IB Biology/Option D - Evolution
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Option D.1 Origin of Life on Earth
D.1.1 Outline the conditions of the pre-biotic Earth, including high temperature, lightning, UV light penetration and a reducing atmosphere.
- In the pre-biotic Earth (the solar system originated around 4.57 billion years ago) there was little to no oxygen in the atmosphere (any oxygen was absorbed by rocks), so there was no oxygen to steal the energy.
The energy for forming the molecules was provided by lightning, volcanic activity, meteorite bombardment, high temperatures due to greenhouse gasses and UV radiation.
- At first, the Earth was cold and later melted from heat produced by compaction, radioactive decay and the impact of meteorites. The molten material sorted into layers of varying density with the least dense material solidified into a thin crust. The present continents are attached to plates of crust that float on the mantle. The first seas formed from rain that began when Earth had cooled enough for water in the atmosphere to condense.
- The prebiotic era dates from 4.4 billion years ago. The earliest life known is dated to 3.8 billion years ago.
D.1.2 Outline the experiments of Miller and Urey into the origin of organic compounds.
- They simulated conditions on the early Earth by constructing an apparatus that contained a warmed flask of water simulating the primeval sea and an atmosphere of water, hydrogen gas, CH4 (methane), and NH3 (ammonia).
- Sparks were discharged in the synthetic atmosphere to mimic lightning. A water was boiled while a condenser cooled the atmosphere, raining water and any dissolved compounds back to the miniature sea. The simulated environment produced many types of amino acids and other organic molecules leading them to conclude the prebiotic synthesis of organic molecules was possible.
- However, the question of the concentration of methane and other chemicals is in doubt so the applicability of the results is uncertain.
D.1.3 Discuss the hypothesis that the first catalysts responsible for polymerization reactions were clay minerals and RNA.
- Polymers are chains of similar building blocks or monomers, synthesized by condensation reactions (H and OH are removed from polymers and H20 is produced). Early polymerization reactions must have occurred without the help of enzymes.
Clay increases the rate of polymerization in these ways:
- Monomers bind to charged sites between close clay layers, concentrating amino acids and other monomers
- Metal ions at binding sites in clay catalyze dehydration reactions
- The clays provide more stable conditions for the formation of molecules.
Functions of RNA in polymerization:
- There is a type of RNA called a ribozyme that can catalyze its own replication. This may be very crucial to the origin of RNA and DNA and therefore for the beginning of life.
D.1.4 Discuss the possible role of RNA as the first molecule capable of replicating.
- Protobionts, or aggregates of abiotically produced molecules, accumulate organic materials like polypeptides from the environment. However, they probably do not possess a mechanism for replicating their particular characteristics so life cannot start. However, an information storage mechanism like RNA would allow them to pass on their characteristics. RNA probably preceded DNA, as DNA is too complex of a molecule to have formed in early protobionts. Short strands of RNA, perhaps most importantly the RNA called ribozymes, could have copied themselves and could have catalysed other reactions. They could then align nucleotides according to a certain pattern when bound to clay. Thus RNA could be replicated and passed on. This is supported by the fact that RNA plays an important role in genetic control in cell life today.
D.1.5 Discuss a possible origin of membranes and prokaryotic cells.
- Living cells may have been preceded by protobionts, aggregates (groups) of abiotically produced molecules. To form life they need to compartmentalise themselves from the surrounding water.
- Some protobionts in the presence of lipids form a molecular bilayer (liposome) around the protobiont droplet when shaken. This resembles the lipid bilayer of modern cells. The liposomes can break, reform and merge with others and mix their contents. Within these droplets reactions could be catalysed and RNA could replicate itself.
- The most successful liposomes at surviving would pass on their characteristics and develop into early prokaryotes.
D.1.6 Discuss the endosymbiotic theory for the origin of eukaryotes.
