Download Descent with Modification and Population Evolution

Descent with Modification
I.
A.
1.
2.
3.
B.
1.
2.
3.
4.
Descent with Modification
Unity of life
All organisms are related
Organisms have descended from ancestral species
Adaptations
a.
Modifications resulting over time as descendants inhabit differing environments
Metaphor depiction of history of life—“Tree”
Tips are contemporary
Branch points represent ancestors or evolutionary lines
Most branch points are dead
Closely related organisms share recent branch points
Terms and Concepts:
II.
Natural selection—inferences and observations
A.
Observations:
1.
If all individuals reproduced successfully, populations would grow exponentially
a.
All species have a great potential fertility
2.
Populations tend to remain constant
a.
Despite tremendous reproductive capacity, population size remains constant
3.
Resources are limited
B.
Inference 1:
Over production of individuals leads to competition for limited resources with only a limited number of
individuals surviving
C.
1.
2.
D.
Observations
Individual characteristics within a population vary extensively
a.
All individuals within a population differ
Variation is heritable
a.
Most variability can be passed to subsequent generation
b.
Excludes traits acquired during individuals lifetime
Inference 2:
Survival depends on the heritable constitution of surviving individuals—individuals with inherited
traits best suited for their environment are more likely to produce offspring
E.
Inference 3:
The differences in ability to survive and reproduce lead to a gradual change in a population, with
favorable characteristics accumulating over generations
This is Descent with Modification. The mechanism for this modification has been the cumulative
effect of natural selection over extended periods of time.
III.
A.
1.
2.
B.
1.
C.
1.
2.
D.
1.
2.
3.
Conditions for Natural Selection
Variation and Overproduction
Resources limitations usually are apparent only there is more demand than supply
a.
Plentiful resources exclude need for competition
i.
Therefore no selection
Without diversity, even with limiting resources, selection would not occur
Fit genes are generally passed on
Over time, environment favors certain variations that promote survival
a.
Variations are optimal for some but not all environments
i.
Adaptation under one condition may be useless or even deleterious under
different conditions
Change occurs gradually—Gradualism
Advantages of some heritable traits are slight
a.
Traits accumulate over generations
Evolution of life did not occur rapidly with dramatic change
a.
Gradual accumulation of small changes
Individuals do not evolve
Individuals interact with their environment
Individuals adapt to their environments
These changes are not heritable
a.
Only traits carried by gametes can be passed on
Population Evolution
I.
A.
1.
B.
1.
C.
1.
2.
3.
4.
Background
Population
Characteristics
a.
Group of interbreeding members of a species
b.
Share common geographical area
Species
A group of populations whose individuals have the capacity to interbreed and produce fertile
offspring
a.
Not evenly distributed
b.
Population centers
c.
Gene flow may occur between population centers
Gene pool
Aggregate of all alleles for all genes in a population at a given time
Genes consist of alleles
Number of alleles
a.
Diploid
b.
Two alleles for each gene
i.
Homozygote
ii.
Heterozygote
There are generally two or more types of alleles for each gene
a.
Each allele has a relative frequency in the gene pool
II.
A.
1.
Quantifying Allelic Frequency—Hardy-Weinberg Theorem
Theoretical description of allelic frequency in non-evolving populations
Mathematical relationship
a.
Hardy-Weinberg equation
*Class discussion will include the derivation and use of this equation
III.
A.
1.
2.
Microevolution
Definition
Generation to generation change in a population’s allelic or genotypic frequencies
H-W equilibrium assumes nonevolving populations
a.
Differences observed represent evolutionary departures
b.
Differences in gene poopl is a very small scale
i.
Microevolution
B.
1.
2.
3.
4.
5.
Conditions that must be met to maintain Hardy-Weinberg equilibrium:
Large populations
Isolation of populations
No net mutations
Random mating
No natural selection
C.
1.
Causes of Microevolution
Genetic drift: change in gene pool of small populations due to chance
a.
Bottleneck effect
i.
Non-selective death of members of a small population
ii.
Reduction in population size
iii.
Survival is random
iv.
Remaining population has a genetic makeup that differs from original
population
b.
Founder effect
i.
Results from colonization
ii.
Genetic makeup of founding population of new colony differs from that of the
original population
Gene flow
a.
Migration of fertile individuals between populations
b.
Transfers gametes
c.
Reduces inter-population differences
d.
Offsets the effects of genetic drift or natural selection
Mutation
a.
Must occur in gametes to affect microevolution
b.
Little quantitative effect in large population
i.
1 mutation/106 gametes
c.
Original source of genetic variation
Nonrandom mating
a.
Increases homozygous loci but does not alter allelic frequency
b.
Types
i.
Inbreeding
ii.
Assortative mating
c.
Inbreeding
i.
Individuals are more likely to mate with close neighbors
ii.
Changes genotypic frequencies but not allelic
iii.
Reduces number of heterozygotes
2.
3.
4.
d.
5.
Assortative mating
i.
