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Biology for AP® Courses

18.2 Formation of New Species

Biology for AP® Courses18.2 Formation of New Species

Learning Objectives

In this section, you will explore the following questions:

  • What defines a species, and how can different species be distinguished from each other?
  • How does genetic variation lead to speciation?
  • What is the role of pre-zygotic and post-zygotic reproductive barriers in speciation?
  • What is the difference between allopatric speciation and sympatric speciation?
  • How does adaptive radiation explain the diversification?

Connection for AP® Courses

Speciation explains the diversity of organisms that inhabit the Earth. Although all life shares various genetic similarities, only certain organisms combine genetic information by sexual reproduction and produce viable and fertile offspring that, in turn, can successfully reproduce. Scientists call such organisms members of a biological species. As we will study in later, changes in allele frequencies within a population over generations result in microevolution. However, macroevolution leads to the evolution of new species when populations diverge from a common ancestor and, for one reason or another, become reproductively isolated from the original population.

Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through mechanisms that occur within a shared habitat (sympatric speciation). In both cases, populations become reproductively isolated. When populations become geographically isolated, the free-flow of alleles is prevented. Over time—and because of different selective pressures—the populations diverge and become genetically independent species. Prezygotic barriers block reproduction prior to formation of a zygote, whereas postzygotic barriers block reproduction after fertilization occurs. Obviously, if two populations are separated by a vast ocean, they will not come in contact with each other to reproduce. However, if speciation has occurred, even when brought back together, they will retain their species identity. There are many examples of this in nature, including Darwin’s finches, northern and Mexican spotted owls, and Hawaiian honeycreeper. Adaptive radiation occurs when a single ancestral species gives rise to many new species. This may occur, for example, when new habitats become available. It can also be seen historically in the rise of mammals following the extinction of dinosaurs. Other examples of prezygotic isolating mechanisms include mating seasons and unique courtship behaviors. Sometimes mating occurs between two different species, resulting in a hybrid such as the mule, which is a cross between a horse and a donkey. However, most hybrids are inviable or sterile.

Sympatric speciation does not require a geographic barrier and explains how many different species can inhabit the same area. One form of sympatric speciation begins with a serious chromosomal error during cell division. As you recall from our exploration of meiosis, sometimes errors occur in the separation of chromosomes or chromatids, resulting in gametes with extra chromosomes (polyploidy). This type of speciation is more common in plants than in animals, though some examples in animals exist. For example, two groups of cichlid fish in Africa’s Lake Victoria, which have distinct morphologies and diets, may be in the early stage of sympatric speciation without polyploidy, as genetic differences arise between the two groups.

Information presented and the examples highlighted in this section support concepts outlined in Big Idea 1 and Big Idea 3 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 1 The process of evolution drives the diversity and unity of life.
Enduring Understanding 1.C Life continues to evolve within a changing environment.
Essential Knowledge 1.C.1 Speciation and extinction have occurred throughout the Earth’s history.
Science Practice 5.1 The student can analyze data to identify patterns or relationships.
Learning Objective 1.20 The student is able to analyze data related to questions of speciation and extinction throughout the Earth’s history.
Essential Knowledge 1.C.1 Speciation and extinction have occurred throughout the Earth’s
Science Practice 4.2 The student can design a plan for collecting data to answer a particular scientific question.
Learning Objective 1.21 The student is able to design a plan for collecting data to investigate the scientific claim that speciation and extinction have occurred throughout the Earth’s history.
Essential Knowledge 1.C.2 Speciation may occur when two populations become reproductively isolated from each other.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 1.22 The student is able to use data from a real or simulated population(s), based on graphs or models of types of selection, to predict what will happen to the population in the future.
Essential Knowledge 1.C.2 Speciation may occur when two populations become reproductively isolated from each other.
Science Practice 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question.
Learning Objective 1.23 The student is able to justify the selection of data that address questions related to reproductive isolation and speciation.
Essential Knowledge 1.C.2 Speciation may occur when two populations become reproductively isolated from each other.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 1.24 The student is able to describe speciation in an isolated population and connect it to change in gene frequency, change in environment, natural selection, and/or genetic drift.
Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.C The processing of genetic information is imperfect and is a source of genetic variation.
Essential Knowledge 3.C.1 Changes in genotype can result in changes in phenotype.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 3.24 The student is able to predict how a change in genotype, when expressed as a phenotype, provides a variation that can be subject to natural selection.
Essential Knowledge 3.C.1 Changes in genotype can result in changes in phenotype.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 3.26 The student is able to explain the connection between genetic variations in organisms and phenotypic variations in populations.

