BIO185

DAY 2

MICROEVOLUTION MECHANISMS



If a population lacks variation, mutations can help add variance, but they take a long time and occur less often.



Genetic Mutation: 

  • Mutations introduce variation into a population

  • The only source of brand-new alleles

  • Not all mutations are bad

  • Individual mutations occur so rarely that mutation alone usually does not change allele frequency much.



Genetic Drift —> Allele Fixation

  • Smaller populations have the most fluctuation (genetic drift), and larger populations have less fluctuation (genetic drift)

  • Genetic Drift is a change in the allele frequencies in a population due to chance.

  • Genetic drift is most pronounced in small populations

  • In a large population, many more generations are required before the allele is eliminated or fixed

  • Sampling size affects sampling error. -


  • For example, when flipping a coin 10 times, you expect 5 heads/5 tails. Since the sample size is too small, the chance of it is lower.



Bottleneck: Severely reduces population size and genetic diversity.

  • For example, cheetahs have <1% genetic variation

  • Northern elephant seals have fixed alleles at 24 loci



Founder Effect:

Small Population of Colonizers —-> Genetic Drift



Gene Flow

  • Movement of alleles among populations

  • Aleles can be transferred through the movement of fertile individuals or gametes (pollen, migration, seed dispersal) 

  • Low Gene flow leads to evolution. 

  • For example, the Disruption of gene flow in Death Valley has resulted in many species of pupfish, each endemic to one pond.



Darwin's Clues

  • The first person to come up with a mechanism to explain how evolution happened (not come up with evolution)

  • Sedimentation suggests that the earth is very old

  • Economic theory: Food and resources limit the human population

  • Plants and animals are artificially selected to reproduce based on desired characteristics 



Darwin's Inferences:

  • Inference 1: Not all offspring that are produced survive and reproduce because of a struggle for resources

  • Inference 2: Some individuals are more likely to survive and reproduce than others because of their heritable traits.

  • Inference 3: Differences in survival and reproduction among individuals are non-random, with some traits being passed on at a higher rate than others and increasing in proportion in the population from one generation to the next. 



Natural Selection (NS) Requirements

  1. Variation 

  2. Heritable 

  3. Effect of trait on survival and reproduction 




  • An organism’s genetic contribution to the next generation

  • Organisms with favorable traits survive longer and reproduce more

  • Indicates the relative reproductive success of an organism in passing its genes to the next generation 



NATURAL SELECTION = ADAPTATION 



Adaptation

  • Morphology, physiology, or behavioral trait that gives organisms an advantage to cope with the environment 

  • - Natural selection produces individuals best adapted to survive in a particular environment



Biodiversity

  • Life is diverse because species adapt in different ways 

  • But they maintain some similarities because they possess the same genes from common ancestors