Question

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

• Humans are regarded as a member of the biological kingdom Animalia (animals). They are bipedal hominin characterized by having a higher and vertical forehead compared with earlier hominins. The brain volume is about 1,400 cc. The teeth and jaw are smaller and the chin is prominent. Humans are the hominins capable of creating and using complex tools, solving problems by sense and reasoning, using symbols and language, and creating complex social structures. Over time, humans have demonstrated behavioral modernity and advancement.
• As humans migrated across the world, they encountered new environments requiring them to adapt to new challenges that presented themselves. The distribution of human phenotypes observed today is the result of this continuous adaptation, via biological/physiological and cultural means, and also by the modification of cultural practices, which leads to biological changes.
• Biological evolution explains both the unity and the diversity of species and provides a unifying principle for the history and diversity of life on Earth. Biological evolution is supported by extensive scientific evidence ranging from the fossil record to genetic relationships among species.
• Evolution, which is continuous and ongoing, occurs when natural selection acts on the genetic variation in a population and changes the distribution of traits in that population gradually over multiple generations.
• Natural selection can act more rapidly after sudden changes in conditions, which can lead to the extinction of species. Through natural selection, traits that provide an individual with an advantage to best meet environmental challenges and reproduce are the ones most likely to be passed on to the next generation.
• Over multiple generations, this process can lead to the emergence of new species. Evolution thus explains both the similarities of genetic material across all species and the multitude of species existing in diverse conditions on Earth, its biodiversity which humans depend on for natural resources and other benefits to sustain themselves.
• The original sources of genetic variation are mutations, which are changes in the nucleotide sequence of DNA. Mutations create new alleles and increase genetic variability. Most mutations do not cause significant changes to the health or functioning of an organism. However, if a mutation reduces the chances of survival, the organism may die before reproducing. Therefore, such harmful mutations are likely to be eliminated by natural selection.
• Individuals in natural populations may also select their mates based on certain characteristics, and thus do not reproduce randomly. In this case, alleles for the traits that are selected against will become less frequent in the population.
• Furthermore, populations can experience gene flow, the transfer of alleles into and out of gene pools, due to migration. A classic example of gene flow is observed in most baboon species. Female baboons mate most frequently with dominant males in a troop. Juvenile male baboons almost always leave their birth troops, likely to avoid inbreeding, and join a new troop, where they may pass their genes to offspring.
• In genetic drift, chance events alter the allele frequencies of a population. A major disturbance, such as a natural disaster, may drastically reduce population size and thereby diminish genetic variation. The resulting composition of the gene pool was selected randomly (i.e., surviving the disturbance was not determined by the genetic make-up of the individual). Such a reduction of genetic diversity is called a genetic bottleneck.
• Sometimes, a population may become fragmented into smaller populations due to urban development or other events. A new population is started by a small group of members of the original population and by chance, a previously rare allele may be relatively frequent. This effect on gene frequencies is known as the founder effect.
• The bottleneck effect is an extreme example of genetic drift that happens when the size of a population is severely reduced. Events like natural disasters (earthquakes, floods, fires) can decimate a population, killing most individuals and leaving behind a small, random assortment of survivors.
• The migration will change gene frequencies by bringing in more copies of an allele already in the population or by bringing in a new allele that has arisen by mutation. Because mutations do not occur in every population, migration will be required for that allele to spread throughout that species.