18.2: Formation of New Species

18.2.1: The Biological Species Concept

Learning Objective

Explain the biological species concept

Key Points

• Members of the same species are similar both in their external appearance and their internal physiology; the closer the relationship between two organisms, the more similar they will be in these features.
• Some species can look very dissimilar, such as two very different breeds of dogs, but can still mate and produce viable offspring, which signifies that they belong to the same species.
• Some species may look very similar externally, but can be dissimilar enough in their genetic makeup that they cannot produce viable offspring and are, therefore, different species.
• Mutations can occur in any cell of the body, but if a change does not occur in a sperm or egg cell, it cannot be passed on to the organism's offspring.

Key Terms

species

a group of organsms that, in nature, are capable of mating and producing viable, fertile offspring

hybrid

offspring resulting from cross-breeding different entities, e.g. two different species or two purebred parent strains

gene pool

the complete set of unique alleles that would be found by inspecting the genetic material of every living member of a species or population

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's 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 .

Interbreeding in Dogs

Dogs of different breeds still have the ability to reproduce. The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo.

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 . 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.

Species Similarity & Reproduction

Species that appear similar may not be able to reproduce. The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species.

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.

18.2.2: Reproductive Isolation

Learning Objective

Explain how reproductive isolation can result in speciation

Key Points

• Reproductive isolation can be either prezygotic (barriers that prevent fertilization) or postzygotic (barriers that occur after zygote formation such as organisms that die as embryos or those that are born sterile).
• Some species may be prevented from mating with each other by the incompatibility of their anatomical mating structures, or a resulting offspring may be prevented by the incompatibility of their gametes.
• Postzygotic barriers include the creation of hybrid individuals that do not survive past the embryonic stages (hybrid inviability) or the creation of a hybrid that is sterile and unable to produce offspring (hybrid sterility).
• Temporal isolation can result in species that are physically similar and may even live in the same habitat, but if their breeding schedules do not overlap then interbreeding will never occur.
• Behavioral isolation, in which the behaviors involved in mating are so unique as to prevent mating, is a prezygotic barrier that can cause two otherwise-compatible species to be uninterested in mating with each other.
• Behavioral isolation, in which the behaviors involved in mating are so unique as to prevent mating, is a prezygotic barrier that can cause two otherwise compatible species to be uninterested in mating with each other.

Key Terms

hybrid sterility

a situation in which a mating between two individuals creates a hybrid that is sterile

hybrid inviability

a situation in which a mating between two individuals creates a hybrid that does not survive past the embryonic stages

postzygotic barrier

a mechanism that blocks reproduction after fertilization and zygote formation

prezygotic barrier

a mechanism that blocks reproduction from taking place by preventing fertilization

behavioral isolation

the presence or absence of a specific behavior that prevents reproduction between two species from taking place

temporal isolation

factors that prevent potentially fertile individuals from meeting that reproductively isolate the members of distinct species

reproductive isolation

a collection of mechanisms, behaviors, and physiological processes that prevent two different species that mate from producing offspring, or which ensure that any offspring produced is not fertile

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 improbable, but if mating did occur, offspring would be non-viable or infertile. Many types of diverging characters may affect reproductive isolation, the ability to interbreed, of the two populations. Reproductive isolation is a collection of mechanisms, behaviors, and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not fertile.

Scientists classify reproductive isolation in 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 .

Temporal isolation

These two related frog species exhibit temporal reproductive isolation. (a) Rana aurora breeds earlier in the year than (b) Rana boylii.

In some cases, populations of a species move to a new habitat and take up residence in a place that no longer overlaps with other populations of the same species; this 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 .

Habitat isolation

Speciation can occur when two populations occupy different habitats. The habitats need not be far apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil, while 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 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 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 of different species have differently-shaped reproductive organs. If one species tries to mate with the female of another, their body parts simply do not fit together. .

Differences in reproductive structures in male damselflies

The shape of the male reproductive organ varies among male damselfly species and is only compatible with the female of that species. Reproductive organ incompatibility keeps the 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 in length and diameter, which prevents the plant from being cross-pollinated with a different species .

Reproductive isolation in plants

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 humming birds.

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. 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.

18.2.3: Speciation

Learning Objective

Define speciation and discuss the ways in which it may occur

Key Points

• For the majority of species, the definition of a species is a group of animals that can potentially interbreed, although some different species are capable of producing hybrid offspring.
• Darwin was the first to envision speciation as the branching of two or more new species from one ancestral species; indicated by a diagram he made that bears a striking resemblance to modern-day phylogenetic diagrams.
• For a new species to be formed from an old species, certain events or changes must occur such that the new population is no longer capable of interbreeding with the old one.
• Speciation can occur either through allopatric speciation, when a population is geographically separated from one another, or through sympatric speciation, in which the two new species are not geographically separated.
• Speciation, the formation of two species from one original species, occurs as one species changes over time and branches to form more than one new species.

Key Terms

sympatric

living in the same territory without interbreeding

speciation

the process by which new distinct species evolve

allopatric

not living in the same territory; geographically isolated and thus unable to crossbreed

Speciation

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: the speciation process may not yet be completed.

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, which bears some resemblance to the more modern phylogenetic diagram of elephant evolution . The 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.

The Evolution of Species

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 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.

