Speciation
Genetic Equilibrium
Disruption of Genetic Equilibrium
Formation of Species
Genetic Equilibrium
• Species: a group of individuals that
look similar and whose members are
capable of producing fertile
offspring in the natural environment
Morphological Species Concept
• Morphological Species Concept: the
idea that organisms can be classified by
differences in their appearance
– Using this concept, scientists can readily
communicate about the characteristics,
behavior, and relationships of organisms.
– The morphological concept of species is
limited because it does not account for the
reproductive compatibility of morphologically
different organisms.
Biological Species Concept
• Biological Species Concept: the idea that
organisms can be classified by their ability
to breed with one another
• Using both the biological and
morphological concepts of species,
modern scientists have arrived at the
current definition of the term species.
Species: a group of
individuals that look similar
and whose members are
capable of producing
fertile offspring in the
natural environment
Variation of Traits in a Population
• Population: all the members of a
species that live in the same area
and make up a breeding group
• Within a population, individuals vary
in many observable traits.
Caribou
Population
Distribution
Variation
is the
Raw
Material
of
Natural
Selection
Variation of Traits in a Population
What causes variation in traits?
• Environmental Factors
• Heredity
– A difference in genotype usually results
in a difference in phenotype.
– Variations in genotypes arise through:
• Mutation
• Genetic Recombination
– Independent Assortment
– Crossing Over
Allele Frequencies and
Genetic Equilibrium
• Gene Pool: all the genes for all the
traits in a population
• Allele Frequency: the percentage of
an allele in a gene pool
• Genetic Equilibrium: the condition in
which allele frequencies in a
population do not change from
generation to generation
Genotypic and Allele Frequencies
Hardy & Weinberg
• Hardy and Weinberg outlined the
conditions necessary for genetic
equilibrium.
Godfrey Hardy
Wilhelm Weinberg
Hardy-Weinberg Principle
• Hardy and Weinberg outlined the
conditions necessary for genetic
equilibrium.
1. no mutations occur
2. individuals neither enter nor leave the
population through migration
3. the population is large
4. individuals mate randomly
5. natural selection does not occur
Hardy-Weinberg Principle
• If one of these conditions does not hold
true, allele frequencies of the
population may change. In other
words, evolution will occur.
Disruption of
Genetic Equilibrium
Disruption of Genetic
Equilibrium
Evolution occurs
when genetic
equilibrium is
disrupted.
1. Mutation
• Mutations affect genetic equilibrium
by producing totally new alleles for
a trait.
• In addition, mutations can change
the frequency of the alleles already
present in the gene pool.
• The occurrence of mutation in most
alleles is too low to cause major
changes in the allele frequencies of
an entire population.
1. Mutation
2. Migration
• Migration: the movement of
individual organisms into or out of a
population, community, or biome
– Immigration: movement of new
individuals into a population
– Emigration: departure of individuals
from a population
• Gene Flow: the movement of genes
into or out of a population
2. Migration
3. Genetic Drift
• Genetic Drift: a shift in allele
frequencies in a population due to
random events or chance
– In small populations, chance can
significantly affect allele frequencies
from one generation to the next.
3. Genetic Drift
3. Genetic Drift
4. Non-Random Mating
• If animals chose their mates then
certain traits would be selected for
over less desirable ones.
5. Natural Selection
• Natural selection is an ongoing
process in nature, and is the single
most significant factor disrupting
genetic equilibrium.
4 Types of Natural Selection
1.
2.
3.
4.
Stabilizing Selection
Directional Selection
Disruptive Selection
Sexual Selection
Stabilizing Selection
• Stabilizing Selection: a type of natural
selection in which the average form of a
trait causes an organism to have an
advantage in survival and reproduction
– The extreme forms of the trait confer a
disadvantage to the organism.
– Stabilizing selection is most effective in a
population that has become well adapted to
its environment.
– Stabilizing selection is the most common type
of NS.
Directional Selection
• Directional Selection: a type of
natural selection in which the
distribution of a trait is shifted toward
one of the extremes
Disruptive Selection
• Disruptive Selection: natural
selection in which individuals with
either of the extreme forms of a trait
have an advantage in terms of
survival and reproduction
– The average form of the trait confers a
selective disadvantage to the
organism.
Sexual Selection
• Sexual Selection: the preferential
choice of a mate based on the
presence of a specific trait
– Sexual selection may be stabilizing,
directional, or disruptive.
Formation of
Species
Speciation
• Disruption of genetic equilibrium leads to
changes in the gene pool of a
population.
• Natural selection causes the most
significant genetic changes in a
population, however, such changes do
not necessarily lead to speciation.
• Speciation: the formation of a new
species
Speciation
Speciation
Isolated Populations
• For a new species to arise, interbreeding
of the population of fertile offspring must
somehow cease among members of a
formerly successful breeding population.
– For this to occur, populations or segments of
populations must somehow become isolated.
• Two Forms of Isolation:
– Geographic Isolation
– Reproductive Isolation
Geographic Isolation
• Geographic Isolation: the
physical separation of
populations
– Examples: population
becomes divided by land or
water barrier, colonization of
new island, river changes
course, highway built across
field, etc.
Reproductive Isolation
• Reproductive Isolation: the inability
of formerly interbreeding organisms
to produce offspring
– Reproductive isolation can occur
through disruptive selection.
Rates of Speciation
• Punctuated Equilibrium: a theory that
speciation may occur during brief periods
of rapid genetic change
• Gradualism: a theory that speciation may
occur through a gradual change of
adaptations
• Extinct: the state of a species that no
longer exists
– For a species to continue to exist, some
members must have traits that allow them to
survive and pass their genes on to the next
generation.