Question
Inflorescences (clusters of flowers) make up a lower proportion of plant biomass in perennial species (reproduce every year) as compared with annual species (reproduce only once). Explain this observation with reasoning that is consistent with evolutionary theory.
▶️Answer/Explanation
Ans:
Natural selection favors minimizing reproductive effort.
Reproductive effort is the proportion of energy and other resources
that an organism allocates to reproduction as opposed to its own
growth and maintenance. Reproductive effort is greater per year in annual
plant species because they have only one opportunity to reproduce. Annual
plants reproduce once and then die.
Perennial plants will reproduce many times over the course of a
lifetime, so each reproductive effort is typically lower than the one
reproductive effort made by the annual species. The cumulative cost of
reproduction in perennial plants increases with time and can exceed that of
an annual species over its lifetime. However, the perennial plant can
continue to acquire resources over its life span, whereas annual species
will typically have less time to obtain the energy to devote to
reproduction.
Question
Explain the mechanism(s) by which new alleles are created.
▶️Answer/Explanation
Ans:
New alleles are made from pre-existing alleles by mutation. The
duplicated gene may diverge in sequence, become a nonfunctional
pseudogene, undergo a deletion, or undergo gene convergence.
Gene duplication is a mutational event (strictly speaking, a mutation is
any change in the DNA sequence). There is nothing inherently positive or
negative about it, and there is no constraint on the size of the change. It
results in a single gene present in two or more loci (locations on a
chromosome). Over time, changes may occur in one or more of the
duplicates (as long as one copy of the gene remains intact, there should be
no harmful effect of the mutations in the duplicates).
If the duplicates are not transcribed and translated, they are called
pseudogenes. They do not code for RNA or proteins and do not perform
regulatory functions, so mutations are presumably selectively neutral. They
may persist as pseudogenes or may eventually provide the sequence
information for a new protein.
Gene duplications often arise from unequal crossing over. This
produces tandem duplication on one chromosome and a deletion on the
other.
Recombination (during crossing over) begins with precise alignment of
the sequences on homologous chromosomes. If the homologous sequences
differ by two or more base pairs, new sequences can be generated by
intragenic recombination. Intragenic recombination is recombination
within a gene. Just as crossing over creates new combinations of alleles on
chromosomes, new combinations of gene segments can arise through
intragenic recombination. Because a particular segment of a gene often
codes for the amino acid sequence of a specific domain of a protein, new
combinations of gene segments can theoretically produce new combinations
of protein domains, creating proteins that have new functions, new
combinations of functions, or the ability to recognize new substrates or
ligands.
Unequal crossing over is one of the processes that has generated the
enormous number of copies of nonfunctional sequences that constitutes
much of our DNA, as well as the DNA of most eukaryotes. It is
extremely important in the evolution of new functional genes and the total
increase in the amount of DNA in eukaryotes as compared with
prokaryotes.
Mutation rates vary among genes and between regions within genes.
Mutation rates are typically measured by the effect on phenotype (which
means many mutations will not be “registered”) and average about \(10^{−6}\) – \(10^{−5}\) mutations per gamete per generation. If measured by the average
mutation rate per base pair, one mutation is introduced in every \(10−^{10}\)
base pairs in prokaryotes and \(10^{−9}\) base pairs in eukaryotes (during meiosis).
Question
Evolution occurs by both natural selection and genetic drift. Compare and contrast these two mechanisms of evolution.
▶️Answer/Explanation
Ans:
Evolution can be defined as a change in allele frequencies over time.
• Genetic drift can change allele frequencies, though the changes
are not driven by “selective pressures” and are, therefore, not
adaptive. Genetic drift is not the main driving force of evolution,
but it has a significant effect nonetheless. (See answers 22, 23, 36,
and 51 for more information on genetic drift.)
• Natural selection changes allele frequencies because the organisms
that possess certain alleles (or allele combinations) have a
reproductive advantage over those that don’t. The reproductive
advantage results in a greater number of members in the following
generation that also possess the alleles (or allele combinations).