Option D

D1 Origins of Life on Earth

Pre-biotic Earth• The Solar System originated 4.57 BYA

• The Earth originated 4.5 BYA– Formed by collisions of materials over 100 MY,

creating a planet with oceans of liquid magma and a hot dense atmosphere

– Cooling of the Earth took over 50 MY and the loss of the dense atmosphere

Pre-biotic Earth• Pre-Biotic Earth 4.4-4.0 BYA

– Continents of solid rock forming– Oceans of water forming– High temperatures– High UV light levels– Reducing atmosphere (no O2)– Frequent storms with lightning

• Life on Earth originated 3.5-4.0 BYA– Earliest organisms were bacteria– Stromatolites- banded domes of sediment strikingly similar

to the layered mats constructed by colonies of bacteria and cyanobacteria

Spontaneous Origin of LifeTopic D.1.1 Describe four processes needed for the

spontaneous origin of life on Earth.• Chemical reactions to produce simple organic

molecules, such as amino acids, from inorganic molecules, such as water, CO2, and ammonia.

• Assembling of these simple organic molecules into polymers, for example, polypeptides from amino acids

Spontaneous Origin of Life• Formation of polymers that can self replicate-

this allows inheritance of characteristics• Development of membranes, to form spherical

droplets, with an internal chemistry different from the surroundings, including polymers that held genetic information.

Spontaneous Origin of Life• The product of these 4 processes would have

been cell-like structures that natural selection could have operated on.

Miller and UreyTopic D.1.2. Outline the experiments of Miller and Urey

into the origin of organic compounds.– In the 1920’s, A.I. Oparin of Russia and J.B.S. Haldane of

Great Britain independently postulated that conditions on the primitive Earth favored chemical reactions that synthesized organic compounds from inorganic precursors present in the early atmosphere and seas. Oparin-Haldane Hypothesis

– It cannot happen in the modern world, because the present atmosphere is rich in oxygen, the oxidizing atmosphere of today is not conducive to the spontaneous synthesis of complex molecules because the oxygen attacks chemical bonds, extracting electrons.

Miller and Urey• In 1953, Stanley Miller and Harold Urey tested the

Oparin-Haldane Hypothesis using the conditions of pre-biotic Earth.

A. A warm flask of water simulated the primeval sea

B. The atmosphere consisted of H2O, H2, CH4, and NH3.

C. Sparks were discharged to mimic lightning

D. A condenser cooled the atmosphere, raining water and any dissolved compounds back to the miniature sea.

E. As material circulated through the apparatus the solution in the flask changed from clear to murky brown

F. After 1 week the contents of the flask were examined and found a variety of organic compounds including some amino acids that make up proteins of organisms.

What are their conclusions?• Organic compounds (amino acids) were formed

from inorganic compounds.• Organic compounds could have existed on pre-

biotic Earth.• Life might have arisen from non-living material.

PanspermiaTopic D.1.3. State that comets may have delivered organic

compounds to Earth.• Panspermia- the theory concerned with the arrival of

material from outer space. • Hundreds of meteorites and comets hitting the early

Earth brought with them organic molecules formed by abiotic reactions in outer space.

• Extraterrestrial organic compounds, including amino acids, have been found in modern meteorites, and it seems likely that these bodies could have seeded the early Earth with organic compounds.

Origin of Life4. Discuss possible locations where conditions

would have allowed the synthesis of organic compounds.

• Miller and Urey’s experiment suggest organic compounds could have synthesized in Pre-Biotic Earth.– Lightning, high temps, oceans forming, Reducing

atmosphere

Origin of Life• There are hydrothermal vents deep in the

oceans, with chemicals welling up from the rocks below.

• Around these vents, there are unusual chemical conditions, which might have allowed the spontaneous synthesis of the first organic compounds.

Origin of Life• Panspermia- Tests have shown that meteorites

do contain organic compounds and proto-cells.• Pre-biotic earth was bombarded with

meteorites, comets and interplanetary dust, which might have brought organic compounds that became organized into the first living organisms.

RNATopic D.1.5. Outline two properties of RNA that

would have allowed it to play a role in the origin of life.

