Population genetics, natural selection & speciation

Population genetics is the study of how the genetic composition in a population changes and how these changes are inherited. As genetic change is the basis of evolution, in its broadest sense population genetics looks at how evolution occurs. 

Population: a group of organisms of the same species living in the same habitat/ecosystem. 

To a population geneticist, members of a population must also be able to mix their genes. i.e. reproduce sexually. 

The genetic makeup of such a population is known as the gene pool.

The gene pool is defined: all the alleles of all the gene loci in a population at a particular time. 

In practice we are usually only considering the alleles at one particular gene locus at any one time. 

A gene pool may be isolated, to a greater, or lesser degree, from other such pools by geographical barriers or other means. In a lake there may be virtually no transfer of fish to other ponds but insects, pollen grains etc may move to other ponds. 

For any particular gene there is usually more than one allele. Therefore in the population, each allele represents a certain proportion of all the alleles in the population. This proportion is the allele frequency. The frequency with which an allele occurs has little bearing on whether it is dominant. 

The general formula to represent the frequency with which the dominant and recessive forms of an allele occur in a gene pool of a population is: p+q=1 (p=frequency of dominant allele) (q=frequency of recessive allele) (where alleles are codominant, they are randomly assigned as either p or q).

Weinberg developed an equation to enable us to estimate expected allele frequencies. This is the Hardy-Weinberg equation: 

p=frequency of the dominant allele in the gene pool

q=frequency of the recessive allele in the gene pool

    = the frequency of the homozygous dominant genotype (individual)

    = the frequency of the homozygous recessive genotype (or phenotype) (individual)

2pq =the frequency of the heterozygous genotype (individual)

In any population the frequency of the homozygous recessive genotype can be worked out simply by working out the proportion who have the recessive phenotype. However, we have no idea how many of the dominant phenotype are heterozygous and how many are homozygous. It's possible to estimate the other frequencies by using the two equations above.

This states: the frequencies of the alleles of a particular gene in a population will stay constant from generation to generation. 

However this will only occur under the following conditoning: 

• The populations must be large (less genetic difference as a result) - in small populatioms, chance events can cause large swings in frequencies. 
• Mating between individuals must be random - this ensures an equal chance of each allele being passed on to the next generation. 
• No mutations must occur - this will change allele frequencies. 
• All genotypes must be equally likely to reproduce, i.e. no selection occurs - this ensures an equal chance of the alleles being passed on to the next generation.
• There must be no migration into the population (immigration) or out of the population (emigration) - migration adds new alleles to, or removes alleles from, the population. This is known as gene flow. 

When these conditions are met and the allele frequency does not chance then the population is said to be in genetic equilibrium (or Hardy-Weinberg equilbrium). The Hardy-Weinberg equation only provides a reliable estimate of allele frequencies if this is the case. 

The Hardy-Weinberg Law is never really valid, as the ideal conditions are never met in a real situations. Gene flow (i.e. migration), selection and non-random mating are all common processes. But the equation is an invaluable way of measuring evolutionary chance. If a population is not in genetic equilibrium (i.e. there is a change in allele frequencies) then one or more of the processes must be at work. Some examples of how the genetic equilibrium can be upset are:

1) A SMALL POPULATION.

• More genetic drift in small population.
• Genetic equilbrium depends on the random assortment of genes. In a small population of, say 10 organims if an allele occurs in 10% of them then only 1 individual will carry it. If this dies before breeding the allele will be lost from the population. The loss of 1 individual will not have a great effect on a population with 500 such carriers. 
• When members of a population leave to set up a new community, the reverse of this effect can be seen. Any unusual genes in the new members of the new population are amplified as the population grows. This is known as the founder effect.

2) NON-RANDOM MATING.

For genetic equilibrium to be maintained, there must be an equal chance of any 2 individuals mating, irrespective of the phenotype. This is unlikely because:

• Birds carry out elaborate courtship displays to attract a female and she will choose the most impressive. 
• Male chimps will fight for the right to be the alpha male who breeds with the females first. Some males will not get to breed at all.
• Self-fertilisation in some plants or other forms of inbreeding ensures mating is non-random. 
• In human populations, non-random mating is the normal state. 

3) MUTATIONS.

