Great Migration: Life for Migrants in the City. By the end of 1919, some 1 million blacks had left the South, usually traveling by train, boat or bus; a smaller number had automobiles or even horse-drawn carts.
Genetic variation is increased by meiosis During fertilisation, 1 gamete from each parent combines to form a zygote. Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA.
The combination of different alleles is an individualâs genotype. Natural selection acts on phenotype, not genotype. Genetic variation and evolution are studied in populations. Members of a population share a common group of genes, called a gene pool.Allele frequency is the number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur.n genetic terms, evolution is any change in the allele frequency in a population. Sources of Genetic Variation The three main sources of genetic variation are mutations, genetic recombination during sexual reproduction, and lateral gene transfer. A mutation is any change in a sequence of DNA.