Saturday, February 21, 2009

The Rest of Chapter 23-The Evolution of Populations

The Hardy-Weinberg Equation can be used to determine or predict the allelic frequencies that exist in populations. In order for a population to be in Hardy-Weinberg equilibrium, it must meet all of these criteria:

1. It must be a very large population in size. If a population is small, a change in the gene pool due to chance will have an inordinate effect on the gene frequencies of a population.
EXAMPLE: If you have a small population of purple (dominant) and pink (recessive) flowers and there is a hurricane, all of the population that shows the recessive trait may die and completely alter the gene frequencies so that there are far fewer pink flowers than there usually would have been.

2. There must be no migration in a population. Gene flow, which is the transfer of alleles between populations, cannot occur because that would alter gene frequencies.
EXAMPLE: If a few deer that usually have large antlers wander into a population of deer that have short antlers and start to breed, over time, the population could change dramatically.
3. There can be no mutations. Mutations can change one allele into another, thus altering the gene pool.
EXAMPLE: One ground squirrel has a mutation that gives him more storage space in his cheeks, and he reproduces and passes the mutation onto other ground squirrels. Eventually, a large portion of the population has more cheek storage space and the population is much different than it was originally.

4. Mating must occur randomly. If individuals mate only with other individuals of a certain genotype, this does not meet the criteria of a random mixing of genes.
EXAMPLE: Lionesses look for mates with larger manes who are therefore stronger and better fit to survive. Genes do not mix randomly, so eventually, because more females mate with large-maned males, more lions have big manes.

5. Natural selection may not be taking place. This would alter gene frequencies and cause a deviation from Hardy-Weinberg equilibrium.
EXAMPLE: Longer necked giraffes can reach the highest leaves and therefore have a better chance of surviving. The long necked giraffes that survive give birth to more long necked giraffes.

***Unless a population is being carefully contained and monitored, it is basically impossible for all of these situations to hold true naturally. There is often migration and mutations, natural selection is always occurring, and mating is rarely random. This is why Hardy-Weinberg equations are used to find out how a population changes. The first set of data obtained provides a baseline and the next set shows that the population is evolving (it is rare to get the same, or nearly the same, data as the baseline).
***Populations that deviate from the Hardy-Weinberg equilibrium are, therefore, said to be evolving.

Microevolution can be defined as a generation to generation change in allele frequency in a population. The main causes of microevolution are genetic drift, natural selection, gene flow, and mutation.
*Not to be confused with macroevolution: evolutionary change above the species level, including the origin of a new group of organisms or a shift in the broad pattern of evolutionary change over a long period of time.

1. Genetic Drift: refers to a change in a population's allele frequencies due to chance.
A) The Bottleneck Effect: occurs when a natural disaster or some other event causes a drastic reduction in the size of a population, which in turn causes genetic drift. Bottlenecking usually reduces the genetic variability in a population, since some alleles are lost from the gene pool.
EXAMPLE: Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.
B) Founder Effect: occurs when a few members of a population colonize an isolated location-the smaller the number of founders, the more limited the variability of the genes in the population.
EXAMPLE: The Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency.
2. Natural Selection: refers to the differing reproductive success of individuals in a population. The individuals best suited to their environment will survive to reproduce and to pass on their alleles to the next generation.
A) Stabilizing Selection: favors intermediate variants by selecting against extreme phenotypes. The trend is toward reduced phenotypic variation and a greater prevalence of phenotypes best suited to relatively stable environments:
EXAMPLE: Birth weight. Babies of low birth weights generally have a lower chance of surviving (now with modern medical technology this actually isn't really the case). Babies with high birth weights are much harder and more dangerous to deliver. Average sized babies are the easiest to give birth to and have the best chance of survival.

B) Directional Selection: favors variants of one extreme. It shifts the frequency curve for phenotypic variants in one direction toward rare variants which deviate from the average of that trait.
EXAMPLE: This is most common when members of a species migrate to a new habitat with different environmental conditions or during periods of environmental change.

C) Diversifying (aka Disruptive) Selection: opposite phenotypic extremes are favored over intermediate phenotypes. This occurs when environmental conditions are variable in such a way that extreme phenotypes are favored.
EXAMPLE: A species of butterfly with characteristics between two noxious model species gain no advantage from their mimicry.

