3.7.3 Evolution may lead to speciation

Individuals within a population of a species often show a wide range of variation in their phenotypes - this is due to both genetic and environmental factors. Variation often results from a mutation and further variation results from meiosis and the random fertilisation of gametes during sexual reproduction. We need to know about a few ways in which we can get variation; genetic factors, and environmental factors.

Genetic factors:
It is important to recognise that within a population all members have the same genes. Genetic differences occur as members have different alleles of these genes. Genetic variation arises from:

• Mutations: sudden changes to genes and chromosomes. These may be passed on to the next generation.
• Meiosis: Produces new combinations of alleles before they are passed onto gametes
• Random fertilisation of gametes: Produces new combinations alleles. The offspring are therefore different to the parents. Which gamete fuses with which at fertilisation is also random adding to the variety of offspring two parents can produce.

Environmental factors:

A lot of variation is due largely to environmental influences. Environmental influences include 

 climatic conditions, pH, and food availability. These influences affect the way the organisms genes are expressed. E.g a flower might have a gene to be tall but if it grows in low light then it will not be tall, it will be short. It is important to realise that some characteristics blend into one another forming a continuum (for example, mass and height). Characteristics that display this type of variation are not controlled by a single gene but by many (polygenes). Environmental factors play a major role in determining where on the continuum an organism lies. E.g. individuals genetically predetermined to be the same height may grow to be different heights due to variations in environmental factors (such as diet). This type of variation is the product of polygenes and the environment. If we measure the heights of a large population of people and plot the number of individuals against heights on a graph we will probably obtain a bell-shaped curve known as a normal distribution curve. 

Natural selection

The environmental factors that limit the population of a species are called selection pressures. These include predation, disease, and competition. Selection pressures determine the frequency of all alleles within the gene pool (a gene pool is the total number of all the alleles of all the genes of all the individuals within a particular population at a given time). The process of evolution by mean of natural selection depends upon a number of factors:

• Organisms produce more offspring than can be supported by the available supply of food/light/space...etc
• There is genetic variety within the populations of all species
• A variety of phenotypes that selection operates against

Populations rarely increase in size at such a rate, meaning that death rates must be high. High reproductive rates have evolved in many species to ensure a sufficient large population survives to breed and produce the next generation. This compensates for a high death rate from e.g. predation, competition (for food and water), extremes of temperature, natural disasters such as earthquakes/fires, and disease. Some species have evolved lower reproductive rates and a higher degree of parental care - lower death rates that result help to maintain their population size. 

Ultimately, there are too many offspring for the available resources meaning there is intraspecific competition amongst individuals for the limited resources available. The greater the numbers of offspring the greater this competition is and the more die int he struggle to survive. These deaths are not random. It follows that in a population individuals best suited to the prevailing conditions (e.g better able to escape from predators/catch prey/obtain light/resist disease/find a mate) are more likely to survive than those less well adapted. These individuals are more likely to breed and pass off their more favourable allele combinations to the next generation which will therefore have a different allele frequency than the previous generation. The population will have evolved a combination of alleles that are better adapted to the prevailing conditions. This selection process does depend on individuals of a population being genetically different from one another.

Variation provides the potential for a population to evolve and adapt to new circumstances. Conditions change over time. Having a wide range of genetically different individuals in the population means that some will have the combination of genes needed to survive in almost any new set of circumstances. Populations sowing little individual genetic variation are often more vulnerable to new diseases and climate changes. It is also imperative that a species is capable of adapting to changes resulting from the evolution of other species (e.g if their predators become faster, they must be better adapted to hiding). All in all, the larger a population is and the more genetically varied the individuals within it the greater the chance that one/more individuals will have the combination of alleles that lead to a phenotype which is advantageous in the struggle for survival. These individuals are more likely to breed to produce offspring and pass on their more favourable allele combinations. 

So there are three types of selection that we need to know about:

