DPTopic10.3:GenePoolsand

Speciation

Reflection:Whatisaspecies?EssentialIdea:GenePoolschangeovertime

RevisionWhatisagene? Whatisanallele? Evolution involves a change in allele frequency in a population’s gene

pooloveranumberofgenerations.

Insmallgroupsandusingthediagramopposite,suggestadefinitionGenePoolandAlleleFrequency

Species&Speciation1. Define a species. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

2. When does speciation begin? ______________________________________________________________________________________________________________________________ ______________________________________________________________________________________________________________________________

3. What does “reproductive isolation” mean? ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

4. At what point do two isolated species (mainland & island) become separate species? ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Exercise 1: Scan the QR code, watch the video andanswerthesequestions

5.Complete details of the three types of isolation in the table below. Explain how each type of isolation prevents birds from interbreeding.

Type of barrier causingspeciation

How does separationoccur?

Examplefromthevideo

Geographic

Volcanicisland

Behavioural

Matingbehavior

Temporal

Breedingseason

SpeciationSpeciation is the formation of a new species by the splitting of an

existingone.Itrequiresthefollowing:

1. Isolationofgenepools

2. Differentialchangesinallelefrequency

IsolatingMechanismsCanyouthinkofanywaysthatgenepoolscanbecomeisolated?

PolyploidyPolyploidyisthechangeinthenumbersofhomologouschromosomes

ofanorganism.

Polyploidy is a form of chromosomal mutation and causes abrupt

speciationbychromosomalchange.

A polyploid will be formed from the fusion of two diploid gametes.

DiploidgametesmaybeformedwhenthespindlesfailinMeiosis.

Polyploidyisverycommoninplantsandhasresultedintheformation

ofmanynewspecies:

exampleistheoriginofthecultivatedpotatoSolanumtuberosum(2n=

48),anautopolyploidofthesmaller,wildSolanumbrevidens(2n=24)

Speciation and Polyploidy

1 © David Faure, InThinking http://www.thinkib.net/biology

Polyploidy in tissue cultures of Allium fistulosum

In an experiment to look at mechanisms of polyploidy in 2002 Andrzej JOACHIMIAK and Tomaz ILNICKI measured the amount of nuclear DNA and the chromosome number of cells in tissue cultures of the species Allium fistulosum (Japanese bunching onion) The figure below shows a karyogram of the diploid chromosome number

1. Deduce the diploid chromosome number of A. fistulosum from the image fig.1 (1 mark)

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2. At what stage of mitosis was the cell cycle stopped in order to photograph the chromosomes, explain how you know? (2 marks)

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2

The researchers grew the A. fistulosum cells from the root tips of germinating seedlings over two years. In each of six collections (every four months) 1000 nuclei were analysed. The graphs below show the results of three of the analyses of the nuclei.

3. Compare the number of sets of chromosome in cells of A. fistulosum from the samples taken after 8 months and 24 months (2 marks)

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4. Evaluate the evidence that the cells of A. fistulosum undergo polyploidy while in tissue culture (2 marks)

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5. Polyploidy is believed to have contributed to the huge variety of species in the Allium genus. Outline how polyploidy can lead to the rapid creation of new species, especially in cultivated varieties of plants. (2 marks)

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3

Polyploid types are known by the number of sets of chromosomes and a more common that you might imagine in many agricultural species.

x triploid (three sets), for example seedless watermelons x tetraploid (four sets), cotton Gossypium hirsutum x hexaploid (six sets), for example wheat, and kiwifruit x octaploid (eight sets), for example dahlias x decaploid (ten sets), for example certain strawberries x dodecaploid (twelve sets), the invasive plant Spartina anglica

or the amphibian Xenopus ruwenzoriensis.

6. Using the data in the graphs suggest which types of polyploid of Allium occurred in the cells

of the experiment ( 1 mark)

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The frequency distribution graphs above show chromosome numbers of many different sizes. Many cells don’t have a simple multiples of the full chromosome set. In their discussion the authors state,

“After two years of culture the tissue culture is composed of cells with different levels of ploidy, but diploid and tetraploid cells are still represented in relatively high numbers. Cells may reduce ploidy levels and produce highly heterogenous cell populations in cultures. Indeed, Allium. fistulosum cultures consist of cells with different DNA content and nuclei sizes. They are accompanied by numerous types of chromosome aberrations”

7. Explain how these observations of chromosome aberrations (“mutations”) could provide an explanation for the variable amounts of DNA seen in the study. (1 mark)

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……………………………………………………………………………………………………………………………………………………… Extracts taken, and consent requested, from Joachimiak, A. and Ilnicki, T., “Nuclear morphology, polyploidy, and chromatin elimination in tissue culture of Allium fistulosum L.,” Acta Soc. Bot. Pol., 72.11 The full paper can be accessed at http://www.biologia.studies.uj.edu.pl/~joachimiak/WWW/Joachimiak/NuclearMorph2003.pdf

ThepaceofSpeciation

Discuss the ideas on the pace of evolution including gradualism andpunctuatedequilibrium

Topic 10: Genetics and evolution

Biology guide99

Essential idea: Gene pools change over time.

10.3 Gene pools and speciation

Nature of science:

Looking for patterns, trends and discrepancies—patterns of chromosome number in some genera can be explained by speciation due to polyploidy. (3.1)

Understandings:

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

Evolution requires that allele frequencies change with time in populations.

Reproductive isolation of populations can be temporal, behavioural or geographic.

Speciation due to divergence of isolated populations can be gradual.

Speciation can occur abruptly.

Applications and skills:

Application: Identifying examples of directional, stabilizing and disruptive selection.

Application: Speciation in the genus Allium by polyploidy.

Skill: Comparison of allele frequencies of geographically isolated populations.

Guidance:

Punctuated equilibrium implies long periods without appreciable change and short periods of rapid evolution.

Theory of knowledge:

Punctuated equilibrium was long considered an alternative theory of evolution and a challenge to the long established paradigm of Darwinian gradualism. How do paradigm shifts proceed in science and what factors are involved in their success?

Utilization:

Many crop species have been created to be polyploid. Polyploidy increases allelic diversity and permits novel phenotypes to be generated. It also leads to hybrid vigour.

Syllabus and cross-curricular links:BiologyTopic 5.1 Evidence for evolution