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. Clear answers to these questions will likely require experimental studies that measure selection at the genomic level to directly quantify how selection acts on the genome. Nevertheless, the integration of geographic, ecological, and new genomic approaches is likely to yield new insight into speciation over the coming decades.
See text for details. Divergent natural selection : Selection that acts in contrasting directions between two populations, usually with reference to ecological differences between their environments e.
Ecological speciation : A speciation process in which divergent natural selection drives the evolution of reproductive incompatibility i.
Mutation-order speciation : A speciation process in which different and incompatible mutations alleles fix in separate populations that are experiencing similar selective regimes. Dobzhansky-Muller Incompatibility : Hybrid dysfunction arising from negative interactions epistasis between alleles at two or more loci: an allelic substitution at a locus causes no reduction in fitness on its own genetic background, but leads to reduced fitness when placed on the alternative background.
Genomic Island : A region of the genome where differentiation between populations is stronger than expected in the absence of divergent selection stronger than occurs via purely neutral processes such as genetic drift alone.
Natural selection : Differential survival of classes of entities such as alleles which differ in some characteristic s. Sexual selection : Differential reproductive success of classes of entities such as alleles which differ in some characteristic s. Reproductive Isolation : Genetically-based differences between populations which reduce or prevent genetic exchange between them i.
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Speciation: The Origin of New Species. Avian Egg Coloration and Visual Ecology. The Ecology of Avian Brood Parasitism. Cladistics 6: Human Physiology 1. 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. Plant Growth 4. Plant Reproduction Genetics 1. Meiosis 2. Inheritance 3. Speciation Animal Physiology 1.
Antibody Production 2. Each species develops differently based on the demands of their unique habitat or the genetic characteristics of the group that are passed on to offspring. When Arizona's Grand Canyon formed, squirrels and other small mammals that had once been part of a single population could no longer contact and reproduce with each other across this new geographic barrier.
They could no longer interbreed. The squirrel population underwent allopatric speciation. Today, two separate squirrel species inhabit the north and south rims of the canyon. On the other hand, birds and other species that could easily cross this barrier continued to interbreed and were not divided into separate populations. When small groups of individuals break off from the larger group and form a new species, this is called peripatric speciation 2.
As in allopatric speciation, physical barriers make it impossible for members of the groups to interbreed with one another. The main difference between allopatric speciation and peripatric speciation is that in peripatric speciation, one group is much smaller than the other.
Unique characteristics of the smaller groups are passed on to future generations of the group, making those traits more common among that group and distinguishing it from the others. In parapatric speciation 3 , a species is spread out over a large geographic area.
Although it is possible for any member of the species to mate with another member, individuals only mate with those in their own geographic region. Like allopatric and peripatric speciation, different habitats influence the development of different species in parapatric speciation. Instead of being separated by a physical barrier, the species are separated by differences in the same environment. Parapatric speciation sometimes happens when part of an environment has been polluted.
Mining activities leave waste with high amounts of metals like lead and zinc. These metals are absorbed into the soil , preventing most plants from growing. Some grasses, such as buffalo grass , can tolerate the metals. Buffalo grass, also known as vanilla grass, is native to Europe and Asia, but is now found throughout North and South America, too.
Buffalo grass has become a unique species from the grasses that grow in areas not polluted by metals. Long distances can make it impractical to travel to reproduce with other members of the species.
Buffalo grass seeds pass on the characteristics of the members in that region to offspring. Sometimes a species that is formed by parapatric speciation is especially suited to survive in a different kind of environment than the original species. Sympatric speciation 4 is controversial. Sympatric speciation occurs when there are no physical barriers preventing any members of a species from mating with another, and all members are in close proximity to one another.
A new species, perhaps based on a different food source or characteristic , seems to develop spontaneously. The theory is that some individuals become dependent on certain aspects of an environment—such as shelter or food sources—while others do not.
A possible example of sympatric speciation is the apple maggot, an insect that lays its eggs inside the fruit of an apple, causing it to rot. As the apple falls from the tree, the maggots dig in the ground before emerging as flies several months later. The apple maggot originally laid its eggs in the fruit of a relative of the apple—a fruit called a hawthorn. After apples were introduced to North America in the 19th century, a type of maggot developed that only lays its eggs in apples.
The original hawthorn species still only lays its eggs in hawthorns. The two types of maggots are not different species yet, but many scientists believe they are undergoing the process of sympatric speciation. Artificial speciation 5 is the creation of new species by people.
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