Individuals mate with partners possessing similar phenotypes
Natural selection
a.
Variation among individuals favor particular individuals
b.
These individuals are more likely to produce offspring
c.
Alleles are passed on in disproportionate numbers to those of the current generation
d.
The process is adaptive
i.
Favorable genotypes accumulate
IV.
A.
1.
2.
3.
Sources of Genetic Variation
Background
Variation is required for natural selection
Subtle genetic differences underlying variation makes natural selection possible
Variation can occur within or between populations
a.
Product of genetics and environmental influences
i.
Only genetic components are adaptive
ii.
Only genetic components are affected by natural selection
B.
1.
2.
3.
Variation within populations
Variations affects the character states (traits) seen within a population
Characters can be either quantitative or discrete
Polygenetic characters vary quantitatively
a.
Trait is controlled by multiple loci
b.
Contribute most to inheritable variation
c.
Example
i.
Height
Discrete characters are controlled by a single locus
a.
Different alleles produce distinct phenotypes
Polymorphism
a.
Two or more different forms—morphs are present in significant frequencies in a
population
b.
Morph
i.
Contrasting character state
c.
Not limited to physical traits
i.
Cellular and biochemical substrates also exist in multiple character states
ii.
Require advanced scientific tools to be distinguished
4.
5.
C.
1.
2.
3.
Variation between populations
Variation seen between two or more populations of a given species
Geographical variation
a.
Environmental factors differ between areas inhabited by populations
b.
Genetic drift may cause variation among different species
Cline
a.
Some variables may be graded
i.
Eg., Incremental temperature differences in neighboring geographical regions
b.
Genetic variation will parallel differences seen along the cline
D.
1.
2.
3.
4.
E.
1.
Mutation
Random changes in genetic composition of a population
a.
Negligible contribution to genetic variation
Produce new alleles
a.
Generally occur in somatic cells
i.
Not heritable
b.
Only mutations in cell lines giving rise to gametes generate heritable genetic variation
Types of mutations
a.
Point
i.
Affect a single base in DNA
ii.
Generally have little effect
iii.
Effects if realized are usually deleterious
b.
Chromosomal
i.
Affect multiple loci
ii.
Generally not beneficial
Mutation produces variation in organisms with short generational time
a.
Allelic frequency of mutation locus can change rapidly
i.
Asexually reproducing microorganisms
2.
Sexual recombination
Nearly all genetic variation results from the sexual recombination of existing alleles
a.
Random segregation and independent assortment during meiosis
b.
Crossing-over during prophase I of meiosis
c.
Random fertilization
Zygotes possess new combinations of existing alleles
VI.
A.
1.
Preserving Variation
Natural selection reduces genetic variability
Selects individuals with particular traits that favor survival
B.
1.
Mechanisms that preserve variability
Diploidy
a.
Recessive alleles are preserved in the heterozygote state
i.
Recessive alleles are often less favorable or sometimes harmful
b.
The more rare the allele the greater proportion hidden by the dominant allele
Balanced polymorphism
a.
Natural selection preserves variations at some gene loci
b.
Heterozygote advantage
i.
Heterozygote has greater reproductive success than either homozygote (e.g.,
sickle-cell anemia)
c.
Frequency-dependent selection
i.
Reproductive success of any one morph decreases if its phenotype becomes too
common ((e.g., papilio dardanus)
Neutral variation
a.
Some genetic variations confer no advantage or disadvantage
i.
Relative frequencies of neutral alleles not affected by natural selection
2.
3.
V.
A.
1.
2.
3.
Mechanisms of Adaptive Evolution: Natural Selection
Background
Adaptive evolution results from the combined effect of chance events that cause genetic
variation and natural selection that favors some variations over
others
Evolutionary fitness
a.
Relative contribution an individual makes to the gene pool of the next generation
i.
Measured by the number of progeny produced--fecundity
b.
Factors affecting fecundity
i.
Reproductive lifespan
ii.
Survival
Effect of selection
a.
Stabilizing selection
i.
Favors intermediates as opposed to extreme phenotypes
ii.
Reduces phenotypic variation
iii.
Best suited when environmental conditions are stable
b.
Directional selection
i.
Favors variants of one extreme
ii.
Shifts frequency curve toward rare variants
iii.
Occurs during periods of environmental change
c.
Diversifying selection
i.
Opposite phenotypic extremes are favored over intermediate phenotypes
Mechanisms of Evolution
I.
A.
1.
2.
B.
1.
2.
3.
C.
1.
Speciation
Types
Anagenesis
a.
Transformation of an unbranched lineage of organisms to a state different from the
ancestral population
i.
Basis for naming a new species
Cladogenesis
a.
Budding of one or more new species from a parent species that continues to exist
b.
More common pattern of speciation
c.
Produces more biological diversity
Biological species concept
Characteristics
a.
Population whose members have the potential to interbreed
i.
Must be in nature
b.
Breeding results in the production of viable and fertile offspring
c.
