10.3 Gene pools and speciation (AHL)

Essential idea: Gene pools change over time.

The two butterfly species of the Heliconius genus above have only recently diverged and consequently differ in twelve regions of their genomes. Other species of the genus diverged earlier show hundreds of genomic changes.http://phys.org/news/2013-10-evolution-species-requires-genetic.html

By Chris Paine

https://bioknowledgy.weebly.com/

Heliconius pachinus Heliconius cydno

photos from http://www.heliconius.net

Understandings, Applications and SkillsStatement Guidance

10.3.U1 A gene pool consists of all the genes and their different alleles, present in an interbreeding population.

10.3.U2 Evolution requires that allele frequencies change with time in populations.

10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.

10.3.U4 Speciation due to divergence of isolated populations can be gradual.

10.3.U5 Speciation can occur abruptly. Punctuated equilibrium implies long periods without appreciable change and short periods of rapid evolution.

10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.

10.3.A2 Speciation in the genus Allium by polyploidy. 10.3.S1 Comparison of allele frequencies of geographically

isolated populations.

http://www.flickr.com/photos/limowreck666/171979083/

al·lele fre·quen·cyAllele frequency is the proportion of all copies of a gene that is made up of a particular gene variant (allele).

gene poolNoun: The total collection of different alleles in an interbreeding population.

ExampleSay if a recessive allele h made up

2% of the total in a human population…

…then the dominant allele H would make up 98%.

The frequency for h would be expressed as 0.02 and for H 0.98Recessive allele frequency + dominant allele frequency = 1

(for characteristics determined by two alleles)

10.3.U1 A gene pool consists of all the genes and their different alleles, present in an interbreeding population.

New combinations of alleles lead to new phenotypes that can then be selected for or against by the environment. This leads to

evolutionary change in the species

10.3.U2 Evolution requires that allele frequencies change with time in populations.

10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.Fr

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Phenotype (fur colour)

Freq

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Phenotype (fur colour)

Freq

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Phenotype (fur colour)

Freq

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Phenotype (fur colour)

Directional, stabilizing & disruptive selection

Disruptive selection

Directional selection

Stabilizing selection

Rabbit image: http://cliparts.co

Key Evolved population Original population Selective pressure

Original population If the selective pressures applied to a population do not change then the population will not evolve.

However if the selective pressures do change then the population will evolve, but how it evolves depends on which phenotypes are experience the greatest pressure.

Medium ground finch Beak shape and size in Geospiza fortis

http://commons.wikimedia.org/wiki/File:Geospiza_fortis.jpg

10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.Fr

eque

ncy

of in

divi

dual

s

Phenotype (fur colour)

Directional selection Key Evolved population Original population Selective pressure

Selective pressure: during dry years small seeds are not adundant.Result: Birds with larger tougher beaks become more frequent

Example from 5.2.A1 Changes in beaks of finches on Daphne Major.

10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.

Stabilizing selectionFr

eque

ncy

of in

divi

dual

s

Phenotype (fur colour)

Key Evolved population Original population Selective pressure

Selective pressures: Babies of low weight lose heat more quickly and get ill from infectious diseases more easily. Babies of large body weight are more difficult to deliver through the pelvis.Result: Medium weight babies have a much lower mortality and hence the frequency of medium weight babies increases.

Human birth weight

Mayumi Paine (aged 1 day) – photo by Chris Paine

10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.

Disruptive selectionFr

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ncy

of in

divi

dual

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Phenotype (fur colour)

Key Evolved population Original population Selective pressure

Grass (Anthoxanthum odoratum)

Selective pressure: soil close to mine workings contaminated with metals, e.g. copper.Result: Two distinct grass populations arise; slower growing metal-tolerant and faster growing non-tolerant populations.

https://commons.wikimedia.org/wiki/File:20150628Anthoxanthum_odoratum3.jpg

The circumstances preventing different species from interbreeding are known as reproductive isolating mechanisms

10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.

https://youtu.be/rlfNvoyijmo

Really good introduction to speciation and

reproductive isolation* by Bozeman Science

Your syllabus focuses on three ways in which populations can be isolated to prevent reproduction:• Temporal – timing• Behavourial (this affects only animals)• Geographic

*This video also looks at other aspects of the topic including polyploidy, but remember it is not an IB course specific resource so make sure that you know what is relevant to you.

The reproductive isolation only promotes selection in sexually reproducing organisms: it doesn’t apply to single-celled organisms.

