Question

Nitrogen fixation is a key component of biogeochemical cycling. It also represents an important inter-species interaction, which is:

	A.	Commensalism between bacteria and animals
	B.	Competition between plants and fungi
	C.	Predation of bacteria by plants
	D.	Mutualism between plant and bacteria

In 1983, a population of dark-eyed junco birds became established on the campus of the University of California, San Diego (UCSD), which is located many miles from the junco's normal habitat in the mixed-coniferous temperate forests in the mountains. Juncos have white outer tail feathers that the males display during aggressive interactions and during courtship displays. Males with more white in their tail are more likely to win aggressive interactions, and females prefer to mate with males with more white in their tails. Females have less white in their tails than do males, and display it less often. (Pamela J. Yeh. 2004. Rapid evolution of a sexually selected trait following population establishment in a novel habitat. Evolution58[1]:166-74.)

The UCSD campus male junco population tails were, on average, 36% white, whereas the tails of males from nearby mountain populations averaged 40-45% white. If this observed trait difference were due to a difference in the original colonizing population, it would most likely be due to ________.

	A.	mutations in the UCSD population
	B.	gene flow between populations
	C.	a genetic bottleneck
	D.	a founder effect

1. Why does the fitness of a phenotype depend on frequency-dependent selection?

   	A.	Because selection favors the least common phenotype.
   	B.	Because the least number of alleles are at that locus.
   	C.	Because sexual selection produces dimorphism.
   	D.	Because frequency-dependent selection acts against extreme phenotypes.

Answer 1

Sol. 1

Symbiotic nitrogen fixation represents a mutualistic type of relationship between plant and bacteria in which plants provide a niche and fixed carbon to bacteria in exchange for fixed nitrogen.

So, nitrogen fixation is a key component of biogeochemical cycling. It also represents an important inter species interaction, which is "mutualism between plant and bacteria".

Hence, option (D) is the correct answer for this question.