Question

. Consider a trait or phenotypic feature or even a recessive disorder with decidedly non-random geographic patterning to frequency of the trait, feature, or allele where it is high frequency in some populations and low frequency in other populations (non-random frequency distribution). What are the mechanisms or ways that you could testable hypotheses to explain how that pattern developed and why it is maintained – i.e. forces of evolution? Population histories? Cultural mechanisms? Developmental adaptation? Acclimatization? Describe and illustrate with one of the disease alleles or phenotypes presented in the textbook and lecture or if there is one of particular interest to you that we didn’t discuss, use that one! (Hint: Discuss all the mechanisms and factors that we consider when developing evolutionary explanations for traits, phenotypes, and alleles. Note whether there is always one mechanism? Or, are there multiple forces or mechansims?)

Answer 1

When considering all the mechanism and factors developing evolutionary explanations for traits, phenotypes and alleles, there are five key mechanisms that cause a population, a group of interacting organisms of a single species, to exhibit a change in allele frequency from one generation to the next. These are evolution by: mutation, genetic drift, gene flow, non-random mating, and natural selection

1. Evolution by mutation occurs whenever a mistake in the DNA occurs in the heritable cells of an organism. In the single-celled asexual organisms, such as bacterial, the whole cell and its DNA is passed on to the next generation because these organisms reproduce via binary fission. For sexual organisms, mutations are passed to the next generation if they occur in the egg or sperm cells used to create offspring. Mutations occur at random in the genome, but mutations of large effect are often so bad for the organism that the organism dies as it develops, so mutations of smaller effect or even neutral mutations are theoretically more common in a population. The variation that is created in a population through the random process of mutation is called standing genetic variation, and it must be present for evolution to occur. For example in Huntington disease, signs and symptoms do not occur until after a person has children, so the gene mutation can be passed on despite being harmful. For some conditions, having one mutated copy of a gene in each cell is advantageous, while having two mutated copies causes disease. The best-studied example of this phenomenon is sickle cell disease: Having two mutated copies of the HBB gene in each cell results in the disease, but having only one copy provides some resistance to malaria.
2. Evolution by genetic drift occurs when the alleles that make it into the next generation in a population are a random sample of the alleles in a population in the current generation. By random chance, not every allele will make it through, and some will be overrepresented while other decline in frequency regardless of how well those alleles encode for phenotypic suitability to the environment, so sometimes drift reduces the average fitness of a population for its environment. Populations are constantly under the influence of genetic drift. The population bottleneck and a founder effect are two examples of random drift that can have significant effects in small populations. Bottleneck effect occurs when there is a sudden sharp decline in a population’s size typically due to environmental factors (natural disasters such as: earthquakes or tsunamis, epidemics that can decimate the number of individuals in the population, predation or habitat destruction, etc.). It is a random event, in which some genes (there is not any distinction) are extinguished from the population. This results in a drastic reduction of the total genetic diversity of the original gene pool. The small surviving population is considerably be farther from the original one in its genetic makeup. Founder effect is the loss of genetic variation that occurs when a new population is established by a small number of individuals that are cleaved from a larger population. This new population does not have the genetic diversity of the previous one. Because the community is very small and also geographical or socially isolated, some genetic traits are becoming more prevalent in the population. This leads to the presence of certain genetic diseases in the next generations. For exampe, Mycosphaerella graminicola causes Septoria tritici leaf blotch on wheat. McDonald and colleagues used restriction fragment length polymorphism (RFLP) markers to determine the genetic structure of this pathogen worldwide and found that all populations collected from different geographic locations had similar frequencies of common alleles except the populations collected from Australia and Mexico. The Australian and Mexican populations had significantly lower gene diversity, fewer alleles at each locus and the gene frequencies were significantly different from populations at other locations. In Australia, this is probably due to a founder effect whereby only a relatively small number of individuals arrived on this continent with the introduction of modern agriculture. The Patzcuaro, Mexico population was sampled from a breeding nursery used by CIMMYT to screen for resistance to this pathogen. This nursery is located far away from wheat production areas (hence, it has a limited potential for influx of natural inoculum) and was inoculated with a limited number of strains, presenting a clear example of genetic drift due to a small founding population and continued geographical isolation.
3. Evolution by gene flow (migration) makes two different populations more similar to each other. Two different populations are often subject to different selective pressures and genetic drift, so they would be expected to have different allele frequencies. When individuals from one population migrate into a different population, they bring those different allele frequencies with them. If enough migration and mating occurs between two populations, then the two populations will experience changes in allele frequencies and such that their allele frequencies become similar to each other.
4. Selecting a mate at random is a pretty risky idea because half of your offspring’s genes come from your mate. Non-random mating with “like” individuals will shift the genotype frequencies in favour of homozygotes, while non-random mating with “unlike” individuals (negative phenotypic assortment) creates an over-representation of heterozygotes. These shifts can occur without changing the proportion of each allele in the population, also called the allele frequency.
5. Natural Selection leads to an evolutionary change when some individuals with certain traits in a population have a higher survival and reproductive rate than others and pass on these inheritable genetic features to their offspring. Evolution acts through natural selection whereby reproductive and genetic qualities that prove advantageous to survival prevail into future generations. The cumulative effects of natural selection process have giving rise to populations that have evolved to succeed in specific environments. Researchers found that the sickle cell gene is especially prevalent in areas of Africa hard-hit by malaria. In some regions, as much as 40 percent of the population carries at least one HbS gene.It turns out that, in these areas, HbS carriers have been naturally selected, because the trait confers some resistance to malaria. Their red blood cells, containing some abnormal hemoglobin, tend to sickle when they are infected by the malaria parasite. Those infected cells flow through the spleen, which culls them out because of their sickle shape -- and the parasite is eliminated along with them.
   

When we consider whether there is always one mechanism or there are multiple forces of mechanism of evolution, Natural Selection and Genetic Drift are the two of the most relevant mechanisms of evolutionary. Natural selection usually predominates in large populations whereas genetic drift does so in small ones.