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Chapter 12
Who Am I?
Species and Races
12.1 What is a Species
 The primary category in Linnean classification is the species
 Species are given binomial (two-part) names
 First part consists of the genus
 Second part is the specific epithet
 Genus name is capitalized, both names are italicized or underlined when used
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Same genus, different species
Panthera leo and Panthera pardus
12.1 What is a Species - The Biological Species Concept
 The biological species concept states that species are reproductively isolated from one
another.
 In nature, members of the same species can potentially interbreed
 Members of different species cannot interbreed
 Sum total of alleles in a species is called gene pool
12.1 What is a Species - The Nature of Reproductive Isolation
 Movement of alleles within a gene pool is called gene flow.
 Gene flow does not occur between species, due to reproductive barriers.
 Two general kinds of reproductive barriers:
 Prefertilization – prevent fertilization from occuring
 Postfertilization – fertilization occurs, but hybrid cannot reproduce
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Five different prefertilization reproductive barriers
 Spatial
 Behavioral
 Mechanical
 Temporal
 Gamete incompatibility
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Spatial reproductive isolation
 Species are separated by distance
 Example: polar bear (Arctic) and spectacled bear (South America)
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Behavioral reproductive isolation
 Differences in mating behavior may interfere with reproduction
 Example: many birds have mating songs or dances
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Mechanical reproductive isolation
 Sexual organs are incompatible
 Example: many insects have “lock and key” genitals
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Temporal reproductive isolation
 Difference in timing of reproduction
 Example: organisms might have different mating or flowering times
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Gamete compatibility reproductive isolation
 Eggs and sperm of different species unable to fuse
 Common among organisms that release gametes into the environment
12.1 What is a Species - The Nature of Reproductive Isolation
 Postfertilization barriers to reproduction:
 Hybrid inviability
 Hybrid sterility
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Hybrid inviability
 Zygote unable to develop because genetic instructions are incomplete
 Example: sheep crossed with goat produces an embryo, but it dies early in development
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Hybrid infertility
 Product of interspecies cross is unable to reproduce
 Example: mule
12.1 What is a Species - Speciation: an Overview
 Three steps necessary for one species to give rise to a new species
1. Isolation of gene pools of populations
2. Evolutionary changes in gene pools of populations
3. Evolution of reproductive isolation between populations
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Once reproductively isolated, how long does the process of evolution take?
Two general explanations
 Gradualism – slow accumulation of small changes over long period of time
 Punctuated equilibrium – rapid change followed by long periods of no change
 Evidence that both processes are at work
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Isolation and divergence of gene pools
 Migration can lead to isolation of a population
 Examples include oceanic islands
 Because migrant populations are small, genetic changes can occur rapidly
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More than 50 species of Hawaiian silversword are descended from a migrant population of
California tarweed.
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Geographic barriers can also intrude between populations
 Isthmus of Panama connects North and South America, but divides an ocean gulf
 6 pairs of snapping shrimp species exist. One species pair is on the Carribean side and the
other is on the Pacific side
 Genetic evidence indicates that each species pair is descended from ancestral species
separated by rise of Panama
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Species separated by barriers or distance are allopatric
Species occupying the same area are sympatric
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Apple maggot flies appear to be speciating sympatrically.
 Apples are not native to N. America, introduced by colonists
 Apple maggot flies infest hawthorns and apples
 Flies mate on fruit where they will lay their eggs
 Hawthorns fruit 1 month after apples
 Apple-preferring and hawthorn-preferring flies appear to have little gene flow
 In plants, speciation can occur instantaneously, with no barriers between populations.
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Hybrids between plant species are usually infertile.
Hybrids can occasionally become fertile through polyploidy.
Many plants produce male and female gametes and can self-pollinate.
Because of change in chromosome numbers, offspring are genetically isolated from their
parent plants.
 Canola developed as a result of polyploidy
 Scientists suspect that this process is responsible for much of plant species diversity.
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The evolution of reproductive isolation
 No rule to tell with certainty when populations are truly isolated
 The dragonflies in the picture below cannot interbreed
 All dogs are capable of interbreeding
12.2 Races and Genealogical Species
 Biologists do not agree on a definition of the term “race”, and some feel the concept is
meaningless
 Any biological definition of race would probably have the following concepts:
 Races are populations of one species that have diverged
 Little gene flow, so any evolutionary changes in one population do not occur in the others
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Possible criteria for defining race
 Genealogical species concept defines species as smallest group of reproductively
compatible individuals descended from a single common ancestor
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Spotted owl has 3 distinct populations that could theoretically interbreed, but are separated
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physically
Are human races like genealogical species?
