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Chapter
16
Evolution of Populations
16-1 Genes and Variation
• As Darwin developed his theory of evolution, he worked under a serious handicap
• He didn’t know how heredity worked
• This lack of knowledge left two big gaps in Darwin’s thinking
1. He had no idea how heritable traits pass from one generation to the next
2. He had no idea how variation appeared, even though variation in heritable traits was central to Darwin’s theory
• During the 1930’s Evolutionary biologists connected Mendel’s work to Darwin’s
• By then biologists understood that genes control heritable traits
How Common Is Genetic Variation?
• Many genes have at least 2 forms or alleles
• Animals such as horses, dogs, mice, and humans often have several alleles for traits such as body size or coat color
Variation and Gene Pools• Genetic variation is studied in
populations
Population• A group of individuals of the same
species that interbreed
• Because members of a population interbreed, they share a common group of genes called a gene pool
Gene pool• All the genes including all the
different alleles, that are present in a population
Relative Frequency• The number of times that the allele
occurs in a gene pool, compared with the number of times other alleles for the same gene occur
• In genetic terms, evolution is any change in the relative frequency of alleles in a population
Sources of Genetic Variation
• The 2 main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction
Mutations• Any change in a sequence of DNA• Mutations can occur because of• Mistakes in DNA replication• Radiation or chemicals in the
environment
• Some mutations don’t affect the phenotype but some do
Gene shuffling during sexual reproduction
• Mutations are not the only source of variation
• Most heritable differences are due to gene shuffling that occurs during the production of gametes
• The 23 pairs of chromosomes can produce 8.4 milliondifferent combinations of genes
• Crossing over further increases the number of different genotypes that can also appear in offspring
Single – Gene and Polygenic Traits
• The number of phenotypes produced for a given trait depends on how many genes control the trait
Single – gene trait• Controlled by a single gene that
has two alleles
• Variation in these genes leads to only 2 distinct phenotypes
Polygenic traits• Traits controlled by two or more genes• Each gene of a polygenic trait has two
or more alleles• As a result one polygenic trait can have
many possible genotypes and phenotypes
Ex.) height
Polygenic traits
16-2 Evolution as Genetic Change
• A genetic view of evolution offers a new way to look at key evolutionary concepts
• If each time an organism reproduces, it passes copies of its genes to its offspring…
• We can therefore view evolutionary fitness as an organism’s success in passing genes to the next generation
• We can also view an evolutionary adaptation as any genetically controlled physiological, anatomical, or behavioral trait that increases an individuals ability to pass along its genes
Evolution as Genetic Change
• Remember that evolution is any change over time in the relative frequency of alleles in a population.
• This reminds us that it is populations, not individual organisms that can evolve overtime
Natural Selection on Single – Gene Traits
• Natural selection on single gene traits can lead to changes in allele frequencies and thus to evolution
Natural Selection on Polygenic Traits
• Natural selection can affect the distributions of phenotypes in any of three ways
1. Directional Selection• When individuals at one end of
the curve have higher fitness than individuals in the middle or at the other end
2. Stabilizing Selection• When individuals near the center
of the curve have higher fitness than individuals at either end of the curve
3. Disruptive Selection• When individuals at the upper and
lower ends of the curve have higher fitness than individuals near the middle
• Can create 2 distinct phenotypes
Genetic Drift• Natural Selection is not the only
source of evolutionary change
• In small populations, an allele can become more or less common by chance
Genetic Drift• A random change in allele frequency
Genetic Drift• These individuals may carry alleles in
different relative frequencies than did the larger population from which they came
• If so, the population that they found will be genetically different from the parent population
• This cause is not natural selection, but chance
Founder effect• A situation in which allele
frequency changes as a result of the migration of a small subgroup of a population
Evolution vs. Genetic Equilibrium
• To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place
Hardy – Weinberg principle
• States that allele frequency in a population will remain constant unless one or more factors cause these frequencies to change
Genetic equilibrium
• The situation in which allele frequencies remain constant
5 conditions are required to maintain genetic equilibrium
1. There must be random mating
2. The population must be very large
3. There can be no movement into or out of the population
4. No mutations
5. No natural selections
16-3 The Process of Speciation
• Factors such as natural selection and chance events can change the relative frequencies of alleles in a population
• But how do these changes lead to speciation?
