The Evolution of Populations

Population genetics begins with understanding evolution and heredity. It describes the changes of populations and species, from small-scale changes among individuals to large-scale changes over paleontological time periods. Scientists can track changes in allele frequencies within populations over time.

The allele frequency within a given population can change depending on environmental factors. Some alleles become more widespread than others during the process of natural selection. Natural selection can alter the population’s genetic makeup. If a genetic phenotype allows an individual to better survive or reproduce, more offspring will carry the beneficial alleles, likely display the corresponding phenotype, and have the potential for more offspring of their own that also carry the genotype to perpetuate the cycle. Over time, the allele will spread throughout the population. Some alleles will quickly become fixed in this way, meaning that every individual of the population will carry the allele, while detrimental mutations may be swiftly eliminated if derived from a dominant allele from the gene pool. The gene pool is the sum of all the alleles in a population.

Sometimes, allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. This phenomenon is called genetic drift. Natural selection and genetic drift usually occur simultaneously in populations and are not isolated events. It is hard to determine which process dominates because it is often nearly impossible to determine the cause of change in allele frequencies at each occurrence. What ultimately interests biologists is not the frequencies of different alleles, but the frequencies of the resulting genotypes.

Both genetic and environmental factors can cause phenotypic variation in a population. Different alleles can confer different phenotypes, and different environments can also cause individuals to look or act differently. Only those differences encoded in an individual’s genes can be passed to its offspring and form the basis of natural selection. Genetic drift stems from the chance occurrence that some individuals in the germ line have more offspring than others. When individuals leave or join the population, allele frequencies can change as a result of gene flow. Mutations to an individual’s DNA may introduce new variation into a population. Allele frequencies can also be altered when individuals do not randomly mate with others in the group.

Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of the accumulation of mutations that occur over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance. Some mutations are unfavorable or harmful and are quickly eliminated from the population by natural selection. Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations do not do anything and can linger in the genome. Some can have a dramatic effect on a genotype and resulting phenotype.

Founder effect is an event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population. Natural selection and founder effects can lead to significant changes in a population.

Genetic drift can also be magnified by natural events, like a natural disaster killing a large portion of the population randomly. The bottleneck effect results in a large portion of the genome suddenly being wiped out. In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population.