3. Gene Flow: refers to the genetic exchange due to the migration of individuals or gametes between populations.

4. Mutations: refers to a change in an organism's DNA. This can alter the gene pool of a population by changing one allele into another.

Genetic Variation, substrate for natural selection.

Genetic Variation exists naturally in populations. Quantitative characteristics, such as height in humans, vary in a continuum in a population.

A population is said to be polymorphic for a character if this character exists in two or more discrete forms in the population-for example, if a plant bears two different kinds of flowers in population. Geographic variation refers to differences in gene pools between populations or parts of a population.

A cline is one type of geographical variation that is a graded change in some trait along a geographic transect.
EXAMPLE: As altitude increases, the size of plants decrease.

Two processes contribute to the variation in the gene pool of a population, one is mutation and the other is sexual recombination.

Since mutations in some somatic cells disappear when the individual dies, only mutations in gametes are passed to offspring.

Most of the genetic differences that exist in a population are due to the genetic recombination of alleles that already exist in a population.

Genetic recombination is the most important factor in producing variability that occurs in each generation of humans. It can be supported by the existence of sex, bacterial conjugation, and the exchange of chromosome regions in meiosis.

***Quick Review
-Genetic recombination is the production of offspring with combinations of traits that differ from those found in either parent.
+If you want to brush up on this subject, reference Chapter 15.
Factors that contribute to the preservation of genetic variation in a population are diploidy (the condition of being diploid, or having two sets of chromosomes), and balanced polymorphism. The fact that most eukaryotes are diploid (like humans), means that they are capable of hiding genetic variation (recessive alleles) from selection. Balanced polymorphism refers to the ability of natural selection to keep stable the frequencies of two or more phenotypes in a population.

***Quick Review:
-Humans are always diploid, however we do have haploid cells (eggs and sperm).
-Fungi and some protists and algae have a diploid zygote. Meiosis occurs to produce haploid cells, and the haploid cells then divide by mitosis to give a multicellular haploid organism (fungus is a multicellular haploid most of its life).
-In plants and some algae, alternation of generations occurs which includes both a multicellular haploid and diploid stage in the life cycle.
+If you don't remember this and want to brush up, reference Chapter 13.

Individuals with heterozygote advantage are heterozygous at a certain locus, and this confers upon them an advantage that enables them to better survive.
EXAMPLE: Sickle Cell Anemia. People who do not have Sickle Cell Anemia but are carriers are resistant to Malaria.
Practice Questions:
1) Most copies of harmful recessive alleles in a population are carried by individuals that are A) heterozygous for the allele. B) polymorphic. C) haploid. D) homozygous for the allele. E) afflicted with the disorder caused by the allele.

2) All of the following are criteria for maintaining a Hardy-Weinberg equilibrium involving two alleles except A) gene flow from other populations must be zero. B) there should be no natural selection. C) the frequency of all genotypes must be equal. D) matings must be random. E) populations must be large.

3) What is the most reasonable conclusion that can be drawn from the fact that the frequency of the recessive trait (aa) has not changed over time? A) There has been a high rate of mutation of allele A to allele a. B) The population is undergoing genetic drift. C) There has been sexual selection favoring allele a. D) The two phenotypes are about equally adaptive under laboratory conditions. E) The genotype AA is lethal.

4) What effect do sexual processes (meiosis and fertilization) have on the allelic frequencies in a population? A) They tend to increase the frequencies of deleterious alleles and decrease the frequencies of advantageous ones. B) They tend to selectively combine favorable alleles into the same zygote but do not change allelic frequencies. C) They tend to reduce the frequencies of deleterious alleles and increase the frequencies of advantageous ones. D) They tend to increase the frequency of new alleles and decrease the frequency of old ones.

5) Which of the following is the unit of evolution? In other words, which of the following can evolve in the Darwinian sense? A) species B) gene C) chromosome D) individual E) population

ANSWERS: 1) A; 2) C; 3) D; 4) E; 5) E

****Here are a few websites where you can practice some Hardy-Weinberg problems:
A Little Background Info...:
The Hardy-Weinberg equation is named after H.G. Hardy (left) and Wilhelm Weinberg (right). To learn more about these two men and their equation, go to either of these sites:

Monday- Hardy-Weinberg Worksheet Due
Two Chapter 23 Quizzes
Tuesday- Chapter 24 Vocab Quiz
Thursday- Community Day (no class)
Friday- Chapter 24 Quiz
Lab 8 Essay

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