Ev
olution
Linnaeus: Taxonomy
History of Thought
- Developed first taxonomic system
- naming and classifying the diverse forms of life “for the greater glory of God”:
Hutton: Gradualism
- profound changes in the Earth’s features; cumulative, slow process
Lamarck: Evolution Through Use & Disuse
- believed, incorrectly, that body parts could change due to use and disuse and that these acquired characteristics could
be inherited.
Malthus: Populations
- human suffering was due to increasing populations
Cuvier: Paleontology
- Paleontologist that recognized that many extinctions occurred... he argued it must have been due to catastrophism
Lyell: Uniformitarianism
- Geologist that argued geologic processes have not changed throughout Earth’s history
Darwin: Evolution by Natural Selection
- Published “The Origin of Species”
- Believed natural selection was the driving force for evolution.
Mendell: laws of inheritance provided explanation for selection process
The Historical Context of Darwin’s Life and Ideas
Charles Darwin
- Darwin originally studied medicine and theology
- Darwin was hired as Naturalist aboard the H.M.S. Beagle
- Traveled 5 years around South America, Europe, Africa, and Australia collecting fossils
- Became famous for work with the Galapagos finches
- he believed that all the finches of these islands were originally part of the same species
(common ancestor)
- Wrote "On the Origin of Species" (1858)
The Voyage of the HMS Beagle (1831 - 1836)
Darwin discovered that most of the animal species on the Galapagos lived nowhere else in the world, but they resembled
species on the South American mainland
Evolution by Natural Selection
- Evolution- a change in the frequency of alleles (genes) in a population over time
- There are two areas of evolutionary study:
1. Microevolution- how populations of organisms change from
and how new species originate
generation to generation
2. Macroevolution- patterns of change in groups of related species over broad periods of
geologic time.
- These patterns determine phylogeny (the evolutionary relationships among species)
- Natural selection- differential survival and reproduction among individuals in a population
based on inheritable traits.
- The most well suited organisms survive and reproduce at a greater rate, thus eventually
changing the allelic frequencies of the population
Evolution by Natural Selection
- How it works:
1. Mutations, crossing over, etc... lead to genetic variation
2. Genetic variation exists in all populations
3. Struggle for existence (survival) - populations will always reproduce at a
greater rate than the environment can support, thus leading to competition
4. Some organisms possess variations that make them better able to survive
and reproduce than others. Darwin would say these organisms are more fit.
(survival of the fittest).
5. Over time, these variations will continue to be passed on and will eventually
change the population
Example of Natural Selection:
Insecticide Resistance in Insect Pops.
Example of Evolution:
Drug Resistance in HIV
- 3TC interferes with reverse
transcriptase
- 3TC mimics cytosine and when
inserted it
terminates further elongation
- HIV viruses that are resistant to
3TC have a different form of
reverse transcriptase, it can
recognize the difference in
cytosines
Patterns of Selection
Natural selection can impact a population in a variety of ways:
- Directional selection- favors traits at one extreme
- Stabilizing selection- favors individuals with the middle phenotype
- Diversifying selection- favors both extremes
- In order for natural selection to operate, there must be genetic
variation among individuals in a population. It comes from:
- Mutations- change in DNA - Sexual reproduction- crossing over
- independent assortment
- random joining of gametes from two individuals
- Diploidy- allows one gene to be “hidden”
Influencers of Natural Selection
Evolution (change in allele frequencies) can be driven by:
- Changes in the environment
- Gene flow - alleles entering or leaving the population due to emigration or immigration
- Nonrandom mating
- In-breeding
- Breeding based on proximity
- Sexual selection
- preferential mating with males or females with specific traits or behaviors
- Usually leads to male competition and female choice.
- Often leads to sexual dimorphism- differences in the appearance of males and females
- Ex: manes in lions, plumage in birds
Influencers of Natural Selection
- Genetic drift- changes in the gene pool of a small population
Three types:
1. Changes due to random chance
2. Founder effect- when a few individuals migrate and start their own population, which doesn’t
represent the genes in the original population
3. Bottleneck effect- when a population undergoes a dramatic decrease in size (due to natural
disasters, for example) and the individuals left don’t represent the original population
The "Bad" Traits...Why Aren't They All Gone?
Natural selection should extinguish a population's variation by getting rid of the unfavorable
phenotypes...so why doesn't variation disappear?
Techniques to Preserve or Restore Variation:
1.) Diploidy – heterozygote condition hides the recessive form; prevents it
2.) Balanced polymorphism – the frequencies of coexisting forms do not
many generations
Causes of balanced polymorphism:
1.) Heterozygote advantage – better reproductive success than
2.) Hybrid vigor – increased vitality of hybrid offspring
ex. Sickle cell anemia - heterozygotes are resistant to malaria
ex. Plant Crops - homozygotes are more sensitive to disease
from being lost
change noticeably over
homozygotes
Gene Pool - all possible alleles of all members of a population
Hardy Weinberg Theorem: the frequencies of alleles in a population's
gene pool will remain constant over generations as long as the following
conditions are met:
- There can be no natural selection
- Mutations cannot occur
- The population must be isolated (no gene flow) - The population must be large (no
genetic drift)
- Mating must be random
- If any of those 5 things are violated- the populations allele frequencies will
change- the population will evolve.
