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Option D:Evolution
D2: Species and Speciation
D 2.1D 2.1 DefineDefine allele frequency and gene pool.allele frequency and gene pool.
• gene pool –
• allele frequency –
D 2.2D 2.2 StateState that evolution involves a change in that evolution involves a change in allele frequency in a population’s gene pool over allele frequency in a population’s gene pool over a number of generations.a number of generations.
• Evolution of populations is best understood in terms of allele frequencies
• If the allele frequencies remain constant from generation to generation, then the population is not undergoing any evolutionary change and is in
• Evolution can be defined as
(change in the genetic makeup of populations over time)
•Biological species concept▫defined by Ernst Mayr▫population whose members can
▫reproductively compatible
Western MeadowlarkEastern Meadowlark
Distinct species:songs & behaviors are different enough to prevent interbreeding
Distinct species:songs & behaviors are different enough to prevent interbreeding
D 2.3 D 2.3 DiscussDiscuss the definition of the term species. the definition of the term species.
Diverse, but same species
•Species are created by a series of evolutionary processes
▫populations become isolated – reduces gene flow
geographically isolated reproductively isolated
▫isolated populations evolve independently
▫Gene flow = the migration of breeding individuals between populations causing a corresponding movement of alleles
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
- obstacle to mating or to fertilization if mating occursPRE-reproduction barriers
behavioral isolation
geographic isolation ecological isolation temporal isolation
mechanical isolation gametic isolation
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
Geographic isolation•Species occur in different areas
▫.▫allopatric speciation
“other country”Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel
Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
Ecological isolation• Species occur in same region, but
▫reproductively isolated
2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial
2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial
lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest
lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
Temporal isolation
• Species that breed
▫reproductive isolation▫sympatric speciation
“same country”
Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer
Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
Behavioral isolation• Unique
▫identifies members of species ▫attract mates of same species �
courtship rituals, mating calls reproductive isolation
Blue footed boobies mate only after a courtship display unique to their species
Blue footed boobies mate only after a courtship display unique to their species
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
http://www.youtube.com/watch?v=PLhOKC6ZDpI
Video clip of courtship:
firefly courtship displaysfirefly courtship displays
courtship display of Gray-Crowned Cranes, Kenyacourtship display of Gray-Crowned Cranes, Kenya
courtship songs of sympatricspecies of lacewingscourtship songs of sympatricspecies of lacewings
Recognizing your own species
http://www.youtube.com/watch?v=luViM0V--EI&feature=related
Mechanical isolation• Morphological (form & structure)
differences can prevent successful mating▫reproductive isolation
Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.
Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.
Plants
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
Mechanical isolation• For many insects, male &
female sex organs of closely related species do not fit together, preventing sperm transfer▫lack of “fit” between sexual organs:
hard to imagine for us… but a big issue for insects with different shaped genitals!
Animals
I can’t even imagine!
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
Gametic isolation• Sperm of one species may not be able
biochemical barrier so sperm cannot penetrate egg receptor recognition: lock & key between egg &
sperm chemical incompatibility
sperm cannot survive in female reproductive tract
Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.
Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
POST-reproduction barriers•Prevent hybrid offspring from
developing into a
▫reduced hybrid viability▫reduced hybrid fertility▫hybrid breakdown
Horse + Zebra = Zebroid
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
liger
Reduced hybrid viability
•Genes of different parent species may interact & impair the hybrid’s development
Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.
Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
Mules are vigorous, but sterile
Reduced hybrid fertility• Even if hybrids are vigorous, they may be
sterile▫chromosomes of parents may differ in
number or structure & meiosis in hybrids may fail to produce normal gametes
Donkeys have 62 chromosomes(31 pairs)
Horses have 64 chromosomes(32 pairs) Mules have 63 chromosomes!
Hybrid breakdown• Hybrids may be fertile & viable in first
generation, but when they mate offspring are feeble or sterile
In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.
In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.
D 2.4 D 2.4 DescribeDescribe three examples of barriers three examples of barriers between gene pools.between gene pools.
D 2.5 D 2.5 ExplainExplain how polyploidy can contribute to how polyploidy can contribute to speciation.speciation.
Changes in chromosome number may cause instantaneous speciation
▫ polyploidy –
▫ may occur when a fertilized egg duplicates its chromosomes but does not divide into two daughter cells – all subsequent divisions may be normal and all cells are now tetraploid
D 2.5 D 2.5 ExplainExplain how polyploidy can contribute to how polyploidy can contribute to speciation.speciation.
