Natural Selection and Speciation Flashcards

A population of finches on an isolated island primarily eats small, soft seeds. After a prolonged drought, the plants that produce these seeds die out, leaving only plants that produce large, hard-shelled seeds. Which of the following patterns of natural selection is most likely to occur in the finch population?

Answer: Directional selection

Directional selection occurs when environmental pressures favor one extreme phenotype over others. In this scenario, the change in food source from small, soft seeds to large, hard seeds favors finches with larger, stronger beaks. Over generations, the average beak size of the population is expected to shift in this direction.

Which of the following is an example of a postzygotic reproductive isolating mechanism?

Answer: The offspring of a horse and a donkey, a mule, is sterile.

Postzygotic isolating mechanisms act after a zygote (fertilized egg) is formed. Hybrid sterility, where the hybrid offspring (like a mule) is viable but cannot reproduce, is a classic example of a postzygotic barrier. The other options are prezygotic barriers, which prevent fertilization from occurring in the first place (temporal, behavioral, and gametic isolation, respectively).

The formation of the Grand Canyon separated a single population of squirrels into two isolated groups. Over thousands of years, these two groups evolved into distinct species, the Kaibab squirrel on the north rim and the Abert's squirrel on the south rim. This is an example of:

Answer: Allopatric speciation

Allopatric speciation occurs when a population is divided by a geographic barrier, leading to the evolution of reproductive isolation and the formation of new species. The Grand Canyon acted as a physical barrier that prevented gene flow between the squirrel populations, allowing them to diverge genetically over time.

In a particular species of snail, snails with very light-colored shells and snails with very dark-colored shells are both camouflaged effectively in their environment of light-colored rocks and dark shadows. However, snails with intermediate-colored shells are easily spotted by predators. This scenario is most likely to result in:

Answer: Disruptive selection

Disruptive (or diversifying) selection occurs when extreme phenotypes are favored over intermediate phenotypes. In this case, both the lightest and darkest snails have a survival advantage, while the intermediate-colored snails are selected against. This can lead to the population dividing into two distinct groups and can be a driver of speciation.

A small group of individuals colonizes a new island. The allele frequencies of this new population are, by chance, very different from the original population. This phenomenon is known as:

Answer: The founder effect

The founder effect is a form of genetic drift that occurs when a new population is established by a small number of individuals whose gene pool differs from that of the source population. This random sampling can lead to a new population with non-representative allele frequencies compared to the original population.

Sympatric speciation is more common in plants than in animals. A primary mechanism for this type of speciation in plants is:

Answer: Polyploidy

Sympatric speciation occurs without geographic separation. In plants, a common mechanism is polyploidy, which is an error in cell division that results in an organism having more than two complete sets of chromosomes. Polyploid individuals are often unable to breed with the original diploid population, creating instant reproductive isolation within the same geographic area.