Transcript Ch. 17 - MissDavisNHSScience

Lesson Overview
17.1 Genes and Variation
Lesson Overview
Genes and Variation
OBJECTIVES:
• Define evolution in genetic terms.
• Identify the main sources of genetic
variation in a population.
• State what determines the number of
phenotypes for a trait.
Lesson Overview
Genes and Variation
Genetics Joins Evolutionary Theory
• Heritable traits are controlled by GENES!!!!
• Changes in GENES / CHROMOSOMES result in VARIATION!!!!
• Genotype  combination of alleles it carries; produces a phenotype
• Phenotype  all physical, physiological, and behavioral characteristics
• Natural selection acts directly on PHENOTYPE, not genotype.
• Ex. Some individuals have phenotypes that are better suited to their
environment, so they will survive and reproduce, passing on more copies
of their genes to the next generation.
Lesson Overview
Genes and Variation
Populations and Gene Pools
• Population
• group of individuals of the same species that mate and produce
offspring.
• gene pool
• all the genes, including all the different alleles for each gene, that are
present in a population
• Ex. B = black fur and b = brown fur
• The relative frequency of an allele is studied in gene pools
• Number of times a particular allele occurs in a gene pool
compared with the number of times the other allele appears in the
gene pool
• Ex. How many times does B appear versus b????
Lesson Overview
Genes and Variation
• Ex. this diagram shows the gene pool for fur color in a population of mice.
Lesson Overview
Genes and Variation
Populations and Gene Pools
• Evolution is any change in the relative frequency of alleles in the gene pool of a
population over time.
• Natural selection acts on POPULATIONS not INDIVIDUALS.
Lesson Overview
Genes and Variation
Mutations
• Mutations
• Can produce changes in genotypes, and therefore phenotypes
• May or may not affect fitness (some are lethal and some are beneficial)
• Can affect evolution ONLY if the mutation is heritable (passed from
generation to generation)  must be in the egg or sperm cell
Lesson Overview
Genes and Variation
Single-Gene Traits
• single-gene trait
• a trait controlled by only one gene
• Have only 2 or 3 distinct phenotypes
• Most common forms are dominant or recessive
Lesson Overview
Genes and Variation
Polygenic Traits
• Polygenic traits
• traits controlled by two or more genes
• Each gene has two or more alleles  leads to MANY possible
genotypes and phenotypes
• Ex. AaBBccDd …..for height
Lesson Overview
17.2 Evolution as Genetic
Change in Populations
Lesson Overview
Evolution as Genetic Change in Populations
OBJECTIVES:
• Explain how natural selection affects single-gene and polygenic traits.
• Describe genetic drift.
• Explain how different factors affect genetic equilibrium.
Lesson Overview
Evolution as Genetic Change in Populations
Natural Selection on Single-Gene Traits
• Can lead to changes in allele frequencies and then leads to evolution
• Ex. Deer Population
Lesson Overview
Evolution as Genetic Change in Populations
Natural Selection on Polygenic Traits
• Can affect the range of phenotypes and hence the shape of the bell curve
• Results in:
• Directional Selection
• Stabilizing Selection
• Disruptive Selection
Lesson Overview
Evolution as Genetic Change in Populations
Directional Selection
• Directional selection
• occurs when individuals at one end of the curve have higher fitness
than individuals in the middle or at the other end.
• The range of phenotypes shifts to one extreme of the bell curve
Lesson Overview
Evolution as Genetic Change in Populations
Stabilizing Selection
• Stabilizing selection
• occurs when individuals near the center of the curve have higher
fitness than individuals at either end.
• Narrows the overall bell curve, but keeps the center of the curve at its
current position
Lesson Overview
Evolution as Genetic Change in Populations
Disruptive Selection
• Disruptive selection
• occurs when individuals at the upper and lower ends of the curve have
higher fitness than individuals near the middle
• acts against individuals of an intermediate type and can create two
distinct phenotypes
Lesson Overview
Evolution as Genetic Change in Populations
Genetic Drift
• An allele becomes more or less common simply by chance or a random change
in allele frequency
• Occurs in small populations
Lesson Overview
Evolution as Genetic Change in Populations
Genetic Bottlenecks
• bottleneck effect
• change in allele frequency following a dramatic reduction in the size of
a population
• Ex. After a natural disaster
Lesson Overview
Evolution as Genetic Change in Populations
The Founder Effect
• founder effect
• occurs when allele frequencies change as a result of the migration of a
small subgroup of a population
Lesson Overview
Evolution as Genetic Change in Populations
Evolution Versus Genetic Equilibrium
• genetic equilibrium
• When a population’s allele frequencies remain the same…..NO
EVOLUTION TAKES PLACE.
• Due to:
• Large populations
• Less likelihood of genetic drift
• No mutations
• No new alleles are introduced into the population
• Lack in Random Mating
• Some species prefer to mate with others having specific traits
• No Movement into or out of a Population
• no alleles flow into or out of the gene pool to change
frequencies
• No Natural Selection
• All phenotypes have equal likelihood of survival and
reproduction
Lesson Overview
17.3 The Process
of Speciation
Lesson Overview
The Process of Speciation
OBJECTIVES:
• Identify the types of isolation that can lead to the formation of new species.
• Describe the current hypothesis about Galapagos finch speciation.
Lesson Overview
The Process of Speciation
Isolating Mechanisms
• Speciation
• formation of a new species (population whose members can
interbreed and produce fertile offspring)
• Due to:
• Reproductive Isolation
• Behavioral Isolation
• Geographic Isolation
• Temporal Isolation
Lesson Overview
The Process of Speciation
Reproductive Isolation
•
Reproductive isolation
– occurs when a population splits into two groups and the two populations no
longer interbreed.
– Can result in evolution of two separate species
Lesson Overview
The Process of Speciation
Behavioral Isolation
• Behavioral isolation
• when two populations that are capable of interbreeding develop
differences in courtship rituals or other behaviors.
Lesson Overview
The Process of Speciation
Geographic Isolation
• Geographic isolation
• occurs when two populations are separated by geographic barriers
• Ex. Rivers, mountains, bodies of water
Lesson Overview
The Process of Speciation
Temporal Isolation
• Temporal isolation
• two or more species reproduce at different times
• Ex. Species of orchids….flowers that last only one day and must be
pollinated on that day to produce seeds…..if they bloom on different
days, they cannot cross-pollinate.
Lesson Overview
The Process of Speciation
Testing Natural Selection in Nature
• Darwin hypothesized that the Galápagos finches had descended from a common
ancestor.
• Natural selection shaped the beaks of different bird populations as they became
adapted to eat different foods.
Lesson Overview
The Process of Speciation
Founders Arrive
• Many years ago, a few finches from
South America—species M—arrived
on one of the Galápagos islands
• founder effect
• allele frequencies of this
founding finch population
could have differed from
those in the South American
population
Lesson Overview
The Process of Speciation
Geographic Isolation
• Populations on one island will
separate to other islands due to
competition for resources
Lesson Overview
The Process of Speciation
Changes in Gene Pools
• Over time, populations on each island
adapted to local environments.
• Natural selection could have caused
two distinct populations to evolve (A
and B), each characterized by a new
phenotype.
Lesson Overview
The Process of Speciation
Behavioral Isolation
• When species B returns to its previous
island, behaviors for mating may have
changes to prevent reproduction with
species A
Lesson Overview
The Process of Speciation
Competition and Continued Evolution
• Birds that are most different from
each other have the highest fitness.
• More specialized birds have less
competition for food.
• Over time, species evolve in a way
that increases the differences
between them, and new species may
evolve (C, D, and E).