▫ caused by nondisjunction –
▫ most tetraploid plants are healthy and vigorous and can go through meiosis
▫ gametes produced can only –
cannot fuse with gametes from original parents
▫ occurs in plants because plants can
http://www.youtube.com/watch?v=SbrVw1jrZxE
D 2.5 D 2.5 ExplainExplain how polyploidy can contribute to how polyploidy can contribute to speciation.speciation.
• Autopolyploids (auto= self) are polyploids with multiple chromosome sets derived from a single species
• Autopolyploids form following fusion of 2n gametes
D 2.5 D 2.5 ExplainExplain how polyploidy can contribute to how polyploidy can contribute to speciation.speciation.
• 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
• That does not stop them from being good crops if they can be propagated asexually
Outcrossing – two plantsSelfing – self-fertilizedVegetative – asexual
D 2.5 D 2.5 ExplainExplain how polyploidy can contribute to how polyploidy can contribute to speciation.speciation.
Allopolyploids (allo= different) come about when a sterile F1 hybrid
For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids were sterile until doubling of the number of chromosomes occurred
+ = Wheat Rye Triticale
For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids were sterile until doubling of the number of chromosomes occurred
•Species are created by a series of evolutionary processes▫populations become isolated
geographically isolated reproductively isolated
▫isolated populations evolve independently
•Isolation▫Allopatric
geographic separation▫Sympatric
still live in same area
D 2.6 D 2.6 CompareCompare allopatric and sympatric allopatric and sympatric speciation.speciation.
D 2.6 D 2.6 CompareCompare allopatric and sympatric allopatric and sympatric speciation.speciation.
• Allopatric speciation – speciation that occurs when
• Most
• Population evolves as a result of natural selection and/or genetic drift
D 2.6 D 2.6 CompareCompare allopatric and sympatric allopatric and sympatric speciation.speciation.
• Often results from physical isolation due to the constant changing of the earth’s surface • Ex.: river shifting
course, glaciers migrating, mountain ranges forming, land bridges forming, etc.
D 2.6 D 2.6 CompareCompare allopatric and sympatric allopatric and sympatric speciation.speciation.
• Also results from a type of genetic drift called founders effect:• Genetic drift is
• Founders effect occurs when a
Parent Population: red, yellow, black
The new population is genetically different than the parent population.
Example of founders effect:
D 2.6 D 2.6 CompareCompare allopatric and sympatric allopatric and sympatric speciation.speciation.
• Sympatric speciation - a new species develops
• More common in
• Usually results from:• Ecological isolation• Polyploidy
Adaptive Radiation
When a species gives rise to many new species in a relatively short period of time
Typically occurs when populations
of a
D 2.7 D 2.7 OutlineOutline the process of adaptive radiation. the process of adaptive radiation.
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 colonized 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 2.7 D 2.7 OutlineOutline the process of adaptive radiation. the process of adaptive radiation.
Insect eaters
Bud eater
Seed eaters
Cactuseater
Warbler
finch
Tree
finc
hes
Ground finches
Adaptive radiation
SeedSeedeaterseaters
FlowerFlowereaterseaters
InsectInsecteaterseaters
Rapid speciation:new species filling new niches,because they inheritedsuccessful adaptations.
D 2.7 D 2.7 OutlineOutline the process of adaptive radiation. the process of adaptive radiation.
Warbler finch
Woodpecker finch
Small insectivoroustree finch
Largeinsectivorous
tree finch
Vegetariantree finch
Cactus finch
Sharp-beaked finch
Small groundfinch
Mediumground finch
Large ground finch
Insect eaters
Bud eater
Seed eaters
Cactuseater
Warbler
finch
Tree
finc
hes
Ground finches
Darwin’s finches•Differences in beaks
▫associated with eating different foods▫survival & reproduction of beneficial
adaptations to foods available on islands
D 2.7 D 2.7 OutlineOutline the process of adaptive radiation. the process of adaptive radiation.
Darwin’s finches•Darwin’s conclusions
▫small populations of original South American finches landed on islands variation in beaks enabled individuals to
▫over many generations, the populations of finches changed anatomically & behaviorally accumulation of advantageous traits in
population emergence of
D 2.7 D 2.7 OutlineOutline the process of adaptive radiation. the process of adaptive radiation.
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. -Flight/gliding in birds, pterosaurs, bats, insects and flying fish!
Convergent evolution describes evolution towards
.
http://commons.wikimedia.org/wiki/File:AlleAlle_2.jpg
http://commons.wikimedia.org/wiki/File:Little_penguin_Eudyptula_minor.jpg
Little Auk
Little Penguin
D 2.8 D 2.8 CompareCompare convergent and divergent convergent and divergent evolution.evolution.
Features that come about by convergent evolution are known as
D 2.8 D 2.8 CompareCompare convergent and divergent convergent and divergent evolution.evolution.
