The Evolution of Populations
Darwin's Origin of Species convinced most biologist of his time that species do indeed evolve; however many still questioned how this evolution could occur. What was needed was an understanding of inheritance. An understanding of inheritance was necessary in order to understand how chance variations arise in populations and how these variations are transmitted from parents to offspring. The answers to these questions were evident in the work of Gregor Mendel who lived about the same time as Darwin. However Mendel's work went unnoticed until the beginning of the 1900's. With the discovery of Mendel's work on genetics and with Darwin's theories of natural selection, the stage was set for the development of the science of population genetics.
Population Genetics "The study of allele behaviour in populations".
A population is a group of interbreeding individuals of a single species. All alleles found in that population make up its "gene pool". A gene pool can be though of as the total aggregate of genes in a population at any one time. That is all the alleles at all the gene loci of all the individuals of the population. Every individual in a population is genetically different, so where does all this genetic variation come from?
Sources of Genetic Variation
1) Ultimately all new genes arise by mutation (i.e. point mutation creates new codons and therefore new protein sequences).
Slight differences in DNA sequences, which produce slightly different protein sequences, are "alleles".
2) BUT: once a variety of alleles are in existence, "recombination" (meiosis and crossing over) becomes the mechanism that provided almost endless genotypic variation in the population. Sexual Reproduction very important.
Important: Together mutation and genetic recombination provide the genetic variation upon which natural selection acts. BUT natural selection can act on genetic variation only when it is expresses as phenotypic variation.
The Hardy-Weinberg Theorem
The Hardy-Weinberg Theorem describes a population that is not evolving. That is this population's allelic frequency remains the same from one generation to the next. At this point it might be useful to explain the concept of "allelic frequency" using an example.
Recall that the genotype of a diploid individual can contain a maximum number of only two alleles for any given gene (one on each homologous chromosome). But, there is no such restriction on the "population's" gene pool. It may contain any number of allelic forms. The gene pool is characterised by the frequencies of the alleles of a gene (or their ratios).
If we have a hypothetical population with allele A and a, as shown in these "Bead critters' below
Therefore the frequencies of A and a are:
A = 0.9
a = 0.1
If these allelic frequencies were to change with time, by definition that would be Evolution.
How can we calculate the genotypic ratios that will be present in the population? If we assume all genotypes have an equal chance of survival then....
90% of all sperm will carry A
10% of all sperm will carry a
90% of all eggs will carry A
and 10% of all eggs will carry a
We could set up a punnett square with these gene frequencies, like the one below,
sperm eggs 0.9 A 0.1 a 0.9 A 0.81 AA 0.09 Aa 0.1 a 0.09 Aa 0.01 aa Eg.: Sperm 0.9A X egg 0.9A = Zygote AA 0.81 etc.
The only difference between this punnett square and the normal cross of two individuals is that here we consider all the sperm produced by all the males of the population (not just from one individual) and the same holds true for the eggs. The original allelic frequency was A = 0.9, and a = 0.1 and the F1 genotypic ratio was AA = 0.81, Aa = 0.09 + 0.09 = 0.18, aa = 0.01
Therefore the allelic frequency now for F1 is still A = 0.9 and a = 0.1 no change.
Therefore evolution is not automatic, but occurs when something disturbs the genetic equilibrium i.e. differential survival, or "natural selection".
"Under certain conditions of stability both allelic frequencies and genotypic ratios remain constant from generation to generation in sexually reproducing populations."
The Equations of The Hardy-Weinberg Equilibrium
The same results we got in the punnet square can be obtained with a simple algebraic formula.
Eg. #1 If P = frequency of allele A = 0.9 and q = frequency of allele a = 0.1
Then expansion of the binomial expression (p + q)2= p2 + 2pq + q2 = 1, Is the "Hardy-Weinberg formula"
If we substitute the allelic frequencies into the formula:
p2 + 2pq + q2 = 1
(0.9)(0.9) + 2(0.9)(0.1) + (0.1)(0.1) = 1
0.81 + 0.18 + 0.01 = 1
Therefore the term p2 = frequency of genotype AA = 0.81. The term 2pq = frequency of genotype Aa = 0.18, and the term q2 = frequency of genotype aa = 0.01
Eg. #2 What if we know the genotypic frequency but wish to calculate the frequency of the alleles in a population? Use the same formula.
