How did one ancestral population give rise to many species, each with different adaptations

Abstract

Abstract: As reflected in the title of his masterwork On the Origin of Species, Darwin proposed that adaptation is the primary mechanism of speciation. On this, Darwin was criticized for his neglect of reproductive isolation, his lack of appreciation for the role of geographic barriers, his failure to distinguish varieties from species, and his typological species concept. Two developments since Darwin, the biological species concept of Ernst Mayr and the methods of Coyne and Orr for estimating the contribution of different barriers to the total reproductive isolation, provide a framework for reconciling Darwin’s view on the primacy of adaptation in speciation with later proposals that emphasize reproductive isolation. A review of the few studies that have estimated the contributions of multiple isolating barriers suggests that habitat isolation and other barriers that operate before hybrid formation are much stronger than intrinsic postzygotic isolation. In light of these data, I suggest that Darwin’s focus on adaptation in the origin of species was essentially correct, a conclusion that calls for future studies that explore the links between adaptation and speciation, in particular, ecogeographic isolating barriers that result from adaptive divergence in habitat use. The recent revival in thinking about ecological factors and adaptive divergence in the origin of species echoes Darwin’s much‐criticized “principle of divergence” and suggests that the emerging views from today’s naturalists are not so different from those espoused by Darwin some 150 years ago.

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Darwin’s finches are a classic example of how evolution happens through natural selection. We will discuss Darwin’s observations on the Galapagos finches and how they helped inform the theory of evolution by natural selection.

Voyage of the Beagle to the Galapagos Islands

From 1831-to 1836, Charles Darwin embarked on a voyage on the H.M.S Beagle. Their mission: survey the South American coastline. Although he was hired as a geologist, Darwin spent a lot of time observing and collecting animals, plants, and fossils.

After five years of the voyage, the Beagle stopped over at the Galapagos Islands, a group of volcanic islands 900 km west of South America. The Galapagos Islands are home to diverse, endemic animal life, including blue-footed boobies, giant land tortoises, and the Galapagos finches.

His observations during his visit to the Galapagos Islands would later become the foundation for his theory of evolution by natural selection.

Darwin’s Finches and Their Beaks

While in the Galapagos Islands, Darwin noticed that organisms were similar from island to island but had certain features that distinguished them from each other.

Finches, in particular, caught Darwin’s attention. He found that over a dozen species of finches inhabited the islands. While the finches were similar in size and color, Darwin observed that their beak shapes varied.

Darwin attributed the unique beak shapes to the food sources available in their specific environment. Their distinct shapes gave them a higher chance of survival. Some finches had broad, blunt beaks, which helped them crack nuts and seeds. Other finches had long, pointed beaks, which helped them snatch insects (Fig. 1).

Darwin's Finches: a Diagram

Fig. 1 - A diagram comparing the beaks of four species of Galapagos finch

Traits and behaviors that help organisms survive and reproduce are called adaptations. Darwin noted that the finches’ beak shapes were adaptations that helped them obtain food in their habitat. By explaining how these unique finch species came to be, Darwin was able to formulate his theory of evolution by natural selection.

Darwin's Finches: a Short Activity

Here's a short activity you can do to understand how different beak shapes are adapted to collect specific food types.

You will need the following materials:

• Food sources, like sunflower seeds, grapes, hard-boiled eggs, and yogurt

• Tools: a toothpick, a pair of tweezers, a spatula, and a fork

• Paper plates

Methodology:

1. Fill a plate with sunflower seeds. This plate will be called Plate A.

2. Leave Plate B empty.

3. For 30 seconds, try to transfer as much content from Plate A to Plate B using each tool.

4. Weigh Plate B and write in the table below how much you were able to transfer.

5. Repeat the process for each of the food sources.

Answer the following questions:

1. What tool was the best for collecting sunflower seeds? Grapes? Hard-boiled eggs? Yogurt? Why?

2. What tool was the worst for collecting each of the food sources? Why?

3. How would you relate this activity to the finches' beak shapes as adaptations?

4. What do you think would happen to finches with beak shapes that are unsuitable for the available food source? How about finches with the most suitable beak shapes?

