Post on 27-Dec-2015
What the rat example illustrates is a concept that turns up again and again in nature and provides proof of evolution.
Many populations contain the traits necessary to survive when the environment turns on them.
Which traits will be good or more common normally can’t be predicted but through variation populations can survive and evolve.
HOW VARIATION ARISES FROM SIMILARITIES…
Evolution is predicated on the concept that genetic variation leads to the environment choosing which version of a trait is best.
What mechanism(s) allow for variation?There has to be a way for nature to allow for the creation of new alleles.
Evolution starts with mutations in individuals; mutation is the mechanism for the creation of new alleles.
The penetrance of this mutation is completely dependent on SEXUAL reproduction.
I. INDIVIDUALS DON’T EVOLVE, POPULATIONS DO
Population Interbreeding individuals of the same species in the
same area.All individuals of a species have to share certain
traitsBut individuals of a population vary in the details of
these shared traits.These variations of course are because of alternative
versions of the alleles that cause these shared traits.
VARIATION IN POPULATIONS
Mutations
Mutations
Where does VARIATION come from?
Mutations
Mutations
Mutations
Mutations
II. HOW TO DETECT EVOLUTION: VARIATION CAN BE CALCULATED
Variation is distributed within a population.This fact is a way evolution can be studied.A distribution is an overview of the relative
frequency and range of a set of values.Often, some values in a range are more
common than others. A normal distribution, or bell curve, is
one that tends to cluster around an average value in the center of the range.
This sets the range of what’s normal and standard for a population.
THE GENE POOL
The variation is founded in alleles Alleles
• Different forms of the same gene
• Determines genotype and phenotype
Gene pool
• All alleles found in one population Within the gene pool, variation is due
to various alleles. Mutations are responsible for the
various alleles.The Black Jaguar6% of the South American population
The dominant phenotype
VARIATION & INHERITANCE
You have to also consider recombination in changes in genetics…
Variation is constantly changing because of natural selection but also genetic shuffling, through new combinations of alleles.
Proving evolution is partially achieved through investigating the change in allelic frequency over several generations.
The change is measured against the population in genetic equilibrium.
Allele frequencies• Relative abundance of alleles of a given gene in a
population• Represented as percentages, much like the %
composition of compounds. Natural populations are never in genetic
equilibrium• Genetic equilibrium = the allele frequency stays the
same.• A theoretical state which occurs when a population is
not evolving
VARIATION & ALLELES ARE TRACKED BY STUDYING ALLELE
FREQUENCIES
Researchers know whether or not a population is evolving by tracking deviations from a baseline of genetic equilibrium.
Five conditions required for a stable gene pool:1. Mutations do not occur2. Population is infinitely large3. No gene flow4. Random mating5. All individuals survive and reproduce equally
This doesn’t happen but the rate these change will affect the rate of evolution.
III. GENETIC EQUILIBRIUM
GENOTYPE FREQUENCIES VS. ALLELE FREQUENCIES
The thing you need to know is that genotypic and allelic frequencies always add up to 1.
Genotype Frequency(frequency of EE) + (frequency of Ee) + (frequency of ee) = 1Allele Frequency: (frequency of E) + (frequency of e) = 1
Genetic equilibrium is affected by several processes that alter allelic frequency.
This is how evolution and speciation happens.
Tracking evolution is studied by and defined by changes in allele frequencies over time.
MICROEVOLUTION
Determining Genetic Equilibrium… the BASELINE
The Hardy-Weinberg formula can be used to determine if a population is in genetic equilibrium
p2(AA) + 2pq (Aa) +q2(aa) = 1.0
The frequency of the dominant allele (A) plus the recessive allele (a) equals 1.0
p + q = 1.0
THE HARDY-WEINBERG FORMULA
Homozygous dominant
Homozygous recessive
Heterozyg
ote
The Hardy-Weinberg principle describes a population that is not evolving.
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving.
