POPULATION GENETICS AND NATURAL SELECTION

Now that you know more about genes, alleles and polypeptide synthesis we can provide a clearer explanation of  how evolution takes place.

I. Evolution = changes in gene (allele) frequencies in a population over time

A. Evolution takes place at the population, not species level. I.e. populations, not species evolve

B. Population = a group of interbreeding individuals of the same species sharing a common geographical area

C. Species = a group of populations that have the potential to interbreed in nature and produce viable offspring

D. Gene pool = sum total of all the alleles within a population

E. Four processes of evolution:

1. mutation - changes in nucleotide sequences of DNA. Mutations provide new alleles, and therefore are the ultimate source of variation

2. recombination - reshuffling of the genetic material during meiosis (prophase I & metaphase I)

3. natural selection - differential reproduction (discussed below)

4. reproductive isolation (discussed shortly)

F. Mutation and recombination provide natural variation, the raw material for evolution.

II. Hardy-Weinberg Law

A. Early in the 20th century biologists believed that natural selection would eventually result in the dominant alleles driving out or eliminating the recessives. Therefore, over a period of time genetic variation would eventually be eliminated in a population

B. Early in this century the geneticist Punnett was asked to explain the prevalence of blue eyes in humans despite the fact that it is recessive to brown. He couldn't do it so he asked a mathematician colleague named Hardy to explain it. Coincidentally, a physician named Weinberg also came up with an explanation similar to Hardy's

C. Hardy-Weinberg law - the frequencies of alleles in a population will remain constant unless acted upon by outside agents or forces (listed below)

D. Hardy-Weinberg law describes the genetics of non-evolving populations. A non-evolving population is said to be in Hardy-Weinberg equilibrium

E. The following will disrupt Hardy-Weinberg equilibrium causing evolution to occur:

1. Mutation - by definition mutations change allele frequencies causing evolution

2. Migration - if new alleles are brought in by immigrants or old alleles are taken out by emigrants then the frequencies of alleles will change causing evolution

3. Genetic drift - random events due to small population size. Random events have little effect on large populations.

E.g., consider a population of 1 million almond trees with a frequency of r at 10%. If a severe ice storm wiped out half, leaving 500,000, it is very likely that the r allele would still be present in the population. However, suppose the initial population size of almond trees were 10 (with the same frequency of r at 10%). It is likely that the same ice storm could wipe the r allele out of the small population.

a. Intense natural selection or a disaster can cause a population bottleneck, a severe reduction in population size which reduces the diversity of a population. The survivors have very little genetic variability and little chance to adapt if the environment changes.

By the 1890's the population of northern elephant seals was reduced to only 20 individuals by hunters. Even though the population has increased to over 30,000 there is no genetic variation in the 24 alleles sampled. A single allele has been fixed by genetic drift and the bottleneck effect. In contrast southern elephant seals have wide genetic variation since their numbers have never reduced by such hunting.

b. Bottleneck effect, combined with inbreeding, is an especially serious problem for may endangered species because great reductions in their numbers has reduced their genetic variability. This makes them especially vulnerable to changes in their environments and/or diseases. The Cheetah is a prime example.

c. Sometimes a population bottleneck or migration event can cause a founder effect. A founder effect occurs when a few individuals unrepresentative of the gene pool start a new population.

E.g., a recessive allele in homozygous condition causes Dwarfism. In Switzerland the condition occurs in 1 out of 1,000 individuals. Amongst the 12,000 Amish now living in Pennsylvania the condition occurs in 1 out of 14 individuals. All the Amish are descendants of 30 people who migrated from Switzerland in 1720. The 30 founder individuals carried a higher than normal percentage of genes for dwarfism.

4. Nonrandom Mating - for a population to be in Hardy-Weinberg equilibrium each individual in a population must have an equal chance of mating with any other individual in the population, i.e. mating must be random.

a. If mating is random then each allele has an equal chance of uniting with any other allele and the proportions in the population will remain the same. However in nature most mating is not random because most individuals choose their partner.

Sexual selection - nonrandom mating in which mates are selected on the basis of physical or behavioral characteristics.

5. Natural Selection - For a population to be in Hardy-Weinberg equilibrium there can be no natural selection. This means that all genotypes must be equal in reproductive success. But recall Darwin's reasoning:

1. all species reproduce in excess of the numbers that can survive

2. yet adult populations remain relatively constant

3. therefore there must be a severe struggle for survival

4. all species vary in many characteristics and some of the variants confer an advantage or disadvantage in the struggle for life

5. the result is a natural selection favoring survival and reproduction of the more advantageous variants and elimination of the less advantageous variants

F. The Hardy-Weinberg principle sets up conditions which probably never occur in nature. One or more of mutation, migration, genetic drift, non-random mating or natural selection are probably always acting upon natural populations. This means that evolution is occurring in that population.

G. The Hardy-Weinberg principle can also be expressed mathematically:

p2 + 2pq + q2 = 1, let p = the dominant allele and q = the recessive.

Construct a Punnett square crossing two heterozygous (pq) individuals.

p2 + 2pq + q2 must equal 1 because the proportion of all alleles in a population must add up to 1 (100%).

III. Natural Selection = differential reproduction. Organisms with more advantageous gene combinations secure more resources, allowing them to leave more progeny. It is a negative force, nature selects against, not for

It is convenient to recognize three types of selection:

A. Stabilizing Selection - selection maintains an already well adapted condition by eliminating any marked deviations from it. As long as the environment remains unchanged the fittest organisms will also remain unchanged.

1. Human birth weight averages about seven pounds. Very light or very heavy babies have lower chances of survival. Fur color in mammals varies considerably but certain camouflage colors predominate in specific environments. Stabilizing selection accounts for "living fossils" - organisms that have remained seemingly unchanged for  millions of years.

B. Directional Selection - favors one extreme form over others. Eventually it produces a change in the population. Directional selection occurs when an organism must adapt to changing conditions.

1. Industrial melanism in the peppered moth (Biston betularia) during the industrial revolution in England is one of the best document examples of directional selection.

The moths fly by night and rest during the day on lichen covered tree trunks where they are preyed upon by birds. Prior to the industrial revolution  most of the moths were light colored and well camouflaged. A few dark (melanistic) were occasionally noted.

During the industrial revolution soot began to blacken the trees and also cause the death of the lichens. The light colored moths were no longer camouflaged so their numbers decreased quite rapidly. With the blackening of the trees the numbers of dark moths rapidly increased.

The frequency of the dark allele increased from less than 1% to over 98% in just 50 generations. Since the 1950's attempts to reduce industrial pollution in Britain have resulted in an increase in numbers of light form.

2. Antibiotic resistance in bacteria is another example of directional selection. The overuse/misuse of antibiotics has resulted in many resistant strains.

3. Pesticide resistance in insects is another common example of directional selection.

C. Disruptive Selection - occurs when two or more character states are favored.

1. African butterflies (Pseudacraea eurytus) range from orange to blue. Both the orange and blue forms mimic (look like) other foul tasting species (models) so they are rarely eaten. Natural selection eliminates the intermediate forms because they don't look like the models.

D. Ultimately natural selection leads to adaptation - the accumulation of structural, physiological or behavioral traits that increase an organism's fitness.

IV. Fitness

A. Darwin marveled at the "perfection of structure" that made it possible for organisms to do whatever they needed to do to stay alive and produce offspring

B. He called this perfection of structure fitness, by which he meant the combination of all traits that help organisms survive and reproduce in their environment

C. Fitness is now measured as reproductive success, i.e. the number of progeny left behind who carry on the parental genes. Those who fail to contribute to the next or succeeding generations are unfit.

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