Question
Two groups of genetically identical plants were grown in two different
environments. At 12 weeks, the average height of one group was larger
than the other. Which of the following statements most accurately
describes the statistically significant difference in plant height between
the two groups?
(A) A \(σ^2\) value of 3.84 means the null hypothesis is correct and the
apparent difference in height between the two groups of plants
was a result of chance.
(B) A \(σ^2\) value of 3.84 means that there is a 95% probability that the
difference in height observed between the two groups is due to
their different environments.
(C) A \(σ^2\) value of 5.99 means that there is a 5% probability that the
difference in height observed between the two groups is due to
chance.
(D) A \(σ^2\) value of 6.64 means that there is a 99% chance that the null
hypothesis is incorrect.
▶️Answer/Explanation
Ans: B
The null hypothesis assumes that the observed difference from
the expected is due to chance, not the different environments. The null
hypothesis is assumed to be true unless there is evidence to indicate
otherwise. The number of degrees of freedom is the total number of “options”
minus one. If there were 2 possible phenotypes, there is one degree of
freedom. You always subtract one from your range of options because there
is an option that is selected, and so that is removed from the pool of options.
If there were four possible phenotypes, there would be three degrees of
freedom because the organism would be one of the four, leaving three
“options” behind.
In this case there is one degree of freedom, so a σ2
value of 3.84 means
that there is a 5% probability that the observed difference in height is due to
chance. It also means there is a 95% probability that the difference in height
is due to the environment.
Question
Refer to an experiment performed by Resnick and
colleagues in Trinidad (1990–2002). Species of cichlid, Crenicichla, prey
on large, sexually mature guppies in some streams. In other streams and
above waterfalls, there are no Crenicichla and predation on the guppies is
much lower or absent.
Which of the following is an acceptable method to estimate
reproductive effort (reproductive investment)?
(A) weight of embryos relative to the weight of the mother
(B) weight of embryos relative to the weight of the father
(C) the average weight of an individual embryo relative to the weight
of the mother
(D) the total number of embryos that live to sexual maturity
▶️Answer/Explanation
Ans: A
Reproductive effort is the proportion of energy and other
resources that an organism allocates to reproduction as opposed to its
own growth and maintenance. In the case of the guppies, the reproductive
effort is measured by comparing the relative weights of the embryos to their
mother. By itself, the embryo weight is not a true indicator of the effort the
mother puts into creating the embryo. For example, a mother of low mass
would put a larger percent of total resources than a higher mass mother into
producing an embryo of the same mass. Maximizing resources is a trait
that will typically be selected for, so a lower reproductive effort for the
same quality of offspring would be selected for.
Question
Refer to an experiment performed by Resnick and
colleagues in Trinidad (1990–2002). Species of cichlid, Crenicichla, prey
on large, sexually mature guppies in some streams. In other streams and
above waterfalls, there are no Crenicichla and predation on the guppies is
much lower or absent.
Which of the following is a reasonable prediction?
(A) Predation by Crenicichla should select for an overall increase in
body mass of the guppies.
(B) Predation by Crenicichla should favor the evolution of early
sexual maturation and reproduction.
(C) Lack of predation by Crenicichla should favor the production of
many, smaller offspring earlier in life.
(D) Lack of predation by Crenicichla should select for guppies that
can choose when to reproduce based on environmental
conditions.
▶️Answer/Explanation
Ans: B
Guppies from the streams where Crenicichla live are smaller,
mature faster, and reproduce more frequently than those that live in streams
where Crenicichla are absent. This hastened development and reproduction
result in a higher reproductive effort for the guppies that live in
Crenicichla-dominated streams but is selected for because those guppies
that have a longer development and reproductive schedule are less likely to
live long enough to reproduce.
In a population where the predator is present, the “cost” of living
long enough to delay reproduction (and the risk that it requires) does
not offset the benefit of delayed reproduction (lower reproductive
effort). In Crenicichla-dominated streams, guppies that reproduced at a
younger age clearly left behind more offspring than guppies that delayed
reproduction, as demonstrated by the decreased size and increased rate of
maturation in later generations.
Question
Refer to an experiment in which guppies were relocated from
their home stream to a new, nearby stream. Their home stream contained a
predator, Crenicichla, which feeds on large, sexually mature guppies. The
new stream did not contain Crenicichla. After several generations,
researchers took guppies from both the home stream and the new stream
and reared their offspring in a laboratory.
Which of the following would be expected from the populations
relieved of predation on large adults?
(A) They acclimated to the laboratory environment by delaying
maturation but tended to have a greater number of smaller
offspring.
(B) They acclimated to the laboratory environment by reproducing
earlier and at a smaller body size, and their reproduction became
semelparous.
(C) They evolved delayed maturation at larger adult sizes and tended
to have fewer, larger offspring and lower reproductive effort.
(D) They evolved earlier maturation at smaller sizes and tended to
have a larger number of smaller offspring with a greater
reproductive effort.
▶️Answer/Explanation
Ans: C
Selection favors smaller size and early maturation and reproduction
in the guppies preyed upon by Crenicichla. However, this requires a larger
reproductive effort from the guppies. Reproductive effort is the proportion
of energy and other resources that an organism allocates to reproduction as
opposed to its own growth and maintenance. Because natural selection
favors a lower reproductive effort, the removal of the predation is
expected to delay maturity and reproduction, which results in a larger
size and lower reproductive effort, which is what was observed in the
experiment. This is an example of evolution occurring on a time scale that
could be observed by humans.
Question
The amount of genetic variation within populations of hunter-gatherers
in southern Africa is much greater than in non-African populations.
The genetic divergence among these African populations as compared
to non-African populations suggests that:
(A) the environment in Africa has not changed much in the past
200,000 years.
(B) populations in southern Africa have been subjected to more
inbreeding.
(C) the members of the population that initially left Africa were all
related by a common ancestor not more that 3 or 4 generations
prior to their departure.
(D) the populations that colonized the rest of Earth carried the genetic
information from their populations of origin almost intact and few
genetic differences have occurred since then.
▶️Answer/Explanation
Ans: D
Genetic similarities and differences among human populations have
been used to trace the dispersal of humans from Africa throughout the rest
of the world. The amount of variation within human populations is
lower the farther they are from Africa, as would be expected from
successive colonizations.
The amount of DNA sequence variation within human populations
decreases with increasing distance from Africa. Fossils of modern humans
date back 195,000–170,000 years ago in Africa. Fossils of modern humans
outside of Africa are not found until 80,000 years ago. The amount of
genetic variation within populations of hunter-gatherers south of the
Saharan desert, particularly in southern Africa, is much greater than
in non-African populations. This is expected if colonizing populations
carried only a sample of genes from the ancestral population (similar to the founder’s effect). The genetic divergence of some of these African
populations suggests they have been diverging for up to 150,000 years.
Genetic difference among non-African populations are mostly quite small.