Evolution, 4th Edition

gENETIC dRIfT: EvolUTIoN AT RANdoM 167

1. Each generation starts with N juveniles. This population size is fixed by the
   limited space that is available.


2. Every individual survives to become an adult, so there is no selection on survival.


3. When adults reproduce, each individual makes 1000 sperm and 1000 eggs.
   (The actual number does not matter so long as it is reasonably large.) Because
   each individual makes the same number of gametes, there is no selection on
   reproductive success. After reproduction, the adults die.


4. Zygotes are formed by random fertilization between sperm and eggs in the
   pool of 2000N gametes. But since space is limited, only N randomly chosen
   zygotes survive to become juveniles. The life cycle then begins again, and the
   simulation returns to step 1.

What happens when we run the simulation? FIGURE 7.2 shows how allele fre-

quencies change over 500 generations in replicate populations that begin with an

allele frequency of p = 0.5. Frequencies fluctuate randomly. In each generation,

allele frequencies change by the random sampling of the genes that start the next

generation (step 4 of the simulation). This is the essence of genetic drift.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_07.02.ai Date 11-14-2016 01-18-17

0

0.2

0.4

0.6

0.8

1

100 200 300 400 500

N = 500,000

0

0.2

0.4

0.6

0.8

1

100 200 300 400 500

N = 50,000

0

0.2

0.4

0.6

0.8

1

100 200 300 400 500

N = 5000

0

0.2

0.4

0.6

0.8

1

100 200 300 400 500

N = 500

Allele frequency,

p

0

0.2

0.4

0.6

0.8

1

100 200 300 400 500

N = 50

0

0.2

0.4

0.6

0.8

1

100 200

Generation

300 400 500

N = 5

FIGURE 7.2 Computer simulations of drift.

In each graph, five identical populations

start with two alleles at equal frequencies

(p = 0.5). Allele frequencies change in later

generations by random genetic drift (no

selection is acting). Smaller population sizes

cause bigger random changes in allele

frequencies in each generation, and lead

to quicker fixation (p = 0 or p = 1) of one

allele or the other.

07_EVOL4E_CH07.indd 167 3/23/17 9:09 AM