Biology I for Non-Majors

1. Biology I for Non-Majors Theory of Evolution

2. Charles Darwin • From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle, including stops in South America, Australia, and the southern tip of Africa • During stops on island chains, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences • Darwin imagined that the island species might be species modified from one of the original mainland species • He realized that the varied beaks of each finch helped the birds acquire a specific type of food • This lead to his idea of natural selection • The more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits • Alsoknown as “survival of the fittest,”

3. Three Principles Lead to Natural Selection • Most characteristics of organisms are inherited, or passed from parent to offspring • Even though genes and genetics were unknown at the time, this was a common understanding • More offspring are produced than are able to survive, so resources for survival and reproduction are limited • There is competition for those resources in each generation • This principle came from reading an essay by the economist Thomas Malthus who discussed this principle in relation to human populations • Offspring vary among each other in regard to their characteristics and those variations are inherited • Offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete

4. Alfred Wallace • Darwin was not alone in developing these ideas • Alfred Russell Wallace was also a naturalist who traveled to island chains and made similar observations • Papers by Darwin and Wallace presenting the idea of natural selection were read together in 1858 before the Linnean Society in London

5. The Theory of Evolution • Natural selection can only take place if there is variation, or differences, among individuals in a population • These differences must have some genetic basis • Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction • Mutation, a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population • Sexual reproduction also leads to genetic diversity: when two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each of the offspring

6. Evolution Requires Adaptation • A heritable trait that helps the survival and reproduction of an organism in its present environment is called an adaptation • Whether or not a trait is favorable depends on the environmental conditions at the time • The same traits are not always adaptive because environmental conditions can change • Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other • When two species evolve in diverse directions from a common point, it is called divergent evolution • Convergent evolution occurs where similar traits evolve independently in species that do not share a recent common ancestry

7. Physical Evidence for Evolution • Fossils provide solid evidence that organisms from the past are not the same as those found today, and fossils show a progression of evolution • Anatomy shows the presence of structures in organisms that share the same basic form as well as the convergence of form in organisms that share similar environments • Embryology shows structures that are absent in some groups often appear in their embryonic forms and disappear by the time the adult or juvenile form is reached

8. Biological Evidence for Evolution • Biogeography • The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time • Molecular biology • Like anatomical structures, the structures of the molecules of life reflect descent with modification • Evidence of a common ancestor for all of life is reflected in the universality of DNA as the genetic material and in the near universality of the genetic code and the machinery of DNA replication and expression • DNA sequences have also shed light on some of the mechanisms of evolution

9. Misconceptions about Evolution • Evolution is just a theory • A scientific theory is understood to be a body of thoroughly tested and verified explanations for a set of observations of the natural world • A “theory” in common vernacular is a word meaning a guess or suggested explanation; this meaning is more akin to the scientific concept of “hypothesis” • Individual evolve • Evolution is the change in genetic composition of a population over time, specifically over generations, resulting from differential reproduction of individuals with certain alleles

10. Misconceptions about Evolution • Organisms evolve on purpose • A changed environment results in some individuals in the population, those with particular phenotypes, benefiting and therefore producing proportionately more offspring than other phenotypes • Species do not become “better” over time; they simply track their changing environment with adaptations that maximize their reproduction in a particular environment at a particular time • Evolution explains the origin of life • This theory does not try to explain the origin of life • It does not shed light on the beginnings of life including the origins of the first cells, which is how life is defined

11. Genetics and Evolution • Genetics was not understood when Darwin developed his ideas of natural selection • Today, we can combine Darwin’s and Mendel’s ideas to arrive at a clearer understanding of what evolution is and how it takes place • Microevolution, or evolution on a small scale, is defined as a change in the frequency of gene variants, alleles, in a population over generations • Microevolution is sometimes contrasted with macroevolution, evolution that involves large changes, such as formation of new groups or species, and happens over long time periods • most biologists view microevolution and macroevolution as the same process happening on different timescales

12. Alleles and Population Changes • An allele is a version of a gene, a heritable unit that controls a particular feature of an organism • Allele frequency refers to how frequently a particular allele appears in a population • It’s also possible to calculate genotype frequencies—the fraction of individuals with a given genotype—and phenotype frequencies—the fraction of individuals with a given phenotype

13. Environment and Evolution • Natural selection only acts on the population’s heritable traits: selecting for beneficial alleles and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency—a process known as adaptive evolution • Natural selection acts at the level of the individual; it selects for individuals with greater contributions to the gene pool of the next generation, known as an organism’s evolutionary (Darwinian) fitness • it is not the absolute fitness of an individual that counts, but rather how it compares to the other organisms in the population, which is called relative fitness

14. Types of Selection • If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo stabilizing selection  • When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation • In diversifying selection, two or more distinct phenotypes can each have their advantages and be selected for by natural selection, while the intermediate phenotypes are, on average, less fit • Frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection) • The selection pressures on males and females to obtain matings is known as sexual selection; it can result in the development of secondary sexual characteristics that do not benefit the individual’s likelihood of survival but help to maximize its reproductive success

16. Sexual Dimorphisms

17. Variation in a Population • Heritability tells us how much phenotypic variation in a population is ultimately due to genetic differences as opposed to acquired differences • The diversity of alleles and genotypes within a population is called genetic variance • Allele and genotypic frequencies can change due to • Natural selection • Genetic drift • Bottleneck effect • Founder effect • Gene flow

18. Scientists Classify Organisms • Biologists classify organisms to make sense of the incredible diversity of life on Earth • Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among organisms • A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms • Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships

19. Phylogenetic Trees • A phylogenetic tree can be read like a map of evolutionary history • Many phylogenetic trees have a single lineage at the base representing a common ancestor = a rooted tree • In a rooted tree, the branching indicates evolutionary relationships • The point where a split occurs, called a branch point, represents where a single lineage evolved into a distinct new one • A lineage that evolved early from the root and remains unbranched is called basal taxon • When two lineages stem from the same branch point, they are called sister taxa • A branch with more than two lineages is called a polytomy • Unrooted trees don’t show a common ancestor but do show relationships among species

20. Rooted and Unrooted Trees

21. Phylogenetic Trees are Limited • It may be easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true • If two closely related lineages evolved under significantly varied surroundings or after the evolution of a major new adaptation, it is possible for the two groups to appear more different than other groups that are not as closely related • Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for length of time, only the evolutionary order • The length of a branch does not typically mean more time passed, nor does a short branch mean less time passed

22. Taxonomy and Binomial Nomenclature • Taxonomy is the science of classifying organisms to construct internationally shared classification systems with each organism placed into more and more inclusive groupings • The taxonomic classification system uses a hierarchical model • Recent genetic analysis and other advancements have found that some earlier phylogenetic classifications do not align with the evolutionary past; therefore, changes and updates must be made as new discoveries occur

23. Quick Review • Why is Darwin considered the father of evolution? • List the key ideas Darwin used as the basis for ‘descent with modification’. • How did Darwin’s work transform into the theory of evolution? • List types of evidence that support the theory of evolution: physical and biological. • What are common misconceptions about evolution and why are they wrong? • How are genetics and evolution connected? • Describe how the environment and selective pressures contribute to evolution. • What types of variation can be found in a population? • Explain the two main types of phylogenetic trees. • What are some limits of phylogenetic trees? • Describe modern taxonomy and binomial nomenclatures.