Speciation

Key Words

-Behavioral Isolation -Geographic Isolation -Reproductive Isolation -Speciation -Species -Temporal Isolation

Objectives

-Identify the types of isolation that can lead to the formation of new species -Describe the current hypothesis about Galapagos finch speciation

Behavioral Isolation

Males of different cricket species have different mating chirps. Female crickets will only respond to the males of their species. Because of their different mating rituals, cricket species have become reproductively isolated. The Galapagos Island finches also appear to have developed some behavioral differences that proaly contributed to their reproductive isolation. Part of the finch's mating process involves a through inspection of the potential mate's beak. Finches tend to prefer partners with the same type of beak they have. Reproductive isolation due to these behavioral differences initially comes about not because the two groups cannot interbreed, but because they don't want to.

Founders Arrive

Many years ago, a few finches from South America--species M--arrived on one of the Galapagos islands. These birds may have gotten lost or been blllown off course by a storm. Once on the island, they survived and reproduced. Because of the founder effect, allele frequencies of this founding finch population could have differed from allele frequencies in the original South American population.

Behavioral Isolation

Now, imagine that a few birds from the second island cross back to the first island. Will population-A birds breed with population-B birds? Probably not. These finches choose mates carefully. During courtship, they closely inspect a potential partner's beak. Finches prefer to mate with birds that have the same-size beak as they do. Big-beaked birds prefer to mate with other big-beaked birds, and smaller-beaked birds prefer to mate with other smaller-beaked birds. Because the populations on the two island have evolved differently sized beaks, they would probably not mate with each other. Thus, differences in beak size, combined with mating behavior, could lead to reproductive isolation. The gene pools of the two bird populations remain isolated--even when individuals live in the same place. The populations have now become two distinct species.

Changes in Gene Pools

Over time, populations on each island adapted to local environments. Plants on the first island may have produced small, thin-shelled seeds, whereas plants on the second island may have produced larger, thick-shelled seeds. On the second island, directional selection would have favored individuals with larger, heavier beaks, These birds could crack open and eat the large seeds more easily. Thus, birds with large beaks would be better able to survive on the second island. Over time, natural selection would have caused that population to evolve larger beaks, forming a distinct population, B, characterized by a new phenotype.

What is one way a group of individuals might become reproductively isolated from the parent group?

A group of individuals may become physically separated from the parent group. Over time, the two groups can develop traits that reproductively isolate them from one another

Species

A group of individuals that can interbreed and produce fertile offspring.

How might the founder effect and natural selection have produced reproductive isolation that could have led to speciation among Galapagos finches?

According to this hypothesis, speciation in Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population's gene pool, behavioral isolation, and ecological competition.

Temporal Isolation

Another reproductive mechanism comes about when two populations breed at different times. Take, for example, the Monterey pine and the bishop pine--two species of pine tree that both live in California. However, the Monterey pine releases its pollen in early February while the bishop pine sheds its pollen in April. These trees are reproductively isolated because they reproduce at different times of the year.

Competition and Continued Evolution

As these two new species live together on the first island, they compete for seeds. During the dry season, birds that are most different from each other have the highest fitness. That is because the more specialized birds have less competition for certain kinds of seeds and other foods. Over time, species evolve in a way that increases the differences between them. The species-B birds on the first island may evolve into a new species, C. the combined processes of genetic isolation on different islands, genetic change, and behavioral isoalation could have repeated itself again and again across the Galapagos chain. Over many generations, the process could have produced the 13 different finch species found there today.

Geographic Isolation

Geographic isolation comes about when two groups are physically separated. The mice and lemurs in the movies, or Darwin's finches, are examples of speciation that occurred due to geographic isolation.

How can genetic drift lead to speciation?

In genetic drift, random changes in allele frequencies may result in reproductive isolation, leading to speciation.

Allopatric Speciation

Speciation by geographic separation

What is the definition of speciation?

Speciation is the formation of a new biological species.

Sympatric Speciation

Speciation without physical barriers

How did the island of Madagascar become home to 40 different lemur species?

The island of Madagascan is geographically separated from the mainland, allowing lemur species to differentiate from mainland relatives. It also contains many different habitat niches, resulting in the evolution of many different lemur species as they adapted to the different habitats available.

Geographic Isolation

The island's environment was different from the South American environment. Some combination of the founder effect, geographic isolation, and natural selection enabled the island finch population to evolve into a new species--species A. Later, a few birds from species A crossed to another island. Because these birds do not usually fly over open water, they move from island to island very rarely. Thus, finch populations on the two islands were geographically isolated from each other and no longer shared a common gene pool.

What does the term reproductive isolation mean?

Two groups are reproductively isolated if they can no longer interbreed