Let say that a recessive allele "a" causes produces blond hair, and 4% of a population have blond hair, therefore what are the allelic frequencies?
If only homozygous recessives have blond hair then aa = blond
There are 4% blond or 0.04 = aa
therefore q2 = aa = 0.04
therefore q = square root of 0.04= 0.2
Recall that p + q = 1 always (since they are frequencies). therefore if q = 0.2
then 1 - 0.2 = p = 0.8 since (p = 1 - q).
Therefore q = 0.2 and p = 0.8
Substituting into the Hardy-Weinberg equation:
p2 + 2pq + q2 = 1
(0.8)2 + 2(0.8)(0.2) + (0.2)2 = 1
AA = 0.64
Aa = 0.32
aa = 0.04
What are the conditions of "The Hardy-Weinberg Theorem"?
In such a stable population there are no processes that act to change the allelic frequencies from one generation to the next. This requires certain conditions:
1) The population must be large enough to make it highly unlikely that chance alone could significantly alter allelic frequencies.
2) Mutations must not occur (or else there must be mutational equilibrium).
3) There must be no immigration or emigration.
4) Reproduction must be totally random.
Since we see that populations do evolve, what forces or processes are at work that violate these conditions?
Condition #1 This would require that a population be of infinite size which is clearly impossible. Chance events may cause changes in allelic frequencies ie. evolutionary changes, in small populations. Such random chance events are called Genetic Drift (Page 421)
One example of such a random chance event is known as the "Founder effect"
A small sample of a large population randomly chosen from the original population, becomes separated and forms a new population. This could happen if a storm was to blow some birds from the mainland out onto a small island. This new population could have a significantly different allelic frequency than the original large population, depending on which birds just happened to be blown out by the storm.
Condition #2 Mutation is always occurring and rarely are they in equilibrium.
i.e. A mutates to ----> a (1)
a mutates to ----> A (2)
Process 1 rarely equals process 2 therefore, A% slowly moves to a % Called mutation pressure.
Condition #3 No new genes can be introduced to the population from outside the population by immigration, and no genes are lost through emigration. That is there is NO GENE FLOW.
This could only happen if the population is extremely isolated.
Condition #4 Reproduction within the population is seldom totally random. It means all genotypes have exactly the same reproductive success. This condition is probably never meet in any real population.
Finally "Selection pressure" or natural selection is always working to upset the Hardy-Weinberg Theorem.
SO WHY go to so much trouble to explain the Hardy-Weinberg Theorem, only to show that it describes a situation that never occurs in nature?
For one compelling reason: The Hardy-Weinberg Theorem sets up the "Null-hypothesis" - NO EVOLUTION. Which because it is so easily disproved provides an indirect demonstration that populations must constantly be evolving.
Natural Selection Effect on Recessive Alleles.
There are many examples of natural selection "favouring" one allele over another, and this "pressure of selection" (selection pressure) causing the frequency of that allele in the population to change. (We will see examples of the latter.)
Important: Therefore it is easily assumed that the more frequent allele in a population will automatically increase in frequency while the less frequent allele will automatically decrease in frequency and eventually be lost from the population. But this assumption is incorrect.
Lets look at an example of how natural selection can actually maintain a deadly allele in a population under the right conditions.
Example: Sickle cell anaemia (Hs incomplete (partial) dominance)
This is a genetic disease which produces a type of haemoglobin (Hs) which under certain circumstances causes red blood cells to collapse "sickle", which can cause blockages in blood vessels etc. Hs is different from HA (normal haemoglobin) at only one amino acid out of 287. Probably due to a single base substitution.
In any population the following genotypes exist: HAHA - both normal alleles, these people produce all normal haemoglobin. HA Hs - heterozygote These people suffer from "attacks" of red blood cell sickling, but these attacks are usually not lethal. HsHs - people with this genotype suffer from severe sickling and often die at a very young age.
Question: If Hs in the homozygous form is lethal should it not disappear from the population, eventually?
