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Ecology 6.docx

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Western University
Biology 2483A
Mark Moscicki

Ecology-Lecture 6 Oct 1 2013 Trophy Hunting/Evolution  Bighorn sheep are a trophy species. They have very large horns for mating  Bighorn sheep populations have been reduced by 90% by hunting, habitat loss, and introduction of cattle. Hunting is now restricted in N.A; permits to kill ONE large "trophy ram" cost over $100 000. Trophy hunting is a selec tive process. When you get a permit, you want to find the largest male with the biggest horns (biggest payoff)  Trophy hunting removes the largest and strongest males-the ones that would sire many healthy offspring (successful males in terms of mating are being removed)  In one population, 10% of the males were removed by hunting each year, the average size of males and their horns decreased over 30 years of study. 10% may not seem like much but the 10% that you remove could be the 10% responsible for the majority of the offspring!  This trend has been observed in other species...  Rattle snake hunters go out and catch hundreds of rattlesnakes each year. It has been selected for rattlesnakes that do not rattle (dangerous and unintentional consequence)  African elephants are poached for ivory; the proportion of the population that have tusks is decreasing  In cod fishing, the largest fish are most desired. This has led to a reduction in the size and age at which these fish mature. This happens because cod that mature at a younger age are more likely to reproduce before they are caught and killed  Rock shrimp are all born male, and become females when they are large enough to carry eggs. Commercial harvesting takes the largest individuals (best in terms of food quality), which are all females. Selection is favouring small females, or in other words males converting to females at a smaller size (genes for switching sex at a smaller size become more common, resulting in more females, but smaller females lay fewer eggs) Egg production decreases which is a negative consequence. Evolution  Genetic change over time/process of descent with modification  Biological evolution is a change in organisms over time  This is apparent in the fossil system. This is a lake system in which there are many Stickelback skeletons. They started with large pelvic bones and over time, selection favours small pelvic bones. At the end, these fish had no pelvic bones!  Evolution includes the small fluctuations that occur in a population (genetic makeup of population changes from one year to the next) and the larger changes (one species gradually becomes different from its ancestors)  We get extreme differences over time. In the leaf pictures, we see basic modification of the normal leaf structure. Evolution is Allele Frequency Change  Evolution is change over time in the frequencies of alleles in a population Evolution is Descent with Modification  As a population accumulates differences over time, and a new species forms, it is different from its ancestors. But the new species resembles their ancestors and has many of the same characteristics  Populations change over time through natural selection. Individuals with certain heritable traits survive and reproduce more successfully than other individuals ( most successful traits are passed on) Natural selection can cause the accumulation of differences between populations. If two populations experience different environmental conditions, individuals with one set of characteristics may be favoured by natural selection in one population, while individuals with a different set of c characteristics may be favoured in the other population. This can cause populations to genetically diverge from one another. Thus, natural selection is responsible for the modification part of "descent with modification" Populations Evolve, Individuals Do Not  Natural selection sorts favourable traits. As generations go on, a greater proportion of the individuals in that population will have the traits favoured by natural selection Mechanisms of Evolution  Mutation is the source of new alleles on which all evolution depends. Individuals differ from one another in part because they have different alleles for genes. We get different alleles by changing the underlying structure of DNA (mutations, damage, CNV) Different alleles arise by mutation which is a change in the DNA. Mutations can result from copying errors during cell division, mechanical damage, exposure to chemicals (mutagens) or high energy radiation. Formation of new alleles is critical to evolution. If mutation did not produce any new alleles, all members of a population would have identical genotypes and evolution could not occur. Mutations are actually rare. In a generation, one mutation would occur once in every 10 000 to 1 000 000 copies of a gene. In one generation, mutation acting alone causes virtually no changes to allele frequencies of a population. However, it is required for the INITIAL SEED.  Natural selection, genetic drift, and gene flow can cause allele frequencies in a population to change over time. Any given gene locus has 2 alleles, and these alleles that code for genetic information are changing in the population over time.  Phenotype: Observable characteristics determined by the genotype. Your genotype codes for your phenotype.  When environment acts on a phenotype, Figure 6.6 Three Types of Natural Selection (Part 1) selection can move things in various ways. There are 3 types of natural selection 1. Directional Selection: Individuals at one phenotypic extreme are favoured (large size) (Drought favoured large beak size in medium ground finches. Plants surviving the drought had larger seeds. Bigger beaks were required to open these seeds) One extreme has a strong advantage. 2. Stabilizing Selection: Individuals with an intermediate phenotype are favored (parasitic wasps select for small gall size of Eurosta flies; while birds select for large gall size. Therefore, it is favourable to produce galls of intermediate size so that fly larvae inside of the galls have an increased chance of survival) 3. Disruptive Selection: Individuals at both phenotypic extremes are favoured. (African seedcrackers have 2 food sources-hard seeds that large beaks are needed to crack, and smaller, softer seeds that smaller beaks are more suited to. Intermediate sized galls are useless) Figure 6.6 Three Types of Natural Selection (Part 2) Genetic Drift  Chance events determine which alleles are passed onto the next generation (trampling)  If you ONLY have genetic drift acting, neither allele may be more beneficial than the other. Thus natural selection cannot act  Only significant for small populations. To see why, compare a wildflower population of 10 vs 10 000. If a moose were to step on 40% of the 10 (4 flowers) there is a greater chance that the flowers removed will have the same alleles than if a moose were to step on 40% of 10 000 (more likely that a variety of different alleles will be decreased)  Genetic drift has 4 effects on small populations 1. It acts by chance alone, thus causing allele frequencies to fluctuate at random. Some may disappear and others may reach 100% fixation. 2. Because some alleles are lost, genetic variation is reduced. Individuals may become more genetically similar 3. Frequency of harmful alleles can increase, if the alleles only have mildly deleterious effects. If in a very small population, and the allele is only slightly deleterious (doesn't kill organism) genetic drift can overrule natural selection (which is trying to reduce harmful alleles) 4. Differences between population can increase. Chance events can cause an allele to reach fixation in one population but be lost in another. **2 and 3 can have dire consequences. Loss of genetic variation reduces the ability of the population to respond to changing environmental conditions. Increasing harmful alleles can reduce survival and reproduction. This can relate to Figure 6.8 Harmful Effects of Genetic Drift (Part 2) species near extinction.  An example of genetic drift is with Prairie chickens in Illinois. Their populations have been reduced to due habitat loss by increased farming. In 1993, the population was <50. DNA from this population was compared with museum specimens from the 1930s and it showed a decrease in genetic variation. 50% of the eggs failed to hatch, suggesting an accumulation of harmful alleles Gene Flow  Alleles move between populations via movement of individuals or gametes (pollen)  Gene flow has 2 effects 1. Populations become more similar. This is why individuals in different populations of the same species
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