• Directional selection changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population. Basically, selection for one extreme phenotype.
• Within a population there is a range of genetically different individuals in respect of any one phenotype. This continuous variation forms a normal distribution curve (a bell shaped curve). This curve has a mean that represents the optimum value for the phenotypic characteristic under the existing conditions.
• If the environmental conditions change the optimum value for survival will change. Some individuals (either left or right of the mean) will pass on a combination of alleles with the new optimum for the phenotypic character. This results in the mean moving to either the left or right of its original position.
• Directional selection results in one extreme of a range of variation being selected against in favour of the other extreme/the average.
• Stabilising selection preserves the average phenotype of a population by favouring average individuals. Basically, selection against the extreme phenotypes.
• Stabilising selection tends to eliminate the extremes of the phenotype range within a population and with it the capacity for evolutionary change.
• Tends to occur where the environmental conditions are constant over a long period of time
• An example is fur length in mammalian species. Individuals with shorter fur will be advantageous hotter years because they can lose body hear more rapidly whilst individuals with longer fur will be advantageous in colder years. If the temperature is a constant 10 degrees neither extreme will be at an advantage and they will be select against those with average fur length.
• The mean is constant but there will be fewer individuals at each extreme
• Disruptive selection favours individuals with extreme phenotypes rather than hose with phenotypes around the mean of the population.
• This is the opposite of stabilising selection
• It favours extreme phenotypes at the expense of the intermediate phenotypes
• This is the most important in bringing about evolutionary change (although it is the least common form of selection).
• It might arise if temperatures alternate between extreme highs and lows in summer and winter, respectively. This might ultimately lead to two separate species of the mammal (one with long fur which is active in winter, and one with short fur which is active in summer).

Allelic frequency

We know that the gene pool is all the alleles of all the genes of all the individuals in a population at a given time. The number of times an allele occurs within a gene pool is referred to as the allelic frequency. The allelic frequency is affected by selection and selection is due to environmental factors. This in turn means that environmental changes therefore affect the probability of an allele being passed on in a population and hence the number of times it occurs within the gene pool. 

NOTE: environmental factors fo not affect the probability of a particular mutant allele arising. All they do is affect the frequency of a mutant allele that is already present in the gene pool.

Speciation

Speciation si the evolution of new species from existing ones. A species is a group of individuals that have a common ancestry and so share the same genes but different alleles and are capable of breeding to produce fertile offspring. Basically, members of a specie are reproductively separated from other species. The most important way i which new species are formed is through reproductive separation followed by genetic change due to natural selection. 

Suppose that a population becomes separated and undergoes different mutations. This will result in it becoming genetically different from the other populations. Each population of the original species will experience different selection pressures as the environment will be slightly different. Natural selection will then lead to changes in the allelic frequencies of each population. The different phenotypes each combination of alleles produces will be subject to selection pressures that will lead to each population becoming adapted to its local environment. This is called adaptive radiation and results in changes to the allele frequencies of each population. As a result of these changes to allele frequencies it is possible that  the populations would no longer be able to breed to produce fertile offspring and each population would not be a different species with its own gene pool.

Genetic drift can take place in smaller populations because the genetic diversity is less. The relatively few members possess a smaller variety of alleles than the members of a large population. As these few individuals breed the genetic diversity of the population is restricted to those few alleles in the original population. As there are only a small number of different alleles in the original population there is not an equal chance of each being passed on. Those passed on will quickly affect the whole population as their frequency is high. Any mutation to one of these alleles that is selectively favoured will also more quickly affect the whole population as its frequency will also be high. The effects of genetic drift will be greater and the population will change relatively rapidly making it more likely to develop into a separate species. In larger populations the effect of a mutant allele will be less because its frequency is less in the larger gene pool. The effects of genetic drift will be less and development into a new species is likely to be slower. 

There are two forms of speciation:

• Allopatric speciation
• Describes the form of speciation where two populations become geographically separated. This may be the result of a physical barrier between two populations which prevents them interbreeding.
• Barriers include oceans/rivers/mountain ranges/deserts
• If environmental conditions either side of the barrier vary, natural selection will influence the two populations differently and each will evolve leading to adaptations to their local conditions.
• These changes may take hundreds/thousands of generations but ultimately may lead to reproductive separation and therefore the formation of a separate species
• Sympatric speciation
• This describes the form of speciation that results within a population in the same area, leading them to become reproductively separated

Lastly, there are a few key terms we need to commit to memory regarding isolating mechanisms:

• Geographical - populations are isolated by physical barriers such as oceans/mountain ranges/rivers
• Ecological - populations inhabit different habitats within the same area and so individuals rarely meet
• Temporal - the breeding seasons of each population do not coincide so they do not interbreed
• Behavioural - mating is often preceded by courtship which is stimulated by the colour/markings of the opposite sex/the call/particular actions of a mate. Any mutation which causes variations in these patterns may prevent mating
• Mechanical - Anatomical differences may prevent mating occurring (e.g it may be physically impossible for the penis to enter the vagina)
• Gametic - the gametes may be prevented from meeting due to genetic/biochemical incompatibility
• Hybrid sterility - Hybrids formed from the fusion of gametes from different species are often sterile because they cannot produce viable gametes.