Cannot produce viable, fertile offspring with other species
Species is the largest unit of a population where gene flow is possible
Biological species are reproductively isolated from other species in nature
Other types of species
Species can be conceptualized by other means
a.
Morphology
b.
Ecology
c.
Evolution
d.
Characteristics that maximize successful mating
II.
A.
1.
2.
3.
Factors that Cause Reproductive Isolation
Background
For discrete species to exist each must be reproductively isolated
Most species are isolated by more than a single type of barrier
Types of barriers
a.
Prezygotic
i.
Impede mating or fertilization
ii.
Intrinsic
b.
Postzygotic
i.
Prevent hybrid from developing into a viable, fertile adult
ii.
Intrinsic
c.
Geographic
B.
Prezygotic barriers
a.
Habitat isolation
i.
Two species live in different habitats within the same geographical range
ii.
Rarely encounter each other
iii.
Technically not geographically isolated
b.
Behavioral
i.
Species-species signals and behavior that attract mates
ii.
Pheromones, song, etc.
c.
Temporal isolation
i.
Two different species breed at different times of the day, season or years
d.
Mechanical isolation
i.
Anatomical incompatibility
ii.
Prevent sperm transfer
e.
Gamete isolation
i.
Gametes either do not survive to meet or lack gamete recognition
C.
Postzygotic barriers
a.
Reduced hybrid vigor
i.
Hybrid development is aborted during embryological development
ii.
If hybrid survives embryological development, it is frail and soon dies
b.
Reduced hybrid fertility
i.
Hybrid is viable, but sterile
ii.
Gene flow between gene pools is not possible
iii.
Meiosis does not generate normal gametes
c.
Hybrid breakdown
i.
First generation hybrids are viable and fertile
ii.
Subsequent generations are feeble or sterile
III.
A.
1.
Modes of Speciation
Background
For a new species to arise, first need a reproductive barrier
a.
Gene pool of one population is separated from the populations of the parent species
b.
Isolated population can now follow its own evolutionary path
i.
Allelic frequencies can change by selection, genetic drift and mutation
B.
1.
2.
3.
4.
5.
C.
1.
2.
3.
4.
IV.
A.
1.
Allopatric speciation
New species result from geographical isolation
Geographical barrier physically isolates the population
a.
Gene flow is not possible
i.
Isolated gene pools accumulate differences by microevolution
ii.
Populations diverge phenotypically
Examples
a.
Emergence of new mountain ranges
b.
Movement of glaciers
c.
Formation of land bridges
Conditions that favor allopatric speciation
a.
Isolated population is small
b.
Geographic isolation occurs at the fringe of the parent population’s range
i.
Peripheral isolate
Characteristics of peripheral isolates that favor speciation
a.
Gene pool of the peripheral isolate probably differs subtly from the parent population
b.
Genetic drift will continue to cause changes in the gene pool of the peripheral isolate
c.
Evolution is likely to affect the isolate in a different way than the parent population
Sympatric speciation
Species arise within the geographical range of the parent population
a.
Speciation occurs without geographical isolation
Chance genetic change creates a reproductive barrier
a.
Rapid process
i.
Occurs within a single generation
Improper cell division creates an extra set of chromosomes
a.
Polyploidy
Source of chromosomes determines what type of polyploidy has occurred
a.
Autopolyploid
i.
All sets of chromosomes are derived from a single species
ii.
Diploid gametes are formed by non-disjunction during cell division
iii.
Self-fertilization would create a tetraploid that could mate with other tetraploids
iv.
Tetraploids cannot mate with diploids
v.
3n progeny are sterile due to impaired meiosis
b.
Allopolyploid
i.
Hybrid results from chromosomes contributed by two different species
ii.
Hybrids are usually sterile but in some species asexual reproduction is possible
iii.
Mechanisms exist that transform sterile allopolyploids into fertile polyploids
Sources of Evolution Novelty
Modification of existing structures
Exaptation
a.
Gradual refinement of existing structures for new function
b.
Structure that evolved in one context is co-opted for another function
c.
Novel designs arise gradually
i.
Intermediate forms each having some function
d.
Natural selection does not anticipate future
i.
Improves an existing structure in context of its current utility
ii.
Improvement contributes to later modification (e.g., honeycombed bones of
birds)
B.
1.
2.
3.
Modification of genes controlling development
Development requires the action of regulatory genes that control the timing and rate of growth
Modifying the function of these genes will have dynamic effects on the adult form
Allometric growth
a.
Differences in relative growth rates of different body parts
b.
Slight changes affect adult form dramatically
4.
Heterochrony
a.
Temporal changes that give rise to evolutionary novelty
b.
Changing the timing or rate of the development of structures changes their adult form
(e.g., human brain)
Paedomorphosis
a.
Retention of ancestral juvenile structures in sexually mature adult
Homeosis
a.
Alteration of the spacial pattern of development
b.
Alteration in placement of different body parts
5.
6.