18.2.4: Allopatric Speciation

Learning Objective

Give examples of allopatric speciation

Key Points

• When a population is geographically continuous, the allele frequencies among its members are similar; however, when a population becomes separated, the allele frequencies between the two groups can begin to vary.
• If the separation between groups continues for a long period of time, the differences between their alleles can become more and more pronounced due to differences in climate, predation, food sources, and other factors, eventually leading to the formation of a new species.
• Geographic separation between populations can occur in many ways; the severity of the separation depends on the travel capabilities of the species.
• Allopatric speciation events can occur either by dispersal, when a few members of a species move to a new geographical area, or by vicariance, when a natural situation, such as the formation of a river or valley, physically divide organisms.
• When a population disperses throughout an area, into new, different and often isolated habitats, multiple speciation events can occur in which the single original species gives rise to many new species; this phenomenon is called adaptive radiation.

Key Terms

adaptive radiation

the diversification of species into separate forms that each adapt to occupy a specific environmental niche

vicariance

the separation of a group of organisms by a geographic barrier, resulting in differentiation of the original group into new varieties or species

dispersal

the movement of a few members of a species to a new geographical area, resulting in differentiation of the original group into new varieties or species

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 continues for a period of time, the two populations are able to evolve along different trajectories. This is known as allopatric speciation. 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 improbable; therefore, speciation would be probably occur.

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

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 .

Allopatric speciation due to geographic separation

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.

Additionally, scientists have found that the further the 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 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, resulting in speciation.

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 .

Adaptive Radiation

The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics.

In Hawaiian honeycreepers, 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.

18.2.5: Sympatric Speciation

Learning Objective

Give examples of sympatric speciation

Key Points

• Sympatric speciation can occur when one individual develops an abnormal number of chromosomes, either extra chromosomes (polyploidy) or fewer, such that viable interbreeding can no longer occur.
• When the extra sets of chromosomes in a polyploid originate with the individual because their own gametes do not undergo cytokinesis after meiosis, the result is autopolyploidy.
• When individuals of two different species reproduce to form a viable offspring, such that the extra chromosomes come from two different species, the result is an allopolyploid.
• Once a species develops an abnormal number of chromosomes, it can then only interbreed with members of the population that have the same abnormal number, which can lead to the development of a new species.

Key Terms

sympatric speciation

the process through which new species evolve from a single ancestral species while inhabiting the same geographic region

allopolyploid

having multiple complete sets of chromosomes derived from different species

autopolyploid

having more than two sets of chromosomes, derived from the same species, as a result of redoubling

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 too many or too few individual chromosomes in a condition called aneuploidy .

Aneuploidy of chromosomes

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

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, or the inability to interbreed with normal individuals, of an individual in the polyploidy state. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy . 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.

The generation of autopolyploidy

Autopolyploidy results when meiosis 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: 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 . Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

The generation of allopolyploidy

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.

Attributions

• The Biological Species Concept
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "hybrid." http://en.wiktionary.org/wiki/hybrid. Wiktionary CC BY-SA 3.0.
  • "gene pool." http://en.wiktionary.org/wiki/gene_pool. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44574/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_01abc.jpg. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_02ab.jpg. OpenStax CNX CC BY 3.0.
• Reproductive Isolation
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44574/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "Reproductive isolation." http://en.wikipedia.org/wiki/Reproductive_isolation. Wikipedia CC BY-SA 3.0.
  • "temporal isolation." http://en.wikipedia.org/wiki/temporal%20isolation. Wikipedia CC BY-SA 3.0.
  • "reproductive isolation." http://en.wikipedia.org/wiki/reproductive%20isolation. Wikipedia CC BY-SA 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_12.jpg. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_11.jpg. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_09ab.jpg. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_10ab.jpg. OpenStax CNX CC BY 3.0.
• Speciation
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44574/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "allopatric." http://en.wiktionary.org/wiki/allopatric. Wiktionary CC BY-SA 3.0.
  • "sympatric." http://en.wiktionary.org/wiki/sympatric. Wiktionary CC BY-SA 3.0.
  • "speciation." http://en.wiktionary.org/wiki/speciation. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_03ab.jpg. OpenStax CNX CC BY 3.0.
• Allopatric Speciation
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44574/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "vicariance." http://en.wiktionary.org/wiki/vicariance. Wiktionary CC BY-SA 3.0.
  • "dispersal." http://en.wiktionary.org/wiki/dispersal. Wiktionary CC BY-SA 3.0.
  • "adaptive radiation." http://en.wiktionary.org/wiki/adaptive_radiation. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_04.jpg. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_05.jpg. OpenStax CNX CC BY 3.0.
• Sympatric Speciation
  • "Boundless." http://www.boundless.com/. Boundless Learning CC BY-SA 3.0.
  • "OpenStax College, Biology. October 16, 2013." http://cnx.org/content/m44574/latest/?collection=col11448/latest. OpenStax CNX CC BY 3.0.
  • "sympatric speciation." http://en.wikipedia.org/wiki/sympatric%20speciation. Wikipedia CC BY-SA 3.0.
  • "allopolyploid." http://en.wiktionary.org/wiki/allopolyploid. Wiktionary CC BY-SA 3.0.
  • "autopolyploid." http://en.wiktionary.org/wiki/autopolyploid. Wiktionary CC BY-SA 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_08.jpg. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_07.jpg. OpenStax CNX CC BY 3.0.
  • "OpenStax College, Formation of New Species. October 16, 2013." http://cnx.org/content/m44574/latest/Figure_18_02_06.png. OpenStax CNX CC BY 3.0.