Teacher Support

The word species means “kind” in Latin. In daily life, we commonly distinguish between kinds of organisms strictly by using the criteria of appearance. Differentiating between kinds of organisms on the basis of bodily form (or morphology) is, of course, both a useful and practical thing to do. As it turns out, it is also largely in line with other means (physiological traits, biochemical patterns, and DNA sequences) of biological differentiation.

The definition of species as a group of individuals whose members have the potential to interbreed and produce viable, fertile offspring is known as the biological species concept. Inherent in this concept of species as populations of reproductively compatible individuals is the related concept of speciation as dependent on reproductive isolation. While this definition of species depends on ideas of separateness, other definitions depend on ideas of unity. For example, the ecological species concept defines species as a set of organisms adapted to a singular niche. In the phylogenetic species concept, a species is defined as the smallest cluster of individuals within which there is a pattern of ancestry and descent.

You may wish to ask students to research and report on different species concepts. Have students compare and contrast the different definitions. Through discussion, elicit which definitions are useful, and in what kinds of situations, and which are less useful, and in what kinds of situations. You might ask questions such as: Which definition or definitions is useful for identifying asexual species? Which definition or definitions is useful for identifying asexual and sexual species? Why might different definitions work in different environments, for different types of organisms, or at different junctures of individuals’ lives?

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.27][APLO 1.8][APLO 1.23]

Species and the Ability to Reproduce

A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

Members of the same species share both external and internal characteristics, which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and therefore share characteristics and behaviors that lead to successful reproduction.

Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce (Figure 18.10).

Photo a shows a poodle with curly short fur. Photo b shows a cocker spaniel with long, wavy fur that has light brown parts and cream-colored markings on the face, forepaws, belly, hind legs, and tail. The poodle has longer legs than the cocker spaniel. The cockapoo in photo c has curly hair, like the poodle, and short legs, like the cocker spaniel.
Figure 18.10 The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo. (credit a: modification of work by Sally Eller, Tom Reese; credit b: modification of work by Jeremy McWilliams; credit c: modification of work by Kathleen Conklin)

In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group (Figure 18.11). If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile—unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development; therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

Photo a shows a picture of the African fish eagle in flight, and photo b shows the bald eagle perched on a post.
Figure 18.11 The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species. (credit a: modification of work by Nigel Wedge; credit b: modification of work by U.S. Fish and Wildlife Service)

Populations of species share a gene pool: a collection of all the variants of genes in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only be passed to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will be passed on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to be passed on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.

Everyday Connection for AP® Courses

Until recently, these three species of short-tailed pythons, Python curtus, Python brongersmai (middle), and Python breitensteini were considered one species. However, due to the different locations in which they are found, they have become three distinct species.

A map shows the distribution of three python species. Python brongersmai, a brown snake with highly contrasting patterns of yellow-beige and black spots, is found in the southern tip of Thailand, Peninsular (West) Malaysia, and the eastern half of Sumatra. Python breitensteini has less contrast in the pattern of its skin, with patches of light yellow to medium brown and dark brown. It is found in western Sumatra. Python curtus is similar to breitensteini, but is much darker, almost black with visible pattern. It is found on the island comprised of Borneo, Brunei, and East Malaysia.
Figure 18.12
Until recently, three species of short-tailed pythons, Python curtus, Python brongersmai, and Python breitensteini were considered one species. However, due to the different locations in which they are found, they have become three distinct species. What is this an example of?
  1. divergent evolution
  2. sympatric speciation
  3. allopatric speciation
  4. variation


The biological definition of species, which works for sexually reproducing organisms, is a group of actually or potentially interbreeding individuals. 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 in nature of hybrids between similar species suggests that they may have descended from a single interbreeding species, and the speciation process may not yet be complete.