The Hardy Weinberg equation is used to evaluate whether or not the population
is evolving. It is determined using the following values:
- Allele frequencies for each allele
- P= % of dominant gene
- Q= % of recessive gene
- Genotypic frequencies:
- P2= % of homozygous dominant individuals
- Q2= % of homozygous recessive individuals
- 2PQ= % of heterozygous individuals
- The
equations:
- P + Q = 1 (all alleles add up to 100%)
- P2 + 2PQ + Q2= 1 (all individuals add up to 100%)
- Species- a group of individuals that can interbreed and produce live, fertile offspring
- Speciation- the formation of a
new species that are
reproductively isolated
- Anagenesis - one species
evolves into a new one
- Cladogenesis - one species
diversifies and gives rise to new species
Temporal Isolation
example
Drosophila persimilis breeds in early morning, while closely related Drosophila pseudoobscura breeds in the afternoon
+
=
Mechanical Isolation
example
The amazing partnership of the Bucket Orchid and Orchid Bee is so precise that if either one went extinct, the other would follow. No other
orchid can possibly cross-pollinate the Bucket Orchid.
Gametic Isolation
example
The Giant Red Urchin (Strongylocentrotus franciscanus and Purple Urchin (Strongylocentrotus purpuratus) cohabit the rocky
intertidal along the western U.S., but they do not interbreed. Their gametes are genetically/chemically incompatible, maintaining
species integrity.
Reproductive Isolation
- The following are all reasons why two organisms may NOT be able to
fertile offspring
- They are therefore NOT part of the same species
reproduce and/or produce live,
- Some are pre-zygotic (before a zygote forms):
- Spatial isolation- species don’t encounter one another
- Temporal isolation- when species mate or flower during different seasons or different times of the day
- Behavioral isolation- when a species doesn’t recognize another species as a mating partner because it doesn’t
perform proper courtship rituals (songs, scents, etc.)
- Mechanical isolation- the male and female genitalia are structurally incompatible
- Gametic isolation- when male gametes are incapable of penetrating the female gamete
- Some are post-zygotic (after a zygote forms):
- Hybrid inviability- when the zygote fails to develop and dies before birth
- Hybrid sterility- when the hybrid is born but isn’t able to have it’s own children
- Hybrid breakdown- when the hybrids live for a generation, but aren’t well suited to the environment and die out
- Can occur in several ways:
- Allopatric speciation- Sympatric speciation- Adaptive radiation-
- Allopatric Speciation- begins when a population is divided by a geographic barrier (such as a
mountain or river).
- The two populations now become different as each evolves to
- EX. Pupfish- lakes/rivers go through drying trend
- EX: Grand Canyon Squirrels
the different environments.
- Sympatric Speciation- occurs without the presence of a geographic
- EX: polyploidy in plants.
barrier.
Autopolyploidy
- more than 2 sets of chromosomes
- result of self-fertilization
- can't interbreed with diploid plants
- may be able to self-fertilize
Allopolyploidy
- more than 2 sets of chromosomes
- hybrid of 2 different species
- very common
- more vigorous than parents
Adaptive Radiation- when one species rapidly
evolves into
many species. - Happens when colonizing
a new, diverse geographic
area
Patterns of Evolution
- Divergent evolution- when two species that originate from a common
ancestor speciate.
- Example- African and Indian elephants speciated
- Convergent evolution- when two unrelated species share similar traits
due to similar environmental conditions (analogous traits)
- Example- the torpedo body shape of penguins and porpoise
- Parallel evolution- two related species that have made similar
evolutionary changes after diverging from a common ancestor.
- Example- placental and marsupial mammals
- Coevolution- when two species evolve in response to each other.
- Examples- predator/prey relationships or relationships between plants
and pollinators
Macroevolution
The Geologic Time Scale
Macroevolution
Macroevolution
Timing of Macroevolution
- There are two distinct theories used to interpret fossil evidence for evolutionary
history:
- Gradualism -argues that evolution occurs
by the gradual accumulation of small
changes.
- Punctuated equilibrium- - argues that
evolutionary history has long periods of stasis
(stability with little or no evolution) followed by
brief periods of rapid evolution.
Artificial Selection
- Artificial Selection- selection carried out by humans for desirable traits.
- Have led to various breeds of dogs.
- Brussel sprouts, broccoli, cabbage and cauliflower all
originated from a single wild plant.
- Paleontology- fossil evidence shows gradual changes
in species, species that are now extinct, and
transitional fossils
- Law of Superposition - allows for relative dating
- deeper fossils are older
- Transitional Fossils - fossils that show
intermediate forms
- Vestigial structures: historical remnants of
structures that had important functions
in ancestors
- Have become smaller over time
- Ex. snakes – pelvis, leg bones
Biogeography- geographic
distribution of species
- species in different regions
of the world look alike,
especially when in
similar environments
Embryology- reveals similar stages in
development among species.
- EX: gill slits form in fish, chicken, pig
and human embryos
Molecular biology- closely related species share similar DNA and proteins.
Comparative Anatomy- looks at morphology (appearance inside and out):
- Homologous structures- body parts that resemble one another in
different species because they have
evolved from a common ancestor
- Example of Descent with Modification
- Analogous structures- body parts that resemble one another in different species because they evolved
independently as adaptations to similar environments
A Cline
Cline: a graded change in some trait along a geographic axis