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EVOLUTION
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Evolution: the change in allele (gene) frequencies in a population over a period of
time.
Microevolution: changes in gene frequencies
Population genetics: branch concerned with heredity in groups of individuals.
Population: community of individuals of the same species linked by bonds of mating
and parenthood that are found in a common geographical area.
Species: a group of organisms that reproduce and are reproductively isolated from
other species. (E. Mayr) - or - a group of organisms that can have fertile offspring
Populations evolve; individuals cannot.
A population has continuity from generation to generation.
The genetic constitution of a population may change over time.
Genetic variation is necessary for this change.
Evidence of evolution (6):
- 1. Fossils: are the best evidence of evolution over the history of the Earth.
- a. Mold fossil: Impression left in surrounding rock by the decay of organic
material.
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b. Cast fossil: Natural filling of a mold left behind after a fossil has been removed
from rock by a solution.
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c. Permineralization: The process by which shell or skeletal material is infiltrated
by mineral matter making the hard part denser and heavier. Also called
petrification.
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d. Replacement fossils: Process of fossilization in which the original mineral
material of a hard part is replaced by another kind of mineral. It is an actual
molecular level replacement: molecule-by-molecule. Eventually, there are no
more organism molecules.
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e. Organic matter: Some organic material can be left behind (such as bones, teeth,
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etc.). Some organic matter can be pressed in layers of sandstone or shale.
f. Preservation: Organisms are trapped quickly and completely in an oxygen free
environment. This prevents decay.
2. Comparative anatomy:
- a. Homologous structures: structures or parts of organisms that have the same
origin but may or may not have the same function.
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b. Analogous structures: parts of different organisms that have similar function,
but not similar origins.
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c. Vestigial structures: Structures that have no apparent function.
- Pigs with two toes that do not reach the ground?
- Wings on birds that do not fly?
- Why do male humans have nipples?
- Why do rabbits have digestive systems that function 'so poorly that they must
eat their own feces'?
- Legless lizards
- Hip bones in whales
- Teeth in embryonic baleen whales
Comparative embryology:
Developmental patterns are similar in organisms with similar evolutionary
relationships.
There are fewer differences in organisms that are closely related.
“Ontogeny Recapitulates Phylogeny”
- Ontogeny is the developmental sequence, recapitulate means to replay or
review, and phylogeny is evolutionary history. Looking at Haeckel's drawing
from Kardong (1999), it certainly seems to be true.
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4. Biogeography:
- Islands have many species of plants and animals that are endemic and that are
closely related to species on the nearest mainland or neighboring island.
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5. Comparative Biochemistry:
- The sequence of DNA and the proteins that the organism produces.
- Evidence suggests that organisms with similar DNA and proteins are closely
related evolutionarily.
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6. Fossil Dating:
- a. Relative dating: Due to different rates of sedimentation in seas/lakes, the rock
form layers of strata. The fossils in a stratum are a local sampling of the
organisms that existed at that time period.
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b. Absolute dating: The determining of the actual age of the fossil.
- Radioactive dating:
- Fossils contain radioactive isotopes accumulated when the organisms were
alive.
- Once dead, the organisms do not accumulate any more of the isotopes.
- Each radioactive isotope has a fixed rate of decay.
- A "half-life" is the time it takes for 50% of the original isotope to decay.
- The decay is unaffected by temp., pressure, etc. ex: C14 = 5,600 yrs.
- Amino acid racemization:
- Amino acids exist in 2 isomer forms:
- Left handed (L) symmetry
- Right handed (D) symmetry.
- Organisms only synthesize L amino acids.
- In a fossil, the ratio of L to D amino acids can be measured.
- Knowing the rate of conversion (racemization), how long the organism
has been dead can be determined.
- This is a temperature sensitive process.
Evolutionary Theory:
- 1. Jean Baptiste de LeMarck
- Two theories of how organisms changed over time.
- a. Inheritance of acquired characteristics: Organisms can change their body
when needed and pass these changes onto their offspring.
- b. Law of use and disuse: if you don't use it, you lose it!
- 2. Charles Darwin
- Was the naturalist on the H.M.S. Beagle, which went on a 5 yr.
voyage.
- In 1859 he published the Origin of Species. In this book, he outlined
the principles of natural selection:
- Principles of Natural Selection:
- Individuals in a species vary.
- Some variations are heritable.
- More individuals are produced than the environment can support.
- Competition for resources occurs.
- Individuals with favorable traits will survive and reproduce, with the traits
passed on to the offspring.
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3. Alfred Russell Wallace (1823-1913)
- He proposed his theory of evolution at the same time. Almost became the father
of evolution.
