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 tos from http:// www.heliconius.net
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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
10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.Fr
eque
ncy
of in
divi
dual
s
Phenotype (fur colour)
Freq
uenc
y of
indi
vidu
als
Phenotype (fur colour)
Freq
uenc
y of
indi
vidu
als
Phenotype (fur colour)
Freq
uenc
y of
indi
vidu
als
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
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
eque
ncy
of in
divi
dual
s
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.
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.
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.
*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.
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.
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"
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?
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)
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)
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