Chapter: Speciation

Speciation is the evolutionary process through which new species arise. This phenomenon can occur through multiple pathways, but at its core, it begins when gene flow between two populations ceases entirely. Once this genetic exchange stops, isolated populations begin to diverge. A compelling example lies in the prehistoric separation of South America and Africa. These continents were once part of a single landmass. The emergence of a rift valley gradually divided them, and over millions of years, their growing geographic separation physically isolated the populations on either side. This isolation eventually resulted in the divergence of species on each continent. In other words, as gene flow ends, distinct species emerge. But to fully understand this process, we must first examine what defines a species.

Biologists have proposed several definitions of what constitutes a species, each with its advantages and limitations. Perhaps the most widely accepted is the biological species concept, which defines a species as a group of organisms that are reproductively isolated from one another. Reproductive isolation can occur in two main ways. In the first scenario, populations of the same species become physically separated due to a natural event, which prevents them from breeding due to geographic distance. The division of the African and South American continents serves as a classic example of this mechanism.

In the second scenario, reproductive isolation is the result of mutations that render offspring nonviable or sterile. Consider the case of horses and donkeys: when they mate, they produce mules. However, mules are sterile and cannot produce offspring of their own. Therefore, the biological species concept classifies horses and donkeys as separate species. If mules were fertile, then horses and donkeys might be considered a single species. One of the biological species concept's greatest strengths is its applicability to ecological and evolutionary research. It also helps distinguish species that are morphologically identical but reproductively isolated. For instance, some species of salamanders look virtually identical and share the same habitat, yet they do not interbreed, which qualifies them as distinct species.

Despite its utility, the biological species concept is not without its flaws. Firstly, it cannot be applied to extinct species, since we cannot observe their reproductive behaviors. Fossils provide morphological data, but the biological species concept does not necessarily consider morphological differences to indicate different species. Conversely, fossils that look identical may belong to entirely distinct species.

Another major limitation is its reliance on sexual reproduction. Many species reproduce asexually, such as bacteria, and these organisms fall outside the scope of the biological species concept. For example, Planaria, a flatworm, reproduces asexually, which makes this concept less useful for classifying such organisms. Furthermore, the concept assumes geographic overlap between populations. It presumes that populations capable of breeding but choosing not to do so are distinct species, but this only applies in cases where the populations share a geographic range.

Another method for defining species is the morphospecies concept. This approach relies on observable physical differences to categorize organisms. Bird watchers, plant collectors, and amateur naturalists typically use this concept when identifying species. It defines species as populations of organisms that appear significantly different from others. In practical fieldwork, biologists often rely on this method due to the infeasibility of conducting experimental breeding trials. The underlying assumption is that structural differences reflect functional and genetic divergence, which results from a lack of gene flow.

The morphospecies concept has two major advantages. It allows scientists to identify and classify fossils, making it a powerful tool in paleontology. Additionally, it facilitates species identification in the field, even when populations do not overlap geographically. However, it also has significant drawbacks. It cannot distinguish between species that are reproductively isolated but morphologically identical. Moreover, it is subject to human bias. Morphological features can be subjective and open to interpretation. For instance, a newly discovered hominid species, Homo floresiensis, found on the island of Flores in the South Pacific, has generated debate. While the original researchers considered them a new species due to their small stature—only about three feet tall—others argue they are simply a subspecies of Homo sapiens, given their many shared characteristics.

Another complication is the variation within a single species. Domestic dogs (Canis familiaris) range from tiny Chihuahuas to towering Great Danes, yet they are all members of the same species. This highlights how physical diversity does not necessarily equate to speciation.

The third and more modern approach is the phylogenetic species concept, which emerged with advances in DNA sequencing and molecular biology. This method defines a species as the smallest monophyletic group on a phylogenetic tree. A monophyletic group includes an ancestor and all of its descendants. According to this concept, all life on Earth shares a common ancestor, placing all organisms into one vast monophyletic group. Within this tree of life, smaller branches represent more specific groups, and the tips—or leaves—of the branches represent species.

Traits shared by all members of a monophyletic group are called synapomorphies. These can be physical characteristics or genetic markers. For example, all animals are multicellular and ingest their food—two defining synapomorphies. The phylogenetic species concept allows for precise classification without requiring experimental breeding. The term "taxa" refers to any grouping of related organisms, whether it be species, genus, family, or even domain.

Over the past thirty years, the phylogenetic species concept has redefined many taxa. In some cases, it has split a previously singular species into multiple distinct species. In others, it has lumped previously separate species into one. This method is particularly advantageous because it relies on data, rather than assumptions.

