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Lecture 6

Lecture 6 - Evolution and Ecology

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Biology 2483A
Hugh Henry

LECTURE 6: EVOLUTION AND ECOLOGY Trophy Hunting and Inadvertent Evolution: A Case Study  Bighorn sheep populations have been reduced by 90% by hunting, habitat loss, and introduction of cattle. Hunting is now restricted; permits for a large “trophy ram” cost over $100,000  Trophy hunting removes the largest and strongest males—the ones that would sire many healthy offspring.  In one population, 10% of males were removed by hunting each year, the average size of males and their horns decreased over 30 years of study.  This is also being observed in other species: o African elephants are poached for ivory; the proportion of the population that have tusks is decreasing. o Rock shrimp are all born male, and become females when they are large enough to carry eggs. Commercial harvesting takes the largest individuals, which are all females.  Genes for switching sex at a smaller size became more common, resulting in more females, but smaller females lay fewer eggs. What is Evolution?  Evolution can be viewed as genetic change over time or as a process of descent with modification  Biological evolution is change in organisms over time  Evolution can be defined more broadly as 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 has many of the same characteristics as its ancestors, and resembles them  Populations change over time through natural selection: o Individuals with certain heritable traits survive and reproduce more successfully than other individuals. Mechanisms of Evolution  Natural selection, genetic drift, and gene flow can cause allele frequencies in a population to change over time  Phenotype: Observable characteristics that are determined by the genotype.  Individuals differ from one another in part because they have different alleles for genes  Different alleles arise by mutation: change in 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 o If mutation did not produce new alleles, all members of a population would have identical genotypes and evolution could not occur  Mutations are actually very rare o In a generation, one mutation would occur in every 10,000 to 1,000,000 copies of a gene o In one generation, mutation acting alone causes virtually no change in allele frequencies of a population o Mutations are not a strong source of genetic variation – more like a seed that introduces it, and other factors such as natural selection or gene flow, act upon it and increase genetic variation Types of Natural Selection  Directional selection: Individuals at one phenotypic extreme (e.g., large size) are favored o Example: Drought favored large beak size in medium ground finches  Stabilizing selection: Individuals with an intermediate phenotype are favored. o Example: Parasitic wasps select for small gall size of Eurosta flies; while birds select for large gall size  Disruptive selection: Individuals at both phenotypic extremes are favored. o Example: African seedcrackers (birds) have two food sources—hard seeds that large beaks are needed to crack, and smaller, softer seeds that smaller beaks are more suited to crack Genetic Drift  Genetic drift occurs when chance events determine which alleles are passed to the next generation o Only significant for small populations  Genetic drift has four effects on small populations: 1. It acts by chance alone, thus causing allele frequencies to fluctuate at random. Some may disappear, other may reach 100% frequency (fixation) 2. Because some alleles are lost, genetic variation of the population is reduced 3. Frequency of harmful alleles can increase, if the alleles have only mildly deleterious effects 4. Differences between populations can increase  Number 2 and 3 can have dire consequences o Loss of genetic variation reduces the ability of the population to respond to changing environmental conditions o Increase of harmful alleles can reduce survival and reproduction o These effects are important for species that are near extinction  Greater prairie chicken populations in Illinois have been reduced by loss of habitat to farmland. o In 1993, population was less than 50. DNA from this population compared with museum specimens from the 1930s showed a decrease in genetic variation. o 50% of eggs failed to hatch, suggesting fixation of harmful alleles Gene Flow  Gene flow: Alleles move between populations via movement of individuals or gametes.  Gene flow has two effects: 1. Populations become more similar – alleles are shared, becomes more uniform 2. New alleles can be introduced into a population  In the 1960s, new alleles that provide resistance to insecticides arose by mutation in mosquitoes in Africa or Asia o Mosquitos with the new alleles were blown by winds or transported by humans to new locations o The allele frequency increases rapidly in populations exposed to insecticides o Resistance to insecticide – became a very high selection pressure Natural selection is the only evolutionary mechanism that consistently causes adaptive evolution  Adaptations are features of organisms that improve their ability to survive and reproduce  Natural selection is not a random process  By consistently favoring individuals with certain alleles, natural selection causes adaptive evolution – traits that confer advantages tend to increase in frequency over time  Example: Soapberry bugs feed on fruits by piercing them with a needle-like beak. o Feeding is most efficient if beak size matches fruit size o In populations with different food sources, Carroll and Boyd (1992) predicted that beak size would evolve to adapt to fruits of introduced tree species o Beak length is a heritable trait, so the observ
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