• RNA is thought to have served as the first genes, not DNA.

• DNA RNA Proteins: the mechanisms for this is too complicated to have evolved all at once.

• Genes cannot be replicated without enzymes, and enzymes cannot be made without genes.

• The first genes were short strands of RNA that began self-replicating in the prebiotic world.

RNA• RNA also has been shown to act as an enzyme,

called ribozyme.

• RNA has catalytic properties

• RNA can catalyze the formation of more RNA (rRNA, tRNA, and mRNA)

• RNA can bind amino acids and form peptide linkages

• RNA can transcribe into DNA using reverse transcriptase

ProtobiontsTopic D.1.6 State that living cells may have been preceded

by protobionts, with an internal chemical environment different from their surrounding.

• This is biochemical evolution.• Coacervate droplets self-assembles when a solution of

polypeptides, nucleic acids, and polysaccharides is shaken.

• Coacervates can contain polynucleotides (RNA)• Assembly of chains of amino acids can form• Formation of proteins• Alignment of lipids and the formation of a membrane• Synthesis of ATP and anaerobic respiration• Asexual reproduction

ProkaryotesTopic D.1.7 Outline the contribution of prokaryotes

to the creation of an oxygen rich atmosphere.

• The first organisms on earth were photosynthetic prokaryotes.

• Oxygen is a waste product of photosynthesis.

• Oxygen concentrations build up over time

Pre

cam

bria

n

First HumansExtinction of dinosaursOrigin of reptilesPlants colonize land

Origin of multicellular organisms(oldest animal fossils)

Oldest eukaryotic fossils

Accumulation of atmospheric oxygenfrom photosynthetic cyanobacteria

Origin of Earth

Origin of life?

Earth cool enough for crust to solidify

Oldest prokaryotic fossils

CenozoicMesozoic

Paleozoic

500

0

1500

2500

3500

4500

Endosymbiotic TheoryTopic D.1.8 Discuss the endosymbiotic theory for the

origin of eukaryotes.

• Eukaryotic cells contain membrane bound organelles.

• According to the Endosymbiotic Theory proposed by Lynn Margulis of the University of Massachusetts, both the Mitochondria and Chloroplasts have evolved from independent prokaryotic cells, which were taken into a larger heterotrophic cell by endocytosis.

• Instead of being digested, the cells were kept alive and continued to carry out aerobic respiration and photosynthesis.

Endosymbiotic Theory

Endosymbiotic Theory• The characteristics of mitochondria and chloroplasts that

support the Endosymbiotic Theory are: • Similar in size to Bacteria• They grow and divide like cells.• They have a circular naked loop of DNA.• They synthesize some of their own proteins using 70S

ribosomes.• They have double membranes, as expected when cells are

taken into a vesicle by endocytosis. • Reproduce by binary fission.• Cristae are similar to mesosomes of prokaryotes.• Thylakoids are similar to structures containing chlorophyll

in photosynthetic prokaryotes.

D2 Species and Speciation

Allele Frequency and Gene PoolTopic D.2.1 Define Allele Frequency and Gene

Pool.• Allele Frequency- is the frequency of an allele,

as a proportion of all alleles of the gene in the population.

• Allele frequency can range from 0.0 to 1.0– Usually expressed as a percentage or a proportion

• Gene Pool- is all the genes in an interbreeding population.

EvolutionTopic D.2.2 State that evolution involves a change

in allele frequency in a population’s gene pool over a number of generations.

SpeciesTopic D.2.3 Discuss the definition of a species.• What is a species?• A species is a potentially interbreeding population

having a common gene pool.• Typological species concept- species are static,

nonvariable assemblages of organisms that conform to a common morphological plan

• Plato and Aristotle– Today we use it as the type specimen

• Problems:– What are type characteristics?

Species• Morphological species concept- Species are

distinguished from each other by their morphological characteristics.

