Forces of Microevolution Examples:
1. In the late eighteenth century, a severe storm devastated a small island and killed
a number of the island's inhabitants. Approximately 20 people survived. After
four generations, the people of the island began exhibiting symptoms of a rare
recessive vision disorder. Of the roughly few thousand people living on the island
today, almost 10% of them are affected by the disorder and about 30% are
carriers. All of these people are able to trace their ancestry to a single male
survivor who researchers believe was the carrier of the disease that emerged when
some of his descendants intermarried. What is this an example of? (Bottleneck
effect, type of genetic drift)
2. Rabbits are either black or white to blend with either black or white stones. Grey
rabbits don’t blend into any background and are easily eaten by predators. What is
this an example of? (Disruptive selection, one outcome of natural selection)
3. Orange throated (homozygous dominant), yellow-throated (homozygous
recessive), and blue-throated (heterozygous) lizards are all found in equal
numbers in the population because if the numbers of one color is elevated, another
color outcompetes them. What is this an example of? (Frequency-dependent
balancing selection; the p and q alleles would be 0.5 each in this scenario)
4. Human babies that are too small at birth are weak and often die. Human babies
that are too large cannot fit through the birth canal and mother/child die. What is
this an example of? (Stabilizing selection, one outcome of natural selection)
5. A small group of Amish people moved from Europe to Lancaster, Pennsylvania.
The allele frequency for a syndrome in which individuals have six fingers per
hand and heart defects was much higher after several generations in Pennsylvania
than it was in Europe. What is this an example of? (Founder effect, type of genetic
drift)
6. The environment in England changed during industrialization, and moths with
black coloration, not white or grey, were more reproductively successful. What is
this an example of? (Directional selection, one outcome of natural selection)
7. A population of flowering plants with yellow and white flowers is located several
miles from another population of these flowering plants. Small rodents in the
forest eat the fruits of these plants, carrying seeds from one plant population to the
other. What is this an example of? (Gene flow)
8. Within an African tribe on the grasslands, one individual does not produce
melanin like the other people and thus has white skin, white hair, and pink eyes.
This individual is partially blind and prone to cataracts and skin cancer. What
kind of allele is this? (Harmful mutation)
9. Birds of paradise with the most flamboyant display of feathers win the female
mate and reproduce. What is this an example of? (Sexual selection, one outcome
of natural selection)
10. Some plants naturally have an herbicide resistance allele, even in environments in
which there has never been herbicide applied. What kind of allele is this? (Neutral
mutation, until herbicide is applied advantageous)
11. An excerpt from the New York Times: Oct 7, 1994
“The cystic fibrosis gene may have survived through hundreds of human
generations because it gives protection against cholera. Studies at the
University of North Carolina School of Medicine at Chapel Hill have
found that laboratory mice carrying [heterozygous for] cystic fibrosis
genes did not suffer the deadly diarrhea that is typically caused by cholera.
This, said a researcher who conducted the study, Dr. Sherif E. Gabriel,
may explain why cystic fibrosis is one of the most common gene defects.
‘The frequency of the CF gene has caused people to speculate for many
years why it was at such a high level,’ Dr. Gabriel said. ‘When you have a
gene that codes for a fatal disorder, you expect the numbers in the
population to continually be decreasing. But this has not happened with
CF.’”
What is this an example of? (heterozygote advantage a form of balancing
selection)