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E8. Natural selection acts on variation in a population.


Student Outcome: E8.1

Know that a gene pool is the sum of all the genes of all the individuals in a population.


Within populations are combinations of genes and different gene types. A peccary (a wild pig) is a diploid organism, which means that there are two copies of each gene in every cell in its body. A mutation can produce a subtle variation in either, or both, of these genes, some lethal and some that simply alter a trait slightly.


Suppose that bristle length on the bodies of the peccaries is controlled by a single gene (written as B), and that a mutation in this gene results in shorter bristles (written as b).


These two varieties of the bristle-length gene are called alleles that, in various combinations, produce the peccary phenotype.


A single peccary may have any one of three different genotypes:


BB (homozygous dominant)

Bb (heterozygous), and

bb (homozygous recessive)


These genotypes produce one of two phenotypes:


long bristles (BB and Bb), or

short bristles (bb).


Thus, individual peccaries can have only 0, 1, or 2 copies of any one gene variant (- -, - b, or bb, respectively), giving a frequency of 0, 0.5 or 1.0 (0%, 50%, or 100%) for that specific gene in an individual peccary.


With 15 peccaries in the population, each with two copies of the bristle-length gene, there are 30 copies, or alleles, in all. When scientists wish to study evolution (a population phenomenon), they have to measure and follow what happens to all these genes, in the whole population of peccaries, all at the same time.


A gene pool is the sum of all the individual genes in a given population.


Source: http://www.brooklyn.cuny.edu/bc/ahp/LAD/C21/C21_GenePool.html


How Stuff Works has a good go at explaining gene pools. Have a look here.


Limited gene pool made bad waves


Sam Jones

Tuesday January 25, 2005

The Guardian


Scientists have long known that some illnesses are hereditary, but a group of researchers has come up with a more general explanation for humankind's susceptibility to disease. They blame the small ancestral gene pool that spawned the species millions of years ago.


A study by researchers from the universities of Bath, Edinburgh and Sussex claims that there was not a big enough population of early humans to develop a wide and varied gene pool that would fight mutations.


They believe that if our ancestors had benefited from the vast choice of mates available to other species such as mice and rats, there would have been fewer of the mutations in our DNA that can trigger illness. More mates would have meant that many of the mutations would have been eradicated through natural selection.


The study, published in PLOS Biology, found that key regions of our DNA controlling when genes are switched on and off have been altered by around 140,000 naturally-occurring mutations over the past six million years. This has left modern humans with "sloppy" gene control mechanisms which can make us susceptible to illnesses.


The researchers suggest most of the "mildly harmful" mutations occurred when there was only a small population of about 10,000 early Hominids - the two legged primates that evolved into both humans and chimpanzees.


Had there been more early Hominids, most of these mutations would have been overridden by natural selection from a larger pool of available DNA.


"We are used to viewing us as the pinnacle of evolution, but seeing that rodents control their genes much more precisely is somewhat sobering," said Martin Lercher from the University of Bath.


"As a species, we have become used to ever increasing health care and nutrition.But if what we found is still ongoing, then these improvements might at some point be offset by the deterioration of our gene control regions."


Source: http://www.guardian.co.uk/uk_news/story/0,3604,1397967,00.html


Here is a fascinting site. Random notes are run through an "ear" and played. If you like it, the note is tagged and evolves. It takes a little time but eventually you should hear a set a notes that you like! It is called EvoMusic.



Student Outcome: E8.2

Know that members of a population show genetic variability.


  • Individuals are Variable

Darwin knew that individuals were variable, that is, each individual in a population carried a unique set of traits. What he did not know is what produced this variability, namely genetic differences. Variation in the genes of individuals arises from several sources. Mutation, the alteration of existing genes to form new alleles, can arise from copying errors during DNA replication, DNA damage, and repair or recombination during cell division. Varation also arises from sexual reproduction, wherein new combinations of DNA are created through the independent assortment of genes.

  • Some Variations are Passed Down

This statement was a truly unique portion of Darwin's theory. In 1856, he did not know about DNA. He did not know about recombination events. He did not even know about genes. He merely understood that for selection to occur, variations must be transmittable from parent to offspring. We now know, that variation is caused by differences in genes and genes are passed on to offspring. More importantly, different genes are passed on to offspring independently of each other (independent assortment) and intact.


Source: http://www.sparknotes.com/biology/evolution/synthesis/section2.rhtml


Go here to see the range of variation in dogs. This shows the potential for variety in all species.



Student Outcome: E8.3

Describe how biotic and abiotic factors contribute to natural selection.


