Lecture 4: Modern Theory of Evolution, Human Variation and Biocultural Evolution

Modern Theory of Evolution

  • aka the Modern Synthesis incorporates the old ideas of Darwin with what we know about evolution now, including the role of variation plus natural selection. Page 60-61.
  1. Variation in a population
  • Is produced by mutation, which is the only way to get a brand new allele. Mutation mixes existing alleles to produce a different new one.
  • The alleles are then distributed by recombination and sexual reproduction, which are a much higher source for diversity than gene flow or genetic drift.
  • Natural selection acts on variation which already exists in a population. Natural selection is the most important long term factor in the direction of evolutionary change. Evolution is the change in the genetic structure of a population, which mostly in macroevolution. Page 65.
  • Reproductive fitness.


  1. Long time scale (macro-evolutionary, which is when there are large visible changes in a population. Ex speciation, this usually takes 100's of generations).
  2. Shorter time scale (micro-evolutionary, which is when there is a change in the allele frequency from one generation to the next. Over time this leads to a shift in the allele frequency). Ex. blood groups:


  • A population is a group of interbreeding individuals in the same species who are marked by a degree of genetic relatedness and share a common gene pool. They can have fertile offspring.
  • Any two people in this world can interbreed, but we have some factors that are geographical, ecological and social which determine mate choice. But genetic material shows us that humans are amazingly interbred for a species that is the most widely distributed.

Sources that Produce and Redistribute Genetic Variation

  1. Produce Variation:
  • Sexual reproduction or meiosis. In sexual reproduction, only the phenotypes change, not the alleles.
  • Recombination
  • Mutation, which is the only source for new alleles.

2. Redistribute Variation:

  • How phenotypes and genotypes spread into new or nearby populations.
  • Some species might theoretically interbreed, but they may be geographically isolated, etc.
  • Gene flow is the exchange of alleles between populations due to the interbreeding of migrants. It includes sexual reproduction and recombination.
  • Genetic drift only occurs in small populations, like on islands which are geographically isolated. The allele frequency changes, which is an evolutionary change. It is caused by random factors. The Founder effect is a type of genetic drift.

3. Allele frequencies:

  • Can only be changed by mutation, gene flow or genetic drift, not sexual reproduction or recombination.

Human Variation - Race

Race and Polytypic Species

  • Humans are a polytypic species because we have different local populations that differ in expression of phenotypic traits. These are called subgroups, because people in different regions look different. The effect now is reduced because of immigration and interracial marriages etc.

Race as Both Social and Biological Concepts

  • Races are subgroups of the same species that could theoretically interbreed but are geographically isolated.
  • Anthropology is about the cultural and physical differences of human beings, such as ethnicity, which is cultural groups, not biological.

Problems With Use of Race and the Definition of Human Biological Groupings

  1. Controversy with social associations: Before the 1800's, people were already aware that skin colour was a bad method for dividing people into groups. Categories for skin colour are overlapping, so it is continuous, making it hard to put people into categories. Instead, they tried measuring head size and shape to find new groups. They also tried placing people into categories based on their nationality, religion etc. 
  2. Races are not fixed biological units
  3. Continuous variation and polygenic traits
  4. There is more variation within populations than between them. Ex. a person's religion, geographic location, ethnicity, etc. We interbred a lot in the past, there was no evidence of isolation
  5. Modern synthesis focused on the interactions.

Human Variation and Biocultural Evolution

New Foci for Human Variation Studies

  1. Variation in allele frequency (genotypic) within and between populations
  2. Adaptive significance of genotypic and phenotypic variation

Variation in Allele Frequency - Polymorphism and Human Biocultural Evolution

  1. Polymorphism is when there is a genetic trait with more than one allele in an appreciable amount in a population. Ex. Table 4.1. There is a reason why the allele for sickle cell anemia is preserved in a population. It may have physical or environmental advantages. Ex. You may be lactose intolerant if there is not much milk available in that environment.
  2. The Hardy-Weinberg equilibrium equation is used to predict what the allele frequency should be under ideal conditions. Compare this to the actual preserved alleles to see if any evolutionary changes occurred in the population. Page 80.
  3. Multivariate approaches (statistics) are when you look at more than one trait.

Genetics an Environment (Physical and Cultural) - Review

  1. Importance of environment
  2. Importance of cultural adaptations
  3. Biocultural evolution
  4. Biocultural adaptations. Ex. the Inuit eat a lot of fish which contains vitamin D because they don't get the vitamin from the sun. It is their way of thriving and adapting to their environment.

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