A striking example involves African elephants. Traditionally, biologists recognized only two species of elephants: African and Asian. However, studies uncovered a smaller, tuskless elephant in African forests, distinct from the larger, savanna-dwelling variety. DNA evidence revealed these two types of African elephants are, in fact, separate species, despite inhabiting overlapping ranges. The forest and savanna elephants had differentiated so thoroughly that they became reproductively isolated.

The main drawback of the phylogenetic species concept is that it dramatically increases the number of recognized species, many of which look nearly identical. This complicates field identification when DNA tools are unavailable. For example, what was once considered a single species has been divided into 16 nearly indistinguishable species. Furthermore, many phylogenies remain incomplete, particularly for less-studied groups like slime molds and leeches.

Once a species is defined, the concept of subspecies becomes clearer. Subspecies are geographically distinct populations within a species that exhibit limited gene flow and may possess unique traits.

Speciation often results from genetic isolation due to physical separation. When gene flow between populations ceases, divergence can begin. Physical separation can occur in two ways: dispersal and vicariance. Dispersal involves a portion of a population migrating to a new location. If these migrants remain isolated, genetic drift and natural selection can rapidly alter their genetic composition, leading to divergence.

Vicariance refers to a scenario in which a natural barrier divides a population, isolating gene flow. The separation of South America and Africa exemplifies this, as does the formation of the Grand Canyon. Around 5–6 million years ago, the canyon split a population of squirrels. Over time, the populations on the north and south rims evolved into distinct species: the Kaibab squirrel and Abert’s squirrel.

These mechanisms underpin allopatric speciation, in which physical separation drives the evolution of new species. Whether through dispersal or vicariance, the result is genetic isolation and eventual divergence.

In contrast, sympatric speciation occurs without geographic isolation. Initially thought impossible due to the assumption that gene flow within a shared environment would prevent divergence, sympatric speciation has now been documented. One mechanism involves habitat preference. For example, Darwin’s finches have evolved different beak sizes suited to different seeds, reducing competition and promoting divergence.

Another example of sympatric speciation involves fly species that inhabit the same area but prefer different host plants. Apple flies pollinate apple trees, while hawthorn flies stick to hawthorn bushes. Though they share a habitat, their resource preferences reduce gene flow and drive speciation.

One particularly rapid form of sympatric speciation is polyploidy, a chromosome-level mutation in which the number of chromosome sets in an organism doubles. This usually happens during mitosis when cytokinesis fails, resulting in cells with double the normal chromosome number. For example, a diploid (2n) cell might become tetraploid (4n).

Polyploidy is rare in animals but common in plants, where it can cause rapid speciation. Organisms with different chromosome numbers cannot produce viable offspring, making polyploidy a potent driver of reproductive isolation. Consider seedless watermelons, which are triploid hybrids produced by mating diploid and tetraploid watermelons. Triploid watermelons are sterile, which is why they lack seeds. These plants are propagated either by asexual reproduction or by repeating the original diploid-tetraploid cross.

There are two types of polyploidy: autopolyploidy and allopolyploidy. Autopolyploidy arises when individuals of the same species produce a zygote from two diploid gametes. This has occurred in plants like maidenhair ferns. The resulting tetraploids cannot breed with diploids, creating reproductive isolation and facilitating sympatric speciation.

Allopolyploidy, on the other hand, involves chromosome duplication following hybridization between different species. This process often yields tetraploid hybrids and has been widely used by horticulturists to develop new flowering plant varieties.

Polyploidy is more common in plants than animals for several reasons. Many plants can self-fertilize, which allows diploid gametes to fuse and form viable offspring. Plants also have a greater capacity for hybridization, increasing the likelihood of allopolyploidy.

When previously isolated populations come into contact, the outcome depends on the extent of their divergence. If divergence is significant, interbreeding is rare, and populations continue to evolve separately. If divergence is minimal, gene flow resumes, and populations may merge.

Sometimes, when isolated species with the potential to interbreed come into contact, a hybrid zone forms. This is a geographic area where hybrid offspring are produced. Typically, these hybrids are less fit than either parent species and eventually die out. However, in rare cases, hybrids are better suited to their environment and outcompete the parent species, giving rise to new species.

Speciation, therefore, is a complex and multifaceted process driven by reproductive isolation, genetic divergence, and environmental pressures. Whether through geographic separation, ecological specialization, or genetic mutation, the origin of new species remains a cornerstone of evolutionary biology.