- Eukaryotic cells were probably symbiotic groupings of prokaryotic cells with smaller species living within larger prokaryotes. The endosymbiotic theory focuses on the origins of chloroplasts and mitochondria.
- The ancestors of these organisms (photosynthetic prokaryotes like algae or respiring bacteria) originally entered the host cell as undigested prey or internal parasites. Normally the invaders would be digested but in this case they were not.
- The host cell would allow them to live and exploit them. These organisms provide food and energy. Those which absorbed prokaryotes which became chloroplast and mitochdria formed plants, those without the chloroplasts became animal cells.
- The evidence that supports this theory is:
- Mitochondria and chloroplasts have bacteria-like RNA and ribosomes (70S as opposed to 80S in eukaryote cytoplasms) that enable them to make their own proteins and divide independently of the host cell.
- Mitochondria and chloroplasts both have double membranes, the inner one probably that of the original cell, and the outer that which was created when the cell was absorbed.
- Thylakoids resemble structures found in blue-green bacteria.
- Chloroplasts and mitochondria have naked DNA like prokaryotes.
- Chlorophyll a is found in both prokaryotes and eukaryotes.
- Christae in mitochondria resemble mesosomes in bacteria.
Option D.2 Origin of Species
D.2.1 Outline Lamarck's theory of evolution by the inheritance of acquired characteristics.
- Lamarcks theory of inheritance of acquired characteristics states that the modifications an organism acquired during its lifetime could be passed along to its offspring. The common idea is that if a giraffe stretches up to reach high leaves, it can pass a long neck onto its offspring.
D.2.2 Discuss the mechanism of, and lack of evidence for, the inheritance of acquired characteristics.
- There is no evidence that acquired characteristics can be inherited. Characteristics that can be passed on are in your genes. If you are not born with genes programming for certain characteristics, you cannot pass down characteristics acquired during your lifetime as these characteristics do not reach sex cells. Example: if you cut off the tails of mice, their offspring will still have tails.
D.2.3 Explain the Darwin-Wallace theory of evolution by natural selection.
- Populations tend to produce more offspring than an environment can support but populations tend to remain constant
- There is a struggle for survival among organisms in a species with varied characteristics
- Those individuals which are best adapted to their environments are most likely to survive
- Only these pass on their positive characteristics in the form of genes to their offspring who also survive
- While on his Beagle voyages, Darwin became intrigued with the different types of finches found in the Galapagos. All the species of birds were very similar, just like a species on the mainland of South America but between the islands they differed in size and beak shape. Darwin found that the birds fed on different types of food. Their beaks were adapted to eat different types of leaves, worms and seeds and other types of diets. Darwin explained all his observations and thoughts about the origin of species by the concept of "natural selection". This theory states that great diversity in a species ensures that some members of a population will be more suited to their environment than others. These individuals will be more likely to live long enough to reproduce and pass on their well-suited genes. Therefore, because those that are best suited (the food on an island) are the ones who have the most children, a population will, over time, adapt itself to its environment. The change in the frequency of characteristics (genes) in a population with a common gene pool is evolution.
D.2.4 Discuss other theories for the origin of species including special creation and panspermia.
- Panspermia is the theory which suggests that life arose elsewhere in the universe and travelled to Earth through space in comets or meteors. Special creation states that a creator(s) formed life directly.
D.2.5 Discuss the evidence for all these theories and the applicability of the scientific method for further investigation.
- Panspermia - Organic compounds and amino acids have been recovered from modern meteorites. This theory is not incompatible with evolution, which would have occurred once life reached Earth. The theory could be tested in the future by looking for life similar to our own beyond Earth but space travel is not yet sufficiently developed.
- Special Creation - most of the evidence for this theory is found in religious faith. There is no real scientific evidence for this theory other than a lack of evidence for alternate theories. The theory cannot be falsified (contradicted) by observations, unlike evolution and panspermia. However, some believe that more and more evidence is being found that points towards creation by a divine being.