You will learn more about the Hardy-Weinberg formula soon…
THE HARDY-WEINBERG PRINCIPLE
Fig. 18-3a, p. 280
Finding out whether a population is evolving. The frequencies of wing-color alleles among all of the individuals in this hypothetical population of morpho butterflies are not changing; thus, the population is not evolving.
Fig. 18-3b, p. 280
490 AA butterflies dark-blue wings
490 AA butterflies dark-blue wings
490 AA butterflies dark-blue wings
420 Aa butterflies medium-blue wings
420 Aa butterflies medium-blue wings
420 Aa butterflies medium-blue wings
90 aa butterflies white wings
90 aa butterflies white wings
90 aa butterflies white wings
Starting Population Next Generation Next Generation
Just as the Hardy-Weinberg principle allows us to see a population that’s not evolving, so we can measure changes, these forces act against equilibrium:
1. Gene Flow2. Genetic Drift3. Mutation4. Non-random Mating5. Natural Selection
IV. MICROEVOLUTIONFORCES OF GENETIC CHANGE
Gene flowPhysical movement of alleles caused by individuals moving into and away from populations Immigration, etc.
Tends to counter the evolutionary effects of mutation, natural selection, and genetic drift on a population
Example: Movement of acorns by blue jays
1. GENE FLOW
Genetic driftThe random drifting of allele frequenciesVery much dependent on population sizeLike a Pendulum…
Leads to…Fixation has occurred when all
individuals in a population are homozygous for one allele
2. GENETIC DRIFT—THE CHANCE CHANGES
BottleneckA drastic reduction in population size brought about by severe pressure
After a bottleneck, genetic drift is pronounced when a few individuals rebuild a population, fumbling to figure out beneficial traits.
Example: Northern elephant sealsExample: The Great Dinosaur Extinction
GENETIC DRIFT & BOTTLENECKS
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Catastrophic Event
New PopulationDynamics
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Founder effectGenetic drift is pronounced when a few individuals start a new population
(Often in conjunction with bottlenecks)
InbreedingBreeding or mating between close relatives who share a large number of alleles
Example: Old Order Amish in Lancaster County, Pennsylvania (Ellis-van Creveld syndrome)
GENETIC DRIFT & THE FOUNDER EFFECT
Mutations are the source of new alleles that give rise to differences in details of shared traits
Lethal mutations usually result in deathDecrease fitness but these rarely survive the gene pool.
Neutral mutations have no effect on survival or reproduction
Often considered “silent” mutations. In humans, the average person undergoes 50-100 mutations in their lifetime.
Less than 3% have any consequence.Remember, ~95% of our genome is “junk” DNA
Beneficial mutations convey an advantage.Rare but does occur… has to.
3. MUTATION REVISITED
With sexual selection, some version of a trait gives an individual an advantage over others in attracting mates
Distinct male and female phenotypes (sexual dimorphism) is one outcome of sexual selection
SEXUAL SELECTION
Females are attracted to males with larger, dark manes
Correlation with higher testosterone levels better nutrition & health more muscle & aggression better sperm count / fertility more successful young
But imposes a cost to male It’s HOT! Is it worth it??
THE LION’S MANE…
The Golden RatioSymmetry
“SEXY” = SUBCONSCIOUS FITNESS MARKERS
This contributes to non-random mating:Humans have preferences, just like all other sexual organisms.
Balanced polymorphismA state in which natural selection maintains two or more alleles at relatively high frequencies
Occurs when environmental conditions favor heterozygotes
Example: SexesExample: Sickle cell anemia and malaria
HbA/HbS heterozygotes survive malaria more often than people who make only normal hemoglobin
BALANCED POLYMORPHISM
Natural selectionThe variable survival and reproduction among individuals of a population that vary in details of their shared traits.
This is a driving force of evolution.Occurs in recognizable patterns depending on the organisms and their environment.
5. NATURAL SELECTION REVISITED
Natural Selection Acts in 3 Major Ways:All Populations Have Variation
Sometimes hard to detect, not so in humansIndividuals Tend To Produce Too Many
Offspring Individuals in a population will always have a hard time surviving
All Populations Depend on the Reproduction of IndividualsFitness = the reproductive success of grandchildren.
