Evolutionary Theory Notes

The Theory of Evolution

  • Evolution is the change in inheritable traits in a population over generations.

  • Change in traits is caused by changes in the genes (in DNA) that code for those traits.

  • Natural selection tends to increase favorable traits in a population and decrease unfavorable traits.

What Evolution is NOT

  • Evolution is not about things getting “better and better” or more & more complex.

  • Selection is about survival and reproduction; organisms with traits that help them survive long enough to reproduce will pass their genes on.

  • Evolution is not about organisms “trying” to adapt; “trying” does not change genes.

  • Only inheritable traits already in a population can be passed on.

  • Evolution does not give organisms what they “need.”

  • If it did, there would never be deaths or species extinctions.

  • If individuals have traits that help them survive environmental change, they may pass those traits on while others die.

  • Evolution does not mean that individuals in a population slowly change.

  • Change happens to populations over generations as those with unfavorable traits are weeded out and those with favorable traits are more likely to reproduce.

  • Evolution is not a theory of how life began.

  • Evolution explains how living organisms change over generations.

Early Ideas

  • Many people associate evolution with Darwin, but other scientists were talking about organic change long before Darwin published his theory of natural selection.

  • Jean de Lamarck (1744 - 1829)

    • Lamarck used the theory of Inheritance of Acquired Characteristics, a widely- held belief at the time, to explain change in organisms.

    • Blacksmiths, for example, were thought to pass their well -developed right arms on to their sons.

  • Lamarck’s Organic Theory of Development

    • Organisms are shaped by their environment.

    • Change is goal-directed -- organisms have an internal drive to become more and more complex.

    • Use and disuse of parts causes change that is passed on from generation to generation.

  • Charles Darwin (1809 - 1882) & Alfred Russell Wallace (1823 - 1913)

    • Darwin and Wallace, working separately, devised a model for organic change that was based on their observations of living and fossil organisms over many years.

    • Natural Selection was the first evidence-based mechanism for evolution that was proposed.

  • Darwin and Wallace observed that change did not happen equally to all members of a population, as Buffon and Lamarck thought.

  • Instead, Darwin and Wallace proposed that selection happens each generation.

  • Some individuals have traits that help them survive and reproduce, while others have traits that put them at a disadvantage.

  • Over generations, the number of individuals with favorable traits increases while those with unfavorable traits decrease.

Natural Selection

  • Evidence from the natural world showed that change happens to organisms over time.

  • The important question was how?

  • Darwin and Wallace’s Theory of Natural Selection provided a natural mechanism for change.

Variation

  • Within any population, individuals differ from one another in many ways.

Inheritance

  • Some differences between individuals are inheritable & can be passed from parent to offspring.

Differences in survival & reproduction

  • In most populations, younger are born than can survive.

  • Many of the young will die.

  • A few will live long enough to reproduce.

Success is not random

  • While random accidents do happen, an individual’s survival depends mainly on the individual’s traits.

  • Those that live and reproduce may pass their traits to the next generation.

  • Selection favors organisms that are best adapted to the environment.

  • Selection does not produce new traits, nor does it make organisms “perfect.”

  • Selection is all about survival, and those organisms with traits that help them survive are more likely to reproduce.

Natural Selection Example

  • The peppered moth

    • Before the industrial revolution in Britain, most peppered moths were of the pale variety & were well camouflaged against the pale birch trees that they like to sit on.

    • Moths with the mutant black coloring were easily spotted and eaten by birds - giving the white peppered variety an advantage.

    • Then the industrial revolution came along in the 19th century.

    • Airborne pollution in industrial areas mottled the birch tree bark with soot, and now the mutant black -peppered moths blended better against the darkened bark, while the white variety became much more vulnerable to predators

    • Over time the mutated black peppered moths were naturally selected to survive and became far more numerous in urban areas than the pale variety.

Types of Natural Selection

  • Natural selection can affect the distributions of phenotypes in any of three ways:

    • directional selection

    • stabilizing selection

    • disruptive selection

Directional Selection

  • When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place.

Examples

  • Giraffe necks:

    • Short neck giraffes died: could not reach leaves on tree

    • Medium neck giraffes died: could not reach leaves on tree

    • Long neck giraffes prospered: leaves were reachable

  • Horse size:

    • Small horses died: very disadvantageous

    • Medium horses died: disadvantageous

    • Large horses prospered: advantageous

Stabilizing Selection

  • When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.

Example

  • Robins typically lay four eggs, an example of stabilizing selection.

  • Larger clutches may result in malnourished chicks, while smaller clutches may result in no viable offspring.

Disruptive Selection

  • When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.

Examples

  • Rabbit color in an environment with black and white rocks

    • White rabbit lives: Camouflaged

    • Grey rabbit dies: Seen by predator

    • Black rabbit lives: Camouflaged

  • Hummingbird beak length in an environment with deep and shallow flowers

    • Short beak hummingbird lives: can get pollen from flower

    • Medium beak hummingbird dies: cannot get pollen from flower

    • Long beak hummingbird lives: can get pollen from flower

Identifying Types of Natural Selection

  • Consider that a flying beetle has been introduced to a new tropical island. Identify the type of natural selection at work in each example. – stabilizing, directional, or disruptive

    • Only the beetles with the darkest bodies thrive in the new environment. (Directional)

    • The smallest beetles cannot compete for food. The largest beetles are easy prey for birds. (Stabilizing)

    • The beetles with the smallest wings a thrive in rotten tree trunks. The beetles with the largest wings thrive along the coastline. (Disruptive)