September 2015 Review Draft hs 4 Course Life Science/ Biology High School Four Course Model – Life Science/ Biology

Background and Instructional Suggestions

This unit ties in with the previous unit and also relates to some of the statistical and mathematical modeling students did in middle school (MS-LS4-6). As students recognize the lines of evidence supporting evolution, they can now connect it to what Darwin postulated in the middle 1800’s. In the early to middle 1800s, Charles Darwin spent his adult life collecting and analyzing data. Interestingly, he was a naturalist on a boat expedition (HMS Beagle) that was sailing the world to map landforms and geologists on this same expedition (and others like it) would contribute data to our understanding of plate tectonics (see below). The result of Darwin’s work is the foundation for the study of evolution. Many of his observations were also noticed by others for example, Alfred Wallace. What Darwin noticed, was that organisms have the potential to reproduce many more offspring than will survive (for example a spider will lay 100’s of eggs). He noticed that despite the potential for large numbers in a population most populations remain fairly constant in numbers over generations. Darwin concluded that there had to be competition for resources and that is part of what helped in keeping population numbers stable over time. He also noticed that while fossils and modern living organisms differed from place to place, in the same area the fossils and modern living organisms were very similar to one another. For example, Darwin saw that several bird species in the Galápagos Islands looked very similar to one species found on the continent of nearby South America. He also knew that offspring looked like their parents but there was slight variation. He understood how animal breeders manipulate the traits in the population of their livestock or dogs by selectively breeding to reinforce or eliminate certain traits. It was all these observations that helped him frame the theory of Natural Selection which states that there is competition over resources and individuals in a population that can get the resources they need are able to reproduce and pass on their traits to their offspring and therefore are the more fit individuals of the population. If no individuals reproduce, than that population ceases to exist and any unique alleles within that population are also eliminated.

Natural selection acts on the phenotype of an individual, for example the size of a shell or beak. The selective pressure that favors one size over another will translate into a change in proportion of individuals with the favored size in the next generation–if the change is a result of an allele type (inheritable). In other words, the individuals that have the favorable phenotype reproduce and pass on the favorable allele that generated that phenotype. The allele frequencies for the genes are ultimately what changes are passed on to the next generation. Over multiple generations genotype change accumulates. This can be mathematically modeled to show the trends in changes over time. Variation also has to exist in the population before selection can act. What this means is that there has to be variation in beak size or shell size before any environmental change has occurred. The environmental change does not “cause” beaks or shells to get bigger or smaller. Individuals with bigger or smaller beaks or shells within the original population had to exist and those individuals who survive the selective pressure and reproduce will pass on their alleles and begin to “fill” the gene pool with the more favorable alleles. These organisms are said to be “biologically fit,” meaning they survive and reproduce living offspring in a given environment. This meaning is different than the modern term for “fit,” which usually alludes to being strong and healthy.