3. (a) TT = 99% = 0.99
Tt = 1% = 0.01
TT = p2
p2 = 0.99
Therefore, p = 0.99
p = 0.995
p+q=1
q=1–p
q = 1 – 0.995
q = 0.005
T = 0.995, t = 0.005
(b)
T
(p = 0.995)
t
(q = 0.005)
T
t
(p = 0.995)
TT
2
p = 0.99
(q = 0.005)
Tt
pq = 0.005
Tt
pq = 0.005
tt
2
q =
0.000025
(lethal)
TT = 0.99
Tt = 0.005 + 0.005 = 0.01
tt = 0.000025 but these individuals do not survive as embryos.
(c) The calculation will be the same as (a). Therefore, T = 0.995, t = 0.005.
(d) They have no significant effect because, even though this combination is lethal and removes tt individuals, they are so
rare that their removal has little influence on the total number and proportion of t alleles.
(e) The trait would gradually reduce the number of lethal alleles in the population. However, once the allele becomes quite
rare, it would likely remain in the population in very low numbers.
12.3 RANDOM CHANGE
Try This Activity: Demonstrating Chance
(Page 552)
(a) Answers will vary. Students are unlikely to obtain the expected ratio. Students should be encouraged to share their results
with the class.
(b) Answers will vary. Students are unlikely to obtain the expected ratio. Students should be encouraged to share their results
with the class.
(c) The gene frequencies of the “dice rolls” will not closely match expected ratios. These results model the actual the changes
that might occur within a gene pool of a small founder population. The alleles in a large sample—modelled by many dice
rolls—would have frequencies close to the expected population frequencies. When a small sample is taken, modelled by a
few rolls, the sample frequencies are much more likely to differ from the expected values. For example, if a die is rolled
10 000 times, one would expect to get a “3” about one sixth of the time and the actual frequency will be quite close to this
value. If one rolls a die only 10 times the frequency may be much higher or lower because of the effects of random chance
and the low sample size. This is called the “founder effect” when this same effect is applied to population gene
frequencies.
Copyright © 2003 Nelson
Chapter 12 Mechanisms of Evolution 249
PRACTICE
(Page 554)
1.
5 000 000 bats × 2 gametes/offspring × 0.4 mutations per gamete
There would be 800 million mutations in 200 years.
× 200 years = 800 000 000
Section 12.2–12.3 Questions
(Page 555)
Understanding Concepts
1.
Evolution can be defined as a change in the genetic make up (gene or allele frequency) of a population over time.
Evolution can be quantified by measuring allele frequencies within a population. Changes in such frequencies are a
measure of the evolution of the population.
2. An allele may become fixed at 100% in a population when it is much more beneficial than other alleles of the same gene.
Individuals possessing the other allele(s) would be less successful in reproducing and therefore contribute fewer copies of
the poor allele(s) to the next generation. A second mechanism is genetic drift, in which random chance alone, typically in
small populations, results in the elimination of other alleles. Genetic drift may result from a founder effect or a bottleneck.
3. Genetic drift is a change in gene/allele frequencies resulting from random chance alone. Small populations resulting from
a founder effect or a bottleneck are highly susceptible to genetic drift. Small populations such as a decimated cricket frog
population or an overhunted elephant seal population are susceptible to genetic drift. Gene flow is the exchange of alleles
between two or more distinct populations through the movement of individuals or gametes. Examples include the
movement of young male prairie dogs from colony to colony during late summer, as well as the potential for movement of
pollen and seeds over very long distances in heavy winds.
4. Many plants with wind-borne seeds could produce founder populations as their seeds are carried to new locations.
Similarly birds may reach new territory, such as islands, much more readily than most species and establish new
populations. Other small organisms, such as insects that can be carried in the wind over long distances, might be able to
reach new habitats. The result is that, by chance alone, the few individuals that become the newly established population
will not have a set of gene frequencies similar to the original.
5. Individuals with harmful mutations have poor reproductive success and therefore pass on few copies of the harmful alleles
to the next generation. Over time these alleles do not accumulate and become common. They can therefore have little if
any effect on the evolution of the species.
6. Extra copies of fully functioning genes can influence evolution in the long run. If one of the copies mutates, the result
may be the production of a new and useful gene product. This new gene may not initially be useful, but it is free to mutate
and potentially gain a new function. The loss of its original function is not significant as the other copy of the gene would
still be fully functional.
7. (a) No. The plant breeder would likely select the individuals to cross-pollinate from generation to generation. This
artificial selection process would reduce the number of undesirable alleles and increase the frequency of favoured
alleles.
(b) Yes. Gene frequencies are likely to be maintained as the population size is enormous and the environment is relatively
stable.
(c) No. The population size would be relatively small and changes in allele frequencies would be expected due to genetic
drift.
(d) No. The population is probably relatively small and would be subject to genetic drift. Some gene frequencies may have
become fixed, however, and these would remain unchanged.
