The Evolution of
Populations and Speciation
Modern Biology Chapter 16
Genetic Equilibrium
Modern Biology
16-1
Variation
How does variation arise?
• Mutations
– changes in the structure of an organism’s
DNA/ genes
– genes are the source of variation
– rates of mutation are quite variable
• may have positive and negative affects
How does variation arise?
• gene shuffling
– when genes are combined in
different ways, different results
occur
• Independent assortment and
crossing over shuffle genes
• How common is genetic
variation?
– Only about 5% of all vertebrate
genes have 2 or more alleles
Genes, fitness and adaptation
• Fitness: success in passing genes on to
the next generation
• Adaptation: any genetically controlled trait
that increases an organism’s ability to
pass along copies of its genes
Genes, fitness and adaptation
• Genes also define species
– species: a group of similar looking organisms
that can breed with one another and produce
fertile offspring
– reproductive isolation: idea that members of
different species cannot reproduce
– gene pool: group of alleles available to a
population
Reproductive Isolation
Once upon a time certain fruit flies
found mates on rotting bananas
and ignored the rotting mangoes.
Two flies were blown away to an
island where there were no
bananas, so they found each other
on mangoes.
When the flies from the mango
island were blown back to the main
land, they found mates on mangoes
while the original flies still preferred
bananas, hence reproductive
isolation.
Speciation
• The branching points on this partial
Drosophila phylogeny represent speciation
events that happened in the past.
• The scene: a population of wild fruit flies is
minding its own business on several
bunches of rotting bananas, cheerfully
laying their eggs in the mushy fruit.
• Disaster strikes: A hurricane washes the
bananas and the immature fruit flies they contain
out to sea. The banana bunch washes up on an
island off the coast of the mainland. The fruit
flies mature and emerge onto the lonely island.
The two portions of the population, mainland
and island, are now too far apart for gene flow to
unite them. At this point, speciation has not
occurred — mainland and island fruit flies can
mate and produce healthy offspring.
• The populations diverge: Ecological
conditions are slightly different on the
island, and the island population evolves
under different selective pressures and
experiences different random events than
the mainland population does.
Morphology, food preferences, and
courtship displays change over the course
of many generations of natural selection.
• So we meet again: When another storm
reintroduces the island flies to the mainland,
they will not readily mate with the mainland
flies since they've evolved different courtship
behaviors. The few that do mate with the
mainland flies, produce unviable eggs
because of other genetic differences between
the two populations. The lineage has split
now that genes cannot flow between the
populations.
Genes, fitness and adaptation
• Swimming in the gene pool
– without mutation or natural selection the
frequency of alleles in the gene pool remains
fairly constant
– genes get resorted, but the frequency does
not change
Modern Biology 16-2
Natural Selection
• The idea that organisms better suited to their
environments will survive to reproduce more
successfully than organisms less well adapted.
Types of natural selection
• Stabilizing selection
– Selection in which the extremes are selected
against
Types of natural selection
• Stabilizing selection
– Selection in which the extremes are selected
against
Types of natural selection
• Stabilizing selection
– Selection in which the extremes are selected
against
Types of natural selection
• Stabilizing selection
– Selection in which the extremes are selected
against
Types of natural selection
• Directional selection
– Selection against one of the extremes
Types of natural selection
• Disruptive selection
– Selection against the middle
Formation of Species
Modern Biology
16-3
The birth of a new species
• speciation: the formation of new species
The birth of a new species
• for speciation to occur, a population must
evolve enough genetic changes so that
breeding cannot occur between the groups
(why dogs are all one species)
The birth of a new species
• usually occurs
when a physical
barrier comes
between different
populations, barrier
may be removed
later
– barriers include
plate tectonics,
mountain ranges
rising, a busy road
Animation
The pace of evolution
• gradualism: a slow and steady change over
long periods of time
– Darwin believed this means of evolution
– Evidence in horse evolution
The pace of evolution
• punctuated equilibrium: species remain
unchanged for a long time and then periods of
evolution occur quickly
– evidence in
– occurs after mass extinctions
Darwin’s finches
• 13 different species of finches within the
Galapagos islands
• all evolved from one common ancestor
Darwin’s
finches
• island theory:
different
conditions will
create different
pressures
Darwin’s finches
• beak shapes are very different, as are body
structures
• different diets have caused the evolution of
different shaped beaks
– seed eaters have sturdy relatively short beaks for
breaking open hard seed coats
– insect eaters have long thin beaks for grabbing
grubs in tree bark
A Galapagos study
• looking for natural selection
– birds became specialists in eating different foods
• what selection does
– when food is plentiful, the variation doesn’t matter
much
– when food is scarce, the variants best able to cope
have a higher fitness
Evolving Differences
• Organisms that are more closely related tend to
have more traits in common
– Not as true as once thought
– Example: sharks and whales
– All vertebrates have many similar characteristics:
backbone, 4 limbs, etc.
Evolving Differences
• Adaptive radiation: occurs when an organism
or group of organisms colonizes a new area
and competition drives changes toward
organisms more suited to that environment
– Homologous structures become less similar
– Example: Darwin’s finches
Evolving Differences
• Convergent evolution: occurs when different
organisms live in similar environments and
adapt to those new pressures in similar ways
– Example: sharks and whales, elephants and
brontosaurs
– Non-homologous structures become more similar
Evolving Differences
• Distant relatives may have converged and
diverged many times making determinations
more difficult