10.3.7

 Speciation in the genus Allium by polyploidy

What is Speciation?

The formation of new species due to evolution.

What are the two ways in which speciation can occur?

Gradualism - small, continuous and slow changes

Punctuated gradualism - Abrupt changes followed by long periods of no change.

What is a Polyploidy?

Haploid = One set of chromosomes = monoploidy

Diploid = 2 sets of chromosomes

3n = triploid       4n = tetraploid    5n = pentaploid

A cell which has three or more sets of chromosomes is called a polyploidy.

This is more common in plants than animals and is caused when cells do not divide properly.

Speciation of the genus Allium by polyploidy:

Allium genus - Plants such as onions, garlic, shallots and leeks

Some have 8 chromosomes, some are tetraploid, some have 24 chromosomes and some have 32 chromosomes. This can improve the quality of the fruit as it is more likely to have advantageous genes.  

Ways of Speciation

10.3.6

10.3.6: Identifying examples of directional, stabilizing, and disruptive selection

Directional Selection:

When a phenotype is favoured by another by natural selection.

-The frequency of one phenotype increases while the other’s decreases.

-Occurs when the environment changes.

-Can be considered “selection away from one extreme”

Example:

Industrial melanism - Noticed in the lighter-coloured pepper moth’s (Biston betularia) frequency decreased and the frequency of the darker phenotype increased during the industrial revolution.

Stabilizing Selection:

When one phenotype is favoured over two extreme phenotypes. i.e. an intermediate phenotype will be favoured.

-Can be considered “selection away from two extremes” or “selection towards a mean”

Example:

If a plant has two types of phenotypes in its flowers: 1. More nectar 2. Some nectar. Flowers with more nectar will drain the plant’s sugar resources and flowers with less nectar will not attract insects or discourage insects from returning. Therefore, for survival, the plant will favour a phenotype in which the flowers have an intermediate quantity of nectar, balancing the two extremes.

Disruptive Selection:

When two phenotypes are favoured over one intermediate phenotype.

- This can be advantageous to the species’ survival.

- If the two disruptive phenotypes are extreme and the two populations occupy different niches, speciation can occur.

- Can be considered “selection against the mean”  

Example:

Tadpoles in spadefoot toads have two different morphologies:

1. One with an omnivorous diet (Spea multiplicata)

2. One with a strictly carnivorous diet (Spea bombifrons) which includes cannibalism during food scarcity.

Two separate morphologies (hence, two different phenotypes) give the species a better chance of survival in places where water and food supplies are constantly changing.

10.3.5

The process of an evolving population changing significantly enough so that the production of offspring with the original population becomes impossible is called speciation. (Pearson)

The theory of punctuated equilibrium was a theory that originated in the late 20th century, which stated that speciation, happens quickly, often in response to an environmental change, like a volcanic eruption, a meteorite impact, or a major climate change. In response, some species are destroyed and others adapt to their new surroundings, exploiting the niches made available by the extinction of competing species. (Pearson)

This process is also called cladogenesis. (Dictionary.com)

The frequency of cladogenesis is very low i.e. once every 500,000 years.

Examples:

The theory of punctuate equilibrium is stated as the reason for the extinction of the dinosaurs. Environmental changes such as asteroid collisions and Deccan traps are believed to have brought about the extinction of dinosaurs in a matter of hours. This demonstrates the rapid nature of cladogenesis. (University of Antwerp)

Critics of punctuated equilibrium argue that the ‘jumpy’ effect of this theory could simply be an artefact of the incompleteness of the fossil record. Discontinuities in the fossil lineages are a challenge for scientists to explain. (Pearson)

Works Cited

Dictionary.com. cladogenesis. 28 3 2015 <http://dictionary.reference.com/browse/cladogenesis>.

University of Antwerp. "Survival in the first hours of the Cenozoic." 28 3 2015 <http://uahost.uantwerpen.be/funmorph/raoul/macroevolutie/Robertson2004.pdf>.