- The Lamarckian theory also lacks any evidence in its favour.
- Evolution fits with commonly held facts and is supported with sufficient amounts of evidence discussed in D.3. It operates based on well understood processes and laws of nature that have not been contradicted by any current observations, and thus is the preferred theory.
Option D.3 Evidence for Evolution
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 animals.
- The two northern continents are only separated by a small sea, the Bering Straight which has been crossable at times in the past. They have very similar mammal life. The three southern continents have been far more isolated from one another and show far greater variety of mammal life.
- Looking at the way in which continents have drifted over time and examining the fossils in them we can map out the development of different sorts of mammals. Monotremes and marsupials developed in Gondwanaland 165mya before the three southern continents were separated. Placental mammals (developed life young) developed later 135mya and replaced almost all of the monotremes (egg laying mammals) and marsupials (undeveloped young born into pouches) in the other continents.
- Australia was separated from the other continents so animals there could no longer mate with animals from other continents and Australian animals could not travel elsewhere. This is why marsupials and monotremes are now found only on Australia. Monotremes and marsupials have evolved to fit the niches within the Australian environment and there are no placental mammals. This is simply because of geography not because Australia is unfit for placental mammals. These observations and other similar examples are expected under the theory of evolution.
D.3.2 Outline how remains of past living organisms have been preserved.
- Study of past evolution - phylogeny
- Sand eroded from the land is first carried by rivers, seas, swamps where the particles settle to the bottom. These deposits pile up and compress the older sediments into rock. Dead organisms are swept into seas and swamps then settle to the bottom with the sediments. When the sediments turn to rock under pressure the hard parts of the animal may remain as fossils. Minerals also leach into the soft parts of the organism, leaving a petrified cast of its shape within the rock.
- Fossils can also be stored in:
- Resins, which become amber
- Frozen in ice or snow
- In acid peat in which they cannot decay
D.3.3 Outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K.
- Fossils contain isotopes of elements that accumulated in the living organisms. If the isotopes are unstable, they will lose protons and break down over time. Since each radioactive isotope has a fixed half-life it can be used to date fossils based on the relative concentrations of the reactant and product of the decay. Half life is the amount of time it takes for half of a sample of a certain substance to break down. Carbon-14 has a half life of 5000 years so useful or dating fossils less than 100,000 years old. Potassium-40 has a half-life of 1.3 billions years so useful for long-term dating. Error of less than 10%.
D.3.4 Define half-life.
- Half-life - The number of years it takes for 50% of a sample to decay.
D.3.5 Deduce the approximate age of materials based on a simple decay curve for a radioisotope.
- Look on the graph at how many times the concentration of the original isotope has halved then multiply that by the half-life.
D.3.6 Outline the palaeontogical evidence for evolution using one example.
- The existence of the many fossils similar but different to current species is expected under evolution.
- The Acanthostega fossil shows an amphibian from 365mya which has eight fingers and seven toes so is different from current organisms. It has four legs and a backbone like mammals, reptiles and amphibians but also a fish like tail and gills. It lived in water. It is a ‘missing link’ fossil demonstrating an intermediate point in evolution between current amphibians and fish.
- In the horse, the number of toes they have has been reduced from four to one. A succession of fossils from the horse ancestor with four toes to a modern horse with one toe shows a trend towards reduced number of toes.
- A clear progression of species is visible in the fossil record over time, from bacteria, to simple water based organisms, to amphibians, to insects, to reptiles and finally to mammals.
D.3.7 Explain the biochemical evidence provided by the universality of DNA and protein structures for the common ancestry of living organisms.
- All living organisms have DNA, use the same 20 amino acids and use left, not right handed amino acids. This suggests that all life forms had a common ancestor which did the same.
- To determine further relationships between organisms, comparing DNA and protein structure can be helpful. DNA Match two single stranded DNA from different species and see how tightly the DNA from one species can bind to DNA from another species. The tighter the bond the greater the similarity and the more closely related the species.