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 18.13a). Compare this illustration to the diagram of elephant evolution (Figure 18.13b), which shows that as one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct.

Image (a) shows a sketch of lines branching into a tree shape. At the bottom are 11 vertical lines labeled A through L. These then are branched out as they move up across the page through fourteen rows labeled with Roman numerals. Some branches make a straight line from the bottom row to the top row, others keep branching out further at each row, and some are straight partway through the rows until they connect to an existing branch or form no connection and instead stop. The top four rows each consists of a single line from a branch tip (there are 6 branch tips at row XI) to one of 15 individual final designations. Illustration B shows the evolution of modern African and Asian elephants from a common ancestor, the Palaeomastodon. The Palaeomastodon was similar to modern elephants; however, it was smaller and had a long nose instead of a trunk. Side branches of the elephant evolutionary tree gave rise to mastodons and mammoths. The mammoth is more closely related to modern elephants than the mastodon.
Figure 18.13 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 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 (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 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 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 a distribution will be similar to the frequency of the allele at the other end. When populations become geographically discontinuous, 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 trajectories. Thus, their allele frequencies at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group.

Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new branch, erosion forming a new valley, a group of organisms traveling to a new location without the ability to return, or 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, 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.

Biologists group allopatric processes into two categories: dispersal and vicariance. Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms.

Scientists have documented numerous cases of allopatric speciation taking place. 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 (Figure 18.14).

The northern spotted owl lives in the Pacific Northwest, and the Mexican spotted owl lives in Mexico and the southwestern portion of the United States. The two owls are similar in appearance but with slightly different coloration.
Figure 18.14 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. (credit "northern spotted owl": modification of work by John and Karen Hollingsworth; credit "Mexican spotted owl": modification of work by Bill Radke)

Additionally, scientists have found that the farther the distance between two groups that once were the same species, the more likely it is that speciation will occur. 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 types of organisms in each ecosystem differ, as do their behaviors and habits; also, the hunting habits and prey choices of the southern owls vary from the northern owls. These variances can lead to evolved differences in the owls, and speciation likely will occur.

Adaptive Radiation

In some cases, a population of one species disperses throughout an area, and each population 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. This is called adaptive radiation because many adaptations evolve from a single point of origin; thus, 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 six shown in Figure 18.15.

The illustration shows a wheel with the founder species at the hub. The spokes of the wheel are six modern honeycreeper species that evolved from the founder species. Five of these birds eat insects and/or nectar and have long, thick beaks: the Apapane, ‘Iwi, Amakihi, Akiapola’au and Maui Parrotbill. The Nihoa Finch has a short, fat beak and eats insects, seeds, and bird eggs.
Figure 18.15 The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics.

Notice the differences in the species’ beaks in Figure 18.15. Evolution 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 seed-eating bird has 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. Darwin’s finches are another example of adaptive radiation in an archipelago.

Link to Learning

Click through this interactive site to see how island birds evolved in evolutionary increments from 5 million years ago to today.

Refer to [link]
Name three examples of adaptive radiation, and provide a brief statement about each one.
  1. Domestic dogs – There are over 300 distinct dog breeds. Cows – There are over 800 cow breeds recognized worldwide. Domestic cats – Cats have changed drastically in just a few thousand years.
  2. Whales and fish – Although it has been roughly 400 million years since fish and mammals diverged, whales and fish are morphologically similar. Birds and butterflies – Although the common ancestor between vertebrates and insects lived even longer ago than the common ancestor between whales and fish, birds and butterflies both developed flight. Rabbits and kangaroos – Although it has been over 150 million years since their divergence, rabbits and kangaroos both developed powerful jumping legs.
  3. Hawaiian silverswords-There are about 30 species evolved from one parent species. Madagascar lemurs-Their common ancestor likely arrived to Madagascar over 60 million years ago. Hawaiian fruit fly-There are 500 species of fruit fly from one parent species.
  4. Beetles – There are about 350,000 species of beetles that we know of. Birds – There are almost 10,000 species of birds in existence. Frogs – There are almost 5,000 species of frogs worldwide.

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; the prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” A number of mechanisms for sympatric speciation have been proposed and studied.