• Useful for fossils• Problems:

– Sexual dimorphism– Cryptic Species– Geographic variation

Species• Biological species concept- a group of actually or

potentially interbreeding populations, with a common gene pool, which are reproductively, isolated from other such groups. Ernst Mayr (1963)

• Problems: – Sibling species- species that cannot interbreed, but show no

significant differences in appearance.– Hybridization between different species– Species that only reproduce asexually– Fossils

Species• Phylogenetic species concept- monophyletic and

genomically coherent clusters of individual organisms that are descended from a single ancestral taxon and show a high degree of overall similarity in many independent characteristics, diagnosable by a discriminative phenotypic property. A species is a monophyletic group.

• Problems:– DNA, Amino acids, or morphology– Different techniques for analyzing sequence data– Gene tree vs. Species tree

Gene PoolsTopic D.2.4 Describe three examples of barriers between gene

pools.Topic D.2.6 Compare Allopatric and Sympatric Speciation.• The formation of a new species is called speciation.• New species are formed when a pre-existing species splits.• This usually involves the isolation of a population of the

remainder of its species and thus the isolation of its gene pool.• The isolated population will gradually diverge from the rest of

the species if natural selection acts differently on it.• Eventually the isolated population will not be able to

interbreed with the rest of the species—it has become a new species.

Speciation• Allopatric speciation- species are isolated

geographically.• Geographic Isolation- When members of a species

migrate to a new area, forming a population that is geographically isolated from the rest of the population.

• Migration- members of a species move to a new location that is geographically isolated from the original territory.

• The Galapagos Finches• Lava lizards in the Galapagos• Adaptive radiation- the evolution of many diversely

adapted species from a common ancestor.

Speciation• Sympatric speciation- species are not isolated

geographically. A subpopulation becomes reproductively isolated in the midst of its parent population.

Speciation• Behavioral Isolation

– Example: Apple Maggot Fly (Rhagoletis pomonella) – It originally laid its eggs on Hawthorn fruits, but some

individuals started to infest non-native apple trees as well. The fruits ripen at different times, thus the adults emerge and mate at different times.

– Now you have two separate breeding populations of the apple maggot fly. There are differences in allele frequencies, but they have not been classified as different species as of yet.

Speciation• Hybrid Infertility- barriers between gene

pools- often due to polyploidy.• Polyploidy- extra sets of chromosomes• Plants may evolve into new species in one

generation by a polyploid event. • Autopolyploid- More than one set of

chromosomes evolves from a single species.• Allopolyploid- More than one set of

chromosomes evolves from different species.

SpeciationTopic D.2.5 Explain how polyploidy can

contribute to speciation.• Rumex- most species have 20

chromosomes.• Rumex obtusifolius has 40• Rumex crispus has 60• Rumex hydrolapathum has 200

SpeciationTopic D.1.7 Outline the process of adaptive

radiation.• Adaptive Radiation- process in which many

related species evolve from one ancestor– Example: Darwin’s Finches– One finch or mating pair makes it to an island – They have no predetors and unlimited resources.– They are able to reproduce as much as possible

Speciation• Variation exists and they can spread out to other

niches.• They adapt to fill all the niches.

Speciation8. Compare convergent and divergent evolution.• Convergent Evolution- similar structures or form

but not closely related– Shark and porpoise

• Divergent Evolution- 2 or more related species that look different because of habitat– All mammals have the same ancestor but look very

different.

Evolution in Process• Homologous- similar feature from the same

ancestor- limbs• Analogous- similar feature because of

function but different ancestor- wing of bird and insect

Pace of EvolutionTopic D.2.9 Discuss ideas on the pace of evolution

including gradualism and punctuated equilibrium.– Gradualism is the slow change from one form to

another. Punctuated equilibrium, however, implies long periods with no change and short periods of rapid evolution. Mention could be made of the effects of volcanic eruptions and meteor impacts in affecting evolution on Earth.

Pace of Evolution• Over long periods of time, many advantageous

alleles will appear and spread through a species. These micro-evolutionary steps together constitute macroevolution.

• Eventually the amount of change becomes so great that the species is no longer the same and one species have evolved into another.

• Gradualism- slow change from one form to another. Evolution proceeds slowly, but over long periods of time larger changes can gradually take place. This does not fit with the fossil record.