12.3 Humans and the Race Concept - The Morphological Species Concept
 The morphological species concept emphasizes physical differences
 A species is defined as a group of individuals with some reliable physical characteristics
that distinguish them from all other species
 Morphological differences are assumed to correlate with isolation of gene pools
12.3 Humans and the Race Concept - Modern Humans: A History
 Immediate predecessor of Homo sapiens was Homo erectus
 H. erectus first appears in fossil record ~1.8 MYA
 H. sapiens first appears in fossil record ~250,000 years ago.
 Debate about precise model of evolution of modern humans, but all ultimately have Africa
as the place that humans came from
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Most evidence suggests that moderns humans descended from African ancestors within the
last 200,000 years.
 Humans have less genetic diversity than any other great ape (indicates young species).
 Among human populations, those in Africa have greatest genetic diversity.
 Physical differences between humans must have arisen within about 10,000 generations
(not very long).
 Thus, all humans share a recent common ancestor.
12.3 Humans and the Race Concept - Genetic Evidence of Divergence
 Evolution results in a change in allele frequency. If a race is isolated from other races, there
are two expectations:
 Some alleles unique to the race
 Differences in allele frequency compared to other races
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Hypothetical example of a race-specific allele and different allele frequencies between races.
12.3 Humans and the Race Concept - Using the Hardy-Weinberg Theorem to Calculate
Allele Frequencies
 The Hardy-Weinberg theorem states that allele frequencies will remain stable in populations
that meet three conditions
 Large size
 Random mating
 No migration
 No natural selection
 Also provides a means of making predictions of what will happen if assumptions are violated
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HW Theorem is expressed as an equation
 p2 + 2pq + q2 = 1
 p and q are alleles of a gene
 p2 and q2 are homozygous condition (i.e. AA or aa)
 2pq is heterozygous condition (i.e. Aa)
12.3 Humans and the Race Concept - Human Races Are Not Biological Groups
 No race-specific alleles have been identified
 Although sickle cell anemia has long been thought of as a “black disease”, it is not found
in all African populations and it is found in non-African populations
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Populations classified in the same race do not have similar allele frequencies
 The distribution of alleles within racial groups is about the same as between racial groups
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Human races have never been truly isolated
 B blood type first evolved in Asia, but is now widespread. There are no clear boundaries in
the human gene pool.
12.4 Why Human Groups Differ - Natural Selection
 Sickle-cell anemia is an adaptation to environments where malaria is common
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Nose shape is correlated with climate factors. Populations in dry climates have narrower noses
than those in moist climates.
12.4 Why Human Groups Differ - Convergent Evolution
 Traits shared by unrelated populations due to similarities of environment are examples of
convergent evolution
 Human skin color appears to be result of convergent evolution
 Strong correlation between skin color and exposure to UV light
12.4 Why Human Groups Differ - Genetic Drift
 Change in allele frequency that occurs due to chance is genetic drift
 Humans are highly mobile
 Small groups colonizing new areas are prone to genetic drift
 Often drift occurs in three different situations
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Founder effect – genetic differences resulting from a small sample
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Population bottleneck – genetic change resulting from a dramatic reduction of population
numbers
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Chance events – small populations are especially prone to loss of alleles though chance
12.4 Why Human Groups Differ - Sexual Selection
 When a trait influences chance of mating it is sexually selected
 Peacock tail sexually selected
 Sexual selection often accounts for male/female differences in many animal species
 There is some evidence that sexual selection accounts for differences in human
male/female body size
12.4 Why Human Groups Differ - Assortative Mating
 Tendency of organism to choose mate that resemble self is assortative mating
 People tend to mate assertively by height (i.e., tall women marry tall men) and skin color
 Positive assortive mating tends to exaggerate differences between groups
12.5 Race in Human Society
 Scientific data indicate that racial categories are biologically meaningless
 Racial categories are socially meaningful and are socially constructed
 BUT, arbitrary groupings are not necessarily bad – we group ourselves into other categories
(religious, sports fans, cat lovers, etc.)