- Most fundamental enzymes present in most living creatures have a similar amino acid structure.
D.3.8 Explain how variations in specific molecules can indicate phylogeny.
- Proteins are genetically determined. Thus a close match in the amino acid sequence of two proteins from different species indicates that the genes in those proteins evolved from a common gene present in a shared ancestor.
D.3.9 Discuss how biochemical variations can be used as an evolutionary clock.
- Mutations are random changes in gene structure but they occur at a roughly predictable rate. In general the more differences between the amino acid sequence of a common protein, the further in the past two species had a common ancestor. For example, the haemoglobin of gorillas only differs by one amino acid from human haemoglobin whereas elephant haemoglobin differs from human haemoglobin by 26 amino acids. Therefore elephants separated as a species from a common ancestor with humans longer ago then did gorillas. Information like this can help to group organisms in trees of descent and suggest how long ago they had a common gene pool.
- First, check a specific protein and know that mutations occur at a certain rate. Count how many mutations there are in that specific protein and then calculate how many years the organism has evolved. For example, if there are 10 mutations and mutations occur every 5,000 years in this protein - 10 times 5,000 = 50,000 years.
D.3.10 Explain the evidence for evolution provided by homologous anatomical structures, including vertebrate embryos and the pentadactyl limb.
- Evolutionary transitions leave signs in fossil records in terms of descent with modifications.
- Homologous anatomical structure is a test for common ancestry. Descent with modification is evident in anatomical similarities between species grouped in the same taxonomical category. For example, the forelimbs of mammals have been modified to fit their function. However similarities in these structures demonstrate that they all originate from a common ancestor.
- A pentadactyl limb describes the same skeletal elements that make up the forelimbs of humans, cats, whales, bats and all other mammals. They have evolved for different functions but the relationships between the bones in the limb are all remarkably similar.
- Even distantly related organisms go through similar stages in their embryonic development. Many of them cannot be told apart in the embryonic stage despite looking entirely different as adults. All vertebrate embryos go through a stage in which they have gill pouches on the sides of their throats. This is all easily explained by evolution.
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.
- In 1981 the finch Geospiza fortis had a short, wide beak and had a diet of mostly large, hard seeds. In 1982-3 there was an el Niño event which brought much rain and meant that the vegetation changed and over the next 5 years there were far more soft small fruits. The population boomed because of the rains and increase in food then dropped back when the rains stopped. By 1987 they had longer, narrower beaks than they had before because those beaks helped the birds to survive the population cutback after the rains.
- In industrial areas of Europe in 1960 more ladybugs showed the black melanic colour allele. The black absorbs more light, and so the black ones have an advantage staying warm when sunlight levels are low. After 1960 as smog levels decreased the proportion of black ladybugs decreased proportionally and both smog and black butterflies reached constant low levels from 1980. Instead they were replaced with a red colour which warned off predators and was more effective at improving survival.
Option D.4 Human Evolution
D.4.1 State the full classification of human beings from kingdom to sub-species.
- Kingdom - Animalia
- Phylum - Chordata
- Class - Mammalia
- Order - Primata
- Family - Hominidae
- Genus - Homo
- Species - Homo sapiens
- subspecies - H. s. sapiens
D.4.2 Describe the major physical features, such as the adaptations for tree life, that define humans as primates.
- Shoulder joints that allow movement in 3 dimension
- Dexterous hands with opposable thumb and long fingers
- Sensitive fingers with nails
- Eyes closer together in front of the face for enhanced depth perception, excellent eye-hand coordination
- Shoulder joints and skull modified for upright posture
- Parental care with usually single births and long nurturing of offspring.
D.4.3 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.
- Humans, like apes, care for their young for a long time. Offspring also have delayed puberty.
- Humans show more physical similarities with young apes than mature ones, so we may be neotenous – have evolved to retain juvenile ape characteristics.
