Notes 7: Evolution
Evolution and ecology are related
o Evolution is a consequence of populations interacting with their environment
o Adaptations have evolved as a direct result of their selective, adaptive,
reproductive value in an environment
o Understand evolution to appreciate why organisms are what they are and why
they do what they do
Darwin
o The Origin of Species book
o Complete argument for evolution by common ancestry as fact
o Evolution as a consequence of
stratigraphic patterns in fossil record
evidence from morphology
evidence from biogeography
o Hypothesis of natural selection
o Evolution and Common Ancestry
Data is most easily explained as consequence of species changing through
time as they diverged from common ancestor
Why new types of organisms appear in fossil record
Not all present in oldest strata, as predicted by model of single
creation
Why, within a lineage, there is progressive change in characters through
the fossil record
Like horses in recent strata more similar to modern horses than
fossil horses in older strata
Why there is “limitation of design” among organisms
Humans, whales, and bats are different but have similar
underlying structure
Homologous structures (limb bones) built out of same parts
(digits, wrist bones, radius, ulna, humerus) but perform different
functions
Homologous: same structure, different function
o Arms of humans and fins of whales
Why organisms radically different in structure can depend on analogous
structures that function the same way but converge on similar types of
structures
Analogous: different structure, same function
o Wings of eagles and wings of bees
Why every species has structures not designed to do anything and bear
the plain stamp of inutility while they are present and do function in
other species
Vestigial structures: do not perform a function in a specific species
o Wisdom teeth in humans
Why embryos of radically different species are similar
, Like mouse and chicken
Why species have structures as embryos that do not exist in the adult
species, but do exist in other species
Like gill slits in humans
Like legs in whale embryos
Observed patterns that correlated with environmental conditions and
geographic distributions
Hypothesized that different environments may cause species to diverge
A common environment may cause different organisms to
become more similar
Explain homologous and analogous structures
Convergent communities: different organisms fill parallel ecological roles
Converge morphologically and ecologically
Communities on islands populated by species found nowhere else
Geographical isolation in a new environment can cause species to
change and diverge from ancestral groups
Island communities: colonists change in response to new
conditions
o Natural Selection
Reproduction more important than survival, so not survival of the fittest
Malthus concluded struggle for existence naturally arises as growing
populations outstrip limited resources
Organisms with traits that increased probability of gaining resources
makes them more likely to survive and reproduce
Passing on beneficial traits
Beneficial traits would accumulate in a population
Changing it in a way that makes it better adapted in its
environment
Populations of a particular species, isolated in different environments,
would be selected for different traits and would diverge
Creates new species descended from the same ancestral species
Observed variation and knew that some variation was heritable
Darwin did not understand how the heritable material produced
variation
Believed heritable material was a fluid and blended, reducing
variation
Darwin’s Theory of Natural Selection
P = premise
C = conclusion
P1: all populations have the capacity to over-reproduce
P2: resources are finite
C: there will be a struggle for existence, and most offspring die
before reproductive age
P3: organisms in a population vary, and some variation is heritable
, C2: as a result of variation, some organisms more likely to survive
and reproduce, so differential reproductive success
C3: population changes through time, as adaptive traits
accumulate in the population
Corollary: two populations, isolated in different environments,
will diverge as they adapt to their own environments
o Eventually may become different species
Genetics
o Hereditary units are particulate and don’t blend together but remain discrete
o Sources of Variation
Mutation
To create new alleles and genes
Recombination
Crossing over
o Genes re-arranged on chromosomes
Independent assortment
o Genes recombined with other chromosomes
Nearly infinite number of combinations in hereditary particles
(genes)
Occur when genes are subselected in meiosis and mixed in
fertilization
o Gametes combine to create novel combinations of genes
Reflected by phenotypic variation
o Agents of Change
Natural selection
Mutation
