BIOL 2400 Study Guide - Final Guide: Burgess Shale, Single-Nucleotide Polymorphism, Allele Frequency

Excerpts from “On the Tendency of Varieties to Depart Indefinitely from the Original Type,” by Alfred Russel Wallace [Read July 1st, 1858].

The life of wild animals is a struggle for existence. The full exertion of all their
faculties and all their energies is required to preserve their own existence and
provide for that of their infant offspring. The possibility of procuring food during
the least favourable seasons, and of escaping the attacks of their most dangerous
enemies, are the primary conditions which determine the existence both of
individuals and of entire species. These conditions will also determine the population of a species; and by a careful consideration of all the circumstances we may be enabled to comprehend, and in some degree to explain, what at first sight appears so inexplicable—the excessive abundance of some species, while others closely allied to them are very rare. ...

... It is also evident that most changes would affect, either favourably or adversely, the powers of prolonging existence. An antelope with shorter or weaker legs must necessarily suffer more from the attacks of the feline carnivora; the passenger pigeon with less powerful wings would sooner or later be affected in its powers of procuring a regular
supply of food; and in both cases the result must necessarily be a diminution of the population of the modified species. If, on the other hand, any species should produce a variety having slightly increased powers of preserving existence that variety must inevitably in time acquire a superiority in numbers. ...

Complete the chart below for the excerpt from Wallace’s paper. Provide examples of evolution by natural selection that Wallace used in his writing. Describe the species, the variations among individuals, the selective pressure, and how species change over time.

2 examples must be provided for each question and species.

Wallace —Natural Selection

Species:

What trait varies within the species?

What is the selective pressure? (In other words, what causes some individuals to produce more offspring than others?)

How would the population change over time?

Excerpt from “On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection,” by Charles Darwin [Read July 1st, 1858].

To give an imaginary example from changes in progress on an
island:—let the organisation of a canine animal which preyed
chiefly on rabbits, but sometimes on hares, become slightly plastic;
let these same changes cause the number of rabbits very slowly to
decrease, and the number of hares to increase; the effect of this
would be that the fox or dog would be driven to try to catch more
hares: his organisation, however, being slightly plastic, those
individuals with the lightest forms, longest limbs, and best eyesight,
let the difference be ever so small, would be slightly favoured, and
would tend to live longer, and to survive during that time of the year
when food was scarcest; they would also rear more young, which
would tend to inherit these slight peculiarities. The less fleet ones
would be rigidly destroyed. I can see no more reason to doubt that
these causes in a thousand generations would produce a marked
effect, and adapt the form of the fox or dog to the catching of hares
instead of rabbits, ... So would it be with plants under similar
circumstances. If the number of individuals of a species with plumed
seeds could be increased by greater powers of dissemination within
its own area (that is, if the check to increase fell chiefly on the seeds), those seeds which were provided with ever so little more down, would in the long run be most disseminated; hence a greater number of seeds thus formed would germinate, and would tend to produce plants inheriting the slightly better-adapted down.

Questions: 2 examples must be provided for each question

Darwin—Natural Selection

Species:

What trait varies within the species?

What is the selective pressure? (In other words, what causes some individuals to produce more offspring than others?)

How would the population change over time?