Pace of Evolution• The fossil record shows periods of periods of stability,

with fossils showing little change, followed by periods of sudden major change.

• The periods of stability may be due to equilibrium where living organisms become well adapted to their environment so natural selection acts to maintain their characteristics.

• The periods of sudden change that punctuated the equilibrium may correspond with rapid environmental change, caused for example by volcanic eruptions or meteor impacts.

• New adaptations would be necessary to cope with new environmental conditions, hence strong directional selection and rapid evolution—Punctuated equilibrium.

Transient PolymorphismTopic D.2.10 Describe one example of transient

polymorphism.• Populations of ladybug that changed from

having red wings with black spots to black wings are an example of transient polymorphism.

Adalia bipunctata• Adalia bipunctata- Ladybeetle- 2 spotted small beetle,

which usually has 2 small black spots on its red wings. • The red is a type of aposematic coloration. Melanic

forms also exist, with solid black wings. • The melanic forms absorb heat more efficiently and

therefore have a selective advantage when sunlight levels are low.

• The melanic form became more common in industrial areas of Britain, but declined again after the 1960’s.

• If the air is dark with smoke the melanic form can warm up faster, but if there is no advantage to the melanin, then the Aposematic coloration is more of an advantage.

Balanced PolymorphismTopic D.2.11 Describe Sickle Cell Anemia as an

example of Balanced Polymorphism.• Sickle cell anemia is an example of balanced

polymorphism.

Hardy-Weinberg• Heterozygotes (HbA HbS) do not develop sickle

cell anemia and are resistant to Malaria.• The sickle cell allele has increased in frequency

to high levels in some areas. Parts of Africa, as many as 40% of the population are carriers of the sickle cell allele.

D3Evidence for Evolution

Geographical DistributionTopic D.3.1 Describe the evidence for evolution as

shown by the geographical distribution of living organisms, including the distribution of placental, marsupial, and monotreme mammals.

Wallace’s Line

There are huge differences in the types of land animals that are found on either side of Wallace’s Line.

Placentals vs. Marsupials vs. Monotremes

• Placental mammals are found on the Asian side • Mainly marsupial (pouched mammals) and monotreme

(egg laying mammals- only three living monotreme species - the platypus, the short-beaked echidna, and the long-beaked echidna) mammals are found on the Australasian side.

Placentals vs. Marsupials vs. Monotremes

• The land masses on the two sides of the boundary separated about 100 MYA and came together by continental drift about 15MYA. The mammals on the separated landmasses followed different evolutionary paths, so different types evolved.

Australian vs. African Moles• In similar habitats where natural selection acts in

the same way on different organisms, the results are sometimes strikingly similar, for example the marsupial mole of Australia and the golden mole of Africa.

Armadillos• Armadillos- only found in the Americas.

Contempory armadillos are modified descendants of earlier species that occupied these continents and the fossil record confirms this.

FossilsTopic D.3.2 Outline how remains of past living organisms have

been preserved. Include petrified remains, prints, and moulds and preservation in amber, tar, peat and ice.

• Sediments that turn to rock- accumulates both in the sea and on land.

• If hard parts of animals such as shells or bones form part of the sediment, they will be preserved as a cast.

• Minerals sometimes seep into the soft parts of an organism as it decays and harden to form a petrified replica of the organism.

• The remains of past living organisms can be trapped and preserved in various ways:

Fossils• Resins-which turn to amber

Fossils• Frozen in ice or snow- besides animals, plants can

be preserved this way---How?

Fossils• Tar pits- form when crude oil seeps to the surface through

fissures in the Earth's crust; the light fraction of the oil evaporates, leaving behind the heavy tar, or asphalt, in sticky pools.

• Found wooly mammoths fossils, plants, mollusks, and insects.

Fossils• Petrified fossils- A cast is formed as minerals or

other sediments fill and harden within the sedimentary cavity formed as the original organism deteriorates.

Fossils• Acid peat, which prevents decay- Peat- soil material

consisting of partially decomposed organic matter; found in swamps and bogs in various parts of the temperate zone.

• It is formed by the slow decay of successive layers of aquatic and semi aquatic plants, e.g., sedges, reeds, rushes, and mosses.

