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Speciation is an evolutionary process that results in the formation of a new species from a pre-existing species
- It occurs when reproductive isolating mechanisms prevent two breeding organisms from producing fertile, viable offspring
There are two basic mechanisms via which speciation can occur:
- Allopatric speciation (geographical isolation)
- Sympatric speciation (reproductive isolation)
The key to one species diverging into two or more different species is establishing a barrier to gene flow. That is, two or more populations of a species must be prevented from sharing the same pool of genes.
If the barrier is geographic (allopatric speciation), over time the populations may acquire their own, unique genetic changes. After enough changes have been introduced, the two populations may no longer be able to interbreed even if they became reunited geographically. In this way, geographic isolation may lead to reproductive isolation and the formation of new species. Allopatric speciation is thought to be the main mechanism of speciation among most groups of organisms.
In the case of sympatric speciation, two populations become reproductively isolated while they live in the same geographic location. These events are considered to be less common than allopatric speciation. However, many new plant species do emerge through sympatric speciation, by simply duplicating their sets of chromosomes and becoming polyploid. Plants with different numbers of chromosomes cannot mate with their parent species to produce fertile offspring and are therefore distinct species.
Sympatric Speciation
Sympatric speciation is divergence of species within the same geographical location (i.e. without a physical barrier)
- Sympatric speciation may result from the reproductive isolation of two populations as a result of genetic abnormalities
- Typically, a chromosomal error may arise which prevents successful reproduction with any organism lacking the same error
Allopatric Speciation
Allopatric speciation occurs when a geographical barrier physically isolates populations of an ancestral species
- The two populations begin to evolve separately as a result of cumulative mutation, genetic drift and natural selection
- Eventually the two populations reach a degree of genetic divergence whereby they can no longer interbreed (speciation)
Video regarding Speciation Mechanisms
Allopatric and Sympatric
The Founder Effect
The Founder Effect
Sometimes, one species diverges into two by a process called speciation. Speciation can occur when a population becomes divided by an insurmountable physical barrier, such as a mountain range or a body of water. This process is called allopatric speciation, and it is thought to be the way that most new species form. Genetic divergence between the two geographically divided populations can arise for a variety of reasons, often because their environments become different from each other.
Allopatric speciation can also result when a small number of individuals from a population cross an existing barrier and create a new, isolated population. Such a population differs from its parent population because of a phenomenon called the founder effect—the small group of founding individuals has only an incomplete representation of the gene pool of its parent population.
Sympatric speciation is most commonly caused as the result of a meiotic failure during gamete formation
- If meiotic cells fail to undergo cytokinesis, chromosomal number will double in the gamete (e.g. diploid instead of haploid)
- This will result in offspring that have additional sets of chromosomes (polyploidy)
- Speciation will result if the polyploid offspring are viable and fertile but cannot interbreed with the original parent population
Adaptive Radiation
Adaptive Radiation
Adaptive radiation Adaptive radiation occurs when many similar but distinct species evolve relatively rapidly from a single species or from a small number of species. This happens as variations within a population allow certain members to exploit a slightly different niche in a more successful way. A niche is a position or role within a community of an ecosystem. By natural selection and the presence of some kind of barrier, a new species can evolve. A barrier separating populations might be a mountain range or a body of water.
An example of this are the primates found in Madagascar and the Comoro Islands off the southeast coast of Africa. Millions of years ago, without competition from monkeys or apes, lemurs on these islands were able to proliferate. Large numbers of offspring meant a greater chance for diversity.
Among the wide range of variation in lemur species, some are better adapted for living on the ground instead of in trees. Others are better adapted for living in lush rainforests, while some can survive in the desert. Most lemurs are active during the day (diurnal) but some are nocturnal. The reason why there are so many different species of lemur with different specialties is because of adaptive radiation.
Not a single species of living lemur has been found anywhere else in the world. And yet fossils of their ancestors have been found on the continents of Africa, Europe, and Asia. What happened? It is believed that lemurs were not successful in competing with apes and monkeys, because as soon as traces of the latter start to become more prevalent in the fossil record, the lemur-like organisms become rare.
How does Natural Selection Lead to Evolution
How does Natural Selection Lead to Evolution
It is crucial that you remember Darwin’s steps of how natural selection leads to evolution. Be sure to memorize the following:
(1) overproduction of offspring
(2) variation within the population, as a result of meiosis, sexual reproduction, and mutations
(3) struggle for survival, because there are not enough resources for all members of the population
(4) differential survival, those individuals best fit for their environment tend to survive better
(5) reproduction, those who survive can pass on their genes to the next generation.
It is through these steps that populations evolve. Remember that, even though the changes can be observed in individuals from generation to generation, what is of importance is what happens at the level of populations rather than at the individual level.
Exam Style Question
Exam Style Question
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