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2.2 Genetic drift - Coggle Diagram
2.2 Genetic drift
Genetic drift is the random sampling that occurs in gametes in the process of reproduction
The smaller the population, the greater this effect
Buri (1956) conducted a study that investigated the effects of genetic drift on
Drosophila melanogaster
These flies had 2 alleles for eye colour:
bw
and
bw^75
bw/bw
have white eyes,
bw/bw^75
have orange eyes, and
bw^75/bw^75
have red eyes
In 107 population, beginning with 100% heterozygous for eye colour flies, Buri mated 8 males and 8 females at random, and again successively over 19 generations
Over the course of the 19 generations, heterozygous flies became less frequent
Over the course of the 19 generations, homozygocity increased
The proportion of populations in which the
bw^75
allele was lost or became fixed increased with each generation, commencing from generation 5
By generation 19, approximately 30 populations had became fixed for the
bw^75
allele, and in approximately 30 populations the
bw^75
allele was lost
As population size increases, oscillations in allele frequency become less amplified
However, even in large populations, an allele will eventually become fixed due to genetic drift
Genetic drift acts to reduce heterozygosity in a population
So, reduction in heterozygosity is used as a measure of genetic drift
This is one of the reasons why the IUCN considers population size to determine the status of species: smaller populations will tend towards lower genetic variation and therefore poorer overall fitness
Genetic drift reduces genetic variation
What is the difference between natural election and genetic drift?
Natural selection is the change in allele frequency that occurs due to phenotypic fitness within an environment
Is not random
Genetic drift is the alteration of allele frequency due to sampling error that occurs during reproduction
Is random
Genetic drift occurs in all populations because no populations are infinitely large
Why does population size affect the rate of genetic drift?
Genetic drift is higher in small populations because it is more likely that, by chance, the allele frequency in an offspring generation differs from that of the parent generation, than is the case in a large population
The differences brought about by sampling error have more of an impact on the allele frequency in a small population than a large one
So, genetic drift will be greater in a small population than a large one
REVIEW THIS WHEN YOU GET THE CHANCE
What is a population bottleneck?
Where a population crashes i.e. a large proportion of members of the population are killed or die, the surviving members carry only a portion of the alleles originally represented in the population.
Population numbers may re-grow to a comparable size to the number before the crash, but the genetic diversity is greatly reduced and many alleles may be permanently lost
An example of this is with the northern elephant seal (
Mirounga angustirostris
Hoelzel (1993) found no genetic variation among samples from 2 large elephant seal colonies at 34 loci
In fact, these elephant seals were extensively hunted in the 1800s, and thought extinct in 1884
The population regrew and was put under protection in 1922
The rarest alleles are usually lost
A similar effect occurs with the founder effect
The total members of a population is known as the census population size (
N-c
)
This is not necessarily the most useful measure when considering population genetics because some population members won't or can't reproduce
Consider instead the effective population size (
N-e
): This is every member of the population that contributes to the next generation
N-e
may be much smaller than
N-c
The smaller the
N-e
, the greater the genetic drift and hence the heterozygosity reduction
N-e
is usually smaller than
N-c
for the following reasons:
Sex ratio, particularly operational sex ratio
variation in reproductive success
Fluctuating
N-c