Allopatric speciation is the emergence of new species due to a geographic barrier. Genetic drift is random variation in population genetic structure of a single species, which is unrelated to the topic at hand. A physical barrier separates a single species, causing two separate populations to form. Over time these two populations adapt to their environments. Eventually, these two populations are no longer able to successfully reproduce with each other.
Allopatric speciation refers to the process by which a physical barrier separates a single population, causing two or more populations to arise and evolve due to environmental differences to become different species. Examples of factors that can lead to allopatric speciation include island formation, canyon and valley formation, and river paths. Hawks with thin, sharp beaks primarily eat fish and small rodents, while hawks with larger beaks tend to eat reptiles and larger birds.
Houseflies from a certain region migrate and interbreed with a different housefly population in a neighboring area. A river separates members of a squirrel population that used to occupy the same geographical area.
A disease ravages a large fox population, killing all members that did not have a genetic resistance to the disease. Allopatric speciation occurs when a geographical barrier, like a river, mountain, or canyon, separates members of a population.
This barrier prevents the individuals on one side from reproducing with the individuals on the other. In addition, selecting forces may act differently on the two sides of the barrier. This separation eventually results in two distinct species. Here, the only example of allopatric speciation is that regarding the squirrels separated by the river. The example with the hawks refers to sympatric speciation, where no geographical barrier exists, but speciation can still occur due to other stressors.
The remaining choices do not describe speciation at all. Which of the following reasons could explain why sympatric speciation is more common in plants than in animals? Sympatric speciation implies a speciation event while the populations exist within the same geographical region.
Animals moving around more often does not really explain why speciation would occur differently, and plants are not necessarily less prone to chromosomal abnormalities. In fact, plants are more likely to be able to reproduce after abnormal chromosomal inheritance nondisjunction, polyploidy, etc. Instead of wandering around to find a mate, the plant can reproduce with itself and potentially create a reproductively isolated species. Which of the following would be considered an example of conditions leading to allopatric speciation?
Allopatric speciation results in the formation of a new species based on geographic separation of a population from its parent population. Once reproductive barriers emerge in the allopatric population the ability to interbreed with the parent population may be prevented or highly impaired, even if the two populations were to come back into contact.
The other answer choices are factors of sympatric speciation, in which a population can give rise to a new species without geographic isolation. A limited separation of members of a population, followed by reintroduction back into the parent population. Sympatric speciation refers to the evolution of a new species from a parent population without geographic isolation.
The divergence into a new species requires the formation of a reproductive barrier that isolates a subset of the population from the rest, thereby blocking gene flow. The formation of a reproductive barrier can result from polyploidy or natural selection. If a subset of a population chooses to only eat fruit that have fallen from trees while the rest climb the trees to eat, then the subset may eventually evolve different traits.
Polyploidy creates a distinct genetic difference between individuals and can lead to difference phenotypes and reproductive barriers. A species is a group of organisms that can reproduce with one another to produce fertile offspring and is reproductively isolated from other organisms. Speciation can be driven by evolution, which is a process that results in the accumulation of many small genetic changes called mutations in a population over a long period of time.
There are a number of different mechanisms that may drive speciation. One of these is natural selection, which is a process that increases the frequency of advantageous gene variants, called alleles, in a population. Indeed, thanks to the Modern Synthesis, much of current research in Evolutionary Biology is strongly tied to genetics, and current methods for studying speciation are no exception.
As discussed below, the Modern Synthesis led to advances not only in the study of evolution within populations, but also changes in the way species were defined, and in how new species were considered to form. Thus, new species form when individuals from diverging populations no longer recognize one another as potential mates, or opportunities for mating become limited by differences in habitat use or reproductive schedules.
In some cases, these pre-zygotic isolating mechanisms fail to prevent inter-breeding among individuals from separate populations. In these cases, viable hybrids may form, or the consequences of a successful mating attempt may end in failure, either due to the production of inviable zygotes or sterile, non-reproductive offspring. These diverse pre- and post-zygotic barriers are of great importance to speciation biologists because they determine how reproductively-isolated populations are from one another, which indicates how far along the often continuous process of speciation that populations are.
For example, reproductive isolation is weak in the early stages of speciation, but changes to strong or complete in later stages of speciation Figure 2. One or more of the many types of isolating mechanisms may play a role in the evolution of species along a continuum Figure 2. But how and why might reproductive barriers to genetic exchange evolve? Figure 2: Schematic illustration of the continuous nature of divergence during speciation, with three arbitrary points along the speciation continuum depicted.
Numerous types of differentiation can vary quantitatively, with the magnitude of differentiation representing a measure of how far speciation has proceeded. Two headed arrows represent mating between individuals. All rights reserved. A major area of debate among speciation biologists is the geographic context in which it occurs Figure 3. Ernst Mayr emphatically defended his view that speciation was most likely when populations became geographically isolated from one another, such that evolution within isolated populations would lead to enough differences among them that speciation would be an eventual outcome.
