Introductory Biology

13.3 Examples of Evolution

Variation is found in every sexually reproducing population. The environment is an obstacle to an successful reproduction but individuals often produce more offsprings than their environment can support. Individuals who are suited to the environment tend to leave more offsprings than the individuals that are less suited to the environment. **Evolution is the change in the gene frequency over time. Variation occurs because of sex and mutations as well. Meiosis, crossing over, and independent assortment produce unique gametes. The gene pool is made up of all the individuals and tend to over reproduce processing natural selection. Those who are successful to reproducing will pass them on to their offsprings. (Evolution occurs as the frequency of alleles change over time: E= af/t) The industrial melanism: industrial change can cause the environment to alter, an advantage for some individuals. Moths cannot simply change its color because individuals do not evolve. The black moths had a disadvantage in the early years because they were easily preyed on. In the 19th century with all the factories, the trees changed into a darker color with the soot which was an advantage for those black moths.

Bacteria is a great way to see evolution because they reproduce every 20 minutes. THEY ARE EVOLVING BEFORE WE ARE. An example of natural selection is tuberculosis: a disease caused by Mycobacterium tuberculosis bacteria. Bacteria interacts with your body, especially with the white blood cells and leaves tubercles (bloody accumulations in the lung).  The bacterias are attacked by white blood cells, the bacterias fight back and produce a tubercle, which usually breaks down (though it does not and keeps reproducing causing tuberculosis to occur). Just by breathing the same air someone with tuberculosis coughs into, can infect you. Some effects are: fevers, coughs, and lose weight. Millions of lives have been saved with 2 antibiotics: isoiazid and rifampin. Strains of Mycobacterium tuberculosis in the 1980’s appeared to be completely resistant to isoiazid and rifampin (resistance occurs by a single base change from cytosine to thymine in a gene called rpoB). Penicillin was discovered by Alexander Fleming which was “borrowed” from Penicillium (mole). Penicillin make their cell walls so breakable, allow them to explode. Rifampin works by binding to Mycobacterium tuberculosis RNA polymerase,  preventing transcription and kills the bacterium. The mutation in polymerase’s rpoB gene prevents rifampin from binding (without destroying its ability to transcribe mRNA + produce proteins). While the rifampin killed the normal Mycobacterium tuberculosis, the mutated version survived and reproduced causing the bacterium’s growth to slow down.

Overuse of antibiotics is a huge problem worldwide. 3 bacterium is all you need to be infected. There are a lot of bacterium that have resistance and so it is important to take a full antibiotics (full 2 weeks). Usually white blood cells bind to the bacteria and kill them but others that fail to destroy the bacteria allow them to reproduce and produce a tubercle. Evolution in all our antibiotics, we’re are helping bacteria to reproduce. Eventually, this will cause a problem for us humans in the future. Each result will show increase in the resistant bacteria since we caused them to evolve and become stronger.

Darwin had collected 31 specimens of Galapagos finches (9 species) and suggested that all 9 species came from a similar ancestor. In 1938, David Lack researched the effect of variation in seed size on finch beack size but lacked evidence of competition. Peter and Rosemary Grant have been studying the effect of seed size on beak size since 1973.

  • Wet years: beak size is not important, since plenty of food is available
  • Dry years: fewer seeds are produced + easily eaten seeds are limited, leaving only harder seeds.  So, birds with larger beaks survive and reproduce at higher rates
  • Several dry years (in succession): the average beak size increases
  • Several wet years (in succession): the average beak size decreases
  • Overall,  beak size changes by natural selection according to the environmental change

Divergence is the gathering of diferences between groups of related populations. Natural selection continuously improves the traits of a species that fit to the environment. The traits that increase organism’s chances of survival and reproduction are selected. If two populations become isolated, they will diverge/separate. They may be separated if animals wind up on different islands or  by mountains, ponds/rivers, forests, new highways that separate the two, and etc. DIfference will start to appear as the two separated groups of the same species adapt to its new environment. Subspecies are formed when two or more populations of the same species differ in recognizable traits. The genetic differences between subspecies may become so different that they can not interbreed with each other, which makes them separate species. Reproductive isolation is the inability of formerly interbreeding groups to mate or produce fertile offspring. The branches of ancestry never grows back together, they always separate. Factors of reproductive isolating include:

  • mating at different times
  • evolved to different environments
  • having different mating behaviors
  • having different physical structures for mating
  • infertility of offsprings
  • reduced fitness of offspring
Biologists often document the stages of speciation. 1991, Jonathan Losos studied populations of lizards descended from a single population, but now living on sepearate islands. He measured hind-limb length of lizards from separate islands and the average perch diameter of island plants. He noticed that as perch diameter increased, so did hind-limb length. In short, as the populations became geographically isolated in different environments (with different plant perch diameters), it produced different ecological races that differ genetically in their hind-limb. Gradualism was predicted by Darwin that the change will be gradual. Punctuated equilibrium is a rapid change. (Equilibrium is very little change over time.) Factors in Natural Selection which apply to all real populations include: 
  • All populations have genetic variation
  • the environment presents challenges to successful reproduction
  • Individuals tend to produce more offspring than the environment can support.
  • Individuals that are better suited to the environment tend to leave more offspring than those that are less suited to the environment do.
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One thought on “13.3 Examples of Evolution

  1. Yurika,

    Great blog for 13.3 on examples of evolution. You’ve described all the important ideas in detail. This should be good practice for the test on 5/6. Good luck!!

    Mr. F.

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