Defining Populations and the Variations within Them
One of the major breakthroughs in understanding the mechanisms of evolutionary change came with the realization that evolution takes place at the level of populations, not within individuals. In the biological sciences, a population is defined as a group of individuals of the same species who are geographically near enough to one another that they can breed and produce new generations of individuals.
For the purpose of studying evolution, we recognize populations by their even smaller units: genes. Remember, a gene is the basic unit of information that encodes the proteins needed to grow and function as a living organism. Each gene can have multiple alleles, or variants—each of which may produce a slightly different protein. Each individual, for genetic inheritance purposes, carries a collection of genes that can be passed down to future generations. For this reason, in population genetics, we think of populations as gene pools, which refers to the entire collection of genetic material in a breeding community that can be passed on from one generation to the next.
For genes carried on our human chromosomes (our nuclear DNA), we inherit two copies of each, one from each parent. This means we may carry two of the same alleles (a homozygous genotype) or two different alleles (a heterozygousgenotype) for each nuclear gene.
In order to understand evolution, it’s crucial to remember that evolution is always studied at the population level. Also, if a population were to stay exactly the same from one generation to the next, it would not be evolving. So evolution requires both a population of breeding individuals and some kind of a genetic change occurring within it. Thus, the simple definition of evolution is a change in the allele frequencies in a population over time. What do we mean by allele frequencies? Allele frequencies refer to the ratio, or percentage, of one allele (one variant of a gene) compared to the other alleles for that gene within the study population (Figure 4.2). By contrast, genotype frequencies are the ratios or percentages of the different homozygous and heterozygous genotypes in the population. Because we carry two alleles per genotype, the total count of alleles in a population will usually be exactly double the total count of genotypes in the same population (with the exception being rare cases in which an individual carries a different number of chromosomes than the typical two; e.g., Down syndrome results when a child carries three copies of Chromosome 21).