Option D.2 – Species and Speciation
D.2.1 – Define allele frequency and gene pool
Allele Frequency – The frequency of an allele, as a proportion of all alleles of the gene in the population. It is measured on a scale from 0.0 to 1.0
Gene Pool – All the genes in an interbreeding population
D.2.2 – State that evolution involves a change in allele frequency in a population’s gene pool over a number of generations
Selective pressures cause certain alleles to be passed onto the next generation with greater frequency that others.
D.2.3 – Discuss the definition of the term species
‘Species’ is a difficult word to define and biologists are yet to settle on any one meaning.
Before biologists had any understanding of genetics, species were defined according to the morphological characteristics of the organisms. Although still useful, this ignores the fact that species can evolve.
When a species has diverged into two forms with slightly different characteristics, it can be challenging to decide whether they are different enough to be classed as different species, or whether they are still close enough to be one species.
Similarity within a species is maintained by interbreeding. On the other hand, organisms of two different species are morphologically distinct because they do not interbreed. Hence, the definition of a species is:
“a group of actually or potentially interbreeding populations, with a common gene
pool, which are reproductively isolated from other such groups.”
The definition works, but has several issues:
- Sibling species – species that cannot interbreed but have very similar appearance. It makes them difficult to identify as being different.
- Interbreeding species – species that, although morphologically very different, can still interbreed. Plant species in particular can reproduce to form hybrids of two species.
- Asexually reproducing species – when organisms reproduce asexually, there is never any interbreeding, the definition does not readily apply.
- Fossils – without living organisms available, it is impossible to determine from morphology whether interbreeding could occur.
D.2.4 – Describe three examples of barriers between gene pools
Geographical isolation – Migration can lead to geographical isolation of populations. This is a barrier between gene pools that can cause two distinct species to develop. It is a form of allopatric speciation.
Hybrid infertility – frequently due to polyploidy. Species with different numbers of chromosome sets will have offspring that is sterile.
Temporal isolation – organisms may reach sexual maturity at different times. Although they exist in the same geographical area as other organisms, they have a temporal barrier between gene pools and cannot interbreed.
Behavioural isolation – very similar to temporal isolation, as behaviour generally affects whether species are available for breeding at the same time. For example, species that hibernate or undergo metamorphosis may emerge for reproducing at different times.
D.2.5 – Explain how polyploidy can contribute to speciation
In plants like the Rumex genus, the species have a wide array of chromosome set numbers which create barriers between the gene pools. Some a diploid with 20 chromosomes, whilst others have 40 chromosomes and are tetraploid. There are even hexaploid and decaploid species.
If a hybrid is produced between the diploid and tetraploid species, its gametes would be triploid and thus infertile, since meiosis cannot be completed in triploid cells.
D.2.6 – Compare allopatric and sympatric speciation
Speciation – the formation of a new species by splitting of an existing species
Sympatric – in the same geographical area
Allopatric – in different geographical areas
D.2.7 – Outline the process of adaptive radiation
Speciation will occur repeatedly so that one ancestral species will give rise to a number of new species. This is an example of divergent evolution. The new species, despite their common ancestry, will diversify to fill different niches and avoid competition with each other: called adaptive radiation.
It occurs in two potential situations: when the species have an advantage over existing ones, or where there are opportunities that no other species are using. The evidence for adaptive radiation can be seen in the bone structure of mammalian pentadactyl limbs.
D.2.8 – Compare convergent and divergent evolution
Divergent evolution is the result of one ancestral species giving rise to many closely-related species with large differences.
Convergent evolution is the opposite: it is when species with different ancestors independently develop similar traits. For example, echidnas, hedgehogs and porcupines all develops prickly protrusions independently of one another. They were both spatially and temporally isolated and had very different ancestry. In plants, carnivorous species have developed, including the Venus fly trap, sundews and butterworts.
D.2.9 – Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium
Gradualism is the slow change from one to another. Darwin describes this in The Origin of Species, showing how large changes can accumulated over long time periods.
However, the fossil record shows that some changes occurred very rapidly. Punctuated equilibrium implies long periods without appreciable change and short periods of rapid evolution. Volcanic eruptions and meteor impacts affecting evolution on Earth may have affected the rate of evolution. The stable equilibrium reflects time periods where organisms were well-adapted to their environment, so the forces of natural selection maintained the same characteristics. When conditions changed, new adaptations would be required, leading to rapid evolution.
D.2.10 – Describe one example of transient polymorphism
When there are two alleles of a gene in the gene pool, it is polymorphic.
Industrial melanism
When the industrial revolution occurred and pollution of the UK and USA increased rapidly, the peppered moth showed a change in colour. The darker-coloured moths were very prominent during the 19th century.
However, the lighter coloured moths have recently become more prominent as control of air pollution has increased.
D.2.11 – Describe sickle-cell anaemia as an example of balanced polymorphism
Balanced polymorphism is when two alleles can persist indefinitely.
Sickle-cell anaemia is an example of balanced polymorphism where heterozygotes have an advantage in malarial regions because they are fitter than either homozygote.
People who are homozygous for sickle cell anaemia will be resistant to malaria but will be very ill from the anaemia. People who are homozygous to not have anaemia will be healthy but very susceptible to malaria.
Heterozygous individuals carrying only one sickle cell anaemia gene (HbAHbS) will be resistant to malaria and will not develop sickle cell anaemia. They are the ‘fittest’ in the population. In areas of Africa where malaria is common, as many as 40% of individuals carry the sickle cell anaemia gene, making them resistant to malaria.