• One of the principal types of peat is moss peat is an acidifying agent.

Half-LifeTopic D.3.1 4 Define Half-life• The number of years it takes for half of an

original sample of radioactive material to decay or undergo radioactive transformation.

• It is unaffected by temperature, pressure, and other environmental variables.

Radioisotopes3. Outline the method for dating rocks and fossils using

radioisotope, with reference to 14C and 40K. (Knowledge of the degree of accuracy and the choice of isotope to use is expected. Details of the apparatus used are not required.)

• Radiometric dating involves the use of isotope series, such as rubidium/strontium, thorium/lead, potassium/argon, argon/argon, or uranium/lead, all of which have very long half-lives, ranging from 0.7 to 48.6 billion years.

• Subtle differences in the relative proportions of the two isotopes can give good dates for rocks of any age.

• Radiometric dating has an error factor of less than 10%.

14C and Uranium 238• 14C has a half-life of 5730 years; it is best used

for young fossils (< 20,000 years old)• Uranium-238 has a half-life of 4.5 BY and can be

used to date rocks hundreds or thousands of millions years old.

Amino Acids• Amino acids exist in two isomers with either left-

handed (L) or right-handed (D) symmetry. • After an organism dies its population of L amino acids

is slowly converted into proteins, resulting in a mixture of L and D amino acids.

• Knowing the rate at which this chemical conversion occurs (Racemization) then allows us to calculate the ratio of L:D and determine how long the fossil has been dead.

• However, racemization is temperature sensitive.

40K• 40K has a half-life of 1.25 BY and it decays to:• 40Ca by emitting a beta particle with no attendant

gamma radiation (89% of the time) • The gas 40Ar by electron capture with emission of

energetic gamma ray (11% of the time)• Potassium-Argon dating is the only viable dating

technique for dating very old archaeological material (550 million years to 3.8 American billion years)

Decay CurveTopic D.3.5 Deduce the approximate age of

materials based on a simple decay curve for a radioisotope.

EvolutionTopic D.3.6 Outline the palaeontological evidence for

evolution using one example.• Fossil of Acanthostega- 365 MYA fossil. It has

similarities to other vertebrates (backbone and 4 limbs), but it has 8 fingers and 7 toes. It is not identical to any existing organism.

• This suggests that vertebrates and other organisms change over time. Acanthostega is an example of a “missing link”.

• Although it has 4 legs like most amphibians, reptiles, and mammals, it also has a fish-like tail and gills and lived in water.

• This shows that land vertebrates could have evolved from aquatic animals.

Acanthostega

DNATopic D.3.7 Explain the biochemical evidence provided by

the universality of DNA and protein structures for the common ancestry of living organisms.

• There are remarkable similarities between living organisms in their biochemistry.

• All use DNA (or RNA) as their genetic material• All use the same universal genetic code, with only a few

insignificant variations• All use the same 20 amino acids in their proteins• All use left, and not right-handed amino acids.• These similarities suggest that all organisms have

evolved from a common ancestor that had these characteristics.

PhylogenyTopic D.3.8 Explain how variations in specific

molecules can indicate phylogeny.• Phylogeny- the evolutionary history of a group of

organisms.• The phylogeny of many groups of organisms has

been studied by comparing the structure of a protein or other biochemical that they contain, usually DNA.

Phylogeny• Ex. Amino acid sequence of the polypeptide of

hemoglobin has been compared in many vertebrates.

• Differences accumulate gradually over long periods of time and this is a roughly constant rate: evolutionary (molecular) clock.

Phylogeny

Evolutionary ClockTopic D.3.9 Discuss how biochemical variations can

be used as an evolutionary clock.• Evolutionary (Molecular) clock- the rate at which

mutations accumulate in a given gene are at a constant rate and therefore you can date how long two organisms have been diverged.

Homologous structuresTopic D.3.10 Explain the evidence for evolution provided

by homologous anatomical structures, including vertebrate embryos and the pentadactyl limb.

• Homologous anatomical structures are structures derived from the same part of a common ancestor.

• There are also remarkable similarities between some groups of organisms in their structure.

