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Learning Goals
By the end of this reading you should be able to:
- Explain how species are defined
- Describe genetic variables that lead to speciation
- Identify prezygotic and postzygotic reproductive barriers
- Compare and contrast allopatric and sympatric speciation
Introduction
Populations are considered to be the key evolutionary unit but what are populations composed of? Populations consist of are of individuals that are of the same species that live in a distinct area or environment and reproduce with each other. To understand the role of populations in the evolution of species we need to understand how we define species. You will find that in some cases it is obvious that organisms are of different species, but in others, it is not as simple.
Species and the Ability to Reproduce
The biological definition of species, which works for sexually reproducing organisms, is a group of actual or potential interbreeding individuals. According to this definition, one species is distinguished from another when, in nature, it is not possible for mating 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 (Fig. 1). 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.
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 (Fig. 3). If humans were to artificially intervene and fertilize a bald eagle’s egg with an African fish eagle’s sperm 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.
While the inability to reproduce or produce viable offspring is the key to the definition of species that is most commonly used there are other ways in which species are defined. These can include separations based on the ecological area where the organism lives, the resources that the organism uses (particularly in bacteria and archaea), the behavioral patterns of the populations, and in some cases the specific sequences of genes in their DNA. It’s important to remember that it is humans who separate organisms into species and not a way that organisms identify themselves.
Review Question:
While the ability to produce fertile offspring is one means of defining species, what other characters can be used? (Multiple Answers)
A) the morphology of the organisms (what they look like)
B) the habitat in which the organisms live
C) behaviors within populations
D) the resources that the organisms use
Speciation
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 in On the Origin of Species (Fig. 4). Compare this illustration to the diagram of elephant evolution, 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.
For speciation to occur, two new populations must form 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. However, there is no reason why more than two species might not form at one time except that it is less likely and multiple events are most likely single splits occurring close in time.
Allopatric Speciation
A geographically continuous population has a gene pool that is relatively homogeneous. This is in part because gene flow, the movement of alleles across a species’ range, is relatively free, individuals can move and then mate with individuals in their new location. Thus, an allele’s frequency at one end of a distribution will be similar to the allele’s frequency at the other end. When populations become geographically discontinuous, it prevents alleles’ free-flow. When that separation lasts for a period of time, the two populations have the potential to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci can gradually become increasingly 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 creating 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 organism’s biology 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 a new lake divided two rodent populations continued gene flow would be unlikely; therefore, speciation would be more likely.
Additionally, scientists have found that the further 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.
Review Question:
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. We call the process of speciation within the same space sympatric. The prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” Scientists have proposed and studied many mechanisms that could lead to sympatric speciation, the formation of separate species in the same area.
One mechanism is the occurrence of a major chromosomal error during cell division. In a normal cell, during division chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs can fail to separate properly and the resulting cell product has too many or too few individual chromosomes, a condition that is called aneuploidy (Fig. 6).
Review Question:
Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. This is the result of an error in meiosis in which all of the chromosomes move into one cell instead of separating into two cells. Scientists have identified two main types of polyploidy that can lead to reproductive isolation (the inability to interbreed) of an individual in the polyploidy state. In autopolyploidy will have two or more complete sets of chromosomes from its own species.
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 (Fig. 7). These new gametes will be incompatible with the normal gametes that this plant species produces. 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 that we call a tetraploid. These individuals would only be able to reproduce with other individuals of this new kind and not those of the ancestral species, thus reproductively isolating them from the ancestral population.
The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring that we call an allopolyploid. Figure 8 illustrates one possible way an allopolyploid (or alloploid) can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.
The cultivated forms of wheat, cotton, and tobacco plants are allopolyploids. Although polyploidy occurs occasionally in animals, it more likely to result in viable offspring in plants. Animals with any of the types of chromosomal aberrations that have been described there are unlikely to survive and produce normal offspring. Scientists have discovered more than half of all plant species studied relate back to a species that evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more like an adaptation than as an error.
Review Question:
Summary
One of the key factors in defining species and in the process of speciation is the ability of populations of organisms to reproduce with each other. When there are populations that are isolated geographically this can prevent the actual movement of individuals between the populations, hence preventing gene flow. Over time if the environments in which the two populations inhabit are different or the populations are subjected to different selection pressures the two populations can become genetically different enough to prevent reproduction between them and allopatric speciation would have occurred. It is not always necessary for a population or populations to be geographically isolated for them to speciate.
End of Section Review Questions:
Review: Defining Species. (Multiple Answer)
1) While the ability to produce fertile offspring is one means of defining species, what other characters can be used? (Multiple Answer)
A) the habitat in which the organisms live
B) the morphology of the organisms (what they look like)
C) behaviors within populations
D) the resources that the organisms use
Review: Speciation Patterns
2) __________ speciation involves geographic separation of a population while __________ speciation occurs in a population in the same location.
Review: Allopatric and Sympatric Speciation
3) A major difference between allopatric and sympatric speciation is whether...?
Attribution
This material is adapted from OpenStax Biology 2nd Edition, Biology 2e. OpenStax CNX. Nov 26, 2018 http://cnx.org/contents/8d50a0af-948b-4204-a71d-4826cba765b8@15.1.
Figure 1. Speciation in cichlid Fishes: courtesy of CNX OpenStax /CC BY 4.0
Figure 2. Dog Breeding; courtesy of CNX OpenStax / CC BY 4.0 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
Figure 3. Eagle species: courtesy of CNX OpenStax / CC BY 4.0 credit a: modification of work by Nigel Wedge; credit b: modification of work by U.S. Fish and Wildlife Service.
Figure 4. Elephant Evolution: courtesy of CNX OpenStax / CC BY 4.0
Figure 5. Owl Speciation: courtesy of CNX OpenStax / CC BY 4.0 credit “northern spotted owl”: modification of work by John and Karen Hollingsworth; credit “Mexican spotted owl”: modification of work by Bill Radk.
Figure 6. Aneuploidy: courtesy of CNX OpenStax / CC BY 4.0
Figure 7. Autopolyploidy: courtesy of CNX OpenStax / CC BY 4.0
Figure 8. Allopolyploidy: courtesy of CNX OpenStax / CC BY 4.0
B, C, D
transition substitution
Or percentage, or...
Rates, frequencies, percentage... all the same thing here.
480
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