The basic reason why mules cannot reproduce is that a mule does not have a even number of chromosomes. They have 63 chromosomes because a horse and a donkey have different numbers of chromosomes. During mitotic cell division, each of the chromosomes copies itself and then distributes these two copies to the two daughter cells. In contrast, when the mule is producing sperm or egg cells during meiosis, each pair of chromosomes need to pair up with each other. Since the mule doesn't have an even number of homologous pairs, meiosis is disrupted and viable sperm and egg are not formed.
WHAT ARE PREZYGOTIC REPRODUCTIVE BARRIERS?
There are two general categories of reproductive isolating mechanisms: prezygotic, or those that take effect before fertilization, and postzygotic, hybrids between members of different populations through ecological, temporal, ethological, mechanical, and gametic isolation.
WHAT IS THE PHYLOGENETIC SPECIES CONCEPT?
The phylogenetic species concept defines a species as a set of organisms representing a specific evolutionary lineage. The concept of a species as an irreducible group whose members are descended from a common ancestor and who all possess a combination of certain traits. It is less restrictive than the biological species concept. Also it permits successive species to be defined even if they have evolved in an unbroken line of descent, with continuity of sexual fertility. However because slight differences can be found among virtually any group of organisms, the concept tends to encourage extreme division of species into ever-smaller groups.
SUMMARY:
Linnaeus used physical characteristics to distinguish species. His binomial system is the basis of taxonomy, the naming and classification of life's forms. The biological species concept defines a species as a group of populations whose members can interbreed and produce fertile offspring with each other but not with members of other species. Most organisms are classified based on observable phenotypic traits; the morphological species by its ecological niche. According to the phylogenetic species concept, a species is the smallest group that shares a common ancestor and forms one branch on the tree of life.
There are two types of reproductive barriers. Prezygotic barriers, which includes temporal isolation, habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation. Also postzygotic barriers, which includes reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.
Geographically separated from other populations, a small population may become genetically unique as its gene pool is changed by natural selection, mutation, or genetic drift in allopatric speciation. A laboratory study has documented the beginning of reproductive isolation as fruit fly populations adapted to a new food source. Researchers have identified the specific genes involved in some cases of speciation, which is the evolution of reproductive barriers. Hybrid zones are regions in which members of different species overlap and produce at least some hybrid offspring. Over time, reinforcement may strengthen prezygotic barriers to reproduction, or fusion may reverse the speciation process as reproductive barriers weaken and extensive gene flow occurs. In stable hybrid zones, a limited number of hybrid offspring continue to be produced. Adaptive radiation can occur when populations are provided with expanded opportunities following mass extinctions, the colonization of a diverse new environment, or the evolution of new structures. The punctuated equilibrium model draws on the fossil record, where many species change most as they arise from an ancestral species and then change relatively little for the rest of their existence. But some species have evolved by the gradual accumulation of changes. The time interval between speciation events varies considerable, from a few thousand years to tens of millions of years.
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A Mule is a hybrid of a horse and a donkey. However mules cannot mate. Therefore a mule is an animal that never reproduce. A mule is not considered as a specie, because species have to be able to inbreed. In order to make a mule, a horse and a donkey needs to mate.
http://www.youtube.com/watch?v=zl1a1n6XR9g
KEY TERMS:
- Species: as a group of organisms whose members can breed and produce fertile offspring, but who do not produce fertile offspring with members of other groups
- Taxonomy: the branch of biology that names and classifies species and groups them into broader categories
- biological species concept: species as a group of populations whose members have the potential to interbreed in nature and produce fertile offspring
- Morphological species concept: classification is based mainly on phenotype
- Ecological species concept: identification of species in terms of their ecological niches, focusing on unique adaptations to particular roles in a biological community
- Phylogenetic species concept: a species as the smallest group of individuals that shares a common ancestor and that forms one branch on the tree of life.
- Reproductive barrier: a biological feature of the organism itself
- Allopatric speciation: populations separated by a geographic barrier
- Polyploid: their cells have more than two complete sets of chromosomes
- Sympatric speciation: a new species arises within the same geographic area as a parent species
5 FACTS:
1) Reproductive barriers keep species separate
2) In allopatric speciation, geographic isolation leads to speciation
3) In sympatric speciation, speciation takes place without geographic isolation
4) Reproductive barriers may evolve as populations diverge
5) Hybrid zones provide opportunities to study reproductive isolation
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