o Units of heredity (genes) do not blend together
Particulate and are passed in new combinations to offspring
o In diploid species, each parent makes 2n combinations of chromosomes in their
gametes without accounting for crossing over
Population Genetics
o Model
Sources of Variation
Mutation
Recombination
o Crossing over
o Independent assortment
Agents of Change
Natural selection
Mutation
o Rates of mutation are low
o Important as source for new genes
o Does not change frequencies a lot
Drift
, Migration
Non-random mating
Selection and drift are major agents of change
Hardy and Weinberg created a simple model of the genetic structure of a
population at one locus
Defining genotypic and gene frequencies at the locus
Showed that equilibrium would be established in one generation if the
population behaved in a probabilistic and constant way
Equilibrium would occur if the population was infinitely large, bred
randomly, had no mutation, no selection, no migration
If any conditions were not met, evolution would occur
Selection and drift can change genetic structure of a population the most
o Drift
Two patterns related to population size
Change can occur randomly, as a function of statistical sampling error
If breeding population was a small subset of initial population, genetic
structure of subset would deviate from genetic structure of original
population
Small samples more likely to deviate than large samples
Multiple small samples deviate more from one another than
multiple large samples
Effects are most likely in genetic bottleneck and founder effect
Genetic Bottleneck
If a population is dramatically reduced in size, ones remaining are
not representative of whole population
Most likely a reason the population collapsed, like predation or
disease
Selection at relative loci means changes at other loci are random
o Relative loci could be selection for stealth, camouflage,
pathogen resistance
If a population crashes, there will be both selection and drift
o Selection for those resistant to whatever happened
(disease, predation, etc.)
Result of drift
o Drift at other loci by reducing size of breeding population
Ex: cheetah have very low genetic diversity
Founder Effect
If a subgroup migrates to a new, unpopulated area, it is not
representative of the original
Ex: Huntington’s Chorea
o Neurodegenerative disorder caused by autosomal lethal
dominant allele
o Homozygous dominant, it is lethal
o Heterozygous, trait expressed later in life
Evolution and ecology are related
o Evolution is a consequence of populations interacting with their environment
o Adaptations have evolved as a direct result of their selective, adaptive,
reproductive value in an environment
o Understand evolution to appreciate why organisms are what they are and why
they do what they do
Darwin
o The Origin of Species book
o Complete argument for evolution by common ancestry as fact
o Evolution as a consequence of
stratigraphic patterns in fossil record
evidence from morphology
evidence from biogeography
o Hypothesis of natural selection
o Evolution and Common Ancestry
Data is most easily explained as consequence of species changing through
time as they diverged from common ancestor
Why new types of organisms appear in fossil record
Not all present in oldest strata, as predicted by model of single
creation
Why, within a lineage, there is progressive change in characters through
the fossil record
Like horses in recent strata more similar to modern horses than
fossil horses in older strata
Why there is “limitation of design” among organisms
Humans, whales, and bats are different but have similar
underlying structure
Homologous structures (limb bones) built out of same parts
(digits, wrist bones, radius, ulna, humerus) but perform different
functions
Homologous: same structure, different function
o Arms of humans and fins of whales
Why organisms radically different in structure can depend on analogous
structures that function the same way but converge on similar types of
structures
Analogous: different structure, same function
o Wings of eagles and wings of bees
Why every species has structures not designed to do anything and bear
the plain stamp of inutility while they are present and do function in
other species
Vestigial structures: do not perform a function in a specific species
o Wisdom teeth in humans
Why embryos of radically different species are similar
, Like mouse and chicken
Why species have structures as embryos that do not exist in the adult
species, but do exist in other species
Like gill slits in humans
Like legs in whale embryos
Observed patterns that correlated with environmental conditions and
geographic distributions
Hypothesized that different environments may cause species to diverge
A common environment may cause different organisms to
become more similar
Explain homologous and analogous structures
Convergent communities: different organisms fill parallel ecological roles
Converge morphologically and ecologically
Communities on islands populated by species found nowhere else
Geographical isolation in a new environment can cause species to
change and diverge from ancestral groups
Island communities: colonists change in response to new
conditions
o Natural Selection
Reproduction more important than survival, so not survival of the fittest
Malthus concluded struggle for existence naturally arises as growing
populations outstrip limited resources
Organisms with traits that increased probability of gaining resources
makes them more likely to survive and reproduce
Passing on beneficial traits
Beneficial traits would accumulate in a population
Changing it in a way that makes it better adapted in its
environment
Populations of a particular species, isolated in different environments,
would be selected for different traits and would diverge
Creates new species descended from the same ancestral species
Observed variation and knew that some variation was heritable
Darwin did not understand how the heritable material produced
variation
Believed heritable material was a fluid and blended, reducing
variation
Darwin’s Theory of Natural Selection
P = premise
C = conclusion
P1: all populations have the capacity to over-reproduce
P2: resources are finite
C: there will be a struggle for existence, and most offspring die
before reproductive age
P3: organisms in a population vary, and some variation is heritable
, C2: as a result of variation, some organisms more likely to survive
and reproduce, so differential reproductive success
C3: population changes through time, as adaptive traits
accumulate in the population
Corollary: two populations, isolated in different environments,
will diverge as they adapt to their own environments
o Eventually may become different species
Genetics
o Hereditary units are particulate and don’t blend together but remain discrete
o Sources of Variation
Mutation
To create new alleles and genes
Recombination
Crossing over
o Genes re-arranged on chromosomes
Independent assortment
o Genes recombined with other chromosomes
Nearly infinite number of combinations in hereditary particles
(genes)
Occur when genes are subselected in meiosis and mixed in
fertilization
o Gametes combine to create novel combinations of genes
Reflected by phenotypic variation
o Agents of Change
Natural selection
Mutation
o Units of heredity (genes) do not blend together
Particulate and are passed in new combinations to offspring
o In diploid species, each parent makes 2n combinations of chromosomes in their
gametes without accounting for crossing over
Population Genetics
o Model
Sources of Variation
Mutation
Recombination
o Crossing over
o Independent assortment
Agents of Change
Natural selection
Mutation
o Rates of mutation are low
o Important as source for new genes
o Does not change frequencies a lot
Drift
, Migration
Non-random mating
Selection and drift are major agents of change
Hardy and Weinberg created a simple model of the genetic structure of a
population at one locus
Defining genotypic and gene frequencies at the locus
Showed that equilibrium would be established in one generation if the
population behaved in a probabilistic and constant way
Equilibrium would occur if the population was infinitely large, bred
randomly, had no mutation, no selection, no migration
If any conditions were not met, evolution would occur
Selection and drift can change genetic structure of a population the most
o Drift
Two patterns related to population size
Change can occur randomly, as a function of statistical sampling error
If breeding population was a small subset of initial population, genetic
structure of subset would deviate from genetic structure of original
population
Small samples more likely to deviate than large samples
Multiple small samples deviate more from one another than
multiple large samples
Effects are most likely in genetic bottleneck and founder effect
Genetic Bottleneck
If a population is dramatically reduced in size, ones remaining are
not representative of whole population
Most likely a reason the population collapsed, like predation or
disease
Selection at relative loci means changes at other loci are random
o Relative loci could be selection for stealth, camouflage,
pathogen resistance
If a population crashes, there will be both selection and drift
o Selection for those resistant to whatever happened
(disease, predation, etc.)
Result of drift
o Drift at other loci by reducing size of breeding population
Ex: cheetah have very low genetic diversity
Founder Effect
If a subgroup migrates to a new, unpopulated area, it is not
representative of the original
Ex: Huntington’s Chorea
o Neurodegenerative disorder caused by autosomal lethal
dominant allele
o Homozygous dominant, it is lethal
o Heterozygous, trait expressed later in life
