A history of life and natural selection

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Evidence for Evolution

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FossilsA fossil is the remains or traces of an organism

that died long agoMany of the oldest fossils we find are of extinct

speciesMost fossils are found in sedimentary rocks

that settle at the bottom of seas, lakes and marshes

All the fossils together have created a geologic record of Earth’s history

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FossilsFossils show us different

organisms lived at different timesFor example, rock strata from

about 2-3 bya would show fossils of only single celled organism

However, rock strata from 150 mya would show fossils of dinosaurs, the first birds, and a wide variety of plant life

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FossilsIf evolution has occurred, we would see different species throughout historyFossils show us that there has been!!

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FossilsFossils also give us clues to

transitional species. Transitional species

show how organisms gradually change over time

For example, scientists believe the whale ancestor’s were once land dwelling

We have found several transitional fossils to support this idea

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Anatomy Anatomy is the study

of the body structureIf organisms have

evolved from common ancestors, then they would have similar anatomical features…RIGHT?!?!?!

Well, THEY DO!

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Anatomy Some organisms have homologous

structures, which are anatomical structures that occur in different species and that originated by heredity from a common ancestorThe function of that structure may differ in

related organismsFinding homologous structures in different

species indicates they have a common ancestor

Ex: Limb bones in mammals

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Homologous Structures

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Anatomy Some organisms have analogous structures,

which are anatomical structures that have closely related functions, but were not derived from the same ancestorAnalogous structures evolve independently, but

have the same function EX: wings in bats, birds, and bugs

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AnatomyMany organisms display vestigial structures, which are anatomical structures that seem to have no function but resemble structures used in ancestorsRemember, that just because something becomes useless, DOES NOT mean it goes away…it must become detrimental to survival

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Vestigial structures

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Eyes on a mole

Hip bones on a whale or snake

EmbryologyEmbryology studies the development of embryosThe early stages of vertebrate development are incredibly similar

The explanation for this is that vertebrates share a common ancestor and inherited common stages of development

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Fish, reptile, bird, and mammal embryos all have a tail and gill slits.

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Biological MoleculesBiologists now have the

technology to compare DNA, RNA, proteins, and other biological molecules in different organisms (molecular homology)Organisms that have the

least amount of differences in these molecules are closely related by a common ancestor

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BiogeographyClosely related species live in the same geographic

regionsThis results from having similar needs so that they

need similar habitatsFurthermore, in isolated areas (islands), you will

find species that are unique in the world (endemic)

Additionally, similar environments will give rise to different species that have similar traitsExample: Flying squirrel (mammal)of North America

is very similar to the Sugar glider (marsupial) of Australia

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Can we see evolution today? Organisms on Earth ARE currently

evolving One familiar example is bacteria- we have to keep coming up with different antibiotics to fight the rapid evolution of bacteria. They evolve to become resistant to the antibiotics

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Can we see evolution today? It is very hard to see evolution in higher organisms- as the process takes hundreds-thousands of years But we CAN observe natural selection in many species in a relatively short period of time (such as with the peppered moths)

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The goal of population genetics is to understand the genetic composition of a population and the forces that determine and change that composition.

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Variation in a Population Population genetics is the study of

evolution from a genetic point of view Evolution at the genetic level is microevolution

Population genetics looks at the alleles (variations) in a population and how they change over time (evolve)

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Variation in a Population Population genetics also looks at

populations as a whole, because population is the smallest unit in which evolution can occur

A population is defined as a group of individuals of the same species that routinely interbreed

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What can cause traits to vary in a population?Mutations-which are random

changes in DNA sequencesRecombination-genes are

reshuffled in meiosis-due to independent assortment and crossing over

Random pairing of gametes-organisms produce many gametes-any one can be involved in fertilization

The environment also influences the outcome of many traits

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The Gene PoolThe gene pool is

the total genetic information available in a populationSo, ALL alleles for EVERY gene in a population

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Genetic EquilibriumHardy-Weinberg Genetic Equilibrium is a principle

that states genotype frequencies tend to remain the same over generations unless acted upon by an outside forceHardy-Weinberg equation(s):

p2 +2pq + q2 = 1p + q = 1

p = dominant alleleq = recessive allelep2 = homozygous dominantpq = heterozygousq2 = homozygous recessive

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ExampleA population of bunnies has the phenotype

of 36% white bunnies(the recessive gene) and the rest are black. Based on this data, what are the frequency of each genotype?Things to consider:

What are the possible genotypes?Do you know any variables? 2pq, p2, or

q2

How do you solve for p or q?

