Which statement best describes how speciation occurs?
A. Gene flow brings in new alleles, increasing variation within the population’s offspring.
B. A changing environment improves conditions by reducing competition for food, water, and shelter, and reproduction is increased.
C. A small population is able to obtain more resources, so more offspring are born and survive each year.
D. A geographically isolated population experiences changes in gene frequency, and individuals with the most suitable adaptations produce the most surviving offspring.