Transcript gene pool

IB Biology
Option D
D2 Species and Speciation
All syllabus statements ©IBO 2007
All images CC or public domain or link to original material.
Jason de Nys
“…if I really have as bad an expression, as
my photograph gives me, how I can have
one single friend is surprising."
D.2.1 Define allele frequency and gene pool
al·lele fre·quen·cy
Allele frequency is the proportion
of all copies of a gene that is
made up of a particular gene
variant (allele).
gene pool
Noun: The total collection of
different alleles in an interbreeding
population.
Example
Say if a recessive allele h made up
2% of the total in a human …then the dominant allele H
population…
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/
D.2.2 State that evolution involves a change in allele frequency in a population’s gene pool over
a number of generations
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
D.2.3 Discuss the definition of the term species
What is a
species?
D.2.3 Discuss the definition of the term species
There are many definitions,
here are five!
Ecological species
A set of organisms adapted to a particular set
of resources, called a niche, in the
environment.
Genetic species
Based on similarity of DNA of individuals or populations. Having a common gene pool.
Evolutionary species
A group of organisms that shares an ancestor; a lineage that maintains its
integrity with respect to other lineages through both time and space. At some
point in the progress of such a group, some members may diverge from the main
population and evolve into a subspecies.
Lots to discuss if you get a
Cladistic Species
question about this!
A group of organisms that shares an ancestor; a lineage that maintains its
integrity with respect to other lineages through both time and space. At some
point in the progress of such a group, members may diverge from one another:
when such a divergence becomes sufficiently clear, the two populations are
regarded as separate species. (This differs from the Evolutionary definition in
that the parent species goes extinct when two new species are recognised).
Breeding Species
Two organisms that are able to reproduce naturally to produce
fertile offspring of both sexes.
http://en.wikipedia.org/wiki/Species
The genetic definition is most widely
used and works well for most
multicellular organisms
The following 4 slides outline the
exceptions:
…is anything ever simple in Biology?
Hors
d’oeuvre?
It could be (ahem)
physically impossible
for members of the
same species to mate.
Therefore they are
genetically isolated.
http://www.flickr.com/photos/lepetitblonde/2247963815/
1 : Lesser Black-backed Gull
2 : Siberian population Black-backed gull
3 : Heuglin's gull
4 : Birula's Gull
5 : East Siberian Herring Gull
6 : American Herring Gull
7 : Herring Gull
Ring Species:
Adjacent populations
can interbreed but the
populations at the “end
of the line” cannot.
1 and 7 cannot
produce offspring.
So
close…
…and yet
so far :’(
http://commons.wikimedia.org/wiki/File:Ring_species_seagull.svg
Hybrids are usually infertile and can not produce offspring together,
for example the mule (63 chromosomes): a cross between a Male
horse (64 chromosomes) and a female donkey (62 chromosomes)
The liger is a hybrid cross between a male Panthera leo (lion), and a female
Panthera tigris (tiger) and is denoted scientifically as:
Panthera tigris × Panthera leo.
…conversely…
Why are 63
chromosomes a
problem when
reproducing?
The tiglon is a hybrid cross between a female Panthera leo
(lion), and a male Panthera tigris (tiger) and is denoted
scientifically as: Panthera leo × Panthera tigris.
Ligers and tiglons sometimes
produce offspring when mated
back with a parent species
e.g. The hybrid of a male lion
and a female tiglon is a li-tiglon!
MADNESS!!
http://xkcd.com/419/
The genetic definition
only applies to sexually
reproducing organisms
and doesn’t apply to
single-celled organisms
Rats!
http://www.flickr.com/photos/microagua/3721497804/
Additionally:
Fossil remains can’t tell us whether
species were able to interbreed or
produce viable offspring so
palaeontologists tend to use the
cladistic definition
http://www.flickr.com/photos/bruce_mcadam/1393218574/
D.2.4 Describe three examples of barriers between gene pools
The circumstances preventing different species from
interbreeding are known as reproductive isolating mechanisms
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/
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/
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.
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/
Mechanical isolation
Animal example:
Different species of bush baby (Galago) have particular shapes for their genitalia
and they are physically incapable of copulation*.
It is like a
lock and
key.
In plants, mechanical isolation occurs
when different species have different
pollinators that are not able to service
the flowers of other species
She says “We’re
not a good fit”.
What is that
supposed to
mean?