Speciation is the formation of new and distinct species in the course of evolution. (Biology-Online)

The reason for abrupt speciation

(1) the sensitive ecological balance between an animal population and its food source;

(2) the ability for a new species to appear suddenly as a result of a gradual accumulation of advantageous recessive genes. (Stagecast.com)

Quantum speciation: This is sudden emergence of new groups by saltation. This speciation is much more rapid and sudden, and produces new species or higher groups. In small, scattered populations or populations that have migrated to new area, genetic drift, rather than natural selection plays an important role in quickly changing the species. Sometimes chance events or mega mutations aid in quick speciation. Disruptive natural selection then makes the species distinct and diverse. (iaszoology.com)  

Articles showing that speciation can occur abruptly:

http://www.kplu.org/post/scientists-washingtons-state-fish-has-remarkable-evolutionary-past

http://www.sciencedaily.com/releases/2014/09/140930090636.ht

Works Cited

"Speciation." - Definition from Biology-Online.org. N.p., n.d. Web. 02 Apr. 2015.

"Sudden Speciation." Sudden Speciation. N.p., n.d. Web. 02 Apr. 2015.

"Speciation | IASzoology.com." IASzoologycom RSS. N.p., n.d. Web. 02 Apr. 2015.

10.3.4

Speciation due to divergence of isolated populations can be gradually

• Gradualism: gradual divergence over long spans of time. The changes are small, continuous and slow. The assumption that large changes occur as an accumulation of many small ones.
• Speciation can occur gradually over long periods of time, with several intermediate forms in between species leading to the current species.
• Can be seen in more complete fossil records; eg, the whale or the horse.
  
  
  
  
  

10.3.3

Reproductive isolation of populations can be temporal, behavioural or geographic

In various situations populations of members of the same species (same gene pool) can be stopped from reproducing together because there is an insurmountable barrier between them. These barriers can be geographical, temporal, behavioural or related to the infertility caused by hybridization.

Geographical Isolation - Occurs when physical barriers (e.g land or water formations) prevent males and females from reaching each other, hence interbreeding is impossible.

Temporal Isolation - Refers to incompatible time frames that prevent populations or their gametes from encountering each other. For example if the female parts of the flowers of one populations of plants reach maturity at a different time compared with the release of pollen of another population, the two will have great difficulty producing offspring together. Hibernation or late return from migration are other temporal barriers between two gene pools.

Behavioural Isolation - Occurs when one populations lifestyle and habitats are not compatible with those of another population. For example many bird species rely on a courtship display in order for one sex to copulate with the other. If the males has a version of courtship display that is significantly different to that of another population, the females might not find it seductive enough to be potential mates. Hence no or little reproduction is likely to occur between the two populations due to behavioural differences.

10.3.2:

 Evolution requires that allele frequencies change with time in populations

Changes within a gene pool occurring from generation to generation is known as microevolution. Allele frequencies in a population may change due to gene flow, genetic drift, natural selection and mutation. These are referred to as the four fundamental forces of evolution. Only mutation can create new genetic variation. the other three forces simply rearrange this variation within and among populations.

Gene pools are usually stable over time, however as a result of a mutation new alleles can be introduced, and old alleles can disappear when the last organism carrying the allele dies. A result of evolution is that after many generations of natural selection, some alleles prove to be advantageous and tend to be more frequent.

Particular alleles can limit the survival of the organisms in the population, and are not passed on to as many offspring. An allele frequency can be estimated at any time, however it is only a snapshot of the alleles at that particular time. In future generations, the proportions of the alleles would have changed.

If specific populations mix because of immigrations, a change in allele frequencies will occur. This remains the same for emigrations, when one group with a particular allele leaves the population it will change. If a gene pool is modified and the allele frequencies change, some degree of evolution has happened. No change in allele frequencies, however, means no evolution.

In order to calculate the frequencies of alleles, genotypes and phenotypes within a population, the Hardy- Weinberg equation is used.  This equation is useful to determine how fast a population is changing, or predicting the outcomes of matings.

10.3.1

A gene pool consists of all the genes, and their different alleles, present in an interbreeding population

The gene pool: All the genetic information present in the reproducing members of
 a population at a given time. A large gene pool occurs in a population that shows considerable variety in its traits, whereas a small gene pool occurs in a population whose members show little variation, due to inbreeding.

Inbreeding: Is the practice of closely related organisms mate with one another.

Allele frequency: Is a measure of the amount of a specific variation of a gene in a population. It is usually expressed as a proportion or percentage.

Figure: allele frequencies of geographically isolated populations.