Ch.10: Principles of Evolution

Section 3: Theory of Natural Selection

Artificial Selection Natural variation exists in all populations & some variation is heritable, meaning it can be passed from one generation to the next For 100s of years, humans have been using artificial selection - the process by which humans change a species by breeding it for certain traits Darwin believed that a process similar to artificial selection could happen in nature

Darwin Publishes his Theory Over 20 years after Darwin’s voyage on the Beagle, he received a short essay from Alfred Russel Wallace that summarized all of Darwin’s thoughts about evolution. This prompted Darwin to publish his own book called On the Origin of Species by Means of Natural Selection. Natural Selection is the mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than do other individuals

4 Main Principles of Natural Selection 1.Variation - natural genetic variation exists in all populations Population is all the individuals of a species that live in an area 2.Overproduction - populations tend to produce more offspring than the environment can sustain, leading to competition for resources

3.Adaptation - some variations allow an individual to better survive & reproduce Fitness is a measure of the ability to survive and produce more offspring relative to other members of the population 4.Descent with modification - Over time, natural selection results in changes in a population, as only those with the highest fitness will pass on their genes

Natural Selection acts on Phenotypes Natural selection doesn’t create new alleles, genetic mutations do Natural selection can only act on traits that already exist in a population As an environment changes, different traits become beneficial

Section 4: Evidence of Evolution

Darwin’s Evidence The most important & convincing evidence of evolution comes from fossils, geography, embryology, & anatomy Fossils - fossil organisms in lower, older layers of rock are more primitive than those in upper, newer layers; this supports descent with modification

Geography - Darwin observed that certain plants & animals in the Galapagos Islands were similar but not identical to mainland species He hypothesized that some individuals from the mainland migrated to the islands & then adapted to each particular island biogeography is the study of the distribution of organisms around the world

Embryology - similar features of embryos in very different organisms suggests evolution from a distant common ancestor Ex. vertebrate embryos all have gill slits as embryos, but not as adults

Anatomy - comparing body parts of different species provides evidence of evolution Homologous structures are features that are similar in structure but appear in different organisms & have different functions (suggest a common ancestor) Analogous structures are structures that perform a similar function but are not similar in origin; they arise due to similar environmental challenges, not common ancestry

Structural Patterns & Evolutionary History Structural patterns are clues to the history of a species Vestigial structures are remnants of organs or structures that had a function in an early ancestor, but serve little or no function in the present Ex. pelvic bones & hind limb bones in snakes Ex. appendix & tail bone in humans

Section 5: Evolutionary Biology Today

Fossil Record Paleontology (the study of fossils or extinct organisms) was a new science in Darwin’s time Our fossil record is incomplete, but it does support the theory of evolution Many transitional fossils have been found that show the change in organisms over time

Molecular & Genetic Evidence DNA Sequence Analysis - The more related two organisms are, the more similar their DNA will be. Pseudogenes - Like vestigial structures, pseudogenes no longer function but are still carried along with functional DNA. Similarities between pseudogenes suggest a common ancestor

Homeobox Genes - control the development of specific structures; found in many organisms & indicate a very distant common ancestor Protein Comparisons - Molecular fingerprinting compares similarities among proteins of organisms. Species that have the same proteins most likely come from a common ancestor

Evolution Unites all Fields of Biology All biological fields contribute to evolutionary theory The theory of natural selection along with genetics is sometimes called the modern synthesis of evolutionary theory The basic principles of evolution are used in many scientific fields such as medicine, geology, geography, chemistry