Question

1. What does it mean biologically to be human? How can evolution explain both the unity and diversity of human phenotypes? Use specific examples of evidence and some version of the following terms in your answer: neutral genetic variation, evolution, natural selection, drift/bottleneck/founder effect, migration, mutation, and species/population/lineage.

Answer 1

Mutation: Mutation is the change in the base sequence of the genome. Due to the high level of accuracy of DNA replication and proofreading, mutations occur very less often such as a chance of 1 in 10,000 or 1 in a million per base. But they are the basis for evolution. without mutations species would not evolve and new species will not appear.

Neutral genetic variation: It is not yet clear if mutations occur randomly or on purpose, however the neutral theory or neutral genetic variation says that mutations occur randomly without purpose.

Natural selection: Natural selection at the genetic level is survival (and hence passing on) of advantageous alleles and genes. Examples are genes for eyesight, cardiovascular capability, intelligence, fertility, altruism are positively selected for (higher chance of survival, higher chance of mating, higher number of offspring leading to passing of these traits).

Drift: One of the conditions for Hardy-Weinberg equilibrium is that the population should be large, ideally infinite. This is to control for genetic drift. Drift has to do with the probability that the allele frequency in the gametes produced will exactly reflect the frequencies of the parent population. During meiosis, there is half a chance of a gamete getting allele A versus allele a. However, the chance for each gamete is not always 50%. It is possible that a heterozygous parent (Aa) may pass both her two children allele A and none gets allele a. When this happens in the entire population, the next generation will have slightly different frequencies than the parents. This is genetic drift. The larger the sample, the less the chance of drift. Smaller populations are thus more likely to experience drift. In some populations, over many generations, a certain allele may completely be lost. And thus the population has evolved.

Migration: Migration is moving of individuals from one geographical area to another.

Founder effect: The founder effect is a result of a small group in a large population migrating to another geographical area. This results in them having largely different allele frequencies from their original population. This results in loss of alleles due to drift and hence loss of variation. A good example is the Japanese population which has the least genetic variation in the world since they are a result of series of migrations and then a final migration to an island resulting in being cut off reproductively from the rest of the world. The African population, which is the main parent population has the highest genetic variation in the world.

Bottleneck: Unlike the founder effect, bottleneck is not caused by migration but by any event that may kill most of population. This also leads to allele frequencies very different from that of the parent population which leads to all the same subsequenct effects on the variation. But unlike founder effect, natural selection may or may not play a role in which individuals survive. For example, suppose a tsunami hits an island. People who have high cardiovascular capability will be able to climb up the hill fast enough to not get washed away and those without will die. Most of the population will die but those remaining will have exceptional cardio capabilities. Their offspring will carry this trait and the subsequent population will have better survival chances in case another tsunami hits. And thus the population has evolved.

Species: There have been many attempts earlier to define species but the most accepted one is that species are a group of individuals that are capable of interbreeding and producing viable offpspring that is also capable of breeding. And that is what makes us human and distinguishes us from all other species.

Hence evolution is a change in the genes, alleles and genetic variation of a species so as best ensure the survival of the species as a whole.