Transcript allele frequency

10.3 Gene pools and speciation (AHL)
Essential idea: Gene pools change over time.
Heliconius cydno
Heliconius pachinus
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/
photos from http://www.heliconius.net
10.3.U1 A gene pool consists of all the genes and their different alleles, present in an interbreeding
population.
allele frequency
Allele frequency is the proportion
of all copies of a gene that is
made up of a particular gene
variant (allele).
Example
Say if a recessive allele h made up
2% of the total in a human
population…
gene pool
The total collection of different alleles
in an interbreeding population.
…then the dominant allele H
would make up 98%.
The frequency for h would be expressed as 0.02 and for H 0.98
Recessive allele frequency + dominant allele frequency = 1
(for characteristics determined by two alleles)
http://www.flickr.com/photos/limowreck666/171979083/
10.3.U2 Evolution requires that allele frequencies change with time in populations.
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.A1 Identifying examples of directional, stabilizing and disruptive selection.
If the selective pressures
applied to a population do
not change then the
population will not evolve.
Phenotype (fur colour)
Frequency of individuals
Stabilizing selection
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.
Key
Evolved population
Original population
Selective pressure
Phenotype (fur colour)
Frequency of individuals
Original population
Directional selection
Phenotype (fur colour)
Disruptive selection
Frequency of individuals
Frequency of individuals
Directional, stabilizing & disruptive selection
Phenotype (fur colour)
Rabbit image: http://cliparts.co
10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.
Directional selection
Key
Frequency of individuals
Evolved population
Original population
Selective pressure
Phenotype (fur colour)
Medium ground finch
Beak shape and size in Geospiza fortis
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.
http://commons.wikimedia.org/wiki/File:Geospiza_fortis.jpg
10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.
Frequency of individuals
Stabilizing selection
Key
Evolved population
Original population
Selective pressure
Phenotype (fur colour)
Human birth weight
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.
Mayumi Paine (aged 1 day) – photo by Chris Paine
10.3.A1 Identifying examples of directional, stabilizing and disruptive selection.
Frequency of individuals
Disruptive selection
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 metaltolerant and faster growing non-tolerant populations.
https://commons.wikimedia.org/wiki/File:20150628Anthoxanthum_odoratum3.jpg
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
The circumstances preventing different species from
interbreeding are known as reproductive isolating mechanisms
Your syllabus focuses on three ways in which
populations can be isolated to prevent
reproduction:
• Temporal – timing
• Behavourial (this affects only animals)
• Geographic
https://youtu.be/rlfNvoyijmo
*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.
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
The reproductive
isolation only promotes
selection in sexually
reproducing organisms: it
doesn’t apply to singlecelled organisms.
Rats!
http://www.flickr.com/photos/microagua/3721497804/
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
Temporal isolation
Pinus radiata (Monterey Pine)
Pinus attenuata (Knobcone pine)
Pollen Production
MAX
J
F M A M J J
Month
A
S O N D
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
http://www.flickr.com/photos/alancleaver/4293345631/
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
I love me
some
CaCO3 in
my soil
Blechhh!
Acidic soils
are more my
thing
Viola arvensis
http://www.flickr.com/photos/annetanne/3035068940/
Viola tricolor
http://www.flickr.com/photos/carinemily/644052381/
10.3.U3 Reproductive isolation of populations can be temporal, behavioural or geographic.
Behavioural isolation
Animals 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.
http://www.pbs.org/wgbh/evolution/library/05/2/swf_pop/l_052_01.
html
Yo! I don’t like
your music!
Its like,
totally
mutual!
http://www.flickr.com/photos/nrk-p3/2333221093/
http://www.flickr.com/photos/rowelbg/2895578034/
10.3.S1 Comparison of allele frequencies of geographically isolated populations.
Comparison of allele frequencies
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.
Use the information and charts to answer the questions on
the following slides…
Key
PanIA
Population #
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.
12
PanIB
10.3.S1 Comparison of allele frequencies of geographically isolated populations.
Comparison of allele frequencies
1. State the two populations with the highest
PanIB allele frequencies. [1]
1. State the population in which the allele
frequencies were closest to 0.5. [1]
1. 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]
Key
PanIA
Population #
Graph and questions from IB Questionbank
12
PanIB
10.3.S1 Comparison of allele frequencies of geographically isolated populations.
Comparison of allele frequencies
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]
Key
PanIA
Population #
Graph and questions from IB Questionbank
12
PanIB
10.3.U4 Speciation due to divergence of isolated populations can be gradual. AND 10.3.U5 Speciation can
occur abruptly.
The rate of speciation varies
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 ‘During rapid’ evolutionary changes.
• 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.
http://www.ib.bioninja.com.au/_Media/pace_of_evolution_med.jpeg
10.3.U4 Speciation due to divergence of isolated populations can be gradual.
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.U5 Speciation can occur abruptly.
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/ideonexus/4022727065/
http://www.flickr.com/photos/mrccos/288136783/sizes/m/in/photostream/
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)
So far you’ve learnt that cells contain two homologous sets of chromosomes.
Well….. that isn’t always the case.
It goes on:
Pentaploid
Hexaploid
Septaploid
Octaploid
Etc.
up to:
84-ploid and 1260
chromosomes
Ophioglossum reticulatum
A 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
Edited from: http://www.slideshare.net/jasondenys/ib-biology-option-d2-species-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)
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.
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
http://www.ib.bioninja.com.au/_Media/polyploidy_med.jpeg
Edited from: http://www.slideshare.net/jasondenys/ib-biology-option-d2-species-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)
Polyploidy in animals and plants
However, polyploidy is a great source of
speciation amongst plants.
There exist few polyploid animals species
(examples include salamanders, goldfish
and salmon).
Polyploidy often leads to increased size,
resistance to disease and overall vigour.
Many agricultural plants are polyploid
(e.g. wheat) due to having bigger fruits,
seeds and storage organs
https://youtu.be/6Jjilc5eqS0
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
Many species of this genus commonly
reproduce asexually and if polyploidy confers
an advantage a new species may arise.
Onion (A. Cepa), 16 Chromsomes
English Leek (A. Cepa), 32 Chromosomes
https://commons.wikimedia.org/wiki/File:Starr_0703135652_Allium_cepa.jpg
https://commons.wikimedia.org/wiki/File:Poireaux_artlibre_jnl.
jpg