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Grade 11 University Biology – Unit 3 Evolution Mechanisms of Evolution Section 9.1 Pages 350-359 Mechanism of Evolution really means “How do we get variation?” Evolution is responsible for both the remarkable similarities we see across all life and the amazing diversity of that life. Yet, how exactly does it work? Evolution that occurs WITHIN A POPULATION is microevolution. Fundamental to that process is genetic variation, and the mechanisms that selective forces act so that evolution occurs. We (…and Darwin…) have defined evolution as DESCENT WITH MODIFICATION from a common ancestor, but exactly what has been modified? For starters, evolution only occurs when there is a change in GENE FREQUENCY within a population over a long period of time. These genetic differences are heritable such that they can be passed to future generations. Moreover, Variation WITHIN A SPECIES is due to the variety and combination of alleles possessed by individuals. Now, what has been modified? An Example – Environment vs Genes We have two populations of beetles that eat the same plant. Imagine a year or two of drought in which there are fewer plants. All the beetles have the same chances of survival and reproduction, but because of food restrictions, the beetles in the population are a little smaller than the preceding generation of beetles. The difference in weight is NOT evolution. It was due to environmental influences (i.e., low food supply). It was not due to a change in the frequency of genes. The smaller body size was not genetically determined, and consequently, this generation of smallbodied beetles will produce beetles that will grow to normal size if they have a normal food supply. Most of the beetles in the population (say 90%) have the genes for green colouration and a few of them (10%) have a gene that makes them brown. Some number of generations later, the frequency has changed: brown beetles are more common than they used to be and make up 70% of the population. The changing colour is evolution. Why? These two generations of the same population are genetically different. The more genetic variation there is in a population, the greater the diversity of the population AND the greater the change of a selective advantage to some individuals in a changing environment. There are five sources of genetic variation. 1. Mutations Mutation is a change in the DNA of an individual, and that a heritable mutation has the potential to impact an entire gene pool. It could be harmful, beneficial or neutral A single mutation can have a very large and significant effect. NOTE: Evolutionary change is usually based on the accumulation of many mutations The mutations that matter to “large-scale” evolution are the mutations that can be passed to offspring. These are GERM LINE MUTATIONS because they occur in reproductive cells Mutations can occur in many ways including during gene replication, Crossing Over (e.g., Duplication, Translocation), Non-disjunction, or due to external factors such as radiation. Mutations are RANDOM Not all mutations matter to evolution. Since all cells in our body contain DNA, there are lots of places for mutations to occur. If the mutation occurs in a nonreproductive somatic chromosome, the mutation will not be passed to offspring. 2. Genetic Drift The change in allele frequency in a gene pool due to chance. In small population, drift can result in significant change. It contradicts Darwin’s “survival of the fittest” Bottleneck Effect Changes in gene distribution that result in a rapid decrease in population size Results in loss of genetic diversity since variation is lost at the time of the bottleneck Founder Effect A change in the gene pool that occurs when a few individuals start a new isolated population A Founder Effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have: (1) reduced genetic variation from the original population and (2) a non-random sample of the genes in the original population. This could explain Darwin’s finches on the Galapagos Islands 3. Gene Flow The movement of alleles from one population to another due to the migration of individuals. It is also called MIGRATION Individuals carry and introduce genes to a different population where those genes may not have previously existed. As a result, genetic diversity in the population may increase. Gene flow can be a chance event (i.e., unrelated to the genotype) If gene flow stops, isolated populations may change enough so they can no longer mate with members of the other population Examples include pollen from plants blowing into a new field OR people moving to a new country 4. Non-Radom Mating Mating among individuals on the basis of mate selection for a particular genotype or due to inbreeding Preferred Phenotypes - In wild animal populations, individuals may choose mates based on their physical and behavioural traits (i.e., their phenotypes) Inbreeding – In wild or controlled environments, inbreeding occurs when closely related individuals breed. Since parents have the same genotypes, the frequency of homozygous genotypes is increased...which negatively increasese the frequency of expression of harmful recessives alleles (e.g., purebred pets have higher incidence of health problems and often earlier death). 5. Natural Selection Selective forces (e.g., predation, competition) mean some individuals are more likely to survive and reproduce. If having a single allele gives a slight and consistent selective advantage, the frequency of the allele in the population increases over generations, and moreover, a greater likelihood of those favoured individuals surviving, reproducing and passing the allele to their offspring. Thus, natural selection changes the allele frequency of a population. The outcome is evolutionary change. There are three types of selection. Stabilizing Selection – Favours an intermediate phenotype and acts against extreme variants. It reduces variation in a population and improves adaptation in a non-changing environment Directional Selection – Favours the phenotypes at one extreme over another, resulting in the distribution of phenotypes shifting toward the extreme. It is common during environmental change or population migration when new environmental conditions are forming Disruptive Selection – Favours the extremes of a range of phenotypes and eliminates the intermediate phenotypes. The diagram represents disruptive selection. Over time, the intermediate phenotypes (middle part of the curve) disappear from the population. Sexual Selection Natural Selection for mating based on competition between males and choices by female. HOMEWORK Page 352, Questions 1-4 Page 356, Questions 8-12 Page 359, Questions 2, 6-9