Question

HOW DO WE KNOW? Population geneticists study changes in the nature and amount of genetic variation in populations, the distribution of different genotypes, and how forces such as selection and drift act on genetic variation to bring about evolutionary change in populations and the formation of new species. From the explanation given in the chapter, what answers would you propose to the following fundamental questions? (a) How do we know how much genetic variation is in a population? (b) How do geneticists detect the presence of genetic variation as different alleles in a population? (c) How do we know whether the genetic structure of a population is static or dynamic? (d) How do we know when populations have diverged to the point that they form two different species? (e) How do we know the age of the last common ancestor shared by two species?

Short answer

Answer: Some common methods used by geneticists to detect the presence of genetic variations in terms of alleles in a population include DNA sequencing, single nucleotide polymorphism (SNP) analysis, and restriction fragment length polymorphism (RFLP) analysis.

Step-by-step solution

(a) Determining genetic variation in a population

Genetic variation in a population is calculated by estimating the degree of heterozygosity which is the proportion of individuals in the population carrying different alleles (variants) at a particular gene locus. Geneticists can use techniques such as DNA sequencing, single nucleotide polymorphism (SNP) analysis, and restriction fragment length polymorphism (RFLP) analysis to determine the presence of different alleles within a population. By analyzing multiple gene loci and comparing the frequencies of the different alleles, geneticists can estimate the overall genetic variation in the population.

(b) Detecting the presence of different alleles

Geneticists detect the presence of genetic variations in terms of alleles in a population by using various molecular techniques. Some common methods include DNA sequencing, single nucleotide polymorphism (SNP) analysis, and restriction fragment length polymorphism (RFLP) analysis. Each of these methods allows scientists to identify differences in the DNA sequence of individuals within a population and thus detect the presence of different alleles.

(c) Determining static or dynamic genetic structure

To determine whether the genetic structure of a population is static or dynamic, geneticists analyze changes in allele frequencies within the population over time. A static genetic structure indicates that the allele frequencies are not changing much, while a dynamic structure indicates that the allele frequencies are changing significantly. Forces such as natural selection, genetic drift, migration, and mutation could lead to changes in allele frequencies over time, resulting in a dynamic genetic structure.

(d) Identifying diverged populations as different species

Determining when populations have diverged to the point that they form two different species often involves using the concept of reproductive isolation. Reproductive isolation occurs when gene flow between populations is restricted due to incompatibility in their genetic makeup or when there are factors, such as geographical barriers, that prevent individuals from the two populations from mating. If individuals from these separate populations are unable to produce viable, fertile offspring when they mate, they are considered separate species. In some cases, the delineation of species boundaries may rely on other factors such as morphological differences, ecological niches, or molecular markers.

(e) Determining the age of the last common ancestor

Estimating the age of the last common ancestor shared by two species can be achieved through molecular clock analysis. This involves using DNA sequence data to calculate the number of sequence differences (mutations) between the two species and then correlating these differences with an estimate of mutation rate (which varies by gene or molecular marker). By dividing the number of sequence differences by the estimated mutation rate, geneticists can calculate the time since the last common ancestor. Additionally, the analysis can be refined by comparing multiple, independently evolving loci or by using a calibrated molecular clock with known divergence times from the fossil record.