Consider the case of a disease that can only spread during the proportion T of the year that temperature is above a threshold level, where 0 ≤T ≤1. During that proportion
of the year, infection occurs at a rate β, such that the average infection rate across the
full year is Tβ. Then there are two ways to “leave” the infected class. First, during
the infectious proportion of the year T, the infected individuals experience disease-
induced mortality at a rate μ. Second, during the non-infectious proportion of the
year 1 −T, infected individuals can recover at a rate γ. There is no immunity, so
recovered individuals become susceptible again. The disease dynamics are very rapid
in comparison to the host populations’ typical (uninfected) lifespan. The dynamics of
the susceptible S and infected I individuals are then:
dS/dt = −TβS (I/S + I) + (1 −T)γI
dI/dt = TβS (I/S + I) −TμI−(1 −T)γI= TβS (I/S + I)−(Tμ + (1 −T)γ) I

TβS (I/S + I) is Infection.
TμI is Mortality
−(1 −T)γI is Recovery

(a) What are the units of all components of this model: β, γ, μ, T, S, I, and
t? (If a value has no dimensions, say “none”.)

How are rocks that cooled within magma chambers found on earth’s surface today? they are pushed up by magma under earth’s surface. they are revealed by erosion of earth’s surface over time. they are pushed to earth’s surface because of their larger size. they are revealed at earth’s surface due to their low density.

Where do the respiratory and circulatory systems meet? a. in the heart b. in the muscles c. in the lungs d. in the brain

To answer this question, researchers studied populations of the dusky salamander (desmognathus ochrophaeus) living on different mountain ranges in the southern appalachian mountains. the researchers tested the reproductive isolation of pairs of salamander populations by leaving one male and one female together and later checking the females for the presence of sperm. four mating combinations were tested for each pair of populations (a and b)—two within the same population (female a with male a and female b with male b) and two between populations (female a with male b and female b with male a). the proportion of successful matings for each mating combination was measured. for example, when all the matings of a particular combination were successful, the researchers gave it a value of 1; when none of the matings were successful, they gave it a value of 0. then the researchers calculated an index of reproductive isolation that ranged from 0 (no isolation) to 2 (full isolation). the reproductive isolation value for two populations is the sum of the proportion of successful matings of each type within populations (aa + bb) minus the sum of the proportion of successful matings of each type between populations (ab + ba). the table provides data for the geographic distances and reproductive isolation values for 27 pairs of dusky salamander populations.