the way to new species, “such facts (would) undermine the stability of Species.”

Like many sailors, Darwin and the crew of the Beagle dined on Galapagos tortoise, a convenient animal to carry live on long voyages. However, locals living on the islands claimed the tortoises varied according to the islands from which they came, and this idea later played an important role in Darwin’s thinking about the origins of species.

Throughout the trip Darwin sent his mentor, Henslow, collections of plants, animals, insects, and fossils – many of which were previously unknown. While Darwin traveled, Henslow promoted his work by sharing his geological writings and fossils with renowned naturalists. By the time the Beagle returned to England in October of 1836, Darwin himself had been accepted as an established naturalist. His father set up investment accounts to fund his son’s career as a “gentleman scientist.” At that time, governments and universities did not fund scientific research, so only independently wealthy individuals could afford to practice pure science. This position gave Darwin the contacts, resources, and freedom he needed to develop his ideas into the theory of evolution by natural selection.

Standing on the Shoulders of Giants

Science, like evolution, builds on the past. Darwin’s theory was a product not only of his own intellect, but also of the times in which he lived and the ideas of earlier great thinkers. Some of these ideas colored Darwin’s perspective during his five years on the Beagle; many contributed to his thinking after the voyage. Not until 23 years after he returned to England did Darwin crystallize his thoughts and evidence sufficiently to publish his theory.

Before Darwin, most people believed that all species were created at the same time and remained unchanged throughout history. History, they thought, reached back just 6,000 years.

One of the first scientists to explore change in species was Jean Baptiste Lamarck. Lamarck believed that organisms improve traits through increased use, and then pass the improved feature on to their offspring. According to this idea of inheritance of acquired characteristics, giraffes have long necks because early giraffes stretched their necks to reach tall trees and then passed the longer necks on to their calves, as shown below. This attempt to explain adaptation was popular during the 19^th century, and undoubtedly influenced Darwin’s thinking. Although Lamarck advanced the proposal that species change, evidence does not support inheritance of acquired characteristics. You can weight-train for years, but unless your children train as hard as you did, their muscles will never match yours!

According to Lamarck’s idea, inheritance of acquired characteristics, giraffes have long necks because earlier giraffes stretched their necks to reach tall trees, and then passed their lengthened necks down to their calves. Evidence does not support this hypothesis, but many credit Lamarck for advancing the idea that species develop and change.

Much as Lamarck questioned the dogma that species do not change, Charles Lyell challenged the belief that the earth was young. In Principles of Geology, he recorded detailed observations of rocks and fossils, and used present patterns and processes as keys to past events. He concluded that many small changes over long periods of time built today’s landscapes, and that the earth must be far older than most people believed. Captain FitzRoy gave Darwin a copy of Principles of Geology just before the Beagle left England, and Darwin “saw through [Lyell’s] eyes” during the voyage. Darwin’s theory that present species developed gradually over long periods of time reflects Lyell’s influence.

The idea that natural laws, rather than miracles, govern life as well as geology grew during the early 19^th century. Charles Babbage wrote that God had the power to make laws, which in time produce species. His close friend, John Herschel, called for a search for natural laws underlying the "mystery of mysteries" of how species formed. Later, Darwin cited Herschel as “one of our greatest philosophers” and then said he intended "to throw some light on the origin of species — that mystery of mysteries.”

Darwin’s idea that individuals in a population compete for resources came from reading Thomas Malthus. Malthus described a human “struggle for existence” due to exponential population growth and limited food. Darwin thought that animal and plant populations might have similarly limited resources. If so, offspring suited to their environment would be more likely to survive, while those less “fit” would perish.

Breeders of pigeons, dogs, and cattle inspired Darwin’s ideas about selection. By choosing which animals reproduced, breeders could achieve remarkable changes and diversity in a relatively short time. Variations in traits were clearly abundant and heritable. Darwin referred to selective breeding by humans as artificial selection. His observations of how artificial selection worked helped him to develop his concept of natural selection.

One of the last individuals to influence Darwin’s theory was Alfred Russel Wallace, a naturalist whose work in Malaysia led him to conclusions similar to Darwin’s. In 1858 - over 20 years since the Beagle returned to England - Wallace sent Darwin a paper which described concepts nearly identical to Darwin’s ideas about evolution and natural selection. Lyell helped arrange a joint presentation to the Linnean society two weeks later. Darwin, shocked by the sudden competition, worked quickly to complete his book by the following year. Although both naturalists had independently come to the same conclusions, the extensive evidence and careful logic Darwin presented in The Origin of the Species earned him the greater share of recognition for the theory of evolution by natural selection.

Standing on the shoulders of the giants who went before him, Darwin was able to see past the countless details of his beloved work in natural history to formulate a unifying theory to explain the diversity of life.