How Humans Evolved Textbook Notes: Part 4 Evolution and Modern Humans Chapter 14, 15

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12 Apr 2012
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Part 4: Evolution and Modern Humans
Chapter 14
Human Genetic Variation
Explaining Human Variation
Human vary in numerous ways
o Language, culture, fashion, technology, architecture etc.
Scientists distinguish 2 sources of human variation: genetic and environmental
o Genetic variation: refers to the differences between individuals that are
caused by the genes that they inherited from their parents
o Environmental variation: refers to the differences between individuals
caused by environment factors (climate, habitat…) on the organism’s
phenotypes
Culture is an important source of environmental variation
It is difficult to determine the relative importance of genetic and environmental
influences for particular phenotypic traits
o Both genetic transmission and shared environments cause parents and
offspring to be similar
Genetic variation is governed by the processes of organic evolution: mutation,
drift and selection
It is important to distinguish variation within human groups from variation among
human groups
o Variation within groups: refers to differences between individuals within
a given group of people (ex: height between two basketball players)
o Variation among groups: refers to the differences between entire groups
of people (ex: height between basketball players and soccer players)
Variation in Traits Influenced by Single Genes
By establishing the connection between particular DNA sequences and specific
traits, scientists have shown that variation in some traits is genetic
We can prove that traits are controlled by genes at a single genetic locus by
showing that their patterns of inheritance conform to Mendel’s principles
Causes of Genetic Variation within Groups
Mutation can maintain deleterious genes in populations, but only at a low
frequency
Many diseases are caused by recessive genes
o Natural selection steadily removes such genes but they are constantly
being reintroduced by mutation
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Selection-mutation balance: refers to when mutation will introduced enough new
mutants to maintain a constant frequency of the gene
Selection can maintain variation within populations if heterozygotes have higher
fitness than either the two homozygotes
When heterozygotes have a higher fitness than either homozygote has, natural
selection will maintain a balanced polymorphism, a steady state in which both
alleles persist in the population
Variation may exist because environments have recently changed and genes that
were previously beneficial have not yet been eliminated (ex: colonization; the
bringing about of new diseases, foods and lifestyle)
Causes of Genetic Variation among Groups
Selection that favours different genes in different environments creates and
maintains variation among groups
o Ex: the geographical distribution of genetic diversity affecting milk and
lactase persistence correspond very closely with each other
The sequencing of the human genome makes it possible to detect selection from
the DNA sequences
Positive selection leaves detectable patterns in the genome
A new beneficial mutation causes a selective sweep in which both the mutation
and DNA linked to the mutation on the same chromosome spread through the
population
o Include several categories:
Reproductive system (genes affecting the protein of the sperm…)
Morphology (skin colour…)
Digestion (metabolism of alcohol, carbs and fatty acids)
Genetic drift creates variation among isolated populations
o Genetic drift caused by the expansion of a small founding population is
sometimes called the founder effect
Overall patterns of genetic variation mainly reflect the history of migration and
population growth in the human species
As human populations expand, local populations become genetically isolated
form one another and begin to accumulate genetic differences
o Gene flow blurs the effects of expansion
Geographical distance is a very good predictor of genetic distance between human
populations
Variations in Complex Phenotypic Traits
Heritability (of phenotypic traits): the measure that computes the proportion of
variation due to the effects of genes
Genetic Variation within Groups
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