Natural Selection

Darwin's Finches

The Galapagos Islands

The Galapagos Islands are an archipelago of volcanic islands in the Pacific Ocean off the coast of Ecuador. There are eighteen main islands as well as many smaller islands. Charles Darwin visited the Galapagos during his voyage on the Beagle and what he studied there became very significant evidence for his theory of evolution. There are a large number of endemic species found in the Galapagos including about fifteen different species of passerine birds commonly known as Darwin's Finches.

How it Works

There are four main steps in the process of Natural Selection:

  1. There is a population that has variation.
  2. Some variations help with survival and the individuals without that trail will die. The individuals with a favourable trait will survive and reproduce.
  3. The genes for the favourable trait are passed down to the offspring.
  4. In the next generation, there will be a higher number of individuals with the favourable trait.


Natural selection works only if the trait is genetic and can be passed down through generations from parents to offspring.

Who Came Up With the Theory

The term 'Natural Selection' was coined by Charles Darwin "The Origin of Species".


"I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection."
Charles Darwin, The Origin of Species


Darwin studied many species and theorised about evolution. Darwin found that natural selection is one of the main mechanisms for evolution, He used the theory of natural selection and his study of different species to support his theory of evolution.

What Are They

Across the Galapagos Islands, there are a large range of Finches known commonly as Darwin's Finches. There are multiple species ranging across each island. It is commonly believed that because the islands are so isolated, the finch species have all evolved from one common ancestor. The different environmental pressures on each different island meant that some traits that came through variation within the species helped those individuals survive and pass their genes on to the next generation. Over time the separate environments caused the species' to evolve in different directions.

Classification

Kingdom: Animalia

Phylum: Chordata

Class: Aves

Order: Passeriformes

Family: Thraupidae

Genus and Species:

"Darwin's" Finches

Although they are named after Darwin and are a strong piece of evidence supporting his theory of evolution, Darwin did not actually study these finches in great detail. During his time at the Galapagos Islands he did collect a few specimens of the finches but did not label which islands they came from or pay much attention to them. His book "The Origin of Species" did not contain any mention of the finches. The only record he kept of them was a small passage in his journal:



"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... Unfortunately most of the specimens of the finch tribe were mingled together; but I have strong reasons to suspect that some of the species of the sub-group Geospiza are confined to separate islands. If the different islands have their representatives of Geospiza, it may help to explain the singularly large number of the species of this sub-group in this one small archipelago, and as a probable consequence of their numbers, the perfectly graduated series in the size of their beaks." (pp403-420)


Darwin, Journal of Researches into the Natural History and Geology of the countries visited during the voyage round the world of H.M.S. Beagle, revised edition, Henry Colburn 1845.



Darwin assumed correctly of course - the different types of finch with different beak shapes and sizes came from separate islands in the Galapagos. Although Darwin didn't have enough data to use the finches as examples of natural selection in his book, they played a significant role in his formulation of the theory.

Different Beak Shape

Finches found on different islands have different shaped beaks and heads. This is because there are different food sources available. It is assumed that all the finches came from a common ancestor. On the islands with a large availability of seeds, the finches with slight variations in their beaks that were better suited to collect and consume seeds thrived. These finches were then able to reproduce. The next generation would inherit the beaks that are better for collecting that particular food source. The different diets available on each island meant that the finches on that island slowly evolved separately in what is known as divergent evolution until they became so different that they weren't thought of as the same species any more.

Different Beak Size

Within a population of finches, even if they are on the same island, micro-evolution is always occurring Because there are slight variations within the species that create advantages and disadvantages in different environmental pressures.

The finches with beaks that are shaped to eat seeds still compete for resources within their species.

During dry years, there is more availability of large seeds so the finches with slightly larger beaks are able to collect and crack through the shells to eat the seeds. They thrive in seasons of drought and have more offspring than the finches with smaller beaks. The ones with smaller beaks are unable to collect and eat the larger seeds and can't survive. In the next generation there will be more finches with larger beaks.

It is the converse for wet years - the finches with smaller beaks are able to collect more food and survive to pass on the gene of smaller beaks to the next generation.

Natural Selection in the Galapagos Video

(n.b. Start the Video at 7:20)
Evolution in the Galapagos
The finches continue to slowly evolve within the separate species'.

As shown from 10:50-12:04 in the video above, finches on the same island with very similar characteristics still continue to compete for resources and the birds with slight variations that help them to collect enough food for survival will pass those variations on to their offspring but those with variations that have a negative effect on how well they are able to compete for resources in the environment, will not survive and therefore will not be able to pass that characteristic on the their offspring.

Why The Finches Are Important

Darwin's Finches are a simple example of adaptive radiation and demonstrate Darwin's theory of Natural Selection very clearly. It is simple to understand that different beaks are better for collecting and eating different food.
The finches can be used as a simple model to demonstrate and explain Natural Selection.

By Brittany Lewis