A central concept of the theory of evolution is natural selection, which arises from three well-established observations: (1) There is some variation in heritable characteristics within every species of organism, (2) some of these characteristics will give individuals an advantage over others in surviving to maturity and reproducing, and (3) those individuals will be likely to have more offspring, which will themselves be more likely than others to survive and reproduce. The likely result is that over successive generations, the proportion of individuals that have inherited advantage-giving characteristics will tend to increase.


  • Biotic factors

Selectable characteristics can include details of biochemistry, such as the molecular structure of hormones or digestive enzymes, and anatomical features that are ultimately produced in the development of the organism, such as bone size or fur length. They can also include more subtle features determined by anatomy, such as acuity of vision or pumping efficiency of the heart. By biochemical or anatomical means, selectable characteristics may also influence behavior, such as weaving a certain shape of web, preferring certain characteristics in a mate, or being disposed to care for offspring.


Source: http://www.project2061.org/publications/sfaa/online/chap5.htm


Read this reasonably heavy account of how natural selection occured in a laboratory environment with guppie fish.


Beautiful here example of convergent evolution. The article is about how three different species of lizard that have evolved white forms to adapt to the white sands of the desert.


Student Outcome: E8.4

Explain how resistant strains of bacteria can evolve by natural selection.


  • Emergence of resistance


The first step in the emergence of resistance is a genetic change in a bacterium. There are two ways that can happen.


  • Spontaneous mutation in the bacterium’s DNA.


Source: http://www.wellcome.ac.uk/assets/wtx026086.jpg Many antibiotics work by inactivating an essential bacterial protein. Genetic change can remove that protein. Also, mutations in the target protein can prevent the antibiotic from binding or it if does bind, prevent it from inactivating the target protein. Genetic change can also lead to increased production of the antibiotic’s target enzyme so that there are too many of them and the antibiotics cannot inactivate them all. Alternatively, the bacterium may produce an antibiotic-inactivating enzyme. As well, the bacterium may alter the permeability of its cell membrane, or wall to the antibiotic.


  • Transfer of antibiotic-resistant genes


The second way for a bacterium to gain resistance is for an existing antibiotic-resistant gene to transfer from one bacterium to another bacterium. Microbiologist, Doctor John Turnidge, says they literally borrow their resistance genes from neighbouring bugs. "They’re the original life forms almost, so for thousands of millions of years they’ve had a chance to work out ways to survive and one of those is to borrow genes from other bacteria to survive."


  • How does resistance spread?


"Antibiotic resistance is an inevitable consequence of antibiotic use, the more you use them the more resistance you will get." Says Associate Professor Collignon.


As well as the transfer of antibiotic resistance genes directly from one bacterium to another, resistance also spreads through the movement of bacteria from one host to another either directly or indirectly, for example, through food, water or even contact between animals - including humans.


Antibiotics, like herbicides or pesticides, select for antibiotic resistant bacteria. When an antibiotic attacks a particular bacterial infection there is always the chance that, within a population of bacteria, there will be some members with resistance. Those not killed are now free to multiply without any competition from the sensitive strains. Antibiotics can also wipe out friendly bacteria, which would otherwise compete with the resistant strain for resources.


And to make matters worse, antibiotics can also increase resistance emerging in harmless bacteria which can, under certain conditions such as in an immune suppressed patient, become aggressive and cause infection. Just the existence of antibiotic resistant bacteria, harmful or not, increases the likelihood of resistance being passed on to other bacteria.


Resistance is a natural phenomenon perhaps as old as bacterium themselves. However, we have contributed to an increase in the rate of antibiotic resistance through the increased transmission of infection and the misuse and abuse of antibiotics.


  • Misuse and abuse of antibiotics


Australia is one of the highest users of antibiotics in the world. There are just over 22 doses of antibiotics prescribed per thousand people, every day. Unlike other developed countries, Australia’s usage has declined since 1994 when doctors wrote 26.1 million antibiotic prescriptions. By 1998 that had declined to 24 million prescriptions for Australia’s 16 million people.


In the US it’s estimated that 50 million of the 250 million prescriptions issued for antibiotics each year are unnecessary. Doctor John Turnidge, Chairman of Australia’s Joint Expert Technical Committee on Antibiotic Resistance, says he believes Australian medicine could safely cut its antibiotic usage by half.


Source: http://www.abc.net.au/science/slab/antibiotics/resistance.htm


Fortunately here is a video to help explain the process. The contents are not so important but they illustrate the process of resistance.

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