92 The Founder Effect

Key Idea: The founder effect is the loss of genetic variation when a new colony is formed from a small number of individuals. The founder population may evolve differently to the parent population.

` Occasionally, a small number of individuals may become isolated from their original large population, e.g. by dispersal, chance events, or as a result of changing geography. The small colonising, or founder, population is unlikely to have a representative sample of the population's alleles and its genetic diversity will be reduced. ` As a consequence of this founder effect, the evolution of the colonising population is likely to differ from that of the parent population. The colonising population will be more susceptible to genetic drift and the selection pressures for the colonisers in the new environment may be very different to those experienced by the parent population. ` It may be possible for certain alleles to be missing altogether from the isolated population (the allele has been lost).

Mainland population Island population

Some individuals from the mainland population are carried at random to the offshore island by natural forces such as strong winds.

Founders can become isolated by migration, e.g. to an island, but also by geological events, such as the formation of mountains or straits.

This population may not have the same allele frequencies as the mainland population.

1. Compare the mainland population to the island population (use the spaces in the tables below): (a) Count the phenotype numbers for the two populations (i.e. the number of black and pale beetles). (b) Count the allele numbers for the two populations: the number of dominant alleles (A) and recessive alleles (a). Calculate these as a percentage of the total number of alleles for each population.

2. How are the allele frequencies of the two populations different?

3. Describe how organisms could become isolated from a parent population:

Allele A Mainland population

Allele frequencies Phenotype frequencies

Actual numbers Calculate % Black Pale

Allele A Colonising island population

Allele frequencies Phenotype frequencies

Actual numbers Calculate % Black Pale

Allele a Allele a

Total Total

The European garden snail (Cornu aspersum, formerly Helix aspersa) is widely distributed throughout the world, both naturally and by human introduction. However because of its relatively slow locomotion and need for moist environments it can be limited in its habitat and this can lead to regional variation. The study below illustrates an investigation carried out on two snail populations in the city of Bryan,Texas. The snail populations covered two adjacent city blocks surrounded by tarmac roads. The snails were found in several colonies in each block. Allele frequencies for the gene MDH-1 (alleles A and a) were obtained and compared. Statistical analysis of the allele frequencies of the two populations showed them to be significantly different (P << 0.05). Note: A Mann-Whitney U test was used in this instance. It is similar to a Student's t test, but does not assume a normal distribution of data (it is non-parametric).

Block A Block B

1

Source: Evolution, Vol 29, No. 3, 1975

15

Block A

Block B

3

2

7 4 5

6 14

13

8

10 9

12 11

Road (not to sclae)

7 1

2

4 3

6

8 10

9 5 11

12

13

Snail colony (circle size is proportional to colony size). Building

Colony 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 MDH-1 A % 39 39 36 42 39 47 32 42 44 42 44 50 50 58 75

MDH-1 a %

MDH-1 A % 81 61 75 68 70 61 70 60 58 61 54 54 47

MDH-1 a %

4. Complete the table above by filling in the frequencies of the MDH-1 a allele:

5. Suggest why these snail populations are effectively geographically isolated:

6. Both the MDH-1 alleles produce fully operative enzymes. Suggest why the frequencies of the alleles have become significantly different.

7. Identify the colony in block A that appears to be isolated from the rest of the block itself: