Genetic Drift and selection pressures

Lyndsae Drury Isle of Wight 6th Form College

Disruptive SelectionDisruptive selectionIf the temperature difference between summer and winter increases, long hair for animals being active during the winter or short hair for animals being active during the summer is advantageous.Intermediate fur length is disadvantageous. Therefore, two subpopulations are formed over time

Directional SelectionDirectional selectionIf the temperature falls, the individuals with longer fur length are at an advantage as they have better insulation against the cold.There is a selection pressure favouring the animals with longer fur so these animals are more likely to survive and thus reproduce.Over several generations, the average fur length increases as more young have inherited the genes for long fur. When the mean fur length has reached the most advantageous length, the selection pressure ceases.

Directional selectionthe increase in antibiotic-resistant strains of bacteria

Directional selection favors those individuals who have extreme variations in traits within a population. A useful example can be found in the breeding of the greyhound dog. Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 64km/h (40mph). The greyhound was originally used to hunt the fastest of game, fox and deer. Their bred dates to Egypt in 3BC.

Stabilising Pressure

Stabilising selectionInitially there is a wide range of fur length about the mean of 1.5cm.Due to rapid breeding in either very cold or very warm weather, animals with extreme fur lengths survive.When the temperature remains constant with little variation, the individuals with very short or very long hair become less numerous and are eventually eliminated from the population.

• Birth weight in human babies is one good example. Too small and babies lose heat rapidly and are also prone to infectious disease. Too large and they endanger themselves and their mothers by the great difficulty they have passing through the birth canal. So, a small range of birth weights have been selected for and this remains stabilized by the constraints listed above.

• Crocodiles evolution Of all the reptiles alive today, crocodiles and alligators may be the least changed from their prehistoric ancestors of the late Cretaceous period, over 65 million years ago--although the even earlier crocodiles of the Triassic and Jurassic periods sported some distinctly un-crocodile-like features, such as bipedal postures and vegetarian diets.

• Effect of malaria on Heterozygous individuals with sickle cell and normal alleles for sickle cell anaemia

Aa Aa aA

AA AA aa

Aa aa A aa

aA aa aa

AA AA aa

Aa aA aa

AA Aa aa

p2 + 2pq + q2 = 1 and p + q = 1

• p2 + 2pq + q2 = 1 and p + q = 1 • p = frequency of the dominant allele in the

populationq = frequency of the recessive allele in the populationp2 = percentage of homozygous dominant individualsq2 = percentage of homozygous recessive individuals2pq = percentage of heterozygous individuals

Hardy Weinberg

• Within a population of butterflies, the color brown (B) is dominant over the color white (b). And, 40% of all butterflies are white. Given this simple information, which is something that is very likely to be on an exam, calculate the following: – The percentage of butterflies in the population that

are heterozygous. – The frequency of homozygous dominant

individuals.

Answers:• The first thing you'll need to do is obtain p and q.• So, since white is recessive (i.e. bb), and 40% of the

butterflies are white, then bb = q2 = 0.4. • To determine q, which is the frequency of the recessive

allele in the population, simply take the square root of q2 which works out to be 0.632 (i.e. 0.632 x 0.632 = 0.4). So, q = 0.63. Since p + q = 1, then p must be 1 - 0.63 = 0.37. Now then, to answer our questions. First, what is the percentage of butterflies in the population that are heterozygous? Well, that would be 2pq so the answer is 2 (0.37) (0.63) = 0.47. Second, what is the frequency of homozygous dominant individuals? That would be p2 or (0.37)2 = 0.14.

Sickle-cell anemia is an interesting genetic disease. Normal homozygous individials (SS) have normal blood cells that are easily infected with the malarial parasite. Thus, many of these individuals become very ill from the parasite and many die. Individuals homozygous for the sickle-cell trait (ss) have red blood cells that readily collapse when deoxygenated. Although malaria cannot grow in these red blood cells, individuals often die because of the genetic defect. However, individuals with the heterozygous condition (Ss) have some sickling of red blood cells, but generally not enough to cause mortality. In addition, malaria cannot survive well within these "partially defective" red blood cells. Thus, heterozygotes tend to survive better than either of the homozygous conditions. If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

• Answer: 9% =.09 = ss = q2. To find q, simply take the square root of 0.09 to get 0.3. Since p = 1 - 0.3, then p must equal 0.7. 2pq = 2 (0.7 x 0.3) = 0.42 = 42% of the population are heterozygotes (carriers).