4.3: Macroevolution and Speciation

Patterns of Evolution

The evolution of species has resulted in enormous variation in form and function. When two species evolve in different directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the reproductive organs of flowering plants, which share the same basic anatomies; however, they can look very different as a result of selection in different physical environments, and adaptation to different kinds of pollinators (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Flowering plants evolved from a common ancestor. Notice that the (a) dense blazing star and (b) purple coneflower vary in appearance, yet both share a similar basic morphology. (credit a, b: modification of work by Cory Zanker, via open stax)

In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. The wings of bats and insects, however, evolved from very different original structures. When similar structures arise through evolution independently in different species it is called convergent evolution. The wings of bats and insects are called analogous structures; they are similar in function and appearance, but do not share an origin in a common ancestor. Instead they evolved independently in the two lineages. The wings of a hummingbird and an ostrich are homologous structures, meaning they share similarities (despite their differences resulting from evolutionary divergence). The wings of hummingbirds and ostriches did not evolve independently in the hummingbird lineage and the ostrich lineage—they descended from a common ancestor with wings.

What is a species and how do new species arise?

The biological definition of species, which works for sexually reproducing organisms, is a group of actually or potentially interbreeding individuals. According to this definition, one species is distinguished from another by the possibility of matings between individuals from each species to produce fertile offspring. There are exceptions to this rule. Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. In fact, the presence of hybrids between similar species suggests that they may have descended from a single interbreeding species and that the speciation process may not yet be completed.

Definition: Species

The biological species concept defines a species as members of populations that actually or potentially interbreed in nature and produce viable fertile offspring. Although appearance is helpful in identifying species, it does not define species.

Given the extraordinary diversity of life on the planet there must be mechanisms for speciation: the formation of two species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the only illustration found in On the Origin of Species (Figure \(\PageIndex{2}\)a). For speciation to occur, two new populations must be formed from one original population, and 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, meaning speciation in “other homelands,” involves a geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation, meaning speciation in the “same homeland,” involves speciation occurring within a parent species while remaining in one location.

Biologists 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 and such multiple events can also be conceptualized as single splits occurring close in time.

Figure \(\PageIndex{2}\): he 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. (CC-BY; via Open Stax)

Video: Watch the video below for an overview of specieation mechanisms

The following video by the Amoeba Sisters covers species, speciation mechanisms, and reproductive isolation.

Allopatric Speciation: Geographic Separation

A geographically continuous population has a gene pool that is relatively homogeneous (the same). Gene flow, the movement of alleles across the range of the species, is relatively free because individuals can move across their range and mate with other individuals. Thus, the frequency of an allele at one side of a distribution will be similar to the frequency of the allele at the other side. Populations can become geographically separated so that free-flow of alleles is prevented. When that separation lasts for a period of time, the two populations are able to evolve along different paths. Thus, their allele frequencies gradually become more and more different as new alleles independently arise by mutation in each population. When they become geographically isolated, it is likely that the environmental conditions, such as climate, resources, predators, and competitors, for the two populations will differ. With different sets of conditions working on an increasingly different set of alleles, natural selection will favor different adaptations in each group. Different histories of genetic drift, enhanced because the populations are smaller than the parent population, will also lead to divergence.

Given enough time, the genetic divergence between populations will likely affect characters that influence reproduction enough that, were individuals of the two populations brought together, mating would be less likely, or if a mating occurred, offspring would be non-viable or infertile. Many types of diverging characters may affect the reproductive isolation (inability to interbreed) of the two populations. These mechanisms of reproductive isolation can be divided into prezygotic mechanisms (those that operate before fertilization) and postzygotic mechanisms (those that operate after fertilization). Prezygotic mechanisms include traits that allow the individuals to find each other, such as the timing of mating, sensitivity to pheromones, or choice of mating sites. If individuals are able to encounter each other, character divergence may prevent courtship rituals from leading to a mating either because female preferences have changed or male behaviors have changed. Physiological changes may interfere with successful fertilization if mating is able to occur. Postzygotic mechanisms include genetic incompatibilities that prevent proper development of the offspring, or if the offspring live, they may be unable to produce viable gametes themselves as in the example of the mule, the infertile offspring of a female horse and a male donkey.

If the two isolated populations are brought back together and the hybrid offspring that formed from matings between individuals of the two populations have lower survivorship or reduced fertility, then selection will favor individuals that are able to discriminate between potential mates of their own population and the other population. This selection will enhance reproductive isolation.

Isolation of populations leading to allopatric speciation can occur in a variety of ways: from a river forming a new branch, erosion forming a new valley, or a group of organisms traveling to a new location without the ability to return, such as seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely 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 that 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 unlikely; therefore, speciation would be more likely.

Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate subspecies 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 (Figure \(\PageIndex{3}\)). The cause of their initial separation is not clear, but it may have been caused by the glaciers of the ice age dividing an initial population into two.