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Solution36 % of the bunnies

are whiteGenotype = bbTherefore, 36% of

the bunnies = q2

q2 = 0.36q = 0.6Since q = 0.6, thenp + q = 11 – q = pp = 0.4 (40%)

ANSWER:

p2 = 0.16 or 16% (BB)

2 pq = 0.48 or 48% (Bb)

q2 = 0.36 or 36% (bb)

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Genetic EquilibriumHardy-Weinberg Genetic Equilibrium

assumptions:No net mutations occur, so alleles remain the

sameIndividuals neither leave nor come into the

population (no gene flow)The population is large (ideally infinite)Natural Selection DOES NOT occur (so random

mating and no environmental pressures)Random mating (no sexual selection)

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Non-Equilibrium If the alleles and genes do not stay the same

over generations, we know populations are out of equilibrium and evolution may occurEquilibrium is no change, evolution is changeWe can tell if frequencies stay the same by

calculating them using the Hardy-Weinberg equation

So, when genes and alleles change, natural selection and evolution are occurring according to Hardy-Weinberg

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Non-Equilibrium Many things can disrupt genetic equilibrium

and in fact populations RARELY stay at equilibrium for very long

What disrupts equilibrium and cause evolution?MutationsGene flowGenetic driftFounder effectNonrandom matingNatural Selection

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MutationsWe have already discussed many

avenues of genetic mutationMutation rates tend to be low in

animals and plants (about 1 mutation in every 100,000 genes per generation)

In sexually reproducing organisms, sexual recombination is a more important vector for change

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Gene flow Immigration (individuals coming in) and

emigration (individuals leaving) naturally occurs in many populations This causes gene flow- which is the

movement of genes from one population to another

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Genetic Drift In small populations,

chance events can change allele frequencies in a population A change in allele

frequencies is called Genetic Drift

Frequency means how often something occurs

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Bottleneck effectA disaster in the environment can lead to a

drastic change in allele frequencyThrough random chance, certain alleles may

be over-represented in a populationThis gives us an avenue to alter the genotypic

and phenotypic expression of the populationThis bottleneck effect would change the

new population into something different from the original population

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Genetic Drift Example In a population of 25 trees, where there are

two alleles for height- tall (T) and short (t) and the allele frequency is 50:50 A natural disaster- such as a fire, wipes out most

if the population Let’s say 2 trees survive, but they are both

homozygous tall (TT)- so now the allele frequency will be 100% for the tall allele

In a population of more (say 1,000), this is less likely to happen- more trees and therefore more of the original alleles would survive

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Genetic drift The random change of allelic frequency in a population

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Founder effectWhen a few individuals from a population become isolated from the source population, they may change to fill new habitats (founder effect)

Darwin observed this phenomena when he observed the finches on the Galapagos islands

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Nonrandom mating Many species do not mate randomly

Some mate with individuals close to them Some mate with individuals that have

similar traits to them Both of these result in increasing certain

allele frequencies For example, very tall birds, may only mate

with other tall birds (not medium or small), this would cause the tall allele to become more prevalent

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Nonrandom matingMany species of birds, such as peacocks

look for specific characteristics when they mate, like elaborate colorsThis is called sexual selection This leads to sexual dimorphism, a

difference between the physical characteristics of males vs. females

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Intersexual vs. IntrasexualIntrasexual

selection:Occurs between the

same sexDirect competition

between individuals of the same sex to maintain the ability to pass on their traits

Example: Male lions and control of the pride

Intersexual selection:Occurs when one sex

is able to select a mate

Becomes a competition to attract a mate

Examples: Birds – showiness of plumage, ability to build a nest, birdsong

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Natural SelectionMost significant

factor in evolution of populations

Nature selects against non-fit individuals

Reduces harmful alleles

Only acts on expressed phenotypes

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Agents of evolutionary change1.Mutation2.Natural selection3.Genetic drift4.Gene flow5.Nonrandom mating

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What is a species?The biological species concept says a

species is a population of organisms that can successfully interbreed

The morphological species concept says a species is a population of organisms that have a similar appearance

Modern Species Concept- biologists use both of these criteria to classify both living and extinct species today

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Other definitions of speciesPaleontological species: focuses on the

morphological characteristics of organisms in the fossil record.

