Evolutionary Mechanisms. Species. A species is a group of individuals that share many characteristics and are able to interbreed under natural conditions to produce fertile offspring. Gene Pools.
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Frequency worldwide is very low, affecting approximately 1 in 500 000 births.
For the Ashkenazi Jews the incidence around 1 in every 2500 births.
Genetic drift suggested as reason frequency so high.
Jewish populations tended to be small and isolated Individuals who are heterozygous, appear to have increased resistance to tuberculosis (TB).
Heterozygotes have advantage in situations where TB is prevalent: homozygotes more susceptible to TB, and would possibly die, heterozygotes would die early in life.
Heterozygotes would have survival advantage and would be more likely to reproduce and pass their alleles on to the next generation.
Due to discrimination, the Ashkenazi Jews often found themselves isolated in overcrowded ghettos under conditions that would increase the threat of TB, situations, where thefrequency of the allele for Tay-Sachs could be maintained in the population
The theory of evolution through natural selection was put forward independently by Charles Darwin (Fig. 15.1) and Alfred RusselWallace 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
Thomas Malthus’s 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. He demonstrated that natural reproduction rates exceeded the available resources
Darwin realised that under these circumstances a struggle for existence would occur, with the favourable variations being preserved and the unfavourableones being gradually lost from the population.
In 1858, Darwin received copy Alfred RusselWallace’s essay, a naturalist On the Tendency of Varieties to Depart Indefinitely from the Original Type, covered the same ideas that Darwin had been working on.
Darwin had been painstakingly collecting evidence and refining his ideas for 20 years but Wallace’s essay was the stimulus for him to publish his views.
A joint essay was prepared by Darwin and Wallace and read before the Linnaean Society in 1858.
A year later Darwin published his first book, On the Origin of Species. Darwin’s theory of natural selection was based on three observations:
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.
Birth rate. Inspired by Malthus, Darwin realised that all living organisms reproduce at a rate far greater than that at which their available food supply and other resources increase. This would normally result in overcrowding
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.
Survival of the fittest is possible because there is variation within any species.
Reproductiveisolation may lead to the development of separate gene pools.
No two environments are exactly the same, so different alleles more favoured in one environment more than another. over time the allele frequencies of each gene pool will change, depending on which characteristics are favouredfor survival.
Over many generations, the populations become less and less alike as they develop characteristics that better suit them to their respective environments