Rats!

http://www.flickr.com/photos/microagua/3721497804/

10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.

Temporal isolation

J F M A M J J A S O N D

MAXPinus radiata (Monterey Pine)Pinus attenuata (Knobcone pine)

Polle

n Pr

oduc

tion

Pinus radiata and Pinus attenuata are prevented from hybridising because they have separate pollination times.They can be made to hybridise by pollinating them manually.

http://www.flickr.com/photos/alancleaver/4293345631/

*Random fact: The Monterey pine is at risk in it’s native range but is one of the most common plantation trees in the world. If you see a pine forest in Australia or NZ, it is probably Pinus radiata

Month

10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.

Ecological isolation

The two species are in the same area, but live in different habitats

http://www.flickr.com/photos/annetanne/3035068940/ http://www.flickr.com/photos/carinemily/644052381/

I love me some

CaCO3 in my soil

Blechhh!Acidic soils

are more my thing

Viola arvensis Viola tricolor

10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.

Behavioural isolationAnimals exhibit courting behaviour (song,

dance etc.) or release pheremones to attract mates. Individuals are only

attracted to, and will only mate with, members of the opposite sex who perform

the appropriate ritual or release the correct chemical.

Yo! I don’t like your music!

Its like, totally

mutual!

http://www.flickr.com/photos/rowelbg/2895578034/http://www.flickr.com/photos/nrk-p3/2333221093/

10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.

http://www.pbs.org/wgbh/evolution/library/05/2/swf_pop/l_052_01.html

10.3.S1 Comparison of allele frequencies of geographically isolated populations.

PanI is a gene in cod fish that codes for an integral membrane protein called pantophysin.

Two alleles of the gene, PanIA and PanIB, code for versions of pantophysin.

Samples of cod fish were collected from 23 populations in the north Atlantic and tested to find the proportions of the alleles in each population.

The proportions of alleles in a population are called the allele frequencies. The frequency can vary from 0.0 to 1.0 with the total frequency of all alleles always being 1.0.

Comparison of allele frequencies

Key

12

PanIA PanIB

Population #

Use the information and charts to answer the questions on the following slides…

Source: RAJ Case et al. 2005. “Macro- and micro-geographic variation in pantophysin (PanI) allele frequencies in NE Atlantic cod Gadus morhua.” MEPS. Vol 301. Pp 267–278. Figs 1 and 3.

10.3.S1 Comparison of allele frequencies of geographically isolated populations.

Comparison of allele frequencies

Key

12

PanIA PanIB

Population #

1. State the two populations with the highest PanIB allele frequencies. [1]

2. State the population in which the allele frequencies were closest to 0.5. [1]

3. Deduce the allele frequencies of a population in which half of the cod fish had the genotype PanIA PanIA, and half had the genotype PanIA PanIB. [2]

Graph and questions from IB Questionbank

10.3.S1 Comparison of allele frequencies of geographically isolated populations.

Comparison of allele frequencies

Key

12

PanIA PanIB

Population #

1. State the two populations with the highest PanIB allele frequencies. [1]

1 and 2

2. State the population in which the allele frequencies were closest to 0.5. [1]

7/11

3. Deduce the allele frequencies of a population in which half of the cod fish had the genotype PanIA PanIA, and half had the genotype PanIA PanIB. [2]

PanIA 0.75 and PanIB 0.25

Graph and questions from IB Questionbank

10.3.S1 Comparison of allele frequencies of geographically isolated populations.

Comparison of allele frequencies

Key

12

PanIA PanIB

Population #

4. Identify an example of two geographically isolated populations. [1]

5. Give Suggestions why the PanIB allele is more common in population 13 than population 22. [2]

Graph and questions from IB Questionbank

10.3.S1 Comparison of allele frequencies of geographically isolated populations.

Comparison of allele frequencies

Key

12

PanIA PanIB

Population #

4. Identify an example of two geographically isolated populations. [1]

any population from 14 – 23 and any population from 1 – 4

5. Give Suggestions why the PanIB allele is more common in population 13 than population 22. [2]

Cooler water temperature favours PanIB;Interbreeding with icelandic/more northern populations;

Graph and questions from IB Questionbank

10.3.U4 Speciation due to divergence of isolated populations can be gradual. AND 10.3.U5 Speciation can occur abruptly.

http://www.ib.bioninja.com.au/_Media/pace_of_evolution_med.jpeg

Phyletic Gradualism• Evolution occurs at a constant pace

over a long period of time (due to the accumulation of mutations).

