Chapter 23 (Evolution of Populations) Concepts Key
1.) What are the 3 mechanism for microevolution? Briefly describe
what they are.
Three mechanisms for microevolution (which is a change in allele
frequency in a population over generations) are 1.) Natural
selection. 2.) Genetic drift and 3.) Gene flow. In natural selection,
due to genetic variation in the population, certain individuals will
have inherited traits that contribute to their increased survival
and reproduction. By reproducing more, these alleles are
preferentially passed on more to the next generation. In genetic
drift, chance events can alter allele frequency. The chance part
implies that there is no particular advantage for one allele
persisting and another not, outside factors that are not specifically
selecting for one allele over another affect allele frequency. In
gene flow, alleles are passed between populations which can then
alter the frequency of existing alleles. This transfer generally
decreases the general genetic variability between populations.
2.) What are the 2 types of genetic variation? What mechanisms
account for genetic variation in sexual reproduction?
Genetic variability can be measured both at the whole gene level
(gene variability) or on a more precise scale of nucleotide
variability. For gene variability, the important measure is how
many individuals in a population are heterozygotes which allows
for the possibility of the passing of different alleles to offspring.
Homozygous individuals only have one version of the gene
therefore, their offspring have a lower genetic variability
possibility. For nucleotide variability, the comparison is how
different the actual DNA nucleotide sequence may be between
individuals in a population. This variability might not show up in
actual phenotypes due to redundancy in the genetic code or if the
nucleotide differences occur in non-coding areas of the genome.
Mutations can introduce genetic variability but in sexually
reproducing organism that form their gametes through meiosis,
there are 3 methods which can increase genetic variation: crossing
over (exchange of genes between non-sister chromatids of
homologous pairs), independent assortment of pairs of
homologues leading to novel combinations of overall
chromosomes inherited from parents and finally random
fertilization which combines the genetic material from a male and
female into a new offspring.
3.) Define a cline and how geographic variation occurs alone the cline.
When populations are separated by geographic features such as
islands or canyons, their genetic makeups can start to diverge.
Sometimes, the divergence is a more gradual process as natural
selection acts on different zone within the geographic area. A cline
is a graded change in a character along this geographic axis. For
example, from the base of a mountain to the top, there is a slow
decrease in average temperatures and while there may not be a
complete barrier to individuals that live along the slope of the
mountain, they will have different adaptive pressures put on them
living in different climates which can manifest itself in genetic
variation along the mountain slope.
4.) How can you use the Hardy-Weinberg equation to determine allele
frequencies in a gene pool?
The Hardy-Weinberg equation is p2 + 2pq + q2 = 1 where p and q
represent the frequencies of the only 2 possible alleles in a
population at a particular locus. You can use this equation to
determine the allele frequencies in a population based on
observed phenotypes for the gene involved.
5.) What are the 5 conditions necessary in a population which uphold
Hardy-Weinberg equilibrium?
The Hardy-Weinberg theorem assumes that a population is NOT
evolving which implies there are no mutations in the genomes of
the population, only random mating occurs (therefore no mate
selection), no natural selection, that the population is very large to
avoid chance events from having any influence and that there is no
gene flow in and out of the populations. In reality, it would be
virtually impossible for all these conditions to be met so we can
use the Hardy Weinberg allele frequencies to compare populations
over time and see how those frequencies have changed in
populations which is evidence for evolution.
6.) How does the founder effect and genetic bottlenecks affect genetic
drift?
Genetic drift occurs because random events (and not natural
selection) may influence the allele frequencies in population. The
founder effect is one example of genetic drift in which a few
individuals in a population may become isolated from the larger
gene pool. By chance, only the alleles present in those founder
individuals will be present in the new, smaller gene pool. If no
gene flow occurs between the founder population and larger,
parent population, the genetic differences will continue between
the 2 populations.
A similar effect is seen with a genetic bottleneck in which some
chance event eliminates many individuals in a population in a
given area and the surviving individuals will possess only their
alleles which will be generally lower in variability compared to the
original, larger population.
7.) How does gene flow effect genetic variation in a gene pool?
Gene flow is the movement of alleles among populations.
Sometimes alleles are moved by whole animal migration such as
what frequently occurs in some animal populations when
members who reach sexual maturity leave their natal group and
move to another geographic area, bringing their alleles with them.
Sometimes it is only the gametes that disperse such as pollen
spreading on the wind or by pollinators to new areas. In either
case, sometimes the addition of new alleles will give a potential
advantage to the new population and therefore natural selection
will increase the frequency of that allele over time in the new
population. Overall however, gene flow tends to reduce the overall
amount of genetic variability between populations if they are
allowed genetic exchange.
8.) Distinguish between intrasexual and intersexual selection. How
does sexual dimorphism occur in sexual selection?
Intrasexual selection is competition within the same gender for
mates of the opposite sex. In most species, it is the males who
compete but it can be females. Many male have elaborate bluffing
challenges and sometimes outright physical confrontations to gain
the right to mate with a female or females in the area. Intersexual
selection or mate choice is when individuals, usually females, are
choosy in selecting their mates. Mating is not random and females
choose some males and reject others so not all males have an
opportunity to mate. Some males have developed complex mating
rituals to show potential mates their relative fitness to mate with
the implication that they have the strength and vigor to
demonstrate these displays and therefore have overall superior
fitness.