Darwin's Finches and His Theory of Evolution by Natural Selection

Darwin theorized that all of the different finch species on the Galapagos Islands came from one parent species (a common ancestor) that first colonized the islands millions of years ago. Darwin explained that, as populations of the parent species spread from one uninhabited island to the next, they adapted to different ecological niches and rapidly evolved into many descendant species.

How Does Natural Selection Cause Evolution?

In any habitat, resources are limited, so organisms have to compete to survive. Organisms with better-suited traits have a higher chance of survival.

For example, there is an area in the Galapagos Islands where cactuses are the most viable food source. What beak shape do you think would be more favorable for the survival of finches in this area: broad, blunt beaks or long, pointed beaks?

Finches with long, pointed beaks have a higher chance of survival because their beaks allow them to probe the cactus flowers and fruits without being pricked by cactus spines.

Because they have a higher chance of survival, organisms with favorable traits can also reproduce and pass on these traits. Darwin called this the process of natural selection, which is more popularly known as "the survival of the fittest.” Natural selection is the process where organisms with better traits tend to survive and reproduce at higher rates than other individuals because of those traits.

This means that organisms with favorable traits will outnumber those with less favorable traits in the succeeding generation. Over time, species with better adaptations to their habitat are formed. This is how natural selection leads to evolution.

Evolution is the gradual and cumulative change in heritable traits of a population of organisms. This change takes place over the course of several generations.

Speciation is the process where a new species is formed in the course of evolution.

What Accounts for the Many Different Species of Galapagos Finch?

The number of different species of finches in the Galapagos presents an interesting case because speciation tends to produce one new species from a parent species over a long period.

In the case of the Galapagos finches, they underwent a burst of evolution, producing a large number of different descendant species in a relatively short time. This is a type of speciation called adaptive radiation. Adaptive radiation is the process by which a single species rapidly evolves into many species adapted to different ecological niches or the role that a species plays in a habitat.

To illustrate how adaptive radiation might have taken place:

• Let’s say Finch Population A occupies one area.

• Over time, Finch Population A adapts to its new environment and becomes different from its parent species.

• Finch Population B from the same parent species enters the same area and competes with Finch Population A.

• There is pressure on both Populations A and B to adapt to different niches.

• They pass on traits suited to each niche.

• When this process happens several times in one area, several new species may be formed from a single parent species in a short period.

This explains how over a dozen different finch species evolved from one parent species in a relatively short time in the Galapagos Islands (Fig. 2).

Fig. 2 - A diagram showing how a parent species of finch rapidly formed several new species of finch with different beak shapes and feeding habits

Significance of Darwin's Finches

Darwin's discovery that different species of Galapagos finch had unique beak shapes adapted to the food sources available in their specific habitats led to the idea that life forms are not perfect and unchanging. Instead, they were changing over time in response to their environment. Such changes manifest in observable traits, such as the beak shapes of Galapagos finches. He argued that traits change in a population because individuals that inherit traits that are better adapted to the environment have better chances of survival and reproduction; thus, these traits have better representation in succeeding generations. These insights became the foundation for Darwin's theory of evolution by natural selection.

Another key insight that Darwin had drawn from his observations is the process of adaptive radiation. Morphological similarities among the species of Galapagos finches led him to believe that they all descended from a common ancestor but rapidly diversified and speciated because each population adapted to a different ecological niche.

Darwin's Finches - Key takeaways

• Charles Darwin’s observations on the Galapagos Finches led to the formulation of his theory of evolution by natural selection.

• Darwin observed that the different finch species on the Galapagos Islands each had unique beak shapes. He noticed that their beak shapes were suited to the food available in their habitat.

• Darwin theorized that organisms with better traits have a higher chance of survival and reproduction; they can pass on these traits to the next generation. He called this natural selection.

• Over time, species with better adaptations to their habitat are formed. This is why natural selection is the mechanism for adaptive radiation.

How did one ancestral population give rise to many different species?

How did one ancestral finch population give rise to 13 species?

How did the Galapagos finches become different species from a common ancestor?

Why many different finch species originated from the single ancestral species?