Formation of New Species
The Biological Species Concept
A species is defined as a group of individuals that, in nature, are able to mate and produce viable, fertile offspring.Learning Objectives
Explain the biological species conceptKey Takeaways
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.
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.
Reproductive Isolation
Reproductive isolation, through mechanical, behavioral, and physiological barriers, is an important component of speciation.Learning Objectives
Explain how reproductive isolation can result in speciationKey Takeaways
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
- 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
- temporal isolation: factors that prevent potentially fertile individuals from meeting that reproductively isolate the members of distinct species
- behavioral isolation: the presence or absence of a specific behavior that prevents reproduction between two species from taking place
- prezygotic barrier: a mechanism that blocks reproduction from taking place by preventing fertilization
- postzygotic barrier: a mechanism that blocks reproduction after fertilization and zygote formation
- hybrid inviability: a situation in which a mating between two individuals creates a hybrid that does not survive past the embryonic stages
- hybrid sterility: a situation in which a mating between two individuals creates a hybrid that is sterile
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.
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.
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.
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.
Speciation
Speciation is an event in which a single species may branch to form two or more new species.Learning Objectives
Define speciation and discuss the ways in which it may occurKey Takeaways
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
- allopatric: not living in the same territory; geographically isolated and thus unable to crossbreed
- speciation: the process by which new distinct species evolve
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.
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.
Allopatric Speciation
Allopatric speciation occurs when a single species becomes geographically separated; each group evolves new and distinctive traits.Learning Objectives
Give examples of allopatric speciationKey Takeaways
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
- vicariance: the separation of a group of organisms by a geographic barrier, resulting in differentiation of the original group into new varieties or species
- adaptive radiation: the diversification of species into separate forms that each adapt to occupy a specific environmental niche
- 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.
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.
Sympatric Speciation
Sympatric speciation occurs when two individual populations diverge from an ancestral species without being separated geographically.Learning Objectives
Give examples of sympatric speciationKey Takeaways
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
- autopolyploid: having more than two sets of chromosomes, derived from the same species, as a result of redoubling
- allopolyploid: having multiple complete sets of chromosomes derived from different species
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
The generation of autopolyploidy: Autopolyploidy results when meiosis is not followed by cytokinesis.
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.
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