- Humans and apes have dexterous hands and similar hips and muscles.
- 98% of human and chimpanzee DNA is exactly the same.
- Fossils have been found of intermediate species in the evolution of humans from apes.
D.4.4 Outline the trends illustrated by the fossils of Australopithecus including A. afarensis, A. africanus and A. robustis and Homo including H. habilis, H. erectus, H. neanderthalensis and H. sapiens.
A. afarensis: 3 - 3.9 million years – ape-like face
A. africanus: 2.3 - 3 million years – flatter face, larger molars for plant based diet
A. robustus: 1.4 - 2.2 million years – very large molars, bones and skull
H. habilis: 1.6 - 2.4 million years – smaller teeth and jaw for meatier diet, first with tools, size like humans
H. erectus: .4 - 1.8 million years – more complex tools so meat significant part of diet and changed teeth.
H. neanderthensis .5 million years – larger brains and bones, larger teeth and jaw, shorter limbs for the cold
H. sapiens: .1 million years – large brain, flat face, reduced teeth, reduced robustness, chin
- Enlargement of brain and taller and more erect structure.
- The spike connection to the skull becomes more central to balance centre of gravity.
- Pelvis changes to support organs in walking.
- Pelvis shorter and broader to attach walking muscles.
- Legs become stronger and longer while arms become shorter and weaker.
- Knee can now be fully straightened.
- Foot forms more of a platform and rigid shape without opposable toe.
D.4.5 Discuss the possible ecology of these species and the ecological changes that may have prompted their origin.
- The climate of Africa became drier with thin woodland instead of forests. The depletion of forests encouraged a ground-adapted species that could move on land for long distances and who could carry scattered food. Thus, the adaptation of a bipedal human.
- Later Africa became much cooler and there was mostly savanna. This may have prompted the evolution of the Homo genus which used tools and group work to hunt large animals for food.
D.4.6 Discuss the incompleteness of the fossil record and the resulting uncertainties with respect to human evolution.
- The fossil record of human ancestry is incomplete and thus we cannot certainly determine when certain species originated and became extinct.
- early fossil record is fragmented and scarce because: they were not buried, many killed by predators so bones were spread and few died in location where they would be preserved
- as there are ‘missing links’ it can not be ensured that the hypothesised evolution of hominids is accurate
D.4.7 Discuss the origin and consequences of bipedalism and increase in brain size.
- Bipedalism - had to adapt to living on the ground and be able to look for food over longer distances.
- Allowed carrying of food and water and looking over brush.
- The increase in brain size allowed the production of tools. This allowed us to hunt larger animals for meat, changing the diet and the teeth structure towards smaller molars.
- Language becomes possible.
- We learned to limit the environment's influence (e.g. clothing, fire and housing).
- This comes at the cost of a longer development period and greater energy use by the brain.
D.4.8 Outline the difference between genetic and cultural evolution.
- Cultural evolution are changes in the behaviour of a species, for example, what tools they use, as where they live, the things they eat etc.
- Genetic evolution is changes in the genetic makeup of a species.
- Cultural evolution has spanned millions of years in three major stages: the nomadic (hunting), agricultural (settled), and industrial ages. However, we have not changed biologically in any significant way. We are probably not any more intelligent than the cave men. Our increased ability is due to the past experience we draw on.
D.4.9 Discuss the relative importance of genetic and cultural evolution in the evolution of humans.
- Evolution of the brain allowed cultural evolution to take off. Since then cultural evolution has been able to change humans far more quickly than genetic evolution has. Human behaviour and life has changed greatly without much genetic evolution at all. We are now able to change our environment instead of changing to suit our environment. However, recent research (Voight and Kudaravalli, et al., 2006) indicates humans are in fact evolving under selective environmental pressures based on genetic analysis.
Option D.5 Neo-Darwinism
D.5.1 State that mutations are changes to genes or chromosomes due to chance, but with predictable frequencies.