Homologous structures• At an early stage, vertebrate embryos are very similar,

despite huge differences in the structure of the adults.

Homologous structures• The limbs of vertebrates show striking similarities in

their bones, despite being used in many different ways. The structure is called the pentadactyl limb.

• The most likely explanation for these structural similarities is that the organisms have evolved from a common ancestor.

Galapagos FinchesTopic D.3.11 Outline two modern examples of

observed evolution. One example must be the changes to the size and shape of the beaks of Galapagos finches.

Galapagos Finches• Galapagos Finches- Daphne Major (one island inhabited by 2 species

of finches). • Geospiza fortis has a short, wide beak and feeds on a variety of

seeds, including large hard ones. • During 1982-1983, there was a severe El Niño, The available food

population increased and the population of G. fortis also increased, reaching a peak in 1983.

• It dropped back in the drier years following and in 1987, was only 37% of its peak population from 1983. The period of heavy rain changed the vegetation and until 1991 there were fewer plants producing large, hard seed and more producing small soft ones.

• The diet of G. fortis changed. The population had longer, narrower beaks than the average in 1983.

• The conclusion that this change in diet caused a change in beak shape is supported by evidence from the other finch on the island. G. scandens’ population rose and fell the same as G. fortis but neither its diet nor the size of its beak changed.

D4-D6

D4Human Evolution

Classification1. State the full classification of human beings from

kingdom to sub-species.• Domain Eukarya• Kingdom Animalia• Phylum Chordata• Class Mammalia• Order Primate• Family Hominidae• Genus Homo• Species Homo sapiens• Subspecies Homo sapiens ssp. sapiens

Primates2. Describe the major physical features, such as the

adaptations for tree life, that define humans as primates.

• Grasping limbs, with long fingers and a separate opposable thumb

• Mobile arms, with shoulder joints allowing movement in three planes and the bones of the shoulder girdle allowing weight to be transferred via the arms

• Stereoscopic vision, with forward facing eyes on a flattened face, giving overlapping fields of view

• Skull modified for upright posture

Anatomical and Biochemical Evidence

3. Discuss the anatomical and biochemical evidence, which suggests that humans are a bipedal and neotenous species of African ape that spread to colonize new areas.

– Attention should be drawn to the main features only. Neoteny in this case is in relation to the delayed onset of puberty leading to the increased period of parental care.

Bipedalism• Bipedalism- walking on 2 legs• Australopithecus afarensis shows partial

bipedalism• Therefore this is an early development in human

evolution.

Adaptations for Bipedalism• The foramen magnum, a hole in the skull

through which the spinal cord and brain connect, moved forwards. This allows the head to balance on the backbone.

• The arms became shorter and less powerful• The legs became longer and stronger• The knee changed to allow the leg to straighten

fully• The foot became more rigid, with longer heel,

shorter toes and a non-opposable big toe.

Consequences of bipedalism• Collecting food from bushes is easier• Walking long distances while carrying food,

water, infants, tools, or weapons is easier.• Tree climbing is more difficult

Neoteny• Neoteny- keeping juvenile characteristics as an adult.• Adult humans show similarities in appearance to baby

apes, with flat faces, large brain to body size ratio, upright heads, and little body hair.

• This suggests that human evolution from an ape ancestor might have involved a slowing down of development, with a long childhood, delayed puberty and retention of juvenile characteristics in adulthood.

Fossil trends4. Outline the trends illustrated by the fossils of

Australopithecus including A. afarensis, A. africanus, and A. robustus, and Homo including H. habilis, H. erectus, H. neanderthalensis, and H. sapiens.

• Many hominid fossils have been found, dated and assigned to a species.

• These fossils show evolutionary trends, including increasing adaptations to bipedalism and increasing brain size.

Australopithecus afarensis

Australopithecus africanus

Australopithecus robustus

Homo habilis

Homo erectus

Homo neanderthalensis

Homo sapiens

Ecological Changes5. Discuss the possible ecology of these species and

the ecological changes that may have prompted their origin.

• 5 MYA Africa became drier and dense forests were replaced by thinner woodland with clearings.