Ecological species: looks at defining species bases on their role in an ecosystem (niche)

Phylogenetic species: how organisms develop from a common ancestor (we will be examining phylogenetic trees later on)

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Forming new speciesIn order for new species to form, you must

have genetic variation (remember from meiosis, mutation, etc)

Yet not all variation is a result of geneticsThere are also numerous environmental

factors that affect phenotypeLack of nutrition/too much nutritionKey thing to remember: environmental

changes to phenotype are NOT heritable

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How does a species form?The process of species formation is called

speciationSpeciation begins with isolation

In order to form a new species, you must begin with some sort of reproductive isolation

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Scale of speciationWhen you talk about changes in a single

gene pool, this is described as microevolutionExample: Peppered moths of England

When you discuss changes over vast tracts of time, this is referred to as macroevolutionExample: Going from the age of reptiles to the age of mammals

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FitnessIn order to determine which species will be

successful (or what traits are passed on to offspring), we must examine adaptive advantages

Fitness: contribution of an individual to the gene pool for the next generation

Relative fitness: contribution of a particular genotype for the next generation

Therefore, for speciation to occur, the new traits must have some sort of fitness “advantage” for them to be passed to a new generation

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How does a species form?Allopatric speciation is a result of geographic isolation (some physical barrier that separates populations)Allopatric means “different homelands”Once one species is separated into two (or more), gene flow between them stops

As each experiences different environmental pressures, genetic drift occurs in different ways

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How does a species form?Sometimes groups of organisms become reproductively isolatedThis may or may not be due to a physical barrier

Reproductive isolation is when two individuals cannot successfully mate (this means mate and produce healthy FERTILE offspring)

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How does a species form?Types of reproductive isolation

Prezygotic isolation- occurs before fertilizationDifferent species do not breed at the same

timeDifferent species have different mating

rituals (such as a mating call or “dance”)Basically this type of isolation means the

different species WILL JUST NOT MATE in nature

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Types of Pre-zygotic Isolation

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1. Geographic isolation 2. Ecological isolation 3. Temporal isolation

4. Behavioral isolation 5. Mechanical isolation

How does a species form?Postzygotic isolation- occurs after

fertilizationGametes are not be compatible and do

not produce healthy offspringIf offspring is healthy it may be infertile

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How does a species form?Sympatric Speciation- occurs when 2 subpopulation become reproductively isolated, but have no physical barriers between them.

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Sympatric speciationExamples of sympatric speciation:

Polyploidy is a mutation that often occurs in plants (can change chromosome number – go from 2n to 4n, autopolyploid)

As a result the 4n plant can no longer breed with the 2n plant . . . different chromosome number

Animals taking advantage of different aspects of the same resources (Darwin’s finches)

An environment that repeatedly, and drastically, changesLake or pond repeatedly drying out and refilling

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Rate of Speciation: Punctuated Equilibrium vs. GradualismGradualism

Evolution occurs much more slowly (gradually) and consistently

Speciation occurs at intervals further apart

Punctuated EquilibriumEvolution that occurs at more intervals and less consistently

Speciation occurs more frequently

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Types of EvolutionConvergent

evolution- the process by which different species evolve similar traitsThis often occurs

due to the different species living in similar types of environments

Sugar Gliders and Flying squirrels both adapted to living in tall trees, but on different continents

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Types of EvolutionDivergent Evolution- a process in

which the descendents of a single ancestor diversify into several different species that fit a variety of habitatsA great example is Darwin’s finchesOne species of finch came from South America and evolved into 13 distinct species-each of which has a different habitat

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Types of EvolutionAdaptive Radiation– when a new

population in a new environment undergoes divergent evolution until it fills many parts of the environmentThe finches evolved in almost every part

of the Galapagos Islands

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Types of EvolutionCoevolution– when two or more

species have evolved adaptations due to each other’s influence

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Coevolution: Pollinators help plants reproduce and

plants give food to pollinators

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Steps of Speciation in Darwin’s finches1. Founding Fathers & Mothers-finches made their

way from South America2. Separation of Populations-finches crossed to

different islands3. Changes in gene pool-Over time, populations

adapted to the needs of their environment4. Reproductive isolation-birds prefer to mate with

birds that have same beak as they do-2 species have evolved.

5. Share same island-co-existance, extinction, or further evolution

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