NATURAL SELECTION
Fig. 18-4, p. 281
population before selection
directional selection
stabilizing selection
disruptive selection
V. Three Patterns of Natural SelectionNatural selection
Takes 3 forms:DirectionalStabilizingDisruptive
We’ll take a quick peak at all three types.
Directional selectionChanging environmental conditions, the selective PRESSURE put on by the environment, can shift allele frequencies in one consistent direction
Forms of traits at one end of a range of trait variation become more common in the population.
The other becomes less or drops out completely.
DIRECTIONAL SELECTION
Fig. 18-5a, p. 282
Directional selection. These bell-shaped curves signify a range of continuous variation in a butterfly wing-color trait. Red arrows indicate which forms are being selected against; green, forms that are being favored.
Fig. 18-5c, p. 282
Once the norm…
Becomes this with directional selection
Casualties of Natural SelectionNow extinct…
Light color is adaptive in areas of low pollution; Dark color is adaptive in areas of high pollution
PREDATION AND PEPPERED MOTHS
Fig. 18-6a (1), p. 283
Natural selection of two forms of the same trait, body surface coloration, in two settings. (a) Light moths (Biston betularia) on a nonsooty tree trunk are hidden from predators. Dark ones stand out. (b) The dark color is more adaptive in places where soot darkens tree trunks.
In rock-pocket mice, two alleles of a single gene control coat color.
Night-flying owls are the selective pressure that directionally shifts the allele frequency.
PREDATION AND ROCK-POCKET MICE
Fig. 18-7b, p. 283
Visible evidence of directional selection in populations of rock pocket mice. (a) Rock pocket mice that have dark fur are more common in these areas of dark basalt rock. (b, c) The two color types of rock pocket mice, each posed on the dark and light rocks of the area.
A typical two-week course of antibiotics can exert selection pressure on over a
thousand generations of bacteria.If ANY remain, they do so because they may have a mutated gene that causes a resistance to these antibiotics.
Antibiotic resistant strains are now found everywhere, in hospitals and schools.
The rate of these superbugs are exacerbated by people not completing their prescriptions.
ANTIBIOTIC RESISTANT BACTERIA
Stabilizing selection Natural selection that favors an intermediate phenotype and eliminates extreme forms
Disruptive selection Natural selection that favors extreme forms of a trait and eliminates the intermediate forms
SELECTION AGAINST OR IN FAVOR OF EXTREME
PHENOTYPES
Time 2
Time 3Fig. 18-8, p. 284
Stepped Art
Stabilizing Selection
Range of values for the trait
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Fig. 18-9b, p. 284
Stabilizing selection in sociable weavers. Graph shows the number of birds (out of 977) that survived a breeding season. Figure It Out: What is the optimal weight of a sociable weaver bird?Answer: About 29 grams
Fig. 18-10a, p. 285
Disruptive selection eliminates midrange forms of a trait, and maintains extreme forms.
DISRUPTIVE SELECTION: BILL SIZE IN AFRICAN FINCHES
In African seedcracker populations, birds with bills that are about 12 or 15 millimeters wide are favored. The difference is a result of competition for scarce food during dry seasons.But the middle phenotypes are not good at eating the food.
Natural selection theory helps explain diverse aspects of nature, including differences between males and females, and the relationship between sickle-cell anaemia and malaria.
MAINTAINING VARIATION
Fig. 18-13a, p. 287
Distribution of malaria cases reported in Africa, Asia, and the Middle East in the 1920s,
Fig. 18-13b, p. 287
Notice the close correlation between the maps.
Individuals with SS (normal RBCs) get malaria and can die.Individuals with ss (sickle cell anemia) usually die early on.The hybrid (Ss) can survive with malaria and suffer little side affects associated with sickle cell anemia.
Fig. 18-13c, p. 287
Malaria and sickle-cell anemia. (c) Physician searching for mosquito larvae in Southeast Asia.