- Mutations are changes to genes or chromosomes due to chance, but with predictable frequencies.
D.5.2 Outline phenylketonuria (PKU) and cystic fibrosis as examples of gene mutation, and Klinefelter's syndrome as an example of chromosome mutation.
- PKU is caused by a gene mutation that ruins the protein that breaks down the amino acid phenylalanine. It is caused by many possible recessive alleles all at very low frequencies. This causes toxic levels of this amino acid in the blood, which causes mental retardation and death.
- Most instances of cystic fibrosis are caused by a specific mutation on a recessive allele. The mutation changes and amino acid and ruins the body’s protein that moves chloride ions in epithelial cells. A lot of thick mucus builds up inside airways, which leads to infection. Untreated, most children will die by age 5.
- Klinefelter's syndrome results from an extra X chromosome due to trisomy (non-disjunction). Thus a person has an XXY pattern of sex chromosomes. Such a person has male sex organs but low testosterone, reduced male characteristic and abnormally small testes. Not significant in evolution as all are sterile.
D.5.3 Explain that variation in a population results from the recombination of alleles during meiosis and fertilisation.
- Meiosis includes independent assortment of different alleles and also crossing over. These two processes make the total possible gene combinations from an individual very large and produce considerable variation in sexual reproduction by mixing up genes in new combinations.
- In fertilisation the genes of two members of a species are mixed, increasing variation further.
D.5.4 State that adaptations (or micro-evolutionary steps) may occur as the result of an allele frequency increasing in a population over a period of time.
D.5.5 Describe how the evolution of one species into another species involves the accumulation of many advantageous alleles in the gene pool of a population over a period of time.
- Evolution involves the change in heritable traits of a population over successive generations, as determined by shifts in allele frequencies of genes
- Over time this results in speciation
D.5.6 State that a species is a potentially interbreeding population having a common gene pool.
- A species is a potentially interbreeding population having a common gene pool.
- A gene pool is all the genes in an interbreeding population.
D.5.7 Discuss the definition of the term species.
- Most species are identified by their physical characteristics alone, but sometimes these are not sufficient.
- Accurately, a species is a groups of organism that are potentially interbreeding and produce fertile offspring. They have a common gene pool and are reproductively isolated from other such groups.
- This definition is somewhat ambiguous, but the main defining factor is the ability to produce fertile offspring. This is why donkeys and horses are different species. They produce mules when they breed, but because their chromosomes don't match completely, mules are infertile.
- However, some species that are clearly different can interbreed and produce hybrids, like some ducks and plants.
- Species which reproduce asexually cannot be classified in this way.
- Fossils cannot be classified this way as we do not know how they bred.
- Some species are almost identical physically but cannot interbreed like the Pipistrelle bat in Britain.
- Ring species like the herring gulls and lesser black backed gulls around the Arctic circle are able to interbreed with the population adjacent to them, but not with those further around the circle.
D.5.8 Discuss the process of speciation in terms of migration, geographical or ecological isolation and adaptation, leading to reproductive or genetic isolation of gene pools.
- As organisms become separated geographically and cannot breed (their gene pools do not mix) these organisms become different enough to be unable to breed. Such organisms have become different species.
- Differences between two populations that have been geographically or ecologically isolated are caused by natural selection. Such speciation usually occurs because of genetic variation. They have different habitats to evolve to, so they evolve differently, and eventually become so different that they cannot breed. Then, even if they are not geographically isolated, they are reproductively or genetically isolated.
D.5.9 Discuss ideas on the pace of evolution including gradualism and punctuated equilibrium.
- Gradualism suggests that evolutionary change occurs at a constant slow rate.
- Punctuated equilibrium implies long periods with no change and short periods with rapid evolution.
