chapter 20 p4 Flashcards
Founder effect
Small populations can arise due to the establishment of new colonies by a few isolated individuals, leading to the founder effect.
The founder effect is an extreme example of genetic drift.
These small populations have much smaller gene pools than the original population and display less genetic variation.
If carried to the new population, the frequency of any alleles that were rare in the original population will be much higher in the new, smaller population and so they will have a much bigger impact during natural selection.
founder effect example
The Afrikaner population in South Africa is descended mainly from a few Dutch settlers.
The population today has an unusually high frequency of the allele that causes Huntington’s disease.
It is thought that just one of the original settlers carried the disease-causing allele.
The Amish people of America have descended from 200 Germans who settled in Pennsylvania in the 18th century.
They rarely marry and have children outside their own religion and are therefore a closed community.
The Amish have unusually high frequencies of alleles that cause the normally rare genetic disorder Ellis-van Creveld syndrome.
People with the syndrome are short, they often have polydactyly (extra fingers or toes), abnormalities of nails and teeth, and a hole between the two upper chambers of the heart.
Ellis-van Creveld syndrome is an example of founder effect caused by one couple, Samuel King and his wife, who settled in the area in 1744.
Evolutionary forces:
The traits or characteristics of all living organisms show variation within populations.
The distribution of the different variants will take the form of a bell-shaped curve if plotted on a graph. This is known in statistics as a normal distribution.
Stabilising selection:
- Taking the birth weight of babies as an example, babies with an average birth weight will be the most common and therefore form the peak of the graph.
- Babies with very low birth weight are more prone to infections and very large babies result in difficult births.
- Both of these extremes in weight reduce the survival chances of babies so the numbers of survivals of very small or very large babies remains low forming the tails on Figure 5.
- This is natural selection, or survival of the fittest, at work.
- Babies with average birth weights are more likely to survive and reproduce than underweight or overweight babies
- It is an example of stabilising selection because the norm or average is selected for (positive selection) and the extremes are selected against (negative selection).
- Stabilising selection therefore results in a reduction in the frequency of alleles at the extremes, and an increase in the frequency of ‘average’ alleles.
Directional selection:
Directional selection occurs when there is change in the environment and the normal (most common) phenotype is no longer the most advantageous.
Organisms which are less common and have more extreme phenotypes are positively selected.
The allele frequency then shifts towards the extreme phenotypes and evolution occurs.
The changes seen in peppered moths during the industrial revolution are a good example of directional selection.
The changes seen in peppered moths during the industrial revolution are a good example of directional selection.
During this period of time a lot of smoke was released from factories, which killed lichens growing on barks of trees, and the soot made the bark black.
Peppered moths were originally light coloured meaning they were camouflaged by the lichen from predation by birds.
There were always a few darker moths present, due to variation, but these were quickly eaten and the allele frequency maintained.
When the lichens died and the trees became black the situation was reversed.
The light-coloured moths were very visible and were eaten and the darker moths were camouflaged.
Over time the allele frequency shifted due to natural selection and the majority of the peppered moths had the darker colour.
The allele frequency had been shifted towards an extreme (less common) phenotype.
As pollution has decreased again the allele frequency of the lighter coloured moths has increased.
Disruptive selection:
- In disruptive selection the extremes are selected for and the norm selected against.
- The finches observed by Darwin in the Galapagos Islands had been subjected to disruptive selection.
- This is opposite to stabilising selection when the norm is positively selected.
- Although examples of disruptive selection are relatively rare, a well-documented example involves feather colour in male lazuli buntings (Passerina amoena), birds which are native to North America.
- The feather colour of young males can range from bright blue to dull brown.
- There are limited nesting sites in their habitat and so there is a lot of competition between male birds to establish territories and attract female birds.
- Dull, brown males are seen as non-threatening and bright, blue males too threatening by adult males.
- Both the brown and blue birds are therefore left alone but birds of intermediate colour are attacked by adult birds and so fail to mate or establish territories.
- The extremes are selected for and the distribution of phenotypes shows two peaks as in Figure 9.
Speciation
is the formation of new species through the process of evolution.The organisms belonging to the new species will no longer be able to interbreed to produce fertile offspring with organisms belonging to the original species.
A number of events happen leading to speciation:
- Members of a population become isolated and no longer interbreed with the rest of the population resulting in no gene flow between the two groups.
- Alleles within the groups continue to undergo random mutations.
- The environment of each group may be different or change (resulting in different selection pressures) so different characteristics will be selected for and against.
- The accumulation of mutations and changes in allele frequencies over many generations eventually lead to large changes in phenotype.
- The members of the different populations become so different that they are no longer able to interbreed (to produce fertile offspring).
- They are now reproductively isolated and are different species.
Allopatric speciation:
Allopatric speciation is the more common form of speciation and happens when some members of a population are separated from the rest of the group by a physical barrier such as a river or the sea - they are geographically isolated.
The environments of the different groups will often be different and so will the selection pressures resulting in different physical adaptations.
Separation of a small group will often result in the founder effect leading to genetic drift further enhancing the differences between the populations.
A famous example of allopatric speciation is the finches inhabiting the Galapagos Islands located in the Pacific Ocean off the coast of South America.
Allopatric speciation example 1
For about two million years, small groups of finches, from an original population on the mainland, have flown to, and been stranded on, different islands.
The finches, separated from finches on other islands and the mainland by the sea, have formed new colonies on the different islands.
The finches have evolved and adapted to the different environments, particularly food sources, present on the islands and are an example of adaptive radiation - where rapid organism diversification takes place.
As the finches are unable to breed with each other, new species have evolved with unique beaks adapted to the type of food available.
Some species have large, blunt beaks that can crack nuts, some have long, thin beaks for getting to the nectar in flowers, and some have medium-sized beaks which are ideal for catching insects.
The honeycreepers (family Drepanidinae) of the islands of Hawaii are birds that are an even larger example of adaptive radiation.
A single ancestor species has led to the evolution of at least 54 species that have filled every available niche in the different islands.
Allopatric speciation example 2
Panama is a narrow strip of land (isthmus) that joins North and South America and separates the Atlantic and Pacific oceans, and was formed about three million years ago.
This was due to the movement of tectonic plates and resulted in the separation of the organisms that had originally occupied the same habitat when the two oceans were joined.
There were originally about 15 species of snapping shrimp present, now there are 15 species present on one side of the isthmus and 15 different species present on the other side.
Although the shrimp from either side appear to be identical if males and females are mixed they will snap at each other rather than mate.
In 1995 15 iguanas, Iguana iguana, survived a hurricane in the Caribbean on a raft of uprooted trees.
They eventually reached the Caribbean island of Anguilla. These iguanas were the first of their species to reach the island.
If these iguanas are successful in colonising the island it could be the start of an allopatric speciation.
Of course, it could take thousands, if not millions, of years before this is known.
Sympatric speciation p1
Sympatric speciation occurs within populations that share the same habitat. It happens less frequently than allopatric speciation and is more common in plants than animals.
It can occur when members of two different species interbreed and form fertile offspring - this often happens in plants.
The hybrid formed, which is a new species, will have a different number of chromosomes to either parent and may no longer be able to interbreed with members of either parent population.
This stops gene flow and reproductively isolates the hybrid organisms.
Examples of sympatric speciation include fungus-farming ants and blind mole rats.
