CHAPTER 15 NOTES
Darwin on the HMS Beagle
 Darwin’s role on the ship was as naturalist and companion to the captain.
 His job was to collect biological and geological specimens during the ship’s travel.
The Galápagos Islands
 Darwin began to collect mockingbirds, finches, and other animals on the four islands.
 He noticed that the different islands seemed to have their own, slightly different varieties of animals.
 Almost every specimen that Darwin had collected on the islands was new to European scientists.
 Populations from the mainland changed after reaching the Galápagos.
Darwin Continued His Studies
 Darwin hypothesized that new species could appear gradually through small changes in ancestral species.
 Darwin inferred that if humans could change species by artificial selection, then perhaps the same process could work in
nature.
Natural Selection
 Individuals in a population show variations.
 Variations can be inherited.
 You don’t inherit an injury.
 Organisms have more offspring than can survive on available resources.
 Variations that increase reproductive success will have a greater chance of being passed on.
 Acts to select the individuals that are best adapted for survival and reproduction
Evidence for Evolution
The Fossil Record
 Fossils provide a record of species that lived long ago.
 Fossils show that ancient species share similarities with species that now live on Earth.
 The ancient Glyptodont have been compared to modern day armadillos.
 No species today are found in the fossil record over a million years ago!!
 Derived traits are newly evolved features, such as feathers, that do not appear in the fossils of common ancestors.
 Ancestral traits are more primitive features, such as teeth and tails, that do appear in ancestral forms.
Homologous Structures
 Anatomically similar structures inherited from a common ancestor are called homologous structures.
Vestigial Structures
 Structures that are the reduced forms of functional structures in other organisms.
 Appendix and hip bones in whales
 Evolutionary theory predicts that features of ancestors that no longer have a function for that species will become
smaller over time until they are lost.
Analogous Structures
 Analogous structures can be used for the same purpose and can be superficially similar in construction, but are not
inherited from a common ancestor.
 Show that functionally similar features can evolve independently in similar environments
 Examples include dorsal fins of dolphins vs. sharks or wings of birds vs. insects
Comparative Embryology
 Vertebrate embryos exhibit homologous structures during certain phases of development but become totally different
structures in the adult forms.
 All vertebrates have a:
 post anal tail
 dorsal nerve cord (becomes the spinal cord)
 pharyngeal slits (gills)
 notochord (becomes the spine)
Comparative Biochemistry
 Common ancestry can be seen in the complex metabolic molecules that many different organisms share
 As species evolved, one change after another should have become part of their genetic instructions. Therefore, more
and more changes in a gene’s nucleotide sequence should build up over time.
Geographic Distribution
 Evolution is intimately linked with climate and geological forces.
 If environments never changed, then the traits best suited to survive would continue and mutations would not be
beneficial
 Examples are alligators, swamps haven’t changed in many years.
 As climates change, new mutations may now be beneficial.
Types of Adaptation
 An adaptation is a trait shaped by natural selection that increases an organism’s reproductive success.
 Organisms cannot change “adapt” to their environment.
 Either they have the genetic traits to survive or they do not.
 Fitness is a measure of the relative contribution an individual trait makes to the next generation.
Camouflage
 Allows organisms to become almost invisible to predators
Mimicry
 One species evolves to resemble another species.
Mechanisms of Evolution
 Hardy-Weinberg principle states that when allelic frequencies remain constant, a population is in genetic equilibrium.
 In order for populations to remain constants, the five components of Hardy-Weinberg must occur:
1. No natural selection
2. Mating is random
3. No mutations
4. Large population
5. No gene flow - migration
Genetic Drift
 A change in the allelic frequencies in a population that is due to chance
 In larger populations, the alleles tend to remain more stable
 In smaller populations, the effects of genetic drift become more pronounced, and the chance of losing an allele becomes
greater.
 Genetic drift reduces genetic variation!!!
Genetic Drift - Founder Effect
 Occurs when a small sample of a population settles in a location separated from the rest of the population.
 Alleles that were uncommon in the original population might be common in the new population.
 For example, one of the founding members of a small group Germans that began an Amish community in Pennsylvania
had an allele for polydactylism (more than 5 fingers or toes)
 After 200 years of isolation, the 8000 Amish have a much higher % of polydactylism than the rest of the world
Genetic Drift - Bottleneck
 Occurs when a population declines to a very low number and then rebounds.
 Certain traits may become more prevalent while others may die out because of the traits that survived.
 Caused by disasters such as floods or hurricanes
Gene Flow
 Increases genetic variation within a population and reduces differences between populations
 Caused by migrations
Nonrandom Mating
 Promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for
particular traits
Sexual Selection
 Sexual selection operates in populations where males and females differ significantly in appearance.
 Qualities of sexual attractiveness appear to be the opposite of qualities that might enhance survival.
 Sexual selection will effect natural selection by increasing certain traits in a population such as big pretty feathers on
peacocks