- The latter is gaining popularity as being more important than once thought. Volcanic eruptions and meteor impacts and climate changes speed up evolution at certain times as they create rapid changes in the environment which require rapid adaptation to survive. Only certain species will survive and evolve. For example, when the Dinosaurs died, there was a sudden punctuation in the evolutionary path of all other organisms because they were now able to adapt to fill newly available niches.
Option D.6 The Hardy-Weinberg Principle
Only Higher Level
D.6.1 Describe an adaptation in terms of the change in frequency of a gene's alleles.
- If an allele increases the chances of survival and reproduction of individuals that possess it, the frequency of the allele in the gene pool will tend to increase.
- If a moth inherits an allele of a gene that would make it bright pink in a dark brown environment - it would stand out in its environment and be eaten. This would cause the bright pink allele to decline in frequency.
D.6.2 Explain how the Hardy-Weinberg equation (p squared + 2pq +q squared = 1) is derived.
- In a population where two alleles occur, p represents the frequency of the dominant allele and q the frequency of the recessive allele. Thus the combined frequency of the alleles must account for 100% of the genes for that particular locus in the population.
p + q = 1
- When gametes combine their alleles to form zygotes, the probability of AA (A being the dominant allele) is p squared. The probability of aa is q squared. There are two ways in which an Aa genotype can arise, depending on which parent contributes the dominant allele. Thus, the frequency of heterozygous individuals is 2pq. The frequency of genotypes must add to 1. Thus,
p² + 2pq + q² = 1 AA + Aa + aA + aa = 1
D.6.3 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation.
- If a population is known to be following the Hardy-Weinberg Principle, the Hardy-Weinberg can be used to calculate unknown frequencies. An example of this is a gene in which two alleles controls the ability to taste phenylthiocarbamide (PTC). The ability to taste PTC is due to the 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 (tt). If q = frequency of t allele, q squared = .288 so q = .537 If p = frequency of T allele, p = (1 - q) = .463 The frequency of homozygous dominants (TT) and heterozygotes (Tt) can be calculated. p squared = frequency of homozygous dominants p squared = (.463)(.463) = .214 2pq = frequency of heterozygotes 2pq = 2 (.463)(.537) = .497
D.6.4 State that the Hardy-Weinberg principle can also be used to calculate allele, genotype and phenotype frequencies for genes with two alleles.
- The Hardy-Weinberg principle can also be used to calculate allele, genotype and phenotype frequencies for genes with two alleles.
D.6.5 State the Hardy-Weinberg principle and the conditions under which it applies.
- If there are two alleles of a gene in a population, there are three possible genotypes - homozygous for each of the two alleles and heterozygous. The frequency of the two alleles in the population is usually represented by the letters p and q. The total frequency of the alleles in the population is 1.0, so
p + q = 1
If there is random mating in a population, the chance of inheriting two copies of the first of the two alleles is (p)(p). The chance of inheriting two copies of the second of the two alleles is (q)(q). The expected frequency of the heterozygous genotype is 2pq. The sum of
p² + 2pq + q² = 1
- If gene frequencies change, then evolution occurs
D.6.6 Describe one example of transient polymorphism and sickle cell anemia as an example of balanced polymorphism.
- Polymorphism is when two or more forms of a phenotype are represented in high enough frequencies to be readily noticeable.
- A balanced polymorphism is a polymorphism in which the frequencies of the characteristics remain fairly constant over time. For example, the recessive allele for sickle cell anemia is very damaging when it is present in the homozygous recessive form because the person only has sickle shaped red blood cells. However, in the heterozygous form, it protects the person from malaria. That is why it continues to exist instead of being bred out. In populations exposed to malaria it will increase in the population until the homozygous recessive form becomes too common. Then it will remain at a balanced frequency.
- A transient polymorphism is one that is changing in frequency over time. For example, if a population with a high incidence of the sickle cell anemia gene moves to a location where malaria is no longer present the gene will begin to disappear because the selective pressure in its favour has disappeared but the pressure against it (the homozygous form) continues. Over time its frequency will fall.