4.2.2: Speciation

 

Learning Objectives

• Define speciation and discuss the ways in which it may occur
• Give examples of allopatric speciation
• Give examples of sympatric speciation
• Discuss how the fitness of a hybrid will lead to changes in the hybrid zone over time

Speciation Defined

A biological species is a group of potentially interbreeding individuals that produce viable, fertile offspring. For the majority of sexually-reproducing species, this holds true. However, there are exceptions to this rule. Some species are similar enough that hybrid offspring are possible and may occur. The presence of hybrids in nature between similar species suggests that they may have descended from a single interbreeding species and that speciation, a process where a single species branches to form two or more new species, may not yet be complete.

There are mechanisms for speciation. Darwin envisioned this process as a branching event. The modern diagram shows that as one species changes over time, it branches repeatedly to form more than one new species as long as the population survives or until the organism becomes extinct.

Figure \(\PageIndex{1}\): The only illustration in Darwin’s On the Origin of Species is (a) a diagram showing speciation events leading to biological diversity. The diagram shows similarities to phylogenetic charts that are drawn today to illustrate the relationships of species. (b) Modern elephants evolved from the Palaeomastodon, a species that lived in Egypt 35–50 million years ago.

For speciation to occur, two new populations must be formed from one original population. They must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories: allopatric speciation and sympatric speciation. Allopatric speciation (allo- = “other”; -patric = “homeland”) involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation (sym- = “same”; -patric = “homeland”) involves speciation occurring within a parent species remaining in one location.

Biologists typically think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why there might not be more than two species formed at one time except that it is less likely; multiple events can be conceptualized as single splits occurring close in time.

Allopatric Speciation

A geographically-continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across the range of the species, is relatively free because individuals can move and then mate with individuals in their new location. Thus, the frequency of an allele at one end of the range will be similar to the frequency of the allele at the other end.

Allopatric speciation occurs when a single species becomes geographically separated from a natural or human barrier, into two or more populations. Each population evolves new and distinctive traits since gene flow is prevented. If the populations are separated for a long enough period of time, the two populations evolve along different trajectories. New alleles arise by mutation and allele frequencies gradually become different in the two populations. Typically, environmental pressures, such as climate, resources, predators, and competitors for each population will differ causing natural selection to select for different adaptations in each group.

Geographic separation of populations can occur in a variety of ways: a river forming a new branch, erosion forming a new valley, a landslide cutting off a mountain pass, or seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends on the biology of the organism and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are individuals from each population would fly back and forth, continuing gene flow. However, if two rodent populations became divided by the formation of a new lake, continued gene flow would be improbable and speciation would occur.

Scientists have documented numerous cases of allopatric speciation. For example, along the west coast of the United States, two separate sub-species of spotted owls exist. The northern spotted owl has genetic and phenotypic differences from its close relative, the Mexican spotted owl, which lives in the south. They have been separated by the emergence of mountain ranges and deserts. Additionally, scientists have found that the further the spatial distance between two groups that once were the same species, the more probable it is that speciation will occur. This seems logical because as the distance increases, the various environmental factors would generally have less in common than locations in close proximity. Consider the two owls: in the north, the climate is cooler than in the south causing the types of prey organisms in each ecosystem to differ. Also, the hunting habits and locations of each would vary. These variances can lead to evolved differences in the owls, resulting in speciation.

Figure \(\PageIndex{2}\): The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with different climates and ecosystems. The owl is an example of allopatric speciation.

Isolated Evolutionary Change

In isolation, changes in the gene pool can occur through some combination of natural selection and genetic drift (the bottleneck or founder effects). These factors may produce distinct subpopulations on the different islands. So long as they remain separate (allopatric) we consider them subspecies. In fact, they might not be able to interbreed with other subspecies but so long as we do not have evidence, we assume that they can.

How much genetic change is needed to create a new species? Perhaps not as much as you might think. Changes at one or just a few gene loci might cause speciation. For example, a single mutation altering flower color or petal shape could immediately prevent cross-pollination between the new and the parental types.

Adaptive Radiation

In some cases, a population of one species disperses throughout an area with each finding a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species. This is called adaptive radiation because many adaptations evolve from a single point of origin, causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island which leads to geographical isolation for many organisms.

The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species, numerous species have evolved. The response to natural selection based on specific food sources in each new habitat led to the evolution of a different beak suited to the specific food source. The seed-eating birds have a thicker, stronger beak which is suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach the nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects.

Figure \(\PageIndex{3}\): The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics.

Optional Activity \(\PageIndex{1}\)

Which situation would most likely lead to allopatric speciation?

a. A flood causes the formation of a new lake.
b. A storm causes several large trees to fall down.
c. A mutation causes a new trait to develop.
d. An injury causes an organism to seek out a new food source.

Answer

a. A flood causes the formation of a new lake.

Sympatric Speciation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. The process of speciation within the same space is called sympatric speciation. Sympatric speciation occurs when two individual populations diverge from an ancestral species without being separated geographically. 