• For example the change in size and hoof of the modern horse

Punctuated Equilibrium• Long periods of stability are interrupted

by ‘rapid’ evolutionary changes.• During periods of stability well-suited

organisms have no reason to evolve until large environmental changes (e.g. meteor strikes) cause selection pressures to shift.

• Gaps in the fossil record show mass extinction events

The rate of speciation varies

Gradualism is the older idea. Darwin is one of the

originators of the concept, borrowing from his friend

Charles Lyell. Darwin recognised however that not all species evolve at the same rate all of the time

"I think case must be that one generation should have as many living as now. To do this and to have as many species in same genus (as is) requires extinction . Thus between A + B the immense gap of relation. C + B the finest gradation. B+D rather greater distinction. Thus genera would be formed. Bearing relation" (next page begins) "to ancient types with several extinct forms"

http://commons.wikimedia.org/wiki/File:Darwin_tree.png

10.3.U4 Speciation due to divergence of isolated populations can be gradual.

Punctuated equilibrium was first proposed by palaeontologists Niles Eldredge and Stephen Jay Gould in 1972. They were the first to suggest that species did not change for long periods of time but were in stasis until events punctuated (disrupted) the equilibrium (balance)

Richard Dawkins is a prominent critic of the theory

TOK - Find out more: • What evidence are the two theories based on?• Gould (deceased) and Dawkins have both become

popular writers. How does this affect the weight of their opinion:

• In the scientific community?• In the wider community?

http://www.flickr.com/photos/mrccos/288136783/sizes/m/in/photostream/

http://www.flickr.com/photos/ideonexus/4022727065/

10.3.U5 Speciation can occur abruptly.

So far you’ve learnt that cells contain two homologous sets of chromosomes.Well….. that isn’t always the case.

It goes on:PentaploidHexaploidSeptaploidOctaploidEtc.up to:

84-ploid and 1260 chromosomes

Ophioglossum reticulatumA small fern.

The incredible thing is that this plant is able to carry out meiosis accurately with

1260 chromosomes to divvy up

http://commons.wikimedia.org/wiki/File:Haploid,_diploid_,triploid_and_tetraploid.svg

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)

Edited from: http://www.slideshare.net/jasondenys/ib-biology-option-d2-species-and-speciation

How polyploidy happens

When non-disjunction occurs during meiosis in humans, an individual can end up with an extra chromosome or missing chromosomes (e.g. An extra chromosome 21 means Downs syndrome).

Total non-disjunction, is when one of the two cells produced during Meiosis I gets all of the chromosomes. The other cell is not viable and is reabsorbed.This results in two (2n) daughter cells from meiosis instead of the usual four (n) daughter cells.

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)

http://www.ib.bioninja.com.au/_Media/polyploidy_med.jpeg

Polyploidy is much more common in plant species - they lack separate sexes and are capable of asexual reproduction (self-pollination)

Tetraploid offspring cannot mate with diploid organisms (triploid offspring tend to be infertile), speciation has occurred

Edited from: http://www.slideshare.net/jasondenys/ib-biology-option-d2-species-and-speciation

There exist few polyploid animals species (examples include salamanders, goldfish and salmon).

Polyploidy often leads to increased size, resistance to disease and overall vigour.

However, polyploidy is a great source of speciation amongst plants.

https://youtu.be/6Jjilc5eqS0

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)

Many agricultural plants are polyploid (e.g. wheat) due to having bigger fruits, seeds and storage organs

Polyploidy in animals and plants

https://commons.wikimedia.org/wiki/File:Wheat_close-up.JPG

Edited from: http://www.slideshare.net/jasondenys/ib-biology-option-d2-species-and-speciation

10.3.A2 Speciation in the genus Allium by polyploidy.

Chromosome number in genus Allium

https://commons.wikimedia.org/wiki/File:Starr_070313-5652_Allium_cepa.jpg

Onion (A. Cepa), 16 Chromsomes

https://commons.wikimedia.org/wiki/File:Poireaux_artlibre_jnl.jpg

English Leek (A. Cepa), 32 Chromosomes

Many species of this genus commonly reproduce asexually and if polyploidy confers an advantage a new species may arise.

Bibliography / Acknowledgments

Bob Smullen

Jason de Nys