The data would indicate that sickle cell anemia and malaria respond to each other.Both alleles have success so natural selection favors a mix of both over one or the other.
Natural Selection supports the best traitsBut Natural Selection can be limited because:
Indirect Force: Natural selection doesn’t act on genes. Natural selection acts on unsuccessful physical traits… Not the genes that make them. Since traits (phenotypes) are determined by genes
(genotypes), sometimes these unsuccessful phenotypes can pop up if they are recessive.
Role of Mutation; Usually recessive alleles Recessive alleles can be hidden from targeting by natural
selection.
NATURAL SELECTION REVISITED
What is microevolution?How populations change due to changes in allele
frequencies.
Which type deals with allele frequencies that result from a population of rats that stow away on a barge that lands on an island?
Founder aff ect… genetic drift.
Some years black moths are favored, in other years white moths are favored. This form of natural selection is what?
Directional stabilization.
CONCEPT CHECK
SO WE’VE DISCUSSED THE MECHANISMS THAT CAUSE ALLELE
CHANGES
Now…Once gene flow ends, reproductive isolation occurs.
Once reproductive isolation occurs, speciation soon follows
SpeciationEvolutionary process by which new species form
Reproductive isolating mechanisms are always part of the process
Reproductive isolationThe end of gene exchange between populations
Beginning of speciation
REPRODUCTIVE ISOLATION
Reproductive isolating mechanisms prevent interbreeding among groups. Heritable aspects of body form, function, or behavior
that arise as populations diverge… they start to change because of the different selection pressures of the different ecosystem and the traits available the founding population.
What are the isolating mechanisms?1. Mechanical2. Behavioral3. Hybridization 4. Allopatric5. Sympatric/ Polyploidy (genetic)6. Parapatric
REPRODUCTIVE ISOLATING MECHANISMS
Reduced hybrid viability (ligers, tigons) Extra or missing genes
Reduced hybrid fertility (mules) Robust but sterile offspring
Hybrid breakdown Lower fitness with successive generations
3. HYBRIDIZATION
Geographic Isolation= “Allopatric speciation”
A physical barrier arises and ends gene flow between populations
Genetic divergence results in speciationExample: llamas, vicunas, and camels
4. ALLOPATRIC SPECIATION
Winds or ocean currents carry a few individuals of mainland species to remote, isolated islands chains (archipelagos) such as Hawaii
Habitats and selection pressures that differ within and between the islands foster divergences that result in allopatric speciation
THE INVITING ARCHIPELAGOS
Fig. 18-21a, p. 293
B Later, a few individuals of a new species colonize nearby island 2. Speciation follows genetic divergence in the new habitat.
C Genetically different descendants of the ancestral species may colonize islands 3 and 4 or even invade island 1. Genetic divergence and speciation may follow.
A A few individuals of a mainland species reach isolated island 1. In the new habitat, populations of their descendants diverge, and speciation occurs.
Populations sometimes speciate even without a physical barrier that blocks gene flow. Sympatric speciationParapatric speciation
OTHER SPECIATION MODELS
In sympatric speciation, new species form within a home range of an existing species, in the absence of a physical barrier
Polyploidy An example when there is a change in chromosome number that can cause instant speciation .
On Lord Howe Island, species of palms are reproductively isolated because of polyploidy
5. SYMPATRIC SPECIATION
In parapatric speciation, populations in contact along a common border evolve into distinct species
Hybrids in the contact zone are less fit than individuals on either side
6. PARAPATRIC SPECIATION
We have looked at microevolution, small allele frequency changes.
Microevolutionary events that occur independently lead to genetic divergences, which are reinforced as reproductive isolation mechanisms evolve
Now let’s look at macro, large-scale changes.Macroevolution refers to the appearance of new
species over timeNew species evolve mainly because of DIVERGENCE.These show how relationships influence the creation
of new species.Speciation, the formation of new species due to
evolution, links the two.