One form of sympatric speciation can begin with a serious chromosomal error 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 extra sets of chromosomes in a condition that we call polyploidy (Figure 18.16).

Visual Connection

Aneuploidy results when chromosomes fail to separate correctly during meiosis. As a result, one gamete has one too many chromosomes (n +1), and the other has one too few (n – 1). When the n + 1 gamete fuses with a normal gamete, the resulting zygote has 2n + 1 chromosomes. When the n – 1 gamete fuses with a normal gamete, the resulting zygote has 2n -1 chromosomes.
Figure 18.16 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
Define aneuploidy.
  1. Aneuploidy is a mutation that causes the genome of a cell to break down.
  2. Aneuploidy is the condition of having too many or too few chromosomes as a result of an error during cell division.
  3. Aneuploidy is the condition of having every chromosome be inactive, resulting in an inviable embryo.
  4. Aneuploidy is a prezygotic barrier in which the gametes of two distinct species will not fertilize each other.

Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy (Figure 18.17). The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.

Autopolyploidy results in offspring with two sets of chromosomes. In the example shown, a diploid parent (2n) produces polyploid offspring (4n).
Figure 18.17 Autopolyploidy results when mitosis is not followed by cytokinesis.

For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes produced by this plant species. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n called a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.

The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring called an allopolyploid. The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine. Figure 18.18 illustrates one possible way an allopolyploid can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

Alloploidy results from viable matings between two species with different numbers of chromosomes. In the example shown, species one has three pairs of chromosomes, and species two has two pairs of chromosomes. When a normal gamete from species one (with three chromosomes) fuses with a polyploidy gamete from species two (with two pairs of chromosomes), a zygote with seven chromosomes results. An offspring from this mating produces a polyploid gamete, with seven chromosomes. If this polyploid gamete fuses with a normal gamete from species one, with three chromosomes, the resulting offspring will have ten viable chromosomes.
Figure 18.18 Alloploidy results when two species mate to produce viable offspring. In the example shown, a normal gamete from one species fuses with a polyploidy gamete from another. Two matings are necessary to produce viable offspring.

The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations described here are unlikely to survive and produce normal offspring.) Scientists have discovered more than half of all plant species studied relate back to a 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.

Science Practice Connection for AP® Courses


Create a visual representation such as a diagram with annotation to explain how island chains provide ideal conditions for allopatric speciation and adaptive radiation to occur. Then design a plan for collecting data to support the claim that speciation has occurred.

Think About It

  • Two species of fish had recently undergone sympatric speciation. The males of each species had a different coloring through which the females could identify and choose a partner from her own species. After some time, pollution made the lake so cloudy that it was hard for females to distinguish colors. What might take place in this situation?
  • In a lake where most fish of a single species exhibit colorful stripes, a few individual animals have muted colors. The local fisherman receives a large order to catch the most colorful fish for a local aquarium store. The fisherman casts wide nets across the lake to catch a large number of the fish. He then keeps the colorful fish for the aquarium and throws back the dull colored fish. How will this single event change the make-up of the fish population?

Teacher Support

  • This activity is an application of AP® Learning Objective 1.21 and Science Practice 4.2 because students are designing a plan to support the claim that islands provide the ideal conditions for allopatric speciation and adaptive radiation.
  • The first Think About It question is an application of AP® Learning Objective 1.22 and Science Practice 6.4 because students are making a prediction based on observations and a model of natural selection. Students also address questions relating to reproductive isolation and speciation (Learning Objective 1.23 and Science Practice 4.1).
  • In the case of the first Think About It question, the females will likely attempt to breed with members of both species. However, because the two species can no longer interbreed, only females that mate with males of the same species will have offspring. This may eventually drive the evolution of other distinguishing traits, such as chemical signals, so that females can better identify males of their own species.
  • The second Think About It question is an application of AP® Learning Objective 1.24 and Science Practice 6.4 because students are making a prediction based on observations and a model of natural selection.
  • In the second Think About It question, the gene frequency here will be modified as a consequence of a random event which drastically changes the composition of the population and is an example of genetic drift and the bottle-neck effect. The fish in the lake represent an isolated population on which genetic drift will act rapidly.