8. Pseudogenes are genes that have almost identical base pair sequences as fully functioning genes. The most logical
explanation for pseudogenes is that they result from duplication mutations followed by additional mutations that alter
their sequence and destroy their function.
9. Five million newborn bats per year × 2 gametes per offspring × 0.4 mutations per gamete × 200 years yields
800 million mutations. If only 1 in every three million mutations is beneficial, this still yields 267 beneficial mutations in
a modest bat population in a mere 200 years!
10. If a severe bottleneck had occurred, this would have produced a very small breeding population with limited genetic
heterozygosity. In such populations random chance resulting in genetic drift will frequently cause alleles to be eliminated
from the gene pool. The remaining alleles become fixed.
Applying Inquiry Skills
11. The immediate founder effect will be to create a new population of geese with a very small different mix of allele
frequencies from the original population as a whole. Over time there is a strong likelihood that many of the genes will be
250 Unit 4
Evolution
Copyright © 2003 Nelson
represented by a single fixed allele, thereby reducing the genetic diversity of the population and reducing their long-term
viability. There would likely be little or no effect on the original population, as removing only three individuals would not
alter the allele frequencies of a very large population, unless one or more of the individuals happened to be carrying a
very rare allele. Note—this is a hypothetical question. Canada geese would not normally stop migrating in response to an
abundant food supply.
12. Evolution would be limited to the various combinations of alleles within the population. Although this may provide great
diversity, it cannot provide a source of new genetic material that does not already exist within the population. As a result,
no novel traits could ever come into existence. Mutations alone account for all new genetic information.
13. This severe reduction in numbers may fragment the population into many small isolated populations, each experiencing a
bottleneck effect. This may lead to the loss of many alleles by genetic drift. A population with reduced genetic diversity is
not as likely to survive in a variable environment as a population with greater numbers and genetic diversity. The loss of
genetic diversity also dramatically reduces the evolutionary potential of a species.
Making Connections
14. Answers will vary. The founder effect is probably responsible for the virtually complete lack of blood group B in North
American Aboriginal Peoples whose ancestors arrived in small numbers across the Bering Strait about 10 000 years ago.
Other examples include the Dunkers and Amish of North America. They arrived as very small groups and have remained
nearly completely isolated. As a result, their blood group gene frequencies are quite distinct. Another example is that of
the Ashkenazim Jew population, which has an increased incidence of several recessive genetic diseases including TaySachs. The following Web page provides a rich discussion of the founder effect:
<http://www.talkorigins.org/faqs/genetic-drift.html>.
15. Evolution is defined as a change in allele frequencies in a population. Therefore, evolution occurs at the population level
and can only be quantified by examining the genetic make up of many individuals within the population. Simple
inheritance concerns the alleles of individuals—their genotypes and phenotypes—while evolution concerns the success of
such alleles among the entire population.
16.(a) The Vancouver Island Marmot population numbers about 100 individuals. Complex population dynamics related to
clear-cut logging practices seem to be the most recent cause of their decline. An excellent Web page for information on
this interesting species is <http://www.islandnet.com/marmot/homepage.html>.
(b) With such low numbers, the main emphasis is on increasing the population size to about 400 to 600 individuals by
using captive breeding programs. During these programs, attempts to maximize genetic diversity will be considered. In
a study by research scientist A. Bryant (1990) of the University of Calgary, he concluded, “long-term survival of M.
vancouverensis requires that additional (populations) be found or established, and that adequate gene flow between
individual colonies be maintained.” The recovery plan calls for the establishment of three populations in different
locations on Vancouver Island. The following report outline concerns over the genetic diversity of this species:
<http://www.marmots.org/subdir/abstract.html#Bryant%201990>.
12.4 PATTERNS OF SELECTION
Case Study: The Evolution of Antibiotic Resistance in Bacteria
(Page 562)
Understanding Concepts
1.
2.
3.
The evidence suggests that the resistance evolved within the patient. The resistant strain differed by only a single base
pair when compared with the patient’s original bacterial strain, yet the strain was not similar to 100 other strains of the
same bacteria from tuberculosis patients living in the city at the same time. This indicates that the strain evolved within
the patient’s lungs. None of the 100 strains looked at was like the rifampin-resistant strain. When rifampin was
administered, the bacteria were exposed to a new environmental selective agent, which gave the mutant strain an
advantage over the original strain.
These bacteria had never been in contact with the antibiotic before and so had not evolved resistance. There would have
been no selective advantage to possessing an unnecessary trait.
Most drug-resistant strains have one or more additional genes and produce one or more gene products when compared
with nonresistant forms. The maintenance and transcription of such genes has a “cost” to the bacteria. In the absence of an
antibiotic, these genes suddenly switch from being advantageous to adding a cost to their host bacteria. They are therefore
out-competed by the nonresistant strains that don’t waste time, energy, and nutrients replicating, maintaining, and
transcribing the added genes.
Copyright © 2003 Nelson
Chapter 12 Mechanisms of Evolution 251