One form of sympatric speciation can begin with a large chromosomal aberration during cell division. In a normal cell division event, chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition called aneuploidy.

Figure \(\PageIndex{4}\): Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis. In the example shown here, the resulting offspring will have 2n+1 or 2n-1 chromosomes (Tweety207; CC-BY-SA 3.0)

Most organisms without enough chromosomes (2n-1) will not develop. However, polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes (2n+1) and some of these organisms can develop and survive. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. The cultivated forms of wheat, cotton, and tobacco plants are all polyploids. Animals with any of the types of chromosomal aberrations described above are unlikely to survive and produce normal offspring. Scientists have discovered more than half of all plant species studied relate back to an ancestral species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.

Reunion

The question of two populations' status - subspecies or true new species - is resolved if the two ever come to occupy the same territory again (become sympatric). If successful interbreeding occurs, the differences will gradually disappear, and a single population will be formed again. Speciation will not have occurred. If, on the other hand, two subspecies reunite but fail to resume breeding, speciation has occurred and they have become separate species.

The medium tree finch Camarhynchus pauper is found only on Floreana Island in the Galapagos. Its close relative, the large tree finch, Camarhynchus psittacula, is found on all the central islands of the Galapagos including Floreana. Were it not for its presence on Floreana, both forms would be considered subspecies of the same species. Because they do coexist and maintain their separate identity on Floreana, we know that speciation has occurred.

Video

Allopatric, sympatric, and parapatric speciation are all summarized in this 7-minute video.
Question after watching: 
Why do island chains provide ideal conditions for adaptive radiation to occur?

Continued Isolating Mechanisms

What might keep two subpopulations from interbreeding when reunited geographically? There are several mechanisms.

Prezygotic isolating mechanisms act before fertilization occurs. On Floreana, Camarhynchus psittacula has a longer beak than Camarhynchus pauper, and the research teams led by Peter and Rosemary Grant have demonstrated that beak size is an important criterion by which Darwin's finches choose their mates. Two subpopulations may occupy different habitats in the same area and thus fail to meet at breeding time. In plants, a shift in the time of flowering can prevent pollination between the two subpopulations. Structural differences in the sex organs may become an isolating mechanism. The sperm may fail to reach or fuse with the egg.

Postzygotic isolating mechanisms act even if fertilization does occur. Even if a zygote is formed, genetic differences may have become so great that the resulting hybrids are less viable or less fertile than the parental types. The sterile mule produced by mating a horse with a donkey is an example. Sterility in the males produced by hybridization is more common than in females. In fact, it is the most common postzygotic isolating mechanism. When Drosophila melanogaster attempts to mate with its relative Drosophila simulans, no viable males are even produced. Mutations in a single gene are responsible.

Speciation by Hybridization

Speciation occurs over a span of evolutionary time. Over time, two species may further diverge or reconnect, depending on the fitness strength and the reproductive barriers of the hybrids.  When a new species arises, there is a transition period during which the closely-related species continue to interact.

After speciation, two species may recombine or even continue interacting indefinitely. Individual organisms will mate with any nearby individual with which they are capable of breeding. An area where two closely-related species continue to interact and reproduce, forming hybrids, is called a hybrid zone. Over time, the hybrid zone may change depending on the fitness strength and the reproductive barriers of the hybrids.

Figure \(\PageIndex{7}\): After speciation has occurred, the two separate-but-closely-related species (green lines) may continue to produce offspring in an area called the hybrid zone (blue area). The thicker the blue line, the more hybridization occurs between the two species.

Reinforcement

When two species that have separated in allopatry become reunited, their prezygotic and postzygotic isolating mechanisms may become more stringent than those between the same species existing apart from each other. This phenomenon is called reinforcement. It arises from natural selection working to favor individuals that avoid interspecific matings, which would produce less-fit hybrids, when the two species are first reunited.

ybrids can have less fitness, more fitness, or about the same fitness level as their purebred parents. Usually, hybrids tend to be less fit; therefore, reproduction to produce hybrids will diminish over time, which nudges the two species to diverge further. This term is used because the low success of the hybrids reinforces the original speciation. If the hybrids are less fit than the parents, reinforcement of speciation occurs, and the species will continue to diverge until they can no longer mate and produce viable offspring.

If the hybrids are as fit or more fit than the parents, or the reproductive barriers weaken, the two species may fuse back into one species. For a hybrid form to persist, it will generally have to be able to exploit the available resources better than either parent species, with which, in most cases, it will have to compete.

Over time, the hybrids themselves can become separate species. Reproductive isolation between hybrids and their parents was once thought to be particularly difficult to achieve; thus, hybrid species were thought to be extremely rare. With DNA analysis becoming more accessible in the 1990s, hybrid speciation has been shown to be a fairly common phenomenon, particularly in plants.

Video

This 8.5-minute animated video explains how species are crated using a fictional bird species that lands on a volcanic island (which mirrors what happened on the Galapagos Islands).
Question after watching:
Given what you have learned about the process of speciation, why do humans not have any sister species? Propose a hypothesis…