*Take care when Googling “Bush baby genitalia”!
http://www.flickr.com/photos/joachim_s_mueller/4113758487/
Hybrid Inviability
Remember: Male Horse + Female Donkey = Mule
Horse
2n = 64 ∴ sperm n = 32
Donkey 2n = 62 ∴ ovum n = 31
Sperm + Ovum = Mule zygote
32 + 31 = 63
Mule
2n = 63 ∴ gamete n = ?
Note: Sad eyes
http://www.flickr.com/photos/pirateparrot/301798478/
D.2.5 Explain how polyploidy can contribute to speciation
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
How it happens:
Remember:
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 (see 4.2.4)
Self
fertilisation
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.
 See animation
http://commons.wikimedia.org/wiki/File:Polyploidization.svg
Few polyploid organisms exist in the animal kingdom.
Can you think of the reasons why not?
Animal polyploid species include salamanders, goldfish and
salmon.
However, polyploidy is a great source of speciation amongst plants.
Polyploidy often leads to
increased size, resistance to
disease and overall vigour.
Many plants used by humans are
polyploid. Including cereal crops
like wheat.
Polyploid crops generally have
bigger fruits, seeds and storage
organs
Two versions of Polyploidy:
• Autopolyploidy*
• Allopolyploidy
*This is not autopolyploidy
http://www.flickr.com/photos/leapkye/3224058317/
Autopolyploidy
(Auto = “self”)
• Autopolyploids are polyploids with multiple chromosome
sets derived from a single species as described a couple of
slides ago.
• Autopolyploids form following fusion of 2n gametes
• Autopolyploidy can be induced in plants using colchicine, a
chemical extracted from the autumn crocus.
• Autopolyploids with odd ploidys eg triploid or pentaploid
have trouble reproducing sexually
WHY?
• That does not stop them from being good crops if they can
be propagated asexually
Allopolyploidy
(Allo = “different”)
Allopolyploids come about when a sterile F1 hybrid doubles all of its
chromosomes and becomes fertile.
+ =
Wheat
Rye
Triticale
For example, Triticale is the hybrid of
wheat (Triticum turgidum) and rye
(Secale cereale). It combines soughtafter characteristics of the parents, but
the initial hybrids were sterile until
doubling of the number of
chromosomes occurred
Remember the poor sterile mule with 63
chromosomes?
Imagine if we could somehow induce sperm and
ova with 126 chromosomes
Voila! The mule born would be fertile.
Of course, it would need to be done a couple of
times to get a few mules to breed together
http://en.wikipedia.org/wiki/File:Wheat,_rye,_triticale_montage.jpg
http://jonathanturley.org/2009/03/01/a-happy-mule/
D.2.6 Compare allopatric and sympatric speciation
Allopatric speciation
(Allo = “different”, patric = “fatherland”)
This arises when a species is subject to geographic isolation.
This can occur when a population is split by:
• A river
• A mountain range
Gene flow is cut off between the two split
• A desert
populations and they can evolve in different
• A road
directions (See animations below)
• The sea etc.
Remember
Darwin’s
finches?
http://commons.wikimedia.org/wiki/File:Darwin%27s_finches_by_Gould.jpg
Once the populations have been
separated into two gene pools they
can diverge through natural selection
or through random genetic drift
http://upload.wikimedia.org/wikipedia/commons/b/b6/Random_sampling_genetic_drift.gif
Allopatric speciation of Drosophila
in the lab
Even when the “geographic barrier”
is removed, the populations are still
genetically isolated
Sympatric speciation
(Sym = “same”, patric = “fatherland”)
The formation of two or more descendant
species from a single ancestral species all
occupying the same geographic location.
Whether it actually happens is still contested.
Find a pair of species that are thought to have diverged by sympatric speciation
http://etc.usf.edu/clipart/2200/2288/salamander_1.htm
Diagrammatic
comparison
http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Speciation_modes.svg/500px-Speciation_modes.svg.png
D.2.7 Outline the process of adaptive radiation
Starting with a recent single ancestor, this process results in the speciation
and phenotypic adaptation of an array of species exhibiting different
morphological and physiological traits with which they can exploit a range
of divergent environments.
Wikipedia
Think Darwin’s finches (AGAIN!)
They originated from a population of an ancestral species that flew or were blown to the
Galapagos islands from mainland South America.
They colonised the islands and (while geographically isolated) evolved via natural selection
to have beaks that suited the types of food available on their islands.