• Mutations introduce new alleles and lead to variation (due to meiosis).
• Mutations alter alleles and thus upset the genetic equilibrium. 
• They occur all the time, although in animals only those occuring in the reproductive organs will be passed on. 
• They do not happen very rapidly however and in humans this means that each of us probably only contains one new mutation. As most of these are recessive they are unlikely to be expressed, but occasionally one will arise which gives benefit to the population and so becomes a permanent part of the gene pool.

All these factors operate to bring about changes in the gene pool and so upset the Hardy-Weinberg equilibrium. The fact that a population deviates from the Hardy-Weinberg equilibrium shows that species are constantly in a state of evolutionary flux and it is possible to see to what extent natural selection and evolutionary changes are taking place by how great these deviations are. 

A key feature of Darwin's theory of natural selectin was that some individuals are more likely to reproduce than others, i.e. they show differential reproductive success (or differential fertility). This is due to their genotype. However, although it is genes that are passed on, it's the phenotype that is subject to selection. It is not the genes themselves that are tested by selection, but their effects. There are several factors involved in natural selection:

• The size of a population is limited by a number of environmental factors e.g. competition for food and breeding sites in animals, and for water, space and light in plants. 
• There is variation in the population. Some organisms inherit characteristics that mean they are better competitors for these scarce resources than others. They are said to be "biologically fit". This may also happen due to mutation (a chance event), which may result in new, beneficial alleles that confer an advantage. 
• These organisms are more likely to survive to reproduce, or at least to have more offspring than others.
• These organisms are therefore more likely to pass on their alleles to the next generation than others. 
• This causes a change in the allele frequency of that population in subsequent generations. 

Selection can be triggered by a change in the environment of the population, resulting in a certain phenotype (and therefore genotype) becoming favourable. The environmental change creates a selection pressure. 

Large pop=less genetic drift=more stable

Natural selection: mutation -> meiosis -> variation -> large no. of offspring -> competition -> struggle for existence -> selection pressure -> advantageous alleles -> allele frequency increases/decreases

Natural selection usually acts against one or more of the extreme in a range of phenotypes. As a result, a certain phenotype (or phenotypes) become rare and an alternative phenotype becomes common. 

1. DIRECTIONAL SELECTION. (Mode shifted one way or another and occurs in a changing environment)

One type of selection is known as directional selection as it results in an increase in the frequency of one phenotype relative to another. Direction selection acts agast ONE of the extreme phenotypes. It arises when environmental change is sustained over several generations and many examples seen today are where species have adapted to human induced changes. Most demonstrate resistance to chemicals released into the environment by human activity. e.g. resistance to pesticides in insects, resistance to antibiotics in bacteria etc.

E.g. giraffes, food shortage=long neck=get leaves higher up=get a mate because they survive 

Example of rats (read in the pack page 10)

Normal distribution becomes narrower and taller but stays in the same position. Ones in the middle (mode) have an advantage.

This type of selection acts against both the extremes in a range of phenotypes, i.e. it favours the middle phenotypes and so acts to prevent change. Most causes of selection are stabilising which should be not surprising if you consider that each living thing is the product of a long line of successful ancestors - otherwise they would not have survived to breed. This does not mean that it only happens in a constant environment; stabilising selection occurs when there is no change in the range of environments successive generations experience. 

Example of stabilising selection of sickle cell anaemia (look in pack page 11)

Speciationn changes gene pools causing a species to change greatly over time. The greatest change occurs though when existing species gives rise to two or more new species, i.e. at the brancing points in the evolutionary tree. This process is called speciation. 

Species defintion: a group of organisms with similar characteristics that can interbreed to produce fertile offspring

Speciation is a process whereby one gene pool gives rise to more than one gene pool. the stages leading to speciation are as follows:

• part of a population becomes isolated in some way. the most usual method is by geographical isolation. this is where a physical barrier such as a mountain range or a river separates populations that were originally from the same gene pool.
• no interbreeding occurs between the 2 populations.
• also differnt mutations may occur in each of the new populations, which will increase variation and may result in new beneficial alleles.
• the 2 isolated populations experience different environmental conditions, and therefore different selection pressures.
• different deatures and alleles will be an advantage in each environment, and so natural selection operates separately in the 2 populations, with different features selected for in each case.
• in each population organisms with advantageous features will be more likely to survive and reproduce than those without, and so the allele frequencies in each population will change in subsequent generations.
• evenetually the genotypes of the 2 populations may become so differnt that, even if members from each group were reunited, they could no longer successfully interbreed to produce fertile offspring
• the two populations are now classified as different species