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Transcript of Evolution
Darwin & Natural Selection
Evidence of Evolution
The Origin of LIfe
Early Earth was a very inhospitable place
No oxygen (a reducing atmosphere)
water vapor (H2O), CO2, N2, NOx, H2, NH3, CH4, H2S
Lightning, UV radiation & volcanic heat
How could life have begun under these conditions?
1953 Experiment that simulated conditions on early Earth
Results showed some of the 20 amino acids esential for life were created so the potential for life was there but they did not prove how life actually formed.
One possible suggestion for how life started is the fomration of spheres which created the first cell-like structures.
They must have...
A membrane-like structure enclosing
Abiotically produced organic molecules
and be able to ...
Reproduce and perform simple metabolism
The First Genetic Material
Many believe that RNA was the first genetic material used by the first cells because RNA...
is single stranded (simpler than DNA)
codes for proteins = structure
can be copied and passed on
is composed of nucleotides very similar to ATP - an Adenine nucleotide with two extra phosphates.
The Endosymbiotic Theory
The evolution from prokaryotes to eukaryotes
Prokaryotes - no true organelles or nucleus BUT able to reproduce, metabolize food, synthesize proteins & respond to their environment.
Eukaryotes - evovled from prokaryotes to have membrane-enclosed structures with specific jobs (organelles).
The Evolution of Eukaryotes
Endocytosis of auto- and heterotrophic prokaryotes into a larger cell.
Evidence: mitochodria & chloroplasts ...
have their own DNA
double membrane (inner one more like prokaryotes, outer like cell membrane),
are the same size as prokaryotes
reproduce like prokaryotes by splitting
contain the same enzymes and transport systems found in living prokaryotes.
He went against accepted ideas of creation to suggest that populations have not always been the same but instead, change over time by natural selection.
formal classification of living things
gradual geologic change
species can change
populations have limits
fossils show extinctions
geologic changes happen at a constant rate
evolution by natural selection
Fossils & Strange Animals
Darwin found fossils of ocean animals along mountain sides (Cape Verde Islands - Africa). What does this suggest?
Temperate species in South America were more similar to tropical species in South America than to temperate species back home in England. Why?
22 year old Naturalist on the HMS Beagle
5-year voyage around the world beginning in 1831
Noted that species characteristics matched their niche in their particular environment
Darwin said that in order for Natural Selection to occur, the following must be true of a population.
variation among individuals
production of more offspring than can survive (overproduction) thus creating...
competition among individuals and...
differential survival (Survival of the Fittest) eliminating certain traits and increasing the frequency of others.
Adaptations & Speciation
Darwin hypothesized that the organisms that survived did so because they had some advantage over those who didn't survive.
Adaptation - trait that helps an organism survive in its environment
Over long periods of time, a sub-group in a population may develop different or specialized adaptations and eventually be different enough from the original population as to be called a new species.
Speciation - an evolutionary process in which one species splits into 2 or more.
Populations affect each other
Divergent Evolution is like Speciation
Convergent Evolution is like a whale's tail and a fish fin.
Parallel Evolution is like mammals in the Americas and marsupials in Australia.
Evolution in Structures
Human interference in the selection process results in traits favored by people NOT nature increasing in frequency over time.
We can destroy species and diversity but we can also increase it.
Descent with modification; the idea that living species are descendants of ancestral species that were different from the present-day ones; also
defined more narrowly as the
change in the genetic
composition of a population
from generation to generation
(a change in allele frequencies).
Natural Selection acts on Phenotype NOT Genotype
Types of Natural Selection
Stabilizing - more of the average
Directional - move toward one extreme
Disruptive - split to both extremes, eventually get 2 new species
Sexual Selection - males & females appear very different
Record of species now extinct
Timeline of evolution based on rock layers & carbon dating
Fossils of the same species found
on 2 different continents. This is evidence of continental drift & proof that Earth has not always been the same and continues to change (geologic processes constantly reshape earth)
If life has changed slowly and steadily over time as Darwin describes, then there must be evidence of these subtle changes.
Fossil found in North America of a fish with the beginning of arms and legs. Shows the transition from sea life to the move onto land.
Dinosaur with feathers
Link between dinosaur ancestors and modern birds!
share a recent common ancestor
similar internal structure due to shared DNA
but different exterior due to different environment
more distant common ancestor
similar environments so similar exterior
but different internal anatomy & DNA
unused body parts, clues to past
still there because of DNA, genes
Evidence of a Common Ancestor
Chapter 21 & 47
How can you use DNA to back up the argument that "if 2 species share common developmental stages, they must be closely related"?
The Human Genome Project (2003) sequenced all the DNA instructions needed to make the human body.
Computerized technology used in this project was then used on other animals to determine similarties & differences.
The greater the number of nucleotide differences, the more distantly related two species are.
Protein sequences can be compared in a similar way (amino acid differences for a given protein like hemoglobin) because DNA (genes) code for these proteins.
A population of white moths lived on the bark of ash trees which are white.
Industrial Revolution created enough pollution to coat the tree bark in soot, making them darker and the white moths more vulnerable to predators.
Microevolution - rapid change in allele frequencies in the moth population.
When Humans interfere with nature, nature will always find a way to survive and adapt.
1928 Alexander Flemming used Penicillin to treat a bacterial infection.
Since then, we have developed numerous antibiotics to kill bacteria but, like all living organisms, the bacteria developed resistance.