Let's see:
If HA were say 0.8 = p
therefore Hs would be 0.2 = q
therefore Hardy-Weinberg eq.
p2 + 2pq + q2 = 1
therefore p2 = 0.64 HAHA
2(pq) = 0.32 HA Hs
q2 = 0.04 HsHs
But before the next generation HsHs would likely die and not be available to reproduce. Therefore the allelic frequency would change.
therefore 0.64 = HAHA and 0.32 = HAHs for a Total of 0.96
and now 0.64/0.96 = 0.66 = HAHA
and 0.32/0.96 = 0.33 = HAHs
the new allelic frequency would be ...
p2 = 0.66, therefore p =square root of 0.66 = 0.812
q = 1 - 0.812 or q = 0.187
Important: The allelic frequency should shift towards all HA.
BUT Hs is still with us and has been for many generation (hundreds), beside it is very common in certain areas of the world. WHY?
Sickle Cell Anaemia and Malaria
Malaria is a disease caused by an infection of the blood by a small "sporozoan' called Plasmodium vivax.
This P. vivax enters the blood stream when an infected mosquito bites a human. It enters the red blood cells and produces hundreds of new P. vivax. The red blood cell ruptures and releases the new P. vivax to infect other red blood cells. Such a severe infection can be fatal and many hundred of thousands of people die of malaria each year. But some people seem to be partly immune to malaria and suffer very little from its effects. These people are carries of the gene for sickle cell anaemia. i.e HAHs. Normal people HAHA die of malaria (when it is around) people with HsHs die of sickle cell anaemia. But in areas where malaria is prevalent HAHs people survive better than HAHA individuals because HAHs people are immune to malaria. How when P. vivax enter a HAHs red blood cell the cell sickles and kills the P. vivax before it can reproduce and cause a malaria infection.
Natural Selection
Patterns of Natural Selection
Natural selection works on the graded, continuous variation in population of any given trait. For any given trait most people (organisms) in the population will be about normal for that trait. Thus for any given measurement there will be relatively few individuals with either very high or very low values for that trait. If we group all individuals in a population according to one trait they should generate a graph which closely resembles a "bell-shaped curve". The statisticians so called normal distribution.
Natural selection or the "selection pressure" will cause these individuals around the mean to survive better or worse than those at either end. There are three (3) types of selection that causes something to happen to the position of the mean.
1) Stabilising Selection
Stabilising selection is usually found in populations which have become well adapted to their particular habitat (surrounding environment). These surroundings are usually rather stable. Any dramatic change (genetic) in the population is likely to be harmful. Genetic variation still exists and in each new generation the extremes are produced but the selection tends to favour the mean and eliminated the extremes. Most populations are well adapted to their environments therefore, stabilising selection tends to be the most common form of selection.
Eg. giraffe's necks and legs well adapt them to their environment. They are no longer subject to other forms of selection. They have no changed at all in over 20 million years.
2) Directional Selection
Directional sectional favours one of the extremes of the phenotypic range, i.e one end of the curve. When planet or animal; breeders try to breed new plants or animals to produce more of a given trait (i.e more protein in the grain or higher milk production in cows) they are practising a form of "artificial" directional selection (as opposed to "natural" selection). In nature directional selection may be a response to a change in the environment that favours one extreme of the phenotype. Such selection is seen when the environment changes markedly over short time period (i.e. several generations) OR the population moves to a new location with a different set of environmental conditions.
EG. Natural Selection and the Pepper Moth
Biston betularia is the British peppered moth. This moth occurs in two colour "morphs". One colour is a light and mottled (peppered) appearance, while the other colour is almost black. Good records were kept on the distribution of two morphs for many years in England. In the early 1840's England was in the gripes of the "Industrial revolution". Industries began spuing forth soot and smoke in the industrial urban centres. It was noticed that the distribution of black moths to peppered moths was changing in the urban vs. rural areas.
Selection of Biston betularia in England
A) As Peppered moths sat on trees covered with light mottled lichens, they blended into background (camouflage) and birds found if difficult to see them. Therefore they survive "better" than the more conspicuous black morph. This accounted or the difference in the distribution.
B) As the industrial pollution in the urban areas blackened the trees, the lighter coloured peppered morph became more conspicuous to the birds against the black tress. While the black morph blended into the new black background.
3) Disruptive Selection
In disruptive selection the intermediate types are selected against and those at the extremes are favoured. This type of selection produces a "bimodal' (two humped) distribution curve.
