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Chapter 12 – Who Am I?
Species and Races
Biological Species
• A species is a group of individuals that, in
nature, can interbreed and produce fertile
offspring but cannot reproduce with members
of other species
• How can species be reproductively isolated?
Reproductive Isolation by Species
1. Spatial Isolation: species don’t come in contact
2. Behavioral: behaviors that prepare partners for
mating are different
3. Mechanical: sex organs are incompatible
4. Temporal: timing of readiness to reproduce is
different
5. Gamete Incompatibility: proteins on egg that
allow sperm binding do not bind with sperm
https://www.youtube.com/watch?v=8yvEDqrc3XE
Postfertilization Barriers
• Hybrid Inviability: zygot cannot complete
development because of incomplete genetic
instructions
• Hybrid Sterility: hybrid organism cannot
produce offspring
Steps in the Process of Speciation
1. Isolation of the gene pools of subgroups, or
populations, of the species
2. Evolutionary changes in the gene pools of
one or both of the isolated populations
3. The evolution of reproductive isolation
between these populations, preventing any
future gene flow
Race & the Genealogical Species Concept
• Biological race: populations of a single species
that have diverged from each other as a result of
isolation of their gene pools
• Genealogical Species Concept:
– Consist of organisms that can interbreed
– Are all descendants of a common ancestor
– Represent independent evolutionary lineages
• Reflects closer shared ancestry-genealogyamong certain individuals within a biological
species
Humans Originated in Africa
• Morphology of Human Ancestors in the fossil
record
• Human Species is ~200,000
Genetics of Division of Species into Races
1. Alleles that are unique
to a particular race
2. Similar allele
frequencies for a
number of genes
among populations
within races
3. Differences in allele
frequencies among
populations in
different races
Hardy-Weinberg Theorem
• Allele frequencies will remain stable in
populations that are large in size, randomly
mating, and experiencing no migration or natural
selection
• Describes the relationship between two allele
frequency and genotype frequency for a gene
with two alleles in a stable population
https://www.youtube.com/watch?v=u9EVOzVJeZc
Hardy- Weinberg Equations
• Groups Quiz Next Week – solving a HardyWeinberg problem (and other questions)
• How to calculate:
– https://www.youtube.com/watch?v=oG7obMtO8c
Why Aren’t Human Races Different Species?
• No alleles are found in
all members of a race
• Populations within a
race are often as
different as
populations compared
across races
• Human races have
never been truly
isolated
Common Traits & Natural Selection
• Sickle-cell allele protects
an individual from malaria
– In Africa and SW Europe,
there is an overlap of sicklecell and malaria distribution
• Skin color is dependent on
the exposure level of UV
light
• Nose width is dependent
on the moisture of climate
– More surface area and
moisture in a narrow nose
Evolution & Genetic Drift
• Genetic Drift: changes
in allele frequency due
to chance events
• Types of chance events:
– Founder effect – small
sample of a larger
population establishes a
new population
– Bottleneck – a small
subset of a population
https://www.youtube.com/watch?v=NJQ-i1EgGps
Selecting a Mate
• Sexual Selection: a trait influences the
likelihood of mating
– Peacocks with large tails must be both physically
strong and smart to survive
• Assortative mating: individuals choose mates
who are like themselves
– Ex. Height in humans