Download ppt - Option D: Evolution D2: Species and Speciation D 2.1 Define allele frequency and gene pool. gene pool – allele frequency –

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


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


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


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


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


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


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!


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.


POST-reproduction barriers•Prevent hybrid offspring from

developing into a

▫reduced hybrid viability▫reduced hybrid fertility▫hybrid breakdown

Horse + Zebra = Zebroid


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.



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 2.5 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


▫ 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


• Autopolyploids (auto= self) are polyploids with multiple chromosome sets derived from a single species

• 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

• That does not stop them from being good crops if they can be propagated asexually

Outcrossing – two plantsSelfing – self-fertilizedVegetative – asexual


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.


• Allopatric speciation – speciation that occurs when

• Most

• Population evolves as a result of natural selection and/or genetic drift


• 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.


• 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:


• 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.


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.


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


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


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


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.


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


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


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.


All images CC Andrew Colvin

The downward facing arrows indicate selection pressure against individuals with that morphology


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/


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


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


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


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
