Chapter 16 - Evolutionary Mechanisms Flashcards Preview

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Facts about Sickle-Cell Anaemia.

- It occurs mainly in black Africans, or in people of black African ancestry.

- In the tropical zone of Africa, up to 40% of some populations carry the allele for sickle-cell anaemia.


What is Sickle-cell trait?

-Individuals with only one allele for sickle-shaped cells show no ill effects unless oxygen is in short supply.

-When this occurs their red blood cells show mild sickling.


How does Sickle-cell trait give certain advantages to those who have it?

-It provides a degree of immunity to malaria, a disease prevalent in parts of the world where the sickle-cell gene is found.

-For this reason, the allele is maintained in areas where malaria is present.


What is Special Creation?

It is the belief that God individually created each species.


What is Evolution?

It is the gradual change in the characteristics of a species.

-The theory of evolution through natural selection was put forward independently by Charles Darwin and Alfred Russel Wallace in 1858.

-However, it is Darwin's name that is usually associated with this theory because of the massive amount of supporting evidence he collected.


What is the Bionomial system?

Carolus Linnaeus established the basis of this system (our very present system) of classification and the binomial system of naming organisms using the generic (genus) and specific (species) names.


What is the general hypothesis put forwards by Lyell who wrote the book The Principles of Geology?

-That the natural forces existing in the past were much the same as those existing in his own time.

-This hypothesis implied that the Earth's surface has been gradually moulded over a very long period of time, by such simple forces as changes in temperature, running water and earth movements.

-Lyell's ideas provided Darwin with a concept of constant change against which he could view his own work.


What idea did Thomas Malthus, a British clergyman and political economist, provided?

-Malthus, in an essay on The Principle of Population, pointed out that the human population was increasing at a rate far exceeding the rate of food production.

-Drawing examples from natural populations of plants and animals, he demonstrated that natural reproduction rates exceeded the available resources; that is, more plants and animals are produced than can possibly survive.

-Darwin realised that under these circumstances a struggle for existence would occur, with the favourable variations being preserved and the unfavourable ones being gradually lost from the population.


What 3 observations were Darwin's theory of natural selection based on?

1. Variation: Darwin noted that all members of a species vary. He made no attempt to explain the source of this variation. However, he did point out that these variations were passed on from one generation to the next, characteristics displayed by the parents being passed on to their offspring.

2. Birth Rate: inspired by Malthus, Darwin realised that all living organisms reproduce at a rate far greater than at which their available food supply and other resource increase. This would normally result in overcrowding.

3. Nature's Balance: Darwin observed that, although the birth rate of organisms was very high, each species tended to maintain its numbers at a relatively constant level.


What are the Interpretations that Darwin made on his observations?

1. Because of the excessive birth rate and limited resources, there must be a struggle for existence.

2. Because there was a range of variations in any species, those with characteristics best suited to their environment were the ones that were more likely to survive. [Survival of the Fittest]


Define Survival of the Fittest.

-More organisms with favourable characteristics survived, while many of those with unfavourable characteristics died before they had an opportunity to reproduce and pass on the unfavourable characteristics.

-This is possible because there is a variation within any species.


What is variation?

-It is when the members of a species differ from one another in their physical characteristics, body functioning and behaviour.


What causes Variation?

-We now know that much of the variation that can be seen in a population is due to the effects of meiosis and fertilisation, and the simple principles first proposed by Mendel.

-Mendel studied characteristics that were determined by one pair of alleles.

-Many characteristics of organisms are controlled by more than on pair of alleles and this leads to a huge range of variation.


What are mutations?

New variations, showing no resemblance to either parent. may occur quite suddenly and purely by chance.


Summaries the principles of evolution through natural selection.

1. There is variation of characteristics within a species.

2. More offspring of a species are produced than can possible survive to maturity.

3. Due to excessive birth rate, and limited resources, there is a struggle for existence - competition for survival.

4. The individuals with characteristics best suited to the environment have more chance of surviving and reproducing - survival of fittest.

5. Favourable characteristics ( those with survival value) are passed on to the next generation.

6. In the gene pool, the proportion of alleles that produce favourable characteristics gradually increases.


Why did the environment of early humans must have had a profound effect upon the characteristics that were selected as the most suitable for survival in the region where they lived?

Because as result, there are features of human body that appear to correlate well with the environments in which they occur.

For eg. human body shape or stature, can be correlated with resistance to the cold.


Why would individuals with long bodies and short limbs have a survival advantage in very cold environments?

This is because they have a smaller surface area in relation to body volume than those with short bodies and long limbs.


Why would many individuals with less favourable characteristics would have died before reproductive age?

-As fewer of the short-bodied, long-limbed individuals would survive in the extreme cold, fewer of the alleles for these characteristics would have been passed on.

-So the frequency of unfavourable alleles in the gene pool would gradually decrease.


What environment suits Anopheles mosquitoes?

- It transmits the malarial parasite.

-It is not normally an inhabitant of tropical forests.

-It needs quiet, stagnant pools of water to raise its young.

-This habitat is more often found in open areas.


How did the incidence of Malaria increased?

-As humans began to clear the forests of Africa for agriculture, they changed the environment in a manner that created additional breeding areas for Anopheles mosquitoes.

-Thus the incidence of malaria increased.


How is Sickle cells formed?

-Because of the Mutation of the gene responsible for the production of normal haemoglobin.

- The mutant allele responsible for the sickle shape of the affected red blood cells causes the substitution of amino acid (valine) for another (glutamic acid) during the formation of the haemoglobin protein.

- The mutation affects only one of the 287 amino acids in the haemoglobin molecule, but this change is enough to affect the functioning of the red blood cell.

-The affected haemoglobin is often referred to as the haemoglobin S, and cells that contain it collapse into sickle shapes at low oxygen concentration.

-Individuals homozygous for this mutant allele suffer from sickle-cell anaemia, a disease that is usually fatal.


What happens when a person with sickle-cell anaemia dies before reproducing?

-The allele that causes the disease would not be passed on to the next generation

- Therefore, you would expect that over many generations the frequency of the sickle-cell allele would gradually decrease until it was eliminated from the population altogether.


What is Speciation?

It is the process of producing two species through the interbreeding of members from 2 different populations whom have been isolated for a very long period of time, and the environmental influences on each are different enough, major changes in the allele frequencies within each population could occur.


Explain the Diagrammatic representation of variation, isolation, selection and speciation.

1. Variation- a population exists on an island.

-> A range of variations exists within the population, which shares a common gene pool.

2. Isolation - the species is divided into two populations.

-> A barrier has formed dividing the population into two. No interbreeding occurs between the two populations. Each population has a separate gene pool.

3. Selection - two subspecies begin to form.

-> Different selection pressures act on each of the two populations over a number of generations. This brings about a change in the gene frequencies of each gene pool. Such changes lead to the evolution of separate subspecies.

4. Speciation - two species now exist.

-> Over a long period of time the changes in the gene frequencies may be great enough to prevent the production of fertile offspring by interbreeding between the two populations from ever occurring again. When this happens, two species exist.