• This could have prompted the evolution of bipedalism, although early hominids probably still lived partly in trees.

• Australopithecus had powerful jaws and teeth indicating a mainly vegetarian diet.

Ecological Changes• 2.5 MYA Africa became much cooler and drier.

Savannah grassland replaced forest. • This change of habitat may have prompted the

evolution of the first species of Homo, with the development of increasingly sophisticated tools and a change in diet that included meat obtained by hunting and killing large animals.

• Homo erectus and later species developed the use of fire and were able to colonize colder areas and survive during ice ages.

Fossil Record6. Discuss the incompleteness of the fossil record and the

resulting uncertainties with respect to human evolution.

– Knowledge of approximate dates and distribution for the named species is expected. Details of sub-species or particular groups (Cro-Magnon, Peking, etc) are not required. Reasons for the incompleteness of the fossil record should be included.

• The question of where the earliest hominid ancestors lived has still not been answered with certainty.

• The closest existing relatives of humans are chimpanzees and gorillas from Africa and orangutans from Southeast Asia.

D5NeoDarwinism

Mutations1. State that mutations are changes to genes or

chromosomes due to chance, but with predicted frequencies.

Gene and Chromosome Mutations2. Outline phenylketonuria (PKU) and cystic fibrosis as examples of

gene mutation, and Klinefelter’s syndrome as an example of chromosome mutation.

• Examples of gene mutations: PKU, Cystic fibrosis, and Sickle cell anemia.

• Phenylketonuria (PKU) - caused by mutations of an autosomal gene that code for phenylalanine hydroxylase.

• This enzyme converts the amino acid phenylalanine into tyrosine. Without it, phenylalanine accumulates in the blood to a harmful level that causes mental retardation and death in young children.

• Over 30 different alleles cause PKU. • Natural selection has kept them at low frequencies in the human

population because, until screening and treatment (recently became possible), children homozygous for PKU alleles died at an early age.

Gene and Chromosome Mutations• Cystic fibrosis- is the commonest genetic disease in

Europe. It is caused by mutations of a gene coding for a chloride channel.

• This protein transports chloride ions across membranes in epithelium cells. Without chloride channels in the plasma membrane, mucus secreted by epithelia becomes thick and sticky and tends to block airways of the breathing system, causing respiratory infections.

• Although mutations of the chloride channel gene can cause cystic fibrosis, 70% of cases are due to one mutant allele, in which 3 bases coding for phenylalanine have been deleted.

• The frequency of cystic fibrosis in Europe is 1 in 2500.

Gene and Chromosome Mutations• Examples of Chromosome mutations: Klinefelter’s syndrome,

Down’s syndrome, and Turner’s syndrome.• Chromosome mutations often cause infertility and so the

variation that they cause is not inherited. They are therefore not usually significant in evolution.

• Klinefelter’s syndrome- Males having one or more extra X chromosomes (XXY).

• Although recognizably male, those with the syndrome have low testosterone levels and so are infertile and do not fully develop the male secondary sexual characteristics. Slight mental retardation and the development of female characteristics may occur.

• Mental retardation usually only occurs in XXXY, XXYY, and XXXXY forms.

• Is among the most common chromosomal mutations occurring in 1 in 500 to 1000 male births, these numbers maybe misleading because not all men exhibit symptoms.

Gene and Chromosome Mutations• Turner’s syndrome- Females having only one X

chromosome (XO). Recognizable female, short stature and lack of ovarian development.

• Other characteristics are a webbed neck, arms that turn out slightly at the elbow, and low hairline in the back of the head.

• No mental retardation and with medical help can carry a fetus to term.

• Is among the most common chromosomal mutations occurring in 1 in 2500 female births.

Variation3. Explain that variation in a population results from the

recombination of alleles during meiosis and fertilization.• A new individual, produced by sexual reproduction

inherits genes from its two parents. • If there is random mating, any two individuals, in an

interbreeding population could be the two parents, so the individual could inherit any of the genes in the interbreeding population.

• These genes are called the gene pool. A gene pool is all the genes in an interbreeding population.

