Notes on Evolution

Notes on Evolution



Evolution is slow change over time.

a. Geologic evolution: changes in the earth over time

b. Organic evolution: changes in living organisms over time


Living things have changed over time. Organic Evolution.

a. Punctuated equilibrium suggests that evolution occurred in skips and jumps after long periods of stable change (due to migration, mutation, open ecological niches)

b. Gradualism suggests that change occurred slowly and steadily over time.


Methods of Change:

a. Adaptive Radiation: a single species, or group of closely related species, evolve into several different species, each adapted to the environment in a slightly different way.

Adaptive Radiation is believed to have lead to the Age of Dinosaurs and the Age of Mammals.

b. Convergent Evolution is believed to be the explanation as to why unrelated species look similar to one another. It is thought that similar environmental factors (pressures) affect different organisms in similar ways producing a similar outcome. These features are best suited to the demands of the environment.

Streamlined animals can move more easily through water: shark (fish), penguin (bird), dolphin (mammal)

c. Co-evolution demonstrates that an interaction (symbiosis) between two organisms causes one organism to change in response to changes in the other organism.

This relationship is easily seen in the shapes of the beaks of birds and the shapes of the flowers they feed on.


Evidence that supports changes in living things overtime can be found in:

a. Homologous Body Structures are body structures that develop from similar embryonic tissues but serve slightly different functions in the related organisms.

Wings of birds and bats, flippers on whales, a human hand are all composed of the same bones, just arranged a bit differently.

(Compared to analogous structures which serve the same function but are derived from different tissues.)

Vestigial structures are also homologous, but have lost their function over time.

b. Embryological Similarities can be observed in the development of organisms that are related.

The embryos of vertebrates have characteristics in common with one another.

c. Biochemical Similarities, such as similar DNA, blood, and proteins show a greater overlap the more closely two organisms are related.

Though a horseshoe crab was once classified as a crab (because it looks like one) is now classified as an arachnid (the spider group) due to a blood chemistry more like other arachnids.

d. Cytology, the study of cell structure, can be used to determine relationships between organisms.

e. Behavioral traits that are shared, such as nesting and mating behaviors are used to suggest relationships between animals.

f. Co-evolution, unrelated organisms developing similar traits based on their similar environments, is often used as evidence for evolution. (Geographic distribution)

g. Fossils appear to show a gradual change over time in organisms once found on Earth and relationships to organisms still living.

Fossils: imprint, ice, amber, petrifaction, sedimentary rock (molds, casts)


Darwin’s Theory of Natural Selection

  1. Organisms produce large numbers of offspring, yet the population remains relatively constant.
  2. Variations occur among the offspring, as well as, members of the same species.
  3. There is competition among individuals for the necessities of life (food, sunlight, shelter).
  4. Adaptations exist among individuals that may help or hinder their survival.

Adaptation: inherited trait that improves an organism’s chance of survival in a particular environment.

5. The environment determines which individuals survive. Those   individuals with advantageous characteristics survive and pass those characteristics on to their offspring, while those

individuals less suited to the environment are less likely to pass their genes to offspring.

6. Speciation, the formation of a new species, can occur over time if enough changes occur in the organism that prevents breeding between members of the two related groups.

Species: a group of individuals that are capable of breding and producing viable offspring.


page 386.  All organisms today are united into a tree of life by common descent. Species alive today have descended with modification from species that lived in the past

Speciation normally occurs following one of three “isolating mechanisms”. Each of these mechanisms results in reproductive isolation, where individuals cannot interbreed and produce viable offspring.

a. Behavioral isolation – different courtship rituals, displays, songs

b. Geographic isolation – physical separation of the individuals

c. Temporal Isolation – seasonal, timing differences for  mating

Types of Natural Selection

Natural selection acts on traits, not specifically on the genes or DNA. There are three types of natural selection (polygenetic traits)

a. Directional: one extreme trait is selected for

b. Stabilizing: the average trait is selected for

c. Disruptive: both extreme traits are selected for


Genetic Drift involves random changes in a small population. It is also called the Founder Effect. A small group of individuals leaves an area, bringing their “selected” gene pool, and eventually new subgroups form.

Genetic equilibrium- no change in a population can occur in a population only if there are no changing factors. Five criteria must be met:

  1. There must be random mating
  2. The population must be large
  3. There can be no movement of individuals into (immigration)or out of (emigration) the population
  4. There can be no mutations.
  5. There can be no selective advantages to any genes in the gene pool (no natural selection)