Figure \(\PageIndex{3}\): The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with different climates and ecosystems. The owl is an example of incipient speciation. (credit “northern spotted owl”: modification of work by John and Karen Hollingsworth, USFWS; credit “Mexican spotted owl”: modification of work by Bill Radke, USFWS)

Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely for speciation to take place. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls; in the north, the climate is cooler than in the south; the other types of organisms in each ecosystem differ, as do their behaviors and habits; also, the hunting habits and prey choices of the owls in the south vary from the northern ones. These variances can lead to evolved differences in the owls, and over time speciation will likely occur unless gene flow between the populations is restored.

In some cases, a population of one species disperses throughout an area, and each finds 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, which is called adaptive radiation. From one point of origin, many adaptations evolve 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, including the eight shown in (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics.. (CC-BY; via OpenStax)

Notice the differences in the species’ beaks in Figure \(\PageIndex{4}\)) Change in the genetic variation for beaks in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The fruit and seed-eating birds have thicker, stronger beaks which are suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach their nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another well-studied example of adaptive radiation in an archipelago.

Sympatric Speciation: Without Geographic Separation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? A number of mechanisms for sympatric speciation have been proposed and studied, and all of them include reproductive isolation.

Definition: Reproductive Isolation

Reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different populations from producing offspring, or reduce the fertility of hybrid offspring. These barriers maintain the integrity of a species by reducing gene flow between related species.

The mechanisms of reproductive isolation have been classified in two broad categories: pre-zygotic for those that act before fertilization (or before mating in the case of animals) and post-zygotic for those that act after it (Figure \(\PageIndex{5}\)). The mechanisms are genetically controlled and can appear in species whose geographic distributions overlap (sympatric speciation) or are separate (allopatric speciation).

Figure \(\PageIndex{5}\): Prezygotic isolating mechanisms are those that occur before fertilization (before mating) and post-zygotic barriers are those that reduce the fitness of hybrids. (Public Domain; Cornell, B. via BioNinja)

Figure \(\PageIndex{6}\): Pre-and post zygotic isolating mechanisms can work individually or in tandem to drive speciation. These mechanisms occur in both allopatric and sympatric speciation. (Cornell, B. via BioNinja)

Multiple mechanisms may drive speciation simultaneously

In general, the barriers that separate species do not consist of just one mechanism.The twin species of Drosophila, D. pseudoobscura and D. persimilis, are isolated from each other by habitat (persimilis generally lives in colder regions at higher altitudes), by the timing of the mating season (persimilis is generally more active in the morning and pseuoobscura at night) and by behavior during mating (the females of both species prefer the males of their respective species). In this way, although the distribution of these species overlaps in wide areas of the west of the United States of America, these isolation mechanisms are sufficient to keep the species separated.

Behavioral Isolation may play an important roles in sympatric speciation

Another example of sympatric speciation is that which has occurred in cichlid fish in Lake Victoria in Africa. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great numbers, but also over a short period of time. And now scientists are finding similar speciation events in progress among a cichlid fish population in Nicaragua (Figure \(\PageIndex{7}\)). In this locale, two types of cichlids live in the same geographic location; however, they have come to have different morphologies that allow them to eat various food sources. Scientists believe these populations may be in an early stage of speciation. Researchers hypothesize that different morphologies specialize on different food sources which occur at different depths of the lake. When foraging each fish is more likely to interact and mate with similar morphologies. Offspring of these fish would likely behave as their parents and feed and live in the same area, keeping them separate from the original population.

Figure \(\PageIndex{7}\): Cichlid fish from Lake Apoyeque, Nicaragua, show evidence of sympatric speciation. Lake Apoyeque, a crater lake, is 1800 years old, but genetic evidence indicates that the lake was populated only 100 years ago by a single population of cichlid fish. Nevertheless, two populations with distinct morphologies and diets now exist in the lake, and scientists believe these populations may be in an early stage of speciation. (CC-BY; via Open Stax)

A well-documented example of ongoing sympatric speciation occurred in the apple maggot fly, Rhagoletis pomonella, which arose as an isolated population sometime after the introduction of the apple into North America (Figure \(\PageIndex{8}\)). The native population of flies fed on hawthorn species and is host-specific: it only infests hawthorn trees. Importantly, it also uses the trees as a location to meet for mating. It is hypothesized that either through mutation or a behavioral mistake, flies jumped hosts and met and mated in apple trees, subsequently laying their eggs in apple fruit. The offspring matured and kept their preference for the apple trees effectively dividing the original population into two new populations separated by host species, not by geography. The host jump took place in the nineteenth century, but there are now measurable differences between the two populations of fly. It seems likely that host specificity of parasites in general is a common cause of sympatric speciation.

Figure \(\PageIndex{8}\): Apple maggot fly. (CC-BY; Joseph Berger via insectimages.org.)

Attribution

This page is a modified derivative of:

• 11.1 Discovering How Populations Change, Concepts of Biology by OpenStax (licence CC-BY)
• 11.4 Speciation, Concepts of Biology by OpenStax (licence CC-BY)
• Reproductive Isolation by Wikipedia (licence CC-BY-SA 3.0