Higher Biology | Key Area 1.7: Evolution

Evolution

The process of gradual change in the characteristics of a population of organisms, occurring over successive generations of genomic variation.

What is variation (ie. all new genes) a result of?

The changes of frequencies of certain genes results in variation, eg. an advantageously mutated gene becoming more frequent in a population, or a disadvantageously mutated gene becoming less frequent.

Evolution (ie. changes to the pool of genes of a population) arises by the processes of:

- Inheritance / gene transfer

- Natural selection

- Genetic drift

- Speciation

Natural selection

The non-random increase in the frequency of DNA sequences that increase survival, and the non-random reduction in the frequency of deleterious sequences, of which is an ongoing process as environments change continuously, and depends on the transfer of genetic information from one organism to another.

Explain how natural selection is a mechanism of evolution:

Natural and sexual selection are the agents through which evolution occurs, and act on a gene pool to alter allele frequency (therefore also phenotype frequency) in populations.

Explain how natural selection favours advantageous mutations:

Phenotypes best suited for the survival of a species are selected, therefore organisms with optimal phenotypes for their particular environment survive at the expense of those less adapted, ie. survival of the fittest.

Changes in phenotype frequency occur as a result of different selection pressures. Name these three:

- Stabilising selection

- Directional selection

- Disruptive selection

Stabilising selection

- An average phenotype is selected for, while extremes of the phenotype range are selected against and possibly disappearing.

- Stabilising effect causes a smaller range of values (mean phenotype unchanged) and results in less variation in a population.

Directional selection

- One extreme of the phenotype range is selected for over both the average and other extreme value as it confers a selective advantage.

- Directional effect still leaves a range of values, but the mean phenotype changes, making the once-rare advantageous phenotype more common.

Disruptive selection

- Two or more extreme values of the phenotype are selected for, while average fitness is selected against.

- Disruptive effect causes multiple new mean phenotypes to arise, and the range of values is altered.

- The population splits into two or more distinct groups with different characteristics, but are still one species. However, over time, this can lead to speciation.

Vertical gene transfer

When genes are transferred down generations from parent to offspring as a result of sexual or asexual reproduction involving one or two parents.

Horizontal gene transfer

When genes are transferred across from an individual to another within the same generation instantaneously, and does not involve reproduction.

Natural selection in prokaryotes:

- Prokaryotes (eg. bacteria) and viruses can exchange genetic material (plasmids) horizontally, which is instantaneous.

- Natural selection occurs more rapidly in prokaryotes since they have faster evolutionary change (potential for rapid evolution) than organisms which only transfer vertically.

- They can therefore adapt quicker to changes in environments, eg. resistance to antibiotics in a gene can be spread in the bacteria population due to horizontal transfer of plasmids carrying resistance genes.

Speciation

The generation of new biological species by evolution as a result of isolation, mutation and selection.

Species

- A group of organisms capable of interbreeding and producing fertile offspring, and which does not normally breed with other groups.

- Therefore, as long as a population has the opportunity to interbreed and exchange genes, they remain one species.

Isolation barrier

A variable which prevents gene flow between sub-populations during speciation.

How is the type of speciation determined?

- The type of speciation that occurs is determined by the type of isolation barrier.

- Geographical barrier → allopatric speciation

- Behavioural / ecological barrier → sympatric speciation

Allopatric speciation

- The most common type of speciation that occurs when populations are geographically (physically) isolated, and gene flow ceases between separated populations.

- Separation caused by rivers, mountain ranges, lakes drying out, etc.

Explain the process of allopatric speciation:

1. Population of organisms becomes separated by a geographical barrier.

2. Random independent mutations occur in both sub-populations.

3. Natural selection favours selective advantages in the sub-populations according to their separate environments.

4. Over a long period of time (millions of years) sub-populations become adapted for their separate environments.

5. Barrier removed; sub-populations can no longer interbreed and so are separate species.

Sympatric speciation

- Speciation that occurs when populations are ecologically or behaviourally (reproductively) isolated despite sharing the same habitat, and gene flow ceases between sub-populations.

- Occurs very rarely, and more often among plants than animals.

- Ecological barriers isolate by differing abiotic factors (eg, pH, salinity) so populations inhabit different breeding grounds.

- Behavioural barriers isolate by carrying out complex mating rituals where different timings, locations, or mating dances can prevent members from mating.

Explain the process of sympatric speciation:

1. Alternative ecological niche becomes available.

2. Some members of the population exploit different resources and no longer interbreed (behaviour acts as isolating barrier).

3. Mutations produce new variation in each group but are not shared.

4. Natural selection acts separately on sub-populations. Mutants better adapted to exploit the alternative niche enjoy selective advantages.

5. Over millions of years, two genetically distinct species form that can no longer interbreed.