Evolution and Natural Selection

Vocabulary Evolution: any change in the heritable traits within a population across generations Theory: a well-supported, testable explanation of events that have occurred in the natural world Fossil: the preserved remains of an ancient organism Struggle for existence: how members of each species compete to obtain food, living space, and other necessities of life Fitness: the ability of an individual organism to survive and reproduce in its specific environment Adaptation: any trait that increases an organism’s chance of survival

Vocabulary Survival of the fittest: the process of more fit (better adapted) organisms surviving and reproducing the most successfully Natural selection: the process by which random evolutionary changes are selected for by nature in a consistent, orderly, non-random way Descent with modification: the observable fact that when parent have children, the children often look and act slightly different from the parents Common descent: the idea that all life on earth is related, that we all came from a single common ancestor Homologous structure: structures that have the same mature forms but develop from the same embryonic tissues Vestigial organ: organs that have become smaller over time and are no longer useful

Voyage of the Beagle Starting in 1831, Charles Darwin sailed all over the world on the H.M.S. Beagle, observing and studying many different organisms. During his travels, Darwin made many observations and collected evidence that led him to propose a hypothesis about the way life changes over time. That hypothesis, which is now supported by a huge amount of evidence, has become the theory of evolution.

Darwin’s Observations Darwin was very interested in the patterns of diversity that are present in living organisms. Specifically, he was fascinated by the way animals seem so well adapted to surviving in their environments. He wondered if there was some process that led to such a wide variety of life forms.

Living Organisms and Fossils Darwin realized that living animals represented just a part of all the species that had ever lived on earth. During his travels, he collected many fossils . Some of those fossils looked very much like organisms that are still alive. Others looked completely different from any known living creature. Darwin wondered why these species disappeared and how they were related to living species.

The Galapagos Islands Darwin traveled to many places, but the Galapagos Islands inspired him the most. While he was there, he noticed that even though the islands were close together, they had very different climates. Some were hot and dry with very little life. Other islands had much more rainfall and a wider variety of plants and animals. Darwin noticed that the traits of many animals and plants varied noticeably among the different islands of the Galapagos. He hypothesized that these separate species were once members of the same species, and that somehow they changed after being isolated from each other in different environments.

Check for Understanding Answer these questions in your notes now. What did Darwin’s travels reveal to him about the number and variety of living species? What is evolution? Why is evolution called a theory?

Inherited Variation One of Darwin’s important observations is that there is natural variation within members of the same species. For example, some apple trees produce bigger, sweeter apples. Some cows produce more milk than others. Some horses run faster than others.

Evolution by Natural Selection Natural selection is the mechanism for evolution that Darwin proposed. There are three conditions that must be met for natural selection to happen. 1. Variation in characteristics: different individuals in a population must have different characteristics. For example, beetles of the same species may come in green or brown. 2. Differences in fitness: the differences in characteristics between individuals must contribute to their probability of surviving and reproducing. For example, the green color beetles might help them survive by making them harder to see and therefore less likely to get eaten. 3. Heritability of characteristics: the different characteristics that affect fitness must be heritable (able to be passed down to offspring)

Check for Understanding Write the answers in your notes now. What is a mechanism? How does variation occur in a species? How is evolutionary fitness different from physical fitness?

Natural Selection in Peppered Moths Peppered moths have two wing color phenotypes: light and dark. A few hundred years ago, the light color was much more common than the dark color. However, during the industrial revolution, many factories began burning coal as a fuel source. This coal left black soot on many nearby trees and other surfaces. Once the trees became covered in soot, the dark moth phenotype became much more common than the light phenotype. Why do you think the dark colored became more common? Write your hypothesis in your notes now.

Survival of the Fittest A key factor in the struggle for existence is how well an organism is adapted to living in its environment. Darwin called this ability to survive fitness. Fitness is a result of adaptations. Successful adaptations allow organisms to be better suited to their environments, and therefore better able to survive and reproduce. Organisms that are less fit are less likely to survive and reproduce, and are therefore less likely to pass on their genes and traits.

Antibiotic Resistance One example of survival of the fittest is how bacteria become resistant to antibiotics. In any population of bacteria, there may be a few that have a random mutation that makes allows them to survive even in the presence of an antibiotic. The antibiotics then kill off all the remaining bacteria, so only the resistant bacteria survive and reproduce. The offspring of these bacteria will also be resistant (remember that bacteria reproduce asexually, so all offspring are identical to the parents unless there was a mutation). Now, you have an entire population of bacteria that are resistant to antibiotics.

Check for Understanding Write the answer in your notes now. Why is antibiotic resistance a problem?

Evidence of evolution Many of you asked what evidence there is for evolution. The short answer is that there is a lot of evidence that supports the theory of evolution. That’s why it is called a theory and not a hypothesis. There is evidence of evolution in homologous structures, embryology, DNA evidence, and fossils. We will examine each of these in more detail the next time class meets.

Evidence of Evolution

Fossil Evidence

Homologous Structures

Similarities in Embryology

DNA Evidence

Next presentation: sources of variation, variation and gene pools, allele freuqency, natural selection on single gene and polygenic traits, different kinds of selection, genetic drift, the founder effect, speciation, darwins finches