Descent with Modification 19 Descent with Modification 1

Concept 19.3: Evolution is supported by an overwhelming amount of scientific evidence

Scientific Theories Theory—a major unifying framework, supported by all evidence currently known Evolution is “so overwhelmingly established that is has become irrational to call it a theory.” Ernst Mayer (1904-2005)

Evidence for Evolution Direct Observations Homology The Fossil Record Biogeography

1. Direct Observations Biologists have documented evolutionary change in thousands of scientific studies. Example: Insecticide and Drug Resistance Insecticide/drug doesn’t kill all individuals. Resistant survivors reproduce. Resistance is inherited. Insecticide/drug becomes less and less effective.

See your book for more specific examples. Keep in mind: Natural selection does not create new traits, but edits or selects for traits already present in the population The local environment determines which traits will be selected for or selected against in any specific population

2. Homology Evolution is a process of descent with modification. Related species can have characteristics with underlying similarity that function differently. Homology is similarity resulting from common ancestry.

Anatomical Homologies Homologous Structures: Similarities in characteristics resulting from common ancestry. Similar structure, similar development, different functions. Evidence of close evolutionary relationship (recent common ancestor).

Examples: Homology in the tetrapod limb plan. Homology in insect head anatomy. Homology in Great Ape skeletons.

Vestigial Structures: Remnants of features that served important functions in the organism’s ancestors Examples: Hipbones and pelvis in whales. Human appendix. Skink legs.

Comparative Embryology Anatomical homologies in embryos not visible in adult organisms. Example: All vertebrate embryos have similar structures at different stages of development.

Molecular Homologies Life uses a universal genetic code! Examples of homologies at the molecular level are genes shared among organisms inherited from a common ancestor. Homologous genes can be found in organisms as dissimilar as humans and bacteria. A bacterium can make a human protein if given the proper “recipe.”

Building “family” trees with molecular evidence: Closely related species (branches) share the same line of descent until their divergence from a common ancestor. As this happens, genetic differences accumulate. Fewer differences in DNA/protein sequences = more recently diverged.

3. The Fossil Record Generally, layers of sedimentary rock contain fossils. New layers cover older ones, creating a record over time. Fossils can be dated using radioactive decay. The fossil record shows that a succession of organisms have populated Earth throughout a long period of time. Support for a VERY old Earth.

Transitional Fossils Show evolutionary links between groups of organisms. Examples: Tiktaalik: a transition fossil between fish and amphibians Archaeopteryx: dinosaurs to birds “Lucy”: an Australopithicus

4. Biogeography Biogeography, the geographic distribution of species, provides evidence of evolution Earth’s continents were formerly united in a single large continent called Pangaea but have since separated by continental drift An understanding of continent movement and modern distribution of species allows us to predict when and where different groups evolved

Biogeography was the original evidence that informed Darwin and Wallace. They noted organisms found in a particular area tended to be more closely related to each other than they were to organisms found in other areas.

Examples: Divergent Evolution or Adaptive Radiation Galapagos tortoises/finches adapted to different habitats as they spread from the mainland to different islands. Many new species formed as their adaptations allowed them to fill different ecological niches.

Convergent Evolution Organisms not closely related independently evolve similar traits as a result of having to adapt to similar environments or ecological niches This results in Analogous Structures. Similarities in adaptations resulting from similar evolutionary pressures. Different structures, different development, similar functions.

Examples: Convergence in squirrel-like mammals. Sugar gliders and flying squirrels share similar adaptations, however, sugar gliders are more closely related to kangaroos than flying squirrels, because they share marsupial ancestors. Wings have evolved multiple times (flying is a good niche). Convergence in dolphin and shark body plans.

Some Things to Remember: Evolution is not goal-oriented. An evolutionary trend does not mean that evolution is goal-oriented. Surviving species do not represent the peak of perfection. Evolution can only work on what is present. There is a lot of compromise in evolved physiology. Since the phenotype as a whole is the target of selection, it is impossible to improve simultaneously all aspects of the phenotype to the same degree. Ernst Mayr