Adaptations4. State that adaptations (or micro-evolutionary

steps) may occur as the result of allele frequency increasing in a population’s gene pool over a number of generations.

Evolution5. Describe how the evolution of one species into

another species involves the accumulation of many advantages alleles in the gene pool of a population over a period of time.

• If an allele increases the chances of survival and reproduction of individuals that posses it, the frequency of the allele in the gene pool will tend to increase.

• Conversely, if the allele reduces the chance of survival and reproduction, it will decrease in frequency.

• These changes are due to natural selection.

D6.

The Hardy-Weinberg Principle

Hardy-Weinberg5. State the Hardy-Weinberg principle and the

conditions which it applies.• Large population (No Genetic Drift)• Random Mating• No mutation• No Gene flow (No Immigration or Emigration)• All phenotypes have equal fitness (No Selection)

Hardy-Weinberg1. Describe an adaptation in terms of change in frequency of a

gene’s alleles.• If an allele increases the chance of survival and reproduction it

should increase in frequency and vice versa.• The Hardy-Weinberg equation can be used to test for natural

selection. If allele and genotype frequencies in a population show that the Hardy-Weinberg principle is being followed for a particular gene, this indicates no natural selection.

• Members of the population all have an equal chance of survival whatever alleles of the gene they posses.

• The allele frequency will not change between one generation and the next.

• If allele and genotype frequencies do not follow the Hardy-Weinberg principle, a possible reason is that natural favors one allele over another.

Hardy-Weinberg• Adaptations develop in populations as a result of

changes in allele frequencies in the gene pool. (Microevolution)

• A population in which there are 2 alleles of a gene in the gene pool is polymorphic. If one allele is gradually replacing the other, the population shows transient polymorphism.

• Populations of ladybug that changed from having red wings with black spots to black wings are an example of transient polymorphism.

Hardy-Weinberg2. Explain how the Hardy-Weinberg equation (p2 +

2pq +q2 = 1) is derived.

• If there are 2 alleles of a gene in a population, there are 3 possible genotypes:

• aa homozygous recessive• AA homozygous dominant• Aa heterozygous

• The frequency of the 2 alleles in the population is usually represented by p and q.

• The total frequency of the alleles is 1. p + q = 1

Hardy-Weinberg• If there is random mating in a population, the

chance of inheriting 2 copies of the first of the two alleles is (p x p).

• The chance of inheriting 2 copies of the second of the two alleles is (q x q).

• The expected frequency of the 2 homozygous genotypes is p2 and q2.

• The expected frequency of the heterozygous genotype is 2pq.

• The sum of all these frequencies is 1.• p2 + 2pq +q2 = 1

Hardy-Weinberg• Example: MN blood group gene in a town in Japan.

The two alleles are codominant.• Allele frequencies of the P1• M= p= 0.525 N= q= 0.475• F1 Predicted F1 Actual• MM p2= 0.276 0.274• MN 2pq= 0.499 0.502• NN q2= 0.225 0.224• The results show that the actual genotypes fit those

predicted by the Hardy-Weinberg equation very closely.• They therefore follow the Hardy-Weinberg Principle.

Hardy-Weinberg3. Calculate allele, genotype and phenotype

frequencies for two alleles of a gene, using the Hardy-Weinberg equation.

• The ability to taste phenylthiocarbamide (PTC) is due to a dominant allele (T) and non-tasting is due to the recessive allele (t).

• 1600 people were tested in a survey. • 461 were non-tasters- a frequency of 0.288. • Their genotype was homozygous recessive (t t).

Hardy-Weinberg• If q= frequency of t allele, q2 = 0.288 so q= 0.537• If p= frequency of T allele, p= (1-q) = 0.463• The frequency of homozygous dominants (T T)

and heterozygotes (T t) can be calculated.• p2 = frequency of homozygous dominant• p2 = 0.463 x 0.463 = 0.214• 2pq = frequency of heterozygotes• 2pq = 2 (0.463 x 0.537) = 0.497

Hardy-Weinberg4. State that the Hardy-Weinberg principle can

also be used to calculate allele, genotype and phenotype frequencies for genes with more than two alleles.