The Process of Speciation

According to the theory of common descent discussed in Chapter 8, all modern organisms descended from a common ancestral species. This evolution of one or more species from an ancestral form is called speciation, and the process of speciation is often referred to as macroevolution.

For one species to give rise to a new species, most biologists agree that three steps are necessary:

1. Isolation of the gene pools of subgroups, or populations, of the species;

2. Evolutionary changes in one or both of the isolated populations; and

3. The evolution of reproductive isolation between these populations, preventing any future gene flow.

Recall that gene flow occurs when reproduction is occurring within a species. Now imagine what would happen if two populations of a species became physically isolated from each other, so that the movement of individuals between these two populations was impossible. Even without prezygotic or postzygotic barriers to mating between these two populations, gene flow between them would cease.

What is the consequence of eliminating gene flow between two populations? New alleles that arise in one population may not arise in the other, so while a new allele may become common in one population, it may not exist in the other. Even among existing alleles, one may increase in frequency in one population, but not in the other. In this way, each population would be evolving independently. Over time, the traits found in one population begin to differ from the traits found in the other population. In other words, the populations begin to diverge (Figure 10.3).

The gene pools of populations may become isolated from each other for several reasons. Often a small population becomes isolated when it migrates to a location far from the main population. This is the case on many oceanic islands, including the Galapagos and Hawaiian Islands. Species on these islands appear to be the descendants of species from the nearest mainland. The original ancestral migrants arrived on the islands by chance. Because it is rare for organisms from the mainland to find their way across hundreds of miles of open ocean to these islands, populations at each site are practically completely isolated from each other (Figure 10.4).

Populations may also be isolated from each other by the intrusion of a geologic barrier. This could be an event as slow as the rise of a mountain range or as rapid as a sudden change in the course of a river. The emergence of the Isthmus of Panama about three million years ago represents one such intrusion event. This land bridge connected the formerly separate continents of South and North America but divided the ocean gulf between them. Scientists have described several pairs of species of snapping shrimp on both sides of the isthmus that appear

Media Activity 10.1A Populations Diverge if Gene Flow Is Cut Off

Population Divergence
Figure 10.3 Isolation of populations leads to divergence of traits. In this hypothetical situation, populations of beetles diverge as each adapts to its own particular environmental conditions.

Figure 10.4 Migration leads to speciation. The ancestor of Hawaiian silverswords was the much smaller and less dramatic California tarweed. Tarweed seeds were blown or carried by birds to the Hawaiian Islands, creating an isolated population. With no gene flow between the two populations, Hawaiian silverswords evolved into a very different group of species.

Figure 10.4 Migration leads to speciation. The ancestor of Hawaiian silverswords was the much smaller and less dramatic California tarweed. Tarweed seeds were blown or carried by birds to the Hawaiian Islands, creating an isolated population. With no gene flow between the two populations, Hawaiian silverswords evolved into a very different group of species.

to have speciated after this event. These shrimp species seem to be related to each other because of similarities in appearance and lifestyle. In each case, one member of each pair is found on the Caribbean side of the land bridge, while the other is found on the Pacific side (Figure 10.5). This geographic pattern indicates that the two species in each pair descended from a single species. These original

Media Activity 10.1B Migration Leads to Speciation

Figure 10.5 Physical separation leads to speciation. Species of snapping shrimp on either side of the Isthmus of Panama can be paired according to similarities in appearance and habit. Pairs are numbered for simplicity—the letter "C" before the number indicates the species is found on the Caribbean side of the Isthmus, while a "P" indicates a Pacific species. This pattern indicates that recent speciation in this group occurred after the Isthmus emerged and divided formerly continuous populations of ancestral species.

Figure 10.5 Physical separation leads to speciation. Species of snapping shrimp on either side of the Isthmus of Panama can be paired according to similarities in appearance and habit. Pairs are numbered for simplicity—the letter "C" before the number indicates the species is found on the Caribbean side of the Isthmus, while a "P" indicates a Pacific species. This pattern indicates that recent speciation in this group occurred after the Isthmus emerged and divided formerly continuous populations of ancestral species.

species were most likely found throughout the gulf before the isthmus arose and were divided into two isolated populations after the land bridge appeared. Once in isolation, the populations diverged into different species.

Isolation of the gene pools of populations may also occur even if the populations are living in physical proximity to each other. This appears to be the case in populations of apple maggot fly, a species that provides one of the clearest examples of macroevolution "in action."

Apple maggot flies are so named because they are notorious pests of apples grown in northeastern North America. However, apple trees are not native to North America—they were first introduced to this continent less than 300 years ago. Apple maggot flies also infest the fruit of hawthorn shrubs, a group of species that are native to North America. Apple maggot flies appear to have descended from hawthorn-infesting ancestors that began to use the novel food source of apples after the fruit began to be cultivated in their home range. Apples and hawthorns live in close proximity, and apple maggot flies clearly have the ability to fly between apple orchards and hawthorn shrubs. At first glance, it does not appear that the apple maggot flies that eat apples and those that eat hawthorn fruit are isolated from each other.

However, upon closer inspection, populations of apple maggot flies on apples and those on hawthorns actually have little opportunity for gene flow between them. Flies mate on the fruit where they will lay their eggs, and hawthorns produce fruit approximately one month after apples do. Each population of fly has a strong preference for which fruit it will mate on, and flies that lay eggs on hawthorns develop much more quickly than flies that lay eggs on apples. There appears to be little mixing between the apple-preferring and hawthorn-preferring populations.

Scientists who have examined the gene pools of the two groups of apple maggot flies find that they differ strongly in the frequency of some alleles, so much so that they refer to them as incipient (or newly forming) species. Thus it appears that divergence of two populations can occur even if those populations are in contact with each other, as long as some other factor—in this case the timing of mating and reproduction—is keeping their gene pools isolated (Figure 10.6).

For incipient species that have diverged in isolation to become truly distinct biological species, they must become reproductively isolated. This may happen when enough divergence between the populations has occurred so that individuals of different populations are no longer genetically compatible. There is no hard-and-fast rule about how much divergence is required—sometimes a difference in a single gene can lead to incompatibility, while other times populations demonstrating great physical differences can produce healthy and fertile hybrids (Figure 10.7).

Once reproductive isolation occurs, each species may take radically different evolutionary paths because gene flow between the two species is impossible. Over thousands of generations, species that derived from a common ancestor can accumulate many differences, even completely new genes.

The period between separation of populations and the evolution of reproductive isolation—that is, the development of two new species from a common ancestral species—could be thought of as a period during which biological races of a species may form. Determining if the racial groupings on Indigo's census form came about via this process is our focus in the next section.

Media Activity 10.1D Subdivision of the Environment Leads to Speciation

Figure 10.6 Incipient speciation in apple maggot flies. This graph illustrates the life cycle of two populations of the apple maggot fly: one that lives on apple trees, and another that lives on hawthorn shrubs. Note that the mating period for these two populations differs by a month. This results in little gene flow between these two populations.

Figure 10.7 How different are two species? There is no true minimum or maximum amount of divergence that must occur before populations become reproductively isolated. (a) These two species of dragonfly look alike but cannot interbreed. (b) Dog breeds provide a dramatic example of how the evolution of large physical differences do not always result in reproductive incompatibility.

Figure 10.7 How different are two species? There is no true minimum or maximum amount of divergence that must occur before populations become reproductively isolated. (a) These two species of dragonfly look alike but cannot interbreed. (b) Dog breeds provide a dramatic example of how the evolution of large physical differences do not always result in reproductive incompatibility.

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