10.3.3 The Perfect Organism

Question 1

There are four reasons natural selection can’t produce the perfect organism:

Answer 1

1. Evolution is based on history.
2. New organisms are compromised in terms of their phenotype.
3. Not all evolution is adaptive.
4. Selection can only act upon existing alleles.

Question 2

note

Answer 2

• The process of evolution by natural selection results in
  organisms that are best suited for their environment at that particular time. But there are four reasons why natural selection can’t produce the perfect organism.

Question 3

1. Evolution is based on history

Answer 3

• Darwin proposed that evolution occurred by descent with modification. In other words, there will be modifications to existing complex structures, enabling the organism to be better suited to its changing environment. These modifications are constrained because of the limitations imposed by the existing structure

Question 4

2. New organisms are compromises.

Answer 4

• Certain adaptations, especially anatomical structures, are mutually exclusive. For example, the wings of birds are adapted for flying but at a cost to other anatomical features, such as a loss in skeletal mass.

Question 5

3. Not all evolution is adaptive

Answer 5

• Genetic drift results from changes in the gene pool of a small population because of chance. Two examples of genetic drift are bottlenecks and the founder effect. Bottlenecks occur when a disaster, such as an earthquake or human impact, greatly reduces a population. The new population may have a high frequency of alleles that do not necessarily increase
  the chance of their reproductive success.

Question 6

4. Selection can act only upon existing alleles.

Answer 6

• In any given population, there will be variations on existing physical traits, such as the variation in color seen among the garter snakes on the left. The process of natural selection chooses from the available variations the one that is best suited for the current environment. An organism that is best suited for the current environment might not be best suited for future environments.
• The peppered moth exhibits two phenotypes for color, light color and dark color. Before the Industrial Revolution in England, natural selection favored light-colored moths because the bark of the trees where the moths alighted was light in color. Predators could not see the light-colored moths as well on the light-colored bark. But pollutants from industrialization eventually blackened the tree bark. Suddenly, natural selection instead favored the less-frequent dark-colored phenotype. White moths became much less frequent because predators easily found them against the dark
  background of the bark. When England attempted to clean up its air, the trees gradually lightened, and the pendulum for favored coloration swung back toward light-colored moths.

Question 7

Consider a variation on the story of the peppered moth in which populations of the moth in pre-industrial England are entirely made up of white moths (in fact the moth would likely be called the white moth in this instance, not the peppered moth). With the advent of industrialization and coverings of soot on the trees, no change is in fact seen in the “white moth” population except lower population sizes. This is because:

Answer 7

• of lack of genetic variation

Question 8

In birds, clutch size (the number of eggs in a brood) is selected to be larger because there are more offspring, however, as clutch size gets larger each offspring is born smaller, with fewer food reserves. This is an example of

Answer 8

• a compromise

Question 9

One of the reasons for there not being “perfect” organisms is that not all evolution is adaptive. What would be an example of non-adaptive evolution.

Answer 9

• change due to sexual selection

Question 10

Genetic variation is the result of what two processes?

Answer 10

• mutation and sexual recombination

Question 11

What is one of the limiting factors as to why there is no perfect organism?

Answer 11

• Selection only works with existing alleles.

Question 12

Why won’t natural selection ever produce the perfect organism?

Answer 12

• Selection can only act upon existing alleles.

- Organisms are compromises.