- “Ontogeny Recapitulates Phylogeny”
Natural Selection: "Survival of the fittest"
Nature is acting upon a phenotype. Genes code for those phenotypes (or traits). If
the organism is adapted then it will survive and reproduce, passing its' genes to the
next generation.
If the organism is not well adapted, it will not produce offspring to survive the next
generation.
There are three "choices" an organism has to its environment:
- 1.
Adapt
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Migrate
- 3.
Die
Darwin's theory does not emphasize survival, but reproductive success.
Organisms can, after all, live their whole life and never reproduce.
Variation: a key concept in evolution, but where does variation come from?
- 1. Mutations
- Permanent, random chemical changes in the DNA molecule that are passed
onto offspring.
- May be:
- Harmful
- Beneficial
- No net effect
- 2. Variation from recombination
- The creation of genetic variation by recombination can occur more swiftly
than it does when due solely to mutations.
- 3. Variation from migration
- Migration of individuals into a population from other populations can
introduce new genes into the population, or remove genes from the
population.
Species of organisms reveal considerable genetic variation (polymorphism) in their
phenotype.
- a. Morphological variations: different body shapes and/or colors.
- b. Chromosomal variations: vary in chromosome number and shape.
- c. Protein variations: AA substitutions in proteins
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Maintenance of Genetic Variation:
- 1. Sexual reproduction
- a. Independent assortment
- b. Crossing over: combination of 2 parental genomes at fertilization
- 2. Mechanisms that promote out breeding:
- a. Plants
- Some plants are diecious
- Anatomical arrangements do not promote self fertilization
- There are genes for self- sterility
- b. Animals
- Hermaphrodites rarely self-fertilize
- Mammals
- a. Males leave population to mate
- b. Innate desire to avoid incest
- 3. Diploidy
- In haploid organisms, genetic variation is directly expressed in the phenotype,
which is exposed to selection process.
- In diploid organisms, the variations may be "stored" as recessive
alleles. Recessive alleles are protected from selection.
- 4. Heterozygote advantage ("hybrid vigor"):
- Recessive alleles may be harmful in the homozygous state but they may make
the heterozygote have greater reproductive success.
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Changes in Gene Frequency
- 1. Gene pool
- All of the genes of any population at a given time.
- Evolution is a change in allelic frequencies in the gene pool.
- Evolution can proceed randomly or it can proceed under the influence of
natural selection.
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2. How gene pools change
- Random changes in the gene pool are forms of evolution without natural
selection.
- a. Gene Flow: the movement in or out of a population (immigration
or emigration).
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b. Genetic Drift: This is a change in the gene pool that takes place as a result
of chance. There are 2 situations where chance plays a role in evolution.
- Founder Effect:
- A small population branches off from a larger one.
- E.g., a recessive allele in homozygous condition causes Dwarfism. In
Switzerland the condition occurs in 1 out of 1,000 individuals.
Amongst the 12,000 Amish now living in Pennsylvania the condition
occurs in 1 out of 14 individuals. All the Amish are descendants of 30
people who migrated from Switzerland in 1720. The 30 founder
individuals carried a higher than normal percentage of genes for
dwarfism.
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Bottleneck: a population is drastically reduced.
- By the 1890's the population of northern elephant seals was reduced to
only 20 individuals by hunters.
- Even though the population has increased to over 30,000 there is no
genetic variation in the 24 alleles sampled.
- In contrast southern elephant seals have wide genetic variation since
their numbers have never reduced by such hunting.
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c. Nonrandom Mating:
- Where individuals prefer to mate with a particular phenotype.
- d. Mutations:
- A mutation itself may not have much effect in a large population in a
single generation.
- However, they are the ultimate source of all genetic variation and is what
natural selection acts on.
- Without variation by mutations, natural selection would not work!
3. Changes in gene frequencies due to Natural Selection:
- Hardy-Weinberg Equilibrium
- Fitness
- Darwin's idea of fitness is measured by the relative contribution that an
individual makes to the gene pool of the next generation.
- Fitness is the contribution of many factors that affect both survival and
fertility.
- 1. Contribution of a genotype to the next generation compared to the
contribution of alternative genotypes for the same locus.
- 2. Selection coefficient: the difference between the most fit genotype's
relative fitness and the less fit genotype's relative fitness is called the
selection coefficient.
- The rate of harmful recessive alleles decreases in a population, but is
never eliminated. Why?
- Selection is greater against harmful dominant alleles because it is
expressed in the heterozygote.
- A new recessive mutation spreads very slowly in a population even if
it is beneficial. A new dominant mutation spreads more quickly
because the parent has a 50% chance of passing on the mutation.
- What is selected in Natural Selection?
- Remember that a phenotype is an expression of genes.
- This includes all observable attributes of an organism.
