Ecological Principles / Populations

The Study of Ecology

• Population ecology (the number and distribution of individual species and the factors that cause those distributions). Population ecology will be the focus of this page.
• Physiological ecology (the response of a species to various environmental conditions)
• Community ecology (the number of species found at a location and how those species interact)
• Ecosystems ecology (the study of all the species that live together in a habitat., as well as all the physical aspects of that habitat). After we finish population ecology, we'll move on to ecosystems ecology.

15.1 Study Guide

• 1Population size - Smaller populations are much more likely to become extinct.
  a. Smaller populations may be wiped out by natural events (fire, flood, etc).
  b. Smaller populations are often subject to inbreeding, which means individuals mating with their relatives. Inbreeding produces a population in which the individuals are all very similar genetically. This can lead to harmful traits, as recessive traits are passed down from both parents to their offspring.
• 2Population Density - This is the number of individuals of a particular species that live in one area. A dense population would have many individuals of that species living in an area, whereas a much less dense population might have only a few individuals living in the same size area. If there are not many individuals living in a particular area, reproduction is less likely to occur.
• 3 Dispersion - is the way that the individuals are arranged or spaced within their environment.
  a. Randomly (no specific pattern)
  b. Evenly (at regular intervals)
  c. Clumped (bunched together in several groups, like a herd here, and another herd over there.)

15.2 Study Guide

• 1mutation
• 2genetic drift (change in the alleles of a population due to random occurrences, such as fire or landslide. Genetic drift usually affects small populations more than it affects larger ones. If the random occurrence wipes out a large number of a small population, some alleles may be wiped out of that population, which means that future offspring will be more similar to one another.)
• 3gene flow (movement of individuals into our out of a populations. If individuals come into a population, they bring new genes with them. Likewise, if individuals leave, they take their genes with them.)
• 4nonrandom mating (inbreeding, mating with others similar to you, selecting mates based on certain qualities, etc.)
• 5natural selection (If certain alleles create individuals that are more likely to survive and reproduce, those alleles are likely to increase in that population)
  
  a. Natural selection works on phenotypes (the physical expression of a trait) rather on genotypes (the specific alleles that are present). So, because there are considerably more individuals carrying a single copy of a recessive gene (and thus not exhibiting that trait) than individuals who carry 2 copies of the gene (and therefore do exhibit the trait), disease causing alleles continue to be passed down from one generation to another, and are thus not eliminated from the gene pool.
  
  b. Natural selection can affect regular traits as well as polygenic traits (traits influenced by many genes - such as height or skin color in humans). For polygenic traits, natural selection can influence those genes that affect the trait the most to a greater degree.)

• 1Disruptive Selection - This is caused by pressure in the middle, which sometimes divides the population in two. A disruptive selection graph has two hills with a valley in between, a little like a very loosely drawn m.
• 2Stabilizing Selection - Stabilizing selection increases the similarity of organisms in that population. Mr. Anderson uses the example of birth size. Babies who are born too small may not survive, and babies that are too large may have trouble being born, so the vast majority of babies are in the middle range. In stabilizing selection, the "hill" on the graph is much steeper than it is on a normal distribution graph. There are less extremes, and more in the middle.
• 3Directional Selection - In directional selection, the "average" or "norm" shifts a little one way or the other. In the video below, Mr. Anderson gives the example of a population of birds that shifted towards having larger beak sizes. It's like taking the hill shaped curve and scooting the whole thing over a little.
  
  

  

  
  
  Clip art credit: Wikipedia