Divergent Evolution is another way of saying (D.2.7). As natural
selection acts on two or more species that have arisen from a common ancestor, they become phenotypically different.
D 2.8 D 2.8 CompareCompare convergent and divergent convergent and divergent evolution.evolution.
Divergent evolution describes evolution towards
.
It gives rise to , features that now look different or have a different purpose for each species that has evolved
D 2.8 D 2.8 CompareCompare convergent and divergent convergent and divergent evolution.evolution.
Gradualism •Gradual divergence
over long spans of time▫assume that big
changes occur as the accumulation of many small ones
D 2.9 D 2.9 DiscussDiscuss ideas on the pace of evolution, ideas on the pace of evolution, including gradualism and punctuated including gradualism and punctuated equilibrium.equilibrium.
Punctuated Equilibrium
•Rate of speciation is not constant▫. ▫long periods of little
or no change▫species undergo rapid
change when they 1st bud from parent population
Time
D 2.9 D 2.9 DiscussDiscuss ideas on the pace of evolution, ideas on the pace of evolution, including gradualism and punctuated including gradualism and punctuated equilibrium.equilibrium.
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 stabilizing selection.The punctuation could be explained by directional selection or disruptive selection.
D 2.9 D 2.9 DiscussDiscuss ideas on the pace of evolution, ideas on the pace of evolution, including gradualism and punctuated including gradualism and punctuated equilibrium.equilibrium.
All images CC Andrew Colvin
The downward facing arrows indicate selection pressure against individuals with that morphology
D 2.9 D 2.9 DiscussDiscuss ideas on the pace of evolution, ideas on the pace of evolution, including gradualism and punctuated including gradualism and punctuated equilibrium.equilibrium.
Coevolution • Occurs when
• Examples include predators
and prey and organisms that live in symbiotic relationships (mutualism, commensalism, parasitism)
• As one evolves a new feature or modifies an old one, the other typically evolves new adaptations in response
Stinging ant & the acacia plant
Ant stings insects that try to eat the plant, plant provides food in the form of nectar & shelter in the form of thorns
Polymorphism is the (Poly = “many”;
morphism = “shapes”)
Transient Polymorphism is
• ex: peppered moth melanism
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
D 2.10 D 2.10 DescribeDescribe one example of transient one example of transient polymorphism.polymorphism.
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 color of the trees that the moths rested on.
Directional selection did the rest.
http://www.flickr.com/photos/naturalhistoryman/817332984/
D 2.10 D 2.10 DescribeDescribe one example of transient one example of transient polymorphism.polymorphism.
Draw a graph of what that might look like.
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 D 2.10 DescribeDescribe one example of transient one example of transient polymorphism.polymorphism.
Balanced Polymorphism
•Two alleles are maintained in stable equilibrium•Heterozygote has
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.
D 2.11 D 2.11 DescribeDescribe sickle cell anemia as an example sickle cell anemia as an example of balanced polymorphism.of balanced polymorphism.
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
D 2.11 D 2.11 DescribeDescribe sickle cell anemia as an example sickle cell anemia as an example of balanced polymorphism.of balanced polymorphism.
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.
Distribution of the sickle cell traitHistorical distribution of malaria
D 2.11 D 2.11 DescribeDescribe sickle cell anemia as an example sickle cell anemia as an example of balanced polymorphism.of balanced polymorphism.
http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html
Heterozygous:
Sickle cell trait
Heterozygous:
Sickle cell trait
Heterozygous:
Sickle cell trait
50% chance
Homozygous:
Sickle Anaemia
25% chance
Homozygous:
‘Normal’25%
chance
S
AA A A
AA
SSSS
S
HbA HbA
Haemoglobin: NormalRBCs: NormalO2 Capacity: NormalMalaria resistance: None
HbA HbS
Haemoglobin: 50% normal, 50% mutantRBCs: Usually normal, sickle when [O2] lowO2 Capacity: Mild anaemiaMalaria resistance: Moderate
HbS HbS
Haemoglobin: mutantRBCs: SickleO2 Capacity: Severe anaemiaMalaria Resistance: High
http://en.wikipedia.org/wiki/File:Autorecessive.svg
People who are homozygous for sickle cell are severely anemic and have less chance of surviving to reproduce.Likewise individuals homozygous for normal hemoglobin are likely to contract malaria and are less likely to survive.
Heterozygous individuals have what is termed
. They are the most likely to survive and reproduce.
Therefore both alleles are maintained in the population
D 2.11 D 2.11 DescribeDescribe sickle cell anemia as an example sickle cell anemia as an example of balanced polymorphism.of balanced polymorphism.