The same has happened in pest populations exposed to pesticides (graph below)
In the moth example, you can see the population transition from mostly white moths to mostly dark ones.
Evolution acts on the phenotypes of the organisms but it ultimately changes the gene pool of the entire population.
Remember organisms that reproduce sexually have 2 alleles (one from each parent) for each gene.
Alleles can be dominant or recessive (A or a)
Homozygous recessive (aa)
Homozygous dominant (AA)
If the allele frequencies change, then the population is undergoing evolution.
Environmental conditions change
Organisms best fit to new conditions survive and reproduce while others die or reproduce less
New population's gene pool has a different percentage of alleles
Mutation & Sexual Reproduction
The raw material of variation from which natural selection occurs must come from somewhere. Where does variation in a population come from?
Mutation - random changes in the genes of an individual which may or may not give them an advantage but must change the phenotype to have an effect on the natural selection of the population.
Sex - mixes up the genes from both parents every time
they produce an offspring. Each individual has a
unique set of genes.
p2 = the frequency of AA individuals
2pq = the frequency of Aa individuals
q2 = the frequency of aa individuals
p + q = 1
p = frequency of the dominant allele
q = frequency of the recessive allele
In order for a population to remain in HW equilibrium, the following must be true...
No Mutations are occuring
Mating is random, not by choice
No natural selection is taking place
The population size is extremely large
No Gene flow (in or out of population)
1 in 1700 US Caucasian newborns have cystic fibrosis. "C" for normal is dominant over "c" for cystic fibrosis
What is the frequency of the cystic fibrosis allele "c" (mutation)?
What is the frequency of the normal allele "C"?
# recessive individuals:
1/1700 = 0.0006
q2 = rec.individuals
q2 = 0.0006
q = square root of 0.0006
q = 0.024 or 2.4%
the frequency of "c" is 0.024 or 2.4%
If q = 0.024 then p = 0.976
the frequency of "C" is 0.976 or 97.6%
How many individuals in the population are heterozygous?
Heterozygous individuals are represented by 2pq so 2(0.024)(0.976) = 0.047 or 4.7% of the population. To calculate individuals in the population, multiply 0.047 times 1700 (total population) = almost 80 people are heterzygous.
Remember natural selection depends on the traits an organism shows (phenotype). So recessive traits may still exist in a population even though you don't see them, they are in heterozygotes.
Agents of Evolutionary Change
Change in DNA, Increases variation
May have no effect on phenotype (silent mutation) if amino acid sequence remains unchanged.
If amino acid sequenc IS changed then phenotype may change and so might the individual's fitness.
a measure of the ability of an individual to pass on their genes to the next generation. The more genes passed on, the more "fit."
Differential survival & reproduction due to changing environmental conditions
Combinations of alleles that provide “fitness” increase in the population
Adaptive evolutionary change
Movement of individuals & alleles in & out of populations
Seed & pollen distribution by wind & insect,
Migration of animals = sub-populations may have different allele frequencies, causes genetic mixing across regions reduce differences between populations.
p. 478 text (summary)
Significant in small populations because they lack diversity and are more prone to allelic changes
Causes random changes in allele frequencies
Can lead to a loss of genetic variation
Can cause harmful alleles to become fixed (only allele present for that gene)
Two examples of Genetic Drift
sudden environmental change (disaster)
reduces population down to only a few survivors who survive by chance alone
new population may have less/different allelic diversity than the original population.
a few individuals become isolated from a larger population
this smaller group's alleles may lack the same diversity as original population and be subject to drift because it is a small population
The fittest individuals produce more offspring that survive.
males who are better at attracting mates, protecting territories, gathering food, etc. mate more successfully
next generation carries more of their genes/traits than other males.
no geographic barrier, same area
A reproductive barrier that impedes mating between 2 different species or hinders fertlilization if interspecific mating is attempted.
Grand Canyon squirrels
Reduced Hybrid Viability
basically hybrid doesn't survive past birth or if it does, it dies soon after
Reduced Hybrid Fertility
even if hybrids are healthy, their chromosome number is odd thus making them steril
Horses have 64 chromosomes (32 pairs) and mules have 62 chromosomes (31 pairs).
Donkeys have 63 chromosomes.
some hybrids may mate with each other and have offspring but the offspring are feeble or sterile.
plant gametes pass on and express double set of chromosomes
fertile hybrid created from 2 different but very similar species
a subgroup of a species starts to occupy a slightly different niche and no longer interacts with original population.
Female preference drives the diversification of males and leads to species isolation (seperate species) that don't mate with each other.
Area in which interbreeding between different species occurs. The zone can be clearly defined based on allele frequencies (both allele versions exist in about equal frequencies in the hybrid zone but not at all in the opposite species' range.
Mainland vs. Galapagos Finches
Over time, there is a strenghtening of reproductive isolation until hybrids are no longer produced. This gradually happens after a split creating a new species from an original population.
Allopatric males - females sometimes confuse one species for the other so there is still hybridization.
Sympatric - males look different so more reinforcement of species separation, no hybridization.
Rate of Speciation
slow and steady change suggested by Darwin & others
short, rapid bursts of change followed by long times of no change (equilibrium)
Weakening of reproductive barriers, two species fuse because the barriers to reproductive isolation are weak.
Continued production of hybrids to a limited area and extent but two populations still remain separate species with a hybrid zone inbetween.
What are some possible benefits of these hybrid zones? Evolutionary Significance?
Cladograms: phylogenetic trees
Which of the cladograms above doesn't belong?