- The entire phenotype of an individual is the unit of selection.
- It is rare that a single allele can determine a successful phenotype.
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Groups of genes if linked are called 'super genes' and have a greater
influence on selection.
At the same time phenotype is not determine by genes alone, but by the
interactions of the phenotype with the environment.
The various types of selection are:
- 1. Stabilizing Selection:
- Extreme individuals are eliminated and intermediate forms are
favored. The mutant forms are eliminated quickly.
- Populations under this type of selection typically experience a
decrease in the amount of additive genetic variation for the trait
under selection.
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2. Disruptive Selection:
- Increases the two extreme types in a population at the expense of
the intermediate forms.
- The result is a marked difference between the two forms.
- Disruptive selection plays an important role in speciation.
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3. Directional selection:
- Directional selection results in an increase in the proportion of
individuals with an extreme characteristic.
- There is a gradual replacement of one allele or group of alleles by
another allele in the gene pool.
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4. Frequency dependent selection:
- Decreases the frequency of the more common phenotype &
increases the frequency of the least common phenotype.
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Positive Frequency Dependent Selection occurs when a trait has
higher fitness when it is common than when it is rare. For example,
bright warning (aposematic) coloration in a poisonous species is
likely to have higher fitness when it is common because if most
individuals are brightly colored, then predators will have learned to
avoid brightly colored individuals and not attack them.
- Negative Frequency Dependent Selection occurs when a trait has
higher fitness when it is rare than when it is common. Negative
frequency dependent selection will turn out to have some
important implications for evolution of other traits. Ex: Fruit Flies
& Eye Color Preference: What is generally true of negative
frequency dependent selection: since rare traits have high fitness,
and will become common again, negative frequency dependent
selection maintains genetic variation in populations
5. Sexual Selection:
- Intrasexual Selection: Competition for a mate within a sex
- Competition can occur:
- Before female(s) arrive
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- Before copulation
- After copulation
- After fertilization
Intersexual Selection:
- Members of one gender exert strong selective pressures on the
characteristics of the opposite gender through the choice of
mates.
6. Mating Systems:
- These 3 differ in the energy investment by the different genders.
- Polygyny: One male mates with many females.
- Monogamy: One male mates with only one female.
- In most species, females only have sex when they are
fertile. This is because sex takes energy, and carries the risk
of disease. But it also means males can easily tell which
females are fertile, so they don't waste time on mates that
won't get pregnant. Indeed, males usually give females no
help in raising their offspring. "The male strategy is to stay
with the female for as long as she is fertile, and then to
leave," says zoologist Magnus Enquist of Stockholm
University.
- But in some species, including birds, porcupines and
humans, the girls have wised up. By cutting down on visual
and chemical cues, and by having sex all the time, they stop
males from telling whether they are fertile. "The male has
no cue," says Enquist. "All he can see is the behavior of the
female." Once males are blind to a female's condition, he
says, it's no longer worth their while chasing lots of
partners, because the one they're with is as likely to be
fertile as any other. "There is a search cost. It takes some
time to find a female."
- Polyandry: The mating of one female with more than one male
while each male mates with only one female.
- What is the result of natural selection?
- Adaptations to the physical environment
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Some phenotypic variations within a species follow
geographic distribution and correlate with gradual
environmental changes.
This graded variation of a trait or complex is called a
cline. The steeper the cline, the greater the variation.
What is the result of natural selection?
- Adaptations to the biological environment
- Co evolution: Occurs when populations of two or more species interact
closely so that each exerts a strong selective force on the other.
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Mimicry:
- Mullerian Mimicry: Unrelated and unpalatable species may
resemble each other in types of colors and or patterns for
protection.
- Batesian Mimicry: Species that are palatable resemble species that
are not palatable.
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Patterns of Evolution
- Convergent: Organisms that occupy similar environments often resemble one
another although they may be distantly related. When they are subjected to similar
selection pressures, they will have similar adaptations.
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Divergent: When populations become isolated from each other, different
selective pressures lead to different phenotypes.
Development and Macroevolution
- Allometric Growth:
- Genes that control when & how changes in an organism's form occur from a
zygote to an adult.
- Allometric growth is a difference in the relative rates of growth between
various body parts and helps shape the organism.
- Even slightly altering the rates of growth & the adult form is changed.
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Paedomorphosis:
- Genetic changes can also affect the timing of developmental events, such as
the sequence in which different organs start and stop developing
- Ex: where sexually mature adults in one species keep structures that were in
the juvenile form of evolutionary ancestors.
Neoteny:
- Retention of juvenile characteristics in the adults of a species, as among
certain amphibians.
- The attainment of sexual maturity by an organism still in its larval stage.