The modern synthesis theory states that evolution involves changes in populations.

The modern synthesis theory, often referred to as the neo-Darwinian synthesis, is a combination of Charles Darwin's theory of evolution by natural selection with Gregor Mendel's theory of genetics as the basis for biological inheritance. According to this theory, evolution is defined as a change in the frequency of alleles within a population over time. Here are the logical steps explaining how evolution involves changes in populations:

1. Variation exists within all populations of organisms. These variations are partly due to mutations, which are changes in the genetic material (DNA), and can also arise from sexual reproduction that shuffles genes.

2. Some variations are better suited to the organisms' environments, giving them a survival advantage. This is known as "adaptive traits."

3. Organisms with adaptive traits tend to survive longer and have more opportunities to reproduce, passing these advantageous traits on to their offspring. This process is called "natural selection."

4. Over time, natural selection can increase the frequency of the adaptive traits within the population, as more individuals inherit these traits.

5. As the frequency of these advantageous alleles increases, the population evolves.

6. The introduction of new alleles through mutation or gene flow from other populations and the recombination of alleles through sexual reproduction can introduce new traits for selection to act upon.

7. Over long periods, the accumulation of these changes can lead to the emergence of new species.

The modern synthesis provides a framework for understanding how evolutionary processes can lead to the diversity of life we observe today and is supported by substantial empirical evidence from observation, experimentation, and the fossil record.