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Evolutionary Forces that Impact Variation Within Species

Updated on July 21, 2016

Variation Among Species

There are several different scientific processes that lead to differences or variation within the same species. This includes differences in traits like eye and hair color, height, and even tolerance to the sun. Both microevolutionary and non-evolutionary forces combined to affect allele frequencies which are responsible for different traits. Evolutionary mechanisms, include genetic drift, gene flow, mutations and natural selection. These evolutionary forces are the reason that people and other different species look so different from each other.

Gene Flow

  • An allele is a different variant of a gene. Offspring receive one allele from each of their parents for any given trait. This causes the different populations to become genetically similar.
  • Gene flow occurs when alleles are passed from one population to another. The variation within the recipient population will increase, but the variation between the two populations will decrease.
  • Geographical distance plays an important role in gene flow. Populations that are close together will be more similar genetically.
  • Culture also plays an important role in gene flow. If a community does not intermix with other populations because of religious or other cultural practices, gene flow does not take place.



  • Mutations are random changes in the DNA that can create new alleles which leads to new genetic variations. Mutations are extremely rare, and only mutations that occur in sex cells are passed on to offspring.
  • Mutations can be beneficial, harmful or neutral. When a mutation does occur in a sex cell, and a new allele is passed on to offspring, the fate of the allele is mostly left to genetic drift. Since it is rare, more than likely the new allele will be wiped out.

Natural Selection

  • New alleles created by mutations can also be spread through a population by natural selection. Natural selection is the phenomena that advantageous alleles are more likely to be passed on.
  • If a new allele gives an individual some type of reproductive advantage, that individual will be more likely to pass on their genetic material containing the allele than an individual without the allele.
  • Over time, the allele will become more common, and the overall allele frequencies will change. There is no guarantee that an advantageous allele will present itself in any given generation.

Genetic Drift

  • Random genetic drift occurs in every population because the passing of alleles from one generation to the next is essentially random. Each parent can only pass one allele to their offspring.
  • Since each reproduction is an independent event, each allele has an equal chance of being passed on every time the parent reproduces.
  • Genetic drift has a particularly important impact on small populations. In small populations, it is possible for some traits to be lost because no one in a generation inherits a particular gene.
  • Mixture of genetic material between populations, or gene flow, will usually counter the effects of genetic drift.
  • However, if there is not mixture between populations there is no other counter to the effects of genetic drift. If a gene is eliminated, the only way a new gene can be introduced into the population is a mutation.

The blue allele was not passed from parent to offspring.
The blue allele was not passed from parent to offspring.


The combination of these forces results in the tapestry of life we see all around us. It is the reason that humans come in so many shapes and sizes. It's the cause of different breeds of dogs. Ultimately these evolutionary forces have been the building blocks of life for millions of years, but their daily impact can be seen all around you every day.


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    • brenda12lynette profile imageAUTHOR


      6 years ago from Utah

      Thanks Vasiliki! I think learning all the intricate details about life makes it so much more fascinating!

    • Vasiliki Bouras profile image

      Vasiliki Bouras 

      6 years ago

      Excellent, and very informative.


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