The central idea here is that when populations are geographically separated, they will diverge from one another, both in the way they look and genetically. These changes might occur by natural selection or by random chance i. This view of speciation of geographically isolated populations — termed allopatric speciation — is still widely held among speciation biologists as playing a major role in the evolution of biodiversity e.
However, speciation might also occur in overlapping populations that are not geographically isolated i. The problem here is how do populations that are living in the same area, and exchanging genes, diverge from one another? This could occur, for example, if insects adapted to living on different plants within the same geographic region Feder et al. It will be interesting to see how many new examples emerge now that the idea of sympatric speciation is becoming less controversial.
Parapatric speciation refers to populations that are situated in geographic proximity to one another, usually with abutting but non-overlapping ranges.
Here, a small proportion of each population are in actual contact with one another, and thus considered in sympatry, whereas the majority of individuals reside far enough apart that frequent encounters with one another are rare Figure 3. There are putative examples of parapatric speciation in salamanders Niemiller et al. Reprinted from Mallet et al. The s saw a reclassification of modes of speciation away from schemes that focus solely on the geographic mode of divergence and towards a focus on the evolutionary process driving genetic divergence i.
This reclassification was motivated — at least in part — by renewed interest in the extent to which the evolutionary processes which cause adaptation within species also tend to create new species. Further, although the geographic mode of divergence has important implications for speciation via patterns of gene flow and sources of selection, speciation research has reached the point where we can directly test the role of different evolutionary process in driving speciation Butlin et al.
We outline several processes that can drive speciation. Recent years have seen renewed efforts to address these questions. For example, populations living in different ecological environments e. These same evolutionary changes can also result in the populations evolving into separate species. For example, adaptation to different environments might cause differences between populations in the way individuals tend to look, smell, and behave. In turn, these differences might cause individuals from different populations to avoid mating with one another, or hybrids exhibit reduced fitness if mating occurs.
Thus, the populations cease exchanging genes, thereby diverging into separate species because of the adaptive changes that occurred via natural selection.
More specifically, ecological speciation is defined as the process by which barriers to gene flow evolve between populations as a result of ecologically-based divergent selection between environments.
This process makes some simple predictions. For example, ecologically-divergent pairs of populations should exhibit greater reproductive isolation than ecologically-similar pairs of populations of similar age Funk Figure 4 illustrates an example that supports this prediction. Other predictions are that traits involved in divergent adaptation will also cause reproductive isolation, and that levels of gene flow in nature will decrease as ecological differences between populations increase.
Figure 4 Ecological speciation in host-plant associated populations of Timema cristinae walking-stick insects individual populations feed on either the Ceanothus spinosus host plant or on Adenostoma fasciculatum. Pairs of populations feeding on the same host plant species, but in different geographic localities, are ecologically similar and assumed to not be subject to divergent selection. In contrast, pairs of populations feeding on different host plant species are ecologically divergent and subject to divergent selection.
This pattern is independent from neutral genetic divergence, a proxy for time since divergence. A current debate is whether sexual selection can lead to speciation in the absence of ecological divergence van Doorn et al.
Indeed, compelling examples that implicate an important role of sexual selection leading to new species sometimes also involve the evolution of different signals used in mate-selection among populations in different ecological contexts, such as light environment Seehausen et al. Here, signals used in mate-selection become adapted to new ecological environments where the transmission of these traits is more perceptible or audible in a new habitat.
Another mechanism of speciation that involves chance events is speciation by polyploidization. Polyploidy, or the presence of three or more complete sets of chromosomes, has been documented in a wide variety of taxa. Because polyploidy can lead to hybrid infertility, it is viewed as a mechanism that can rapidly lead to the formation of new species, potentially without selection for the divergence of other characters.
Recent advances in genomics now allow such studies to be taken to the genome-wide level, where biologists can examine hundreds of thousands of gene regions, rather than just a handful. A genomic island is any gene region, be it a single nucleotide or an entire chromosome, which exhibits significantly greater differentiation than expected under neutrality i. The metaphor thus draws parallels between genetic differentiation observed along a chromosome and the topography of oceanic islands and the contiguous sea floor through which they are connected.
Following this metaphor, sea level represents the threshold above which observed differentiation is significantly greater than expected by neutral evolution alone. Thus, an island is composed of both directly selected and tightly linked loci. Major remaining questions concern the size, number and distribution i. Digestion 2. The Blood System 3. Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2.
Transcription 3. Translation 8: Metabolism 1. Metabolism 2. Cell Respiration 3. Photosynthesis 9: Plant Biology 1. Xylem Transport 2. Phloem Transport 3.
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