Assignment #2a: The Life Table

1. Build a hypothetical life table and
2. Understand how modifications to the life table can generate different fitness outcomes.

I) Start by putting together a model life table.

Open the 'Examples' workbook from your diskette. Go to the 'Life Table' worksheet.

You see an Abridged Life Table with six variables: Age Category, Death Rate, Survival Rate, Survivorship, Fertility, and Reproductive Value. The only variables you will modify in this exercise are Death Rate and Fertility. In terms of fitness, only Mortality and Fertility really matter.

Enter the Mortality Rates and Fertility Rates according to the outline below.



Survival rate is the chance of dying during an age interval. Calculate survival rate as 1 minus the Mortality Rate. Enter this calculation into the Survival Rate column.

Survivorship is the chance of surviving from birth until the BEGINNING of the age category in question. Set Survivorship to the first age class equal to 1. This means everyone survives to birth.

The Survivorship for all subsequent age classes equals the chance of surviving during the previous age class multiplied by the survivorship to the previous age class. This may seem strange. For example, the chance of surviving to your 30th birthday equals the chance of surviving to your 20th birthday multiplied by the chance of your surviving the age interval of 20 to 29 years.

As a check, make sure Survivorship gets smaller as age class increases. Logically, you can't have a greater chance of surviving from birth to 30 than of surviving from birth to 20.

Now calculate Reproductive Value. Recall that Reproductive Value represents the number of babies that an average person will have in a lifetime. This equals the product of survivorship and fertility summed over all ages.

Place your pointer in the appropriate cell (E17). Click on the Function Wizard. Choose the Math & Trig function category in the left box. Scroll down the list of functions in the right box. Choose the SUMPRODUCT function. Click on the NEXT button.

The SUMPRODUCT dialogue box appears with a set of entry fields listed ARRAY 1, ARRAY 2, etc. Make sure you can see your life table. This may require moving the dialogue box around.

Block the Survivorship data (starting at the top of the column). The first and last cell, separated by a colon, will appear under the ARRAY 1 entry field. Click on the ARRAY 2 entry field. Block of the Fertility data. Click the finish button.

Modifying the Life Table:

Now you will examine the ramifications of various changes to the life table. Try and think of the life table as a description of an organism. You can also think of it as a life course of, for example, humans. Changing the Fertility and Mortality levels will describe different kinds of organisms or life courses. In the next class you will see what happens when various life courses compete against one another in a mixed population.

Start by copying and pasting your database onto another area of the worksheet. This will allow you to make any corrections by referring back to the original life table.

Now go to the "Assignment #2a Answer Sheet" tab in the Examples workbook. Answer Question 1. Return to the "LifeTable" tab.

Set all of the fertility levels to zero. Not surprisingly, when fertility is zero throughout life, reproductive value will also be zero.

Starting with the youngest age class, set the fertility level to 5. Record the reproductive value in the appropriate location on the "Assignment #2a Answer Sheet". Set the youngest age class fertility back to zero and then change the next age class to 5. Again, record the reproductive value. Set the next youngest age class fertility back to zero.

Continue setting the next age class fertility to 5, recording the reproductive value, and resetting to zero until all age classes are done. Then reset the fertility levels to their original values.

Change all of the mortality levels to 0.1 except for that of the very last age class. Leave that at 1 (everyone dies by their 100th year of life).

In sequence, as you did with fertility, change each mortality level to 0.3, record the reproductive value, and then set it back to 0.1.

Now go to the "Assign #2a Answer Sheet" and answer Questions 3 and 4. Print the answer sheet when you are finished.