Brent Cornell

    
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) 

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)

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 versus Sympatric Speciation

    
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

Polyploidy

Fertile polyploid offspring will typically require two polyploid parents (unless allopolyploidy occurs)

• This is because reproduction with the original parent population results in offspring with an uneven number of chromosome sets
• Example:  diploid gamete + haploid gamete = infertile triploid zygote (cannot halve an uneven number when forming gametes)

Consequently, polyploidy is far more common in plant species as they may lack separate sexes or can reproduce asexually

• Self-pollination – many plant species possess both male and female reproductive parts (monoecious) and can hence self fertilise
• Asexual reproduction – infertile polyploids can still reproduce asexually via vegetative propagation

Polyploid crops may be particularly desirable to farmers for a number of reasons:

• Allows for the production of seedless fruits (e.g. triploid watermelons are infertile and hence do not produce seeds)
• Polyploid crops will typically grow larger and demonstrate improved longevity and disease resistance (hybrid vigour)

Consequently, farmers may induce polyploidy in certain plant species by treating plants with certain drugs (e.g. colchicine)

Hybrid Vigour in Polyploids as a Result of Increased Gene Dosage

Speciation in Allium

The genus Allium is comprised of monocotyledonous flowering plants and includes onions, garlic, chives and leeks

• In many of these species polyploidy has occurred, resulting in reproductively isolated populations with distinct phenotypes

Examples of polyploidy in Allium species includes:

• Diploid (2n) = ~ 16 chromosomes (e.g. Allium cepa – garden onion)
• Triploid (3n) = ~ 24 chromosomes (e.g. Allium carinatum – keeled garlic)
• Tetraploid (4n) = ~ 32 chromosomes (e.g. Allium tuberosum – chinese chives)
• Pentaploid (5n) = ~ 40 chromosomes (e.g. Allium oleraceum – field garlic)
• Octoploid (8n) = ~ 66 chromosomes (e.g. Allium nutans – blue chives)