Their beaks are
homologous structures in
that they have evolved
from a common structure
to have different functions.
D.2.8 Compare convergent and divergent evolution
What do humans,
octopi and box
jellyfish have in
common?
http://www.flickr.com/photos/jlambus/2303592201/
We all have complex
camera* eyes.
They evolved
independently in
organisms only very
distantly related.
They are an example
of convergent
evolution
Complex eyes
have evolved 50 to
100 times!
*Camera means ‘room’
Convergent evolution describes the acquisition of
the same biological trait in unrelated lineages.
Other (random!) examples include:
- Penguins in the southern hemisphere
and Auks in the northern hemisphere
both use wings as flippers
- Echolocation in bats, toothed whales
and shrews to capture prey. It even
evolved independently twice
amongst the bats
- Super strong jaws on different
genuses of ants (Trapjaw )
- Flight/gliding in birds, pterosaurs,
bats, insects and flying fish!
Little Auk
http://commons.wikimedia.org/wiki/File:AlleA
Little Penguin
http://commons.wikimedia.org/wiki/File:Little_penguin_Eudyptula_minor.jpg
I’m including this image
because I mentioned bats
twice on the last slide and
bats are awesome!
*
*Whatever he’s saying, its ultrasonic
http://www.flickr.com/photos/furryscalyman/673915993/
Features that come about by convergent evolution are known as
analogous structures
http://www.flickr.com/photos/sniffette/6705872/
http://www.flickr.com/photos/volk/1038089969/
http://www.flickr.com/photos/jaybock/4006029348/
http://www.flickr.com/photos/martynr/76538849/sizes/o/in/photostream/
Divergent Evolution is another way of saying adaptive radiation (D.2.7).
As natural selection acts on two or more species that have arisen from a
common ancestor, they become phenotypically different.
It gives rise to homologous structures, features that now look different
or have a different purpose for each species that has evolved
http://commons.wikimedia.org/wiki/File:Evolution_pl.png
Convergent evolution
Time
Divergent evolution
Parent species
(common ancestor)
Parent
species
Parent
species
D.2.9 Discuss ideas on the pace of evolution including gradualism and punctuated evolution
Phyletic Gradualism, as
the name suggests, is the
idea that evolution
occurs at a slow-butsteady pace.
Punctuated Equilibrium is
the idea that, for most of the
time, species are stable. But
every now and then there is
a disruptive event that
prompts rapid change.
The slope of the line indicates rate of change.
• Vertical lines = little/no change
• Horizontal lines = very rapid change
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
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/
Revisiting the tree for punctuated
equilibrium it should be noted that the
“sudden” speciation events are only
sudden in terms of geological time. They
would still take many generations and
possibly thousands of years.
The periods of stasis may be
explained by stabilising selection
The punctuation could be
explained by
directional selection or
disruptive selection
You should be able to understand and interpret these diagrams.
Practise sketching them.
The downward facing arrows indicate selection pressure against individuals
with that morphology
Directional
Disruptive
After
Before
Stabilising
All images CC Andrew Colvin
Darwin’s Finches (again!) are an example of disruptive selection
A
B
Short-beaked birds (A) and long-beaked birds (B) were able to exploit different
food sources and this selection pressure led to the evolution of two species
http://www.flickr.com/photos/kookr/2917861361/
Lake Turkana (Kenya,
Ethiopia) contains several
species of snails that have
a fossil record showing
long periods with little
change followed by
sudden change
(punctuated equilibrium)
The periods of change
coincide with times
when the water level of
the lake dropped and it
became a series of
smaller lakes.
What happens then?
http://en.wikipedia.org/wiki/File:Lake_turkana_satellite.jpg
That’s right: geographic isolation
Smaller gene pools are
more susceptible to
directional selection
So evolution of the
isolated populations may
be faster than when they
were one big happy gene
pool
By the time lake levels
recovered and the
populations were
united, isolating
mechanisms were in
place that prevented
hybridisation
After each extinction event, the number of genera has bounced back
Phanerozoic_Biodiversity.svg
The K/T extinction event (250 MA at the
Cretaceous-Tertiary boundary) wiped out
over half the genera, including most of
the dinosaurs.
A layer of iridium has been found in
sediments laid down at that time all over
the globe. Iridium is in higher
concentrations in meteorites than on
Earth generally.