Speciation• The formation of new species
Isolating Mechanisms• Since members of the same species
share a common gene pool, in order for a species to evolve into 2 new species, the gene pools must be separated into 2
• As new species evolve, populations become reproductively isolated from each other
Reproductive isolation• When the members of 2
populations cannot interbreed and produce fertile offspring
Behavioral Isolation• When two populations are
capable of interbreeding but have differences in courtship rituals or other reproductive strategies
Eastern & Western Meadowlark
Geographical Isolation• When two populations are
separated by geographic barriers such as rivers, mountains, or bodies of water
Albert & Kaibab Squirrels
Temporal Isolation• When 2 or more species reproduce
at different times
Rana aurora - breeds January - March
Rana boylii - breeds late March - May
Testing Natural Selection in Nature
Q: Can evolution be observed in nature?
A: YES
The Grants
Testing Natural Selection in Nature
• Darwin hypothesized that finches had descended from a common ancestor and overtime, natural selection shaped the beaks of different bird populations as they adapted to eat different foods
• The Grants, realized that Darwin’s hypothesis relied on two testable assumptions
1. There must be enough heritable variation in these traits to provide raw materials for natural selection
2. Differences in beak size and shape must produce differences in fitness that cause natural selection to occur
Variation• The Grants identified and
measured every variable characteristic of the birds on the island
• Their data indicated that there is a great variation of heritable traits among the Galapagos finches
Natural Selection• During the…
• Rainy season – enough food for everyone, no competition
• Dry season – some foods become scarce
• At that time, differences in beak sizes can mean the difference between life and death
• Birds become feeding specialists
Natural Selection• The Grants discovered that individual
birds with different size beaks had different chances of survival during a drought
Speciation in Darwin’s Finches
• Speciation in the Galapagos finches occurred by founding of a new population, geographical isolation, changes in the new population’s gene pool, reproductive isolation and ecological competition
Founders Arrive• Many years ago, a
few finches from South American mainland
• Species A, flew or were blown to one of the Galapagos Islands
Geographic Isolation• Later on, some birds from
species A crossed to another island in the Galapagos group
• The finches then became unable to fly from island to island and become isolated from each other and no longer share a common gene pool
Changes in the Gene Pool
• Overtime, populations on each island became adapted to their local environments
Reproductive Isolation• Now imagine that a
few birds from the second island cross back to the first island
• Q: Will the population A birds, breed with the population B birds?
• A: Probably not
Ecological Competition• As these two new
species live together in the same environment, they compete with each other for available seeds
• The more different birds are, the higher fitness they have, due to less compitition
Continued Evolution• This process of isolation
on different islands, genetic change, and reproductive isolation probably repeated itself time and time again across the entire Galapagos island chain
• Over many generations, it produced the 13 different finch species found there today
Studying Evolution Since Darwin
• It is useful to review and critique the strength and weakness of evolutionary theory
• Darwin made bold assumptions about heritable variation, the age of the Earth, and the relationships among organisms
• New data from genetics, physics, and biochemistry could have proved him wrong on many counts, and they did not
• Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the past
Limitations of Research• The Grants data shows how
competition and climate change affects natural selection
• However, they did not observe the formation of a new species
Unanswered Questions• Many new discoveries have led to
new hypotheses that refine and expand Darwin’s original ideas
• No scientist suggests that all evolutionary processes are fully understood. Many unanswered questions remain
Why Understanding Evolution is Important?
• Evolution continues today
Ex.)
• Drug resistance in bacteria and viruses
• Pesticide resistance in insects
• Evolutionary theory helps us understand and respond to these changes in ways that improve human life