Evolution: How species have changed over time
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Transcript of Evolution: How species have changed over time
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Evolution: How species have changed over time
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First a Perspective of Time
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Those who influenced Darwin
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Charles DarwinWas a Naturalist – mostly observed
organisms in their natural habitats rather than conducting experiments.
Made most of his observations on the Galapagos Islands
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Charles Darwin
Did much of his work in the Mid-1800’s
** Keep in mind this is BEFORE Mendel, Watson and Crick***
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Charles Darwin
Introduced the idea of Natural Selection as a way for new species to form (speciation).
Published The origin of Species in 1859
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The Theory of Natural SelectionAssumptions:
There are not enough resources for all to survive
genetic variation exits in all populations.
Results:
1. Competition
2. Survival of the fittest
3. Descent with modification
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Assumption 1: Not enough resources
What resources are we talking about?
Are there enough for everyone?
Food ShelterSuitable Mates
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Assumption 2: Genetic variation exists
Where do these differences come from?
Remember it doesn’t have to be a NEW gene, just a new combination of genes
Mutations Genetic Recombination
Sexual reproduction
Migration
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Result 1. Competition
What are we competing over?
Who wins? What is the prize?
What happens to those that don’t win?
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Result 2. Survival of the FittestIn nature are we all really equal?
What do we mean by “fittest”?
Is it enough to survive?
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Result 3. Descent with ModificationBreak it up, what does it mean?
What happens to the frequency of fit genes and unfit genes?
What do we see in future generations?
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3. Descent with ModificationNew generations will resemble
previous generations (descent) BUT
more individuals will have the “best” variation PLUS new mutations and combinations (with modification)
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An Example
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Example:
What is the genetic variation? What is the selective pressure? Who has the advantage? What would we predict for the next
generation? Why might the “unfit” phenotype stick
around?
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Rules of EvolutionMutations and their phenotypes are
random. Meaning?
Variation must exist in the population BEFORE selective pressure occurs
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Rules of EvolutionIndividuals can not evolve, only
species
A fit trait in one environment might be eliminated as a weakness in another
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Types of Selection
Natural SelectionWhat determines which variation gets passed on?What is the outcome?
Artificial Selection (a.k.a. selective breeding)What determines which variation gets passed on?What is the outcome?
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Types of Selection
Directional Selection: One extreme or the other is “favored” and increases in frequency while midrange and other extreme decrease
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Types of Selection
Stabilizing Selection: Midrange is favored and increases in frequency while both extremes decrease.
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Types of Selection
Diversifying/disruptive Selection: Both extremes are favored and increase while midrange decreases.
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At what point is a new species formed?
Evolution – change in allele frequency
Speciation – such change that new population is a different species
– two organisms that can successfully reproduce and produce viable, fertile offspring
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Examples:
Cross between a Pug and a Beagle - different breeds but SAME species
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Examples:
Offspring: Puggle! Both viable (obviously) and fertile
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Examples:
Cross between a Horse and Donkey - different species
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Examples:
Offspring: Mule! Viable but infertile
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Gene Pool Isolation
Two populations become separated so their genes are no longer mixed
Mutations appear independently in each population
Selection happens independently in each population
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Mechanisms of IsolationGeographic – Physical barrier separates two
populations
Behavioral – mating behaviors of some are not attractive to others.
Temporal – fertility occurs at different times
Mechanical – different physical means of reproduction
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Principle of a Common Ancestor
Descent with Modification – over generations descendents can look quite different from ancestors.
Thus, organisms that seem very different might share a common ancestor
Suggests if you go far enough back, we are all related!
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Phylogenetic tree: Family Tree of Life
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Common ancestor
Humans and chimps have a common ancestor.
THAT IS NOT THE SAME AS SAYING WE WERE ONCE CHIMPS!!!
Think about it: Do you and your cousin share a common ancestor? Does that mean you are your cousin? Does that mean that either of you are that ancestor?
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Evidence of Common ancestry
Comparative AnatomyComparative EmbryologyComparative Biochemistry
See Determining evolutionary relationships assignment
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Evidence of a Universal Common Ancestor
What do we ALL have in common
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Additional Evidence of Evolution (but not necessarily common ancestry)
Fossil RecordPreserved
remains of ancient life in sedimentary rock
Even of species no longer in existence (most!)