Therefore it is postulated that a large
meteor or comet hit the Earth and
caused the extinction.
Individuals in the species that survived could
move into the empty ecological niches and
directional selection led to rapid evolution
http://www.flickr.com/photos/53402955@N08/4928503884/in/photostream/
D.2.10 Describe one example of transient polymorphism
Darwin's finches…. Have little to do with this point (for a change!).
Instead, the peppered moths (Biston betularia) are the best known example
Polymorphism is the existence of two or more different forms
of a species
Poly = “many” morphism = “shapes”
Prior to 1840 peppered moths in Britain were light grey with dark spots
to blend in with the grey lichen that grew on the trees in their habitat
http://www.flickr.com/photos/wildhastings/4720082589/
The first dark variant was
reported in 1848 and by
1895 most of them were
black.
The term industrial
melanism was coined.
Soot and acid rain from
the burning of coal
changed the colour or
the trees that the moths
rested on.
Directional selection did
the rest.
http://www.flickr.com/photos/naturalhistoryman/817332984/
Before long the majority were
dark.
This situation reversed after 1956
when Britain instituted the clean
air act. Less coal was burnt and
most trees returned to their
original colour.
Now in polluted areas most
moths are dark and in rural areas
most moths are light.
They are not distinct species
because they still interbreed.
The theory that natural selection
due to predation was the cause
of these changes has been
confirmed experimentally by
Dr HBD Kettlewell
D.2.10 Describe sickle cell anaemia as an example of balanced polymorphism
Sickle cell anaemia occurs when a single-base mutation in the gene that
codes for haemoglobin causes the amino acid valine to be produced in a
particular spot rather than glutamic acid.
Valine is non-polar, unlike glutamic acid,
and this causes the mutant variety of
haemoglobin (haemoglobin S) to crystallise
at low concentrations of oxygen.
This in turn pulls the red blood cell into a
sickle shape. It is less able to carry oxygen
and can get stuck in small capillaries,
causing blockages, pain and damage.
Homozygous individuals (HbS HbS) are
subject to a debilitating condition and have
a shortened life expectancy
On the brighter side, while individuals who are heterozygous (HbA HbS) will have
some mutant haemoglobin. They can lead normal lives. As a benefit, they are
resistant to malaria as the plasmodium parasite that causes it is not able to use
sickle cells to reproduce.
Individuals that are homozygous normal (HbA HbA) have no sickle cells
and no resistance to malaria.
Historical distribution of malaria
Distribution of the sickle cell trait
Heterozygous:
Sickle cell trait
Heterozygous:
Sickle cell trait
A S
A A
A S
A S
A S
S
S
HbA HbA
Haemoglobin: Normal
RBCs: Normal
O2 Capacity: Normal
Malaria resistance: None
HbA HbS
Haemoglobin:
50% normal, 50% mutant
RBCs: Usually normal, sickle
when [O2] low
O2 Capacity: Mild anaemia
Malaria resistance: Moderate
HbS HbS
Haemoglobin: mutant
RBCs: Sickle
O2 Capacity: Severe anaemia
Malaria Resistance: High
Homozygous:
‘Normal’
25% chance
Heterozygous:
Sickle cell trait
50% chance
Homozygous:
Sickle Anaemia
25% chance
http://en.wikipedia.org/wiki/File:Autorecessive.svg
This is an example of balancing selection and balanced polymorphism
People who are homozygous for sickle cell are severely anaemic
and have less chance of surviving to reproduce.
Likewise individuals homozygous for normal haemoglobin are likely
to contract malaria and are less likely to survive.
Heterozygous individuals have what
is termed heterozygote advantage.
They are the most likely to survive
and reproduce.
Therefore both alleles are
maintained in the population
http://en.wikipedia.org/wiki/File:Plasmodium.jpg
http://en.wikipedia.org/wiki/File:Sicklecells.jpg
http://commons.wikimedia.org/wiki/File:Simple_balance_scales-01.jpg
Hypotheticals:
France has the greatest number of sickle cell sufferers in Europe because
of immigration from its former African and Caribbean colonies.
What do you expect will happen to the sickle cell allele in France over
time given:
1. no more immigration,
2. modern medicine,
3. and the absence of malaria?
What do you expect will happen to the sickle cell allele in West Africa over
time if:
• We eradicate malaria?
Or
• We develop medication that helps all sickle cell sufferers live
normally
Further information: