Download Evolution, Natural Selection, and Speciation A. Adaptation B

Evolution, Natural Selection, and Speciation
D. L. A. Underwood
Biology 316 - Entomology
A. Adaptation
1. Three definitions:
a. “Any behavioral, morphological, or physiological trait that is assumed to be the result
of natural selection.”
b. “Any physiological or morphological feature or form of behavior used to explain the
ability of an organism to live where it does.”
c. “A change in the population mean of a physical, physiological, or behavioral trait that
results from some current environmental pressure.”
2. Team project -D takes notes; B speaks.
a. Find three things wrong with the following two statements: Individuals evolve to
become better adapted to their environment. This evolution occurs because it is good
for the species.
3. Populations are adapted to historical conditions.
a. They appear to be adapted to their present environment because environmental
conditions usually do not change rapidly.
b. What is adaptive today may not be tomorrow if the environment changes.
4. “Adaptedness” is at the phenotypic level.
B. Phenotypic Variation
1. The phenotype is what you see in an organism. It includes all of the characteristics
of that individual. For example:
a. Color of skin, feather, exoskeleton, etc.
b. Behavior
c. Physiological responses to stimuli
d. Metabolism
e. Abilities
2. Types of variation
a. Discontinuous or discreet characters have definite states.
i. Number of toes
ii. Number of flower petals
iii. Some fur, plumage, and petal colors
iv. Generally under the control of a few genes
b. Continuous or quantitative characters vary along a wide range of states that blend into
one another.
i. Human height
Biology 316 – Evolution, Natural Selection, and Speciation – Page 1 of 6
ii. Human weight
iii. Generally under the control of many genes
c. Can the phenotype of an individual change over time? Yes!
d. Does this mean the individual is evolving? NO!
i. An individual cannot evolve in the biological sense.
ii. Evolution is the change in allele frequencies in a population over time.
e. Can a particular aspect of the phenotype be a product of something other than
genotype (i.e. genes)? Yes!
f. Phenotypic plasticity
i. The expression of the genotype is often influenced by environmental variables
leading to a variable phenotype.
• Snowshoe rabbits change fur color as winter sets in.
• Flower petal color can be determined by soil pH.
• The behavior of monarch butterflies to either fly north or south.
3. Team project - C takes notes, A speaks
a. Why can't an individual evolve in a strict biological sense (not in the colloquial
sense)?
4. Phenotype is the product of genetics and environment
5. Environmental conditions
a. What is an individual's "environment?"
i. Abiotic factors - anything not alive.
ii. Biotic factors - anything alive.
b. Diurnal variability - Changes in the environment that occur over the course of the
day/night.
c. Seasonal variability - Changes in the environment that occur over the course of a
year.
d. Spatial variability - Within an area there are differences due to the physical outlay,
abiotic, and biotic factors.
e. The size and habits of the organism will influence the relative importance of any
particular aspect of the organism’s environment.
f. An organism’s environment is defined by the organism. This is referred to as the
individual's habitat.
C. Artificial Selection
1. Humans have induced changes in animal and plant phenotypes over the centuries
a. Breeds of dogs, chickens, cats, horses, etc.
b. Milk production in cattle
c. Leanness of pork
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D. Natural Selection
1. Definition of evolution
a. Evolution is the change in allele frequencies in a population over time.
b. Technically, this is the definition of "micro-evolution." Speciation and extinction
events are considered "macro-evolution." We won't worry about this distinction.
2. Review - Conditions necessary for evolution by natural selection
a. Individuals vary with respect to phenotype.
b. Offspring phenotypes resemble those of their parents due in part to shared genetics.
c. Different phenotypes exhibit differential survival and reproduction.
3. The classic case of natural selection - Industrial Melanism and the Peppered Moth
a. The peppered moth exists in two color morphs, peppered and black. These two
morphs are the same species and freely interbreed.
b. Before the Industrial Revolution (before air pollution), most trees were covered with
grayish lichens and the most frequent color morph of the moth was peppered.
c. By the late 1800's, industrial pollution in the form of soot blackened the bark of trees
and killed the lichens in areas near cities. The frequency of the black morph
increased during this period, with some populations becoming 100% of the black
morph.
d. Industrial practices now produce less pollution and many areas now host trees with
lichens once again. The frequency of the peppered morph is now increasing.
4. The evolution of insecticide resistance
5. The phenotypes
that are most
represented in the next
generation are said to
be the most fit in the
population.
6. Fitness
a. Individual fitness
is the
proportionate
contribution of
alleles to future
generations
relative to other
individuals in the
population. The
fittest individuals are those that leave the greatest number of descendants relative to
other individuals in the population.
b. In practice, fitness is often applied not to a single individual, but to a phenotype or to
the alleles of a particular aspect of the phenotype.
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i. We may say that in sand dunes, yellow-colored snakes have a higher fitness than
brown-colored snakes (i.e. they are more likely to survive and leave larger
numbers of descendants than brown -colored snakes).
ii. We may also say that the alleles that lead to yellow color have a greater fitness
than those that lead to a brown color because these alleles increase in frequency
through time.
c. Fitness is a relative term. Numbers of offspring produced is not a direct measure of
fitness. Fitness is the proportionate contribution; you must compare numbers of
offspring produced within types.
7. Natural selection as a process or an outcome, not a 'thing' or a force
a. If conditions x, y, and z are present, A is the outcome.
b. A does not force or influence conditions x, y, or z.
c. A is merely the product of these events happening.
8. An analogy - the events
a. Individuals vary phenotypically.
b. Reproduction and survival vary with respect to some phenotypic characters.
c. Some proportion of the phenotype is genetically based (h2).
d. OUTCOME: Alleles that code for the phenotype that reproduces more than other
phenotypes will increase in a population through time. This is natural selection.
e. The phrase "selection favors a given trait" implies some thing is choosing individuals
with the trait. This is
incorrect!!
f. Natural selection is the
result of heritable
biological differences
among individuals that
result in some
individuals leaving
more offspring than
others in the
population.
9. The relationship between
evolution and the
mechanisms resulting in evolution.
a. Natural selection can occur without evolution.
b. Genetic drift or mutation or non-random mating can occur without evolution.
c. Evolution can occur by natural selection or other processes like mutation, meiotic
drive, genetic drift, etc.
d. A population may have natural selection, genetic drift, and evolution occurring at the
same time for different traits.
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10. Team Project - C takes notes, D speaks
a. Identify what is wrong with the following statement: Polar bears evolved to
become white because they needed to be camouflaged against the white snow to
ensure successful hunting.
E. Speciation
1. What is a species?
a. A species is a population or a series of populations whose members can interbreed
and produce viable offspring.
b. Actual interbreeding is not necessary, only the potential of interbreeding is necessary
for a group of populations to be called the same species.
c. Members of populations separated geographically may never encounter one another;
for many species, we have no way to actually test whether interbreeding and the
production of viable offspring is possible or not.
d. Different species are characterized by being reproductively isolated from one another.
2. Reproductive isolation
a. Members of a population of the same species must recognize one another as
potentially viable mates.
b. Breeding with an individual not within the same species will not lead to viable
offspring. Hence, interspecific mating is at an extreme selective disadvantage.
c. Species recognition is accomplished by a variety of mechanisms.
i. Species specific courtship dances
ii. Species specific pheromones
iii. Species specific coloration
d. For many species, interbreeding with a different species is not possible due to
physical differences in the genitalia.
3. Interspecific hybridization
a. Members of closely related species may still be able to interbreed.
b. Sometimes this leads to offspring that cannot reproduce, e.g. mules.
c. Sometimes these matings do produce reproductive offspring, e.g. gulls.
4. Team Project - B takes notes, D speaks
a. Can interspecific hybridization lead to two species becoming one? How?
5. Population differentiation
a. In order for speciation to occur, two populations of the same species must become
reproductively isolated.
b. Generally, geographic separation over a long period of time is required.
i. The different environments of the two places leads to selection for different traits
in the two populations.
ii. Over time the accumulation of different traits will produce populations whose
members are so different that if they encounter one another, they do not recognize
each other as potentially viable mates.
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iii. Sometimes the traits that differ are related to the timing of mating, one may mate
in the spring while the other mates in the fall. Interspecific mating is prevented
by temporal differences in the timing of reproduction.
c. There are four general models.
i. Allopatric speciation – a population is split into two whose members do not
encounter each other.
ii. Founder effect – a small number of individuals become physically separated from
the original population. This could be viewed as a special case of allopatric
speciation.
iii. Parapatric speciation – a population of organisms that do not move much
experiences different selection pressures along its range until differentiation
occurs.
iv. Sympatric speciation – a population splits into two along ecology, e.g. hosts. A
classic example of this is speciation that occurred in flies that feed on apples.
F. Extinction
1. Extinction occurs on multiple levels
a. Local extinction is when a population disappears.
b. Regional extinction is when numerous populations occupying a region disappear.
c. Global extinction is when all populations of a species disappear.
2. Populations become extinct due to a variety of factors.
a. All populations must contain a sufficient number of reproductively active members to
overcome natural death rates.
b. Not all individuals in a population mate. Some are too young while others are too
old.
c. The density of reproductively active members must also be sufficient for individuals
to successfully find one another during the mating season.
d. Specific resources required of the young must also me in sufficient supply. For
example, monarch caterpillars only eat the leaves of milkweed plants. Even if there
are enough adult monarchs for successful mating, not enough milkweed plants in the
area for larval growth can cause a local extinction.
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