Darwin’s Voyage

Chapter 5: Section 1

In 1831, a 22-year old Charles Darwin left for a 5-year long trip on the HMS Beagle to study living things on the voyage.

It was by observing such different species from what he had ever seen in England that he eventually developed the theory of evolution by natural selection.

On the ship…

Darwin observed a variety of species, but current scientists have now identified 1.7 million species of organisms on Earth.

A species is a group of similar organisms that can mate with each other and produce fertile offspring.

Darwin’s Observations

From his observations of South America and the Galapagos Islands, Darwin inferred that a small number of plant and animal species had traveled to the Galapagos Islands, reproduced, and their offspring became different from the original species in South America and different from each other on island to island.

Similarities and Differences

Darwin noticed that these different offspring was well suited to the life it led.

Finches that ate insects had sharp, needlelike beaks, while finches that ate seeds had strong, wide beaks.

Beak shape is an example of adaptation, a trait that helps an organism survive and reproduce.

Adaptations

Darwin spent the next 20 years consulting with other scientists, gathering more information, and thinking through his observations.

Darwin reasoned that the species gradually changed over many generations and became better adapted to the new conditions.

The gradual change in a species over time is called evolution.

Evolution

A scientific theory is a well-tested concept that explains a wide range of observations.

Darwin’s ideas are often referred to as the theory of evolution.

Evolution

In 1859, Darwin published his ideas in a book entitled The Origin of Species.

In this book, he explained that evolution occurs by means of natural selection, the process by which individuals that are better adapted to their environment are more likely to survive and reproduce than other members of the same species.

Natural Selection

Darwin identified a number of factors that affect the process of natural selection:OverproductionCompetitionVariations

Natural Selection

In many species, so many offspring are produced that there are not enough resources (water, food, shelter, etc.) for all of them.

Example: A female sea turtle lays more than 100 eggs, but if they all survived, the sea with be full of turtles and it is not.

Overproduction

Since resources are limited, the offspring must compete with each other to survive.

Competition is usually indirect (not fighting with each other).

Example: Only some will find food first, only some will be fast enough to avoid predators, etc.

Competition

Any difference between individuals of the same species is called a variation.

Example: some newly hatched turtles are able to swim faster than other turtles, giving them an advantage in catching food and avoiding predators.

Variations

Some variations make certain individuals better adapted to the environment. These are more likely to survive and reproduce, passing on the alleles necessary to keep the variation alive.

Over generations, most of the species that survived will have the traits that made their species survive.

Selection

Basically, nature has “selected” organisms with helpful traits to be parents of the next generations. Hence the term “natural selection.”

Over a long period of time, natural selection can lead to evolution. Helpful variations gradually accumulate in a species, while unfavorable ones disappear (because the carriers of those traits will not survive to reproduce).

Selection