Reproductive Isolation

Given enough time, the genetic and phenotypic divergence between populations will affect characters that influence reproduction: if individuals of the two populations were to be brought together, mating would be less likely, but if mating occurred, offspring would be non-viable or infertile. Many types of diverging characters may affect the reproductive isolation (the inability to interbreed) of the two populations.

Reproductive isolation can take place in a variety of ways. Scientists organize them into two groups: prezygotic barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of the development of an organism that reproduces sexually. Therefore, a prezygotic barrier is a mechanism that blocks reproduction from taking place; this includes barriers that prevent fertilization when organisms attempt reproduction. A postzygotic barrier occurs after zygote formation; this includes organisms that don’t survive the embryonic stage and those that are born sterile.

Some types of prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times of the year, often just annually. Differences in breeding schedules, called temporal isolation, can act as a form of reproductive isolation. For example, two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May (Figure 18.19).

Photo a shows Rana aurora, a beige frog with green spots. Photo b shows Rana boylii, a brown frog.
Figure 18.19 These two related frog species exhibit temporal reproductive isolation. (a) Rana aurora breeds earlier in the year than (b) Rana boylii. (credit a: modification of work by Mark R. Jennings, USFWS; credit b: modification of work by Alessandro Catenazzi)

In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the other populations of the same species. This situation is called habitat isolation. Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically independent. For example, a cricket population that was divided after a flood could no longer interact with each other. Over time, the forces of natural selection, mutation, and genetic drift will likely result in the divergence of the two groups (Figure 18.20).

Illustration A shows the black Gryllus pennsylvanicus cricket on sandy soil, and illustration B shows the beige Gryllus firmus cricket in grass.
Figure 18.20 Speciation can occur when two populations occupy different habitats. The habitats need not be far apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil, and the cricket (b) Gryllus firmus prefers loamy soil. The two species can live in close proximity, but because of their different soil preferences, they became genetically isolated.

Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction from taking place. For example, male fireflies use specific light patterns to attract females. Various species of fireflies display their lights differently. If a male of one species tried to attract the female of another, she would not recognize the light pattern and would not mate with the male.

Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place; this is called a gametic barrier. Similarly, in some cases closely related organisms try to mate, but their reproductive structures simply do not fit together. For example, damselfly males and females of different species have differently shaped reproductive organs. If one species tries to mate with another, their body parts simply do not fit together. (Figure 18.21).

Illustrations show four different types of damselfly reproductive organs. Each organ has a hook, but the shape and length of the hook varies, as does the shape of the organ itself.
Figure 18.21 The shape of the male reproductive organ varies among male damselfly species, and is only compatible with the female of the same species. Reproductive organ incompatibility keeps each species reproductively isolated.

In plants, certain structures aimed to attract one type of pollinator simultaneously prevent a different pollinator from accessing the pollen. The tunnel through which an animal must access nectar can vary widely in length and diameter, which prevents the plant from being cross-pollinated with a different species (Figure 18.22).

Illustration a shows a bee collecting pollen from a bright purple foxglove flower. The bee’s body fits inside the bell-like flower. Illustration B shows a hummingbird drinking nectar from a long tube-like trumpet creeper flower.
Figure 18.22 Some flowers have evolved to attract certain pollinators. The (a) wide foxglove flower is adapted for pollination by bees, while the (b) long, tube-shaped trumpet creeper flower is adapted for pollination by hummingbirds.

When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages. This is called hybrid inviability because the hybrid organisms simply are not viable. In another postzygotic situation, reproduction leads to the birth and growth of a hybrid that is sterile and unable to reproduce offspring of their own; this is called hybrid sterility.

Habitat Influence on Speciation

Sympatric speciation may also take place in ways other than polyploidy. For example, consider a species of fish that lives in a lake. As the population grows, competition for food also grows. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that was unused by the other fish. What if this new food source was found at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish; therefore, they would breed together as well. Offspring of these fish would likely behave as their parents: feeding and living in the same area and keeping separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them.

This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. Figure 18.23 shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location but have come to have different morphologies that allow them to eat various food sources.

Illustrations show two species of cichlid fish which are similar in appearance except that one has thin lips, and one has thick lips.
Figure 18.23 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.
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