### force | drift:

Genetic drift
* acts faster and has more drastic results in smaller populations.
* affects smaller populations more than it affects larger populations.
* becomes very important in small populations.
* can also cause increases in alleles that lead to low fitness, just by chance
  - population changes, especially in small populations
  - result in the loss of genes from a population
* can be a real problem for threatened and endangered species
  - cause big losses of genetic variation for small populations
  - lead to populations with reduced genetic variation
  - then lead to an accumulation of late-onset harmful genes that cause senescence
* cause of allelic frequency change within populations of a species.
* causes alleles to disappear from populations thereby reducing genetic variation
  - loss of favorable alleles
  - random changes in allele frequencies in small populations
* change in allele frequencies in a population due to random events.
* involves random changes in allele frequency caused entirely by chance
* is another way that scientists define evolution
  - caused by random sampling of alleles
  - due to a phenomenon called sampling error
* is loss of an allele from a population
  - variation due to a random chance event
* is most likely to occur in a. small populations
  - powerful as an evolutionary cause when operating on small populations
  - often almost negligible in a large population
  - one method by which variability occurs in populations
* is the change in gene frequencies due to chance
  - force of evolution that is most powerful when acting on very large populations
  - name given to random changes which remove variation from a population
* is the random change in allele frequency
  - process of alleles being passed from parents to offspring
  - sampling error of gametes
  - result of chance combinations of different characteristics
* leads to fixation of alleles or genotypes in populations
  - loss of genetic diversity more quickly in large than in small populations
* occurs in a population
  - finite populations and leads to homozygosity in multiple-alleled traits
  - most rapidly in small populations
  - under two special conditions
* random change in the allele frequency of a population
  - walk
* refers to changes in gene frequency that occur by chance alone
  - the composistion of the gene pool due to chance
* tends to get rid of genetic variation.
* very significant factor in changing gene frequencies at small Ne.
+ Evolution, Darwin's theory, Genetic drift and its effect: History of science
  - Modern evolutionary synthesis

Glacial drift
* covers bedrock, which is only locally exposed.
* is discontinuous near the shorelines of Lake Michigan and Lake Huron
  - eroded rock from from one area deposited in other places
  - generally several hundred feet thick
  - material transported by glaciers and deposited directly on the land
  - the loose and unsorted rock debris distributed by glaciers and glacial meltwaters
  - underlain by Precambrian bedrock in the east

Pesticide drift
* can be a serious problem since airblast sprayers are broadcast sprayers.
* hazard to local communities and bodies of water.
* is thought to affect ant populations in adjacent habitat.
* major concern of aerial applicators
  - land application of pesticides
* solvable problem.

Snow drift
* can close roads when large enough, but there are other hidden dangers in snow drifts.
* can cover signs and landmarks and block corners, crosswalks, and handicap access ramps
  - result in a rock outcrop looking different from one image to another

Drive force
* Most drive forces move air
  - combustion air
  - flue gases
* affect components.
* increase pressure.