Lecture 2BIOLOGI POPULASISHES1203DrSARGenetic driftThe Hardy-Weinberg principle is only applicable to large populations.In small populations a situation called genetic drift arises.Consider an allele which occurs 1% of the members of a species

- in a population of one million, 10,000 individuals may be expected to possess this allele.- even if some of these fail to pass it on to their offspring, the vast majority are likely to do so.- the proportion of individuals with the allele will not be significantly altered in the next generation.- this population however, if it is much smaller, say 1000 individuals, only 10 will carry the allele.- the effect of some of these failing to pass it on will have a significant effect on its frequency in the next generation.- this drift in the frequency of an allele is greater the smaller the population.

In an extreme example,

In a population of just 100 will have only a single individual with the allele If this individual fails to breed, the allele will be lost from the population.

Hence genetic drift is the change in gene frequency that results from some chance event.It occurs in population of all sizes, but the effects are often more pronounced in small populationsExample:Between 2 populations of western Grebes (a species of bird)-Grebes of one very tiny populations and a large population with many individuals.-Grebes are fantastic individuals. They have the unusual ability to run atop water, propelled by rapidly-beating, lobed feet. This behaviour is used during courtship and also helps Grebes escape danger. Usually, the larger the foot size, the greater a Grebes ability to skitter across water. Therefore alleles for large feet are more favourable for survival and reproduction than those for small feet.Lets assume that within both Grebe populations, foot size varies continuously from small to large.

While migrating to their breeding ground, a late snow storm kills 99% of the Grebes in both populations.

In the small population, by chance all of the large-footed Grebes are killed, removing the more favourable alleles from the gene pool. the next generation of Grebes from this population will all have small feet. a chance event (the snow storm) increased the frequency of a characteristic that has little adaptive value. Therefore genetic drift has occurred.In the more numerous population, the probability that all large-footed Grebes will be accidentally killed by the storm is less because of the larger number of individuals. The favourable genes for large feet are more likely to get passed on to the next generation

therefore genetic drift is less likely to occur in the larger population.

The example above illustrates how small populations are more susceptible to genetic drift.

Therefore genetic drift can cause dramatic changes in allele frequencies when a few individuals colonize a new habitat (called a founder population), or when the population size becomes drastically reduced during a population crash.When a population crashes, a phenomenon called a bottleneck is produced.

Bottlenecks

The size of populations fluctuates naturally as the environment changes during the year or as conditions change from one year to the next

during extreme periods, many individuals may be killed, greatly reducing the population size

fewer individuals in the population reduces the diversity of available alleles in the gene pool

this produces a genetic bottleneck that retards the populations ability to re-establish its former richness

Species that are near extinction have very small populations

thus creating a bottleneck that often prevents the species from reversing its path to extinction.However, if steps are taken (e.g. by the government or NGOs) to protect those species that are near extinction and allow the population to grow and increase for many generations, then there will come a time when the population is again large enough for random drift to be insignificant.Founder effect

When one or a few individuals split off from the main population and establish a new population in a different area, the founding population has a more limited gene pool than the larger parental population.Often, genetic drift in the founding populations changes allele frequency so dramatically that phenotypes quickly become different from the parental population. Sometimes forming a new species

This is called founder effect.Example:

The founder effect is probably responsible for the relatively high frequency of certain inherited disorders among human populations established by a small number of colonists.

In 1814, fifteen people founded a British colony on Tristan da Cunha, a group of small islands in the Atlantic Ocean, midway between Africa and South America.

Apparently, one of the colonists carried a recessive allele for retinitis pigmentosa, a progressive form of blindness that afflicts individuals who are homozygous for the allele.

Of the 240 descendants who still lived in the island in the late 1960s, four had retinitis pigmentosa and at least 9 others were known, based on pedigree analysis to be carriers.

the frequency of this allele is much higher on Tristan da Cunha than in the populations from which the founders came.

Although inherited diseases provided striking examples of the founder effect, this form of genetic drift also alters the frequency of many alleles in the gene pool that affect more subtle characteristics.

Gene flow

In species with more than one population, it is common for individuals to migrate from one population to another, or for gametes, pollen, or spores to disperse between populations

these movement transfer alleles from gene pool to gene pool

the transfer of alleles between populations is called gene flowGene flow decreases the differences between the gene pools, thus reducing the distinctions between separated populations.

When a gene flow is prevented, the gene pools of the isolated populations will diverge over time because of differences produced by mutations, genetic drift, and natural selection within each population.Diverging gene pools eventually produce individuals with such unique characteristics that the populations are recognized as different species.

Isolated population diverges and

forms new species

Barrier isolates one population

Population of a species

gene flow a) Unrestricted gene flow

b) No gene flow to

c) Isolated population

between all population isolated population

diverges while population

with gene flow do not diverge

Time

Gene flow maintains the similarity of gene pools divergence in species composed of more than one population.

However, if a barrier isolates populations, separated gene pools diverge, eventually producing separate species.

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