Descent with Modification

Evolution is the change in the frequency of genes within a population from one generation to the next. Natural selection explains how that change happens.

A brief explanation of natural selection:

  • Within any population there is some inherent variation. One giraffe’s neck is going to be slightly longer than another giraffe’s. One cheetah is going to run slightly faster than another cheetah.
  • Because there are limited resources (food and preferred mates) there is always going to be competition among individuals in a population for access to those resources.
  • As that competition plays out, some variations are going to prove more beneficial than others.
  • Which variation is the most beneficial depends on the environment and other selection pressures (such as predators).
  • Some individuals are going to be more successful in this competition because they posses those beneficial variations.
  • Success shall be measured in terms of reproductive success – having offspring that reach reproductive maturity and can themselves have children. This is termed fitness.
  • Over time the frequency of beneficial traits will become more prevalent within a population and detrimental traits become less frequent.
  • Evolution never stops and natural selection is always relevant because (1) your competitors are evolving too and (2) the environment is constantly changing. This is termed the Red Queen.

What does this have to do with genetics? Pretty much everything. The problem Darwin struggled to solve was: Where does variation come from? There are a number of factors that produce and redistribute variation, the most important of which are sex and mutation.

  • Sexual reproduction involves the combination of genetic material from two parents to make a unique individual – the offspring.
  • Because the biological process that produces sperm and eggs involves a shuffling of a parent’s genes siblings have a different genetic make-up (genotype) from one another. One’s body and outward appearance (phenotype) is an expression of the interaction between genotype and environment.
  • Sexual reproduction is like a shuffling of the genetic deck of cards. Each shuffle rearranges the deck so that when you deal the cards they come out different. Sex always increases the amount of variation within a population.
  • Mutation is a natural process that is inherent in DNA replication.
  • Every time a cell splits into two cells it must also copy its DNA. In this process of copying sometimes mistakes happen. Usually they are corrected, but sometimes they are not. Mutations are simply uncorrected mistakes arising from DNA copying.
  • The only mutations of any consequence to evolution are those that occur in the sperm and eggs.
  • Mutation is the origin of all variation. If sex is shuffling the deck, mutation is like creating entirely new cards.

The great advantage of sexual reproduction is that it produces variation faster than mutation. Thus a population of sexual reproducers has greater variation than a population of asexual reproducers and can adapt more quickly to changing environments.


About Matt Thompson

Matt Thompson is a project cataloger at The Mariners' Museum library. He has a Ph.D. in anthropology from the University of North Carolina - Chapel Hill and was formerly a professor at ODU. You can find him on Twitter @m4ttTh0mps0n.
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