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Fossils
Fossils are often found in the layers of sedimentary rock.
See changes in fossils over time
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Dating FossilsAbsolute Dating: Using radioactive
organic material in a sample we can get a more accurate age of a fossil
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Dating FossilsRelative Dating: Fossils found in
lower levels are older than upper levels.
Can’t provide exact age, just which is older
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Dating FossilsAbsolute Dating: Radioactive
organic material is used to get a more accurate age of a specimen.
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Radioactive material decays into a non-radioactive decay product at a steady rate.
Half life = time it takes half a sample to decay.
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Example: Some carbon is naturally radioactive – C14.
Half life of C14 – 5,730 years
Decay product is N14
If we look at the sample and determine the ratio of C14 to N14 we can get an idea of how much time has passed
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C-14 remaining
C14:N14 Years from start date
100g 1:0 0
50g 1:1 5,730
25g 1:3 11,460
12.5 1:7 17,190
Assume we start with a sample that is 100g of C14
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Geographic DistributionBiogeography and Convergent
Evolution:
See Determining evolutionary relationships assignment
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Vestigial Organs
Structures that serve little to no purpose NOWSnake skeletons with leg bones and
pelvisBlind, cave-dwelling fish have eye-
sockets but no eyes.
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Vestigial OrgansGives insight into PAST needs of
organism as well as where this organism has come from
What happens first: Need for organ disappears? Or mutated organ appears?
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Genetics in EvolutionDarwin did his work before Mendel
and didn’t understand genes or how inheritance worked.
Thanks to Mendel we know how/why traits get passed from parent to offspring
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Phenotypes NOT genotypesNatural selection acts on phenotypes
NOT genotypes
But in turn will influence allele frequency.
Why aren’t all bad alleles eliminated??
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Mechanisms of Evolution
Remember, it is variation that proposes and selection that disposes
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Mechanisms of Evolution
Genetic DriftEvolution without natural selection
Chance occurrences change allele frequency
More common in small populations
What if more of the “unfit” survive?
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Genetic Drift Founder Effect
Founding Population B
DescendantsSample of
Original Population
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Mechanisms of Evolution
Endosymbiotic theory• Mitochondria and chloroplasts evolved
from free living prokaryotic organisms
• A larger cell engulfed them
• A symbiotic relationship formed
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Endosymbiotic theory
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Evidence of endosymbiosisBoth have their own DNA and produce
their own proteins
Both reproduce independently from the cell through a process like binary fission (bacterial reproduction)
Double membranes of both are similar to prokaryotic membranes
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Patterns of EvolutionMass Extinction
Periodic large-scale extinction events
Dramatically changes landscape eliminating or creating selective pressures
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Patterns of Evolution
Adaptive RadiationSingle species evolves into
several different species that live in different ways (adaptations)
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Patterns of EvolutionCo-evolution
Due to close relationship two species share with each other, change in one organism results in a change with the other.
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Patterns of Evolution
GradualismWhat Darwin subscribed toTiny changes accumulate over
huge period of time to yield large changes.Think Grand Canyon only organisms
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Patterns of Evolution
Punctuated EquilibriumMore modern theory proposed by Gould
and Eldridge
Proposed change occurs in spurts followed by periods of stasis
More support in fossils!
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Are organisms always evolving?Hardy Weinberg Equilibrium – suggests no!
Under certain conditions, populations won’t evolveConditions:
1. Large population
2. No migration in or out
3. No natural selection
4. Random Mating
5. No net mutations
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How do we tell? Determine allele frequencies over different
generations and see if they changep = frequency of dominant alleleq = frequency of recessive allele
p + q = 1
p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant
q2 = frequency of recessive genotype
2pq = frequency of heterozygote
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Example problem:
A population of aphids can either be brown or green. Green is recessive. In a population of 1000 aphids 250 are green. What are the allele frequencies for the green and brown alleles?
Then figure out the homozygous dominant and heterozygote populations too.