KEY CONCEPTS HOW SPECIES ARISE
Macroevolution Large-scale patterns of evolutionary change Includes patterns of change such as one species
giving rise to multiple species, the origin of major groups, and major extinction events
Coevolution Stasis Exaptation
Adaptive radiationKey innovationExtinctions
VII. MACROEVOLUTION
MACROEVOLUTION…THE PATTERNS OF EVOLUTION
Patterns of Macroevolution: that aff ect species. Co-evolution: Organisms are part of one other’s
environment, so they can aff ect one another’s evolution. Species that live in close contact often have clear adaptations to one another’s existence.
Adaptive Radiation: Over time, species may split into two or more lines of descendants, or lineages. As this splitting repeats, one species can give rise to many new species. The process tends to speed up when a new species enters an environment that contains few other species (meaning less competition). Helps explain fi nches.
Extinction: If all members of a lineage die off or simply fail to reproduce, the lineage is said to be extinct. The fossil record shows that many lineages have arisen and radiated, but only a few of their descendants survived and evolved into the species present today. Scientist estimate that 99% of all species that have lived, went extinct.
Two species in close ecological contact act as agents of selection on each other (coevolution)Predator and preyHost and parasitePollinator and flower
Over time, the two species may come to depend on each other.
COEVOLUTION
Fig. 18-25, p. 296
proboscis
nectar tube
10 cm
CoevolutionHawk moth & Orchid
Coevolved species. The orchid Angraecum sesquipedale, discovered in Madagascar in 1852, stores its nectar at the base of a floral tube 30-centimeters (12 inches) long. Charles Darwin predicted that someone would eventually discover an insect in Madagascar with a proboscis long enough to reach the nectar and pollinate the flower. Decades later, the hawkmoth Xanthopan morgani praedicta was discovered in Madagascar. Its proboscis is 30–35 cm long.
A niche is the entire collective habitat where a population lives.
When populations inhabit different niches, divergence often follows…because there are different selection pressures.
ECOLOGICAL NICHES
Adaptive radiation A burst of speciation that
occurs when a lineage encounters a new set of niches because of a Key Innovation: A structural or functional adaptation that allows individuals to exploit their habitat in a new way
Again, caused by variation and natural selection
Fig. 18-26a, p. 297
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Evolutionary tree diagram showing the adaptive radiation of mammals following the K–T extinction event. Branch widths indicate the range of biodiversity in each group at different times. We show only a sample of modern mammals. The entire mammalian lineage includes more than 4,000 modern species. The photograph shows a fossil of Eomaia scansoria (Greek for ancient mother climber), complete with the imprint of its fur. About 125 million years ago, this mouse-sized insect-eater crawled on low branches. It is thought to be an offshoot of the lineage that led to mammals.
How do Species Evolve?There are two main schools of though…1. Gradualism2. Punctuated Equilibrium
PATTERNS OF MACROEVOLUTION
MACROEVOLUTION
Gradualism : In Darwin’s day, the idea of slow, gradual change was new to geology as well as biology. Darwin had argued that large scale changes, such as the formation of new species, must require many small changes to build up gradually over a long period of time. This model is called gradualism.
MACROEVOLUTION
Gradualism conflicts with another more modern theory that suggests evolution can happen in bursts over a very fast period of time.
Punctuated Equilibrium: Some biologists argue that species do not always evolve gradually. Species may remain stable for long periods until environmental changes drastically and suddenly create new pressures. Then, many new species may “suddenly” appear. This model is called punctuated equilibrium.
Gradualism Punctuated Equilibrium
Small evolutionary changes accumulate over long periods of time. These result from mutations present in alleles present in population.
Catastrophic events, like volcanoes or meteor strikes, suddenly change the environment, creating new selection pressures. The fittest survive.
Which way is correct?Probably both but there is a lot of evidence that PE is the
more correct.
Stasis A lineage exists for millions of years with little or no
change (e.g. coelacanth)
Exaptation (preadaptation) Some complex traits in modern species held different
adaptive value in ancestral lineages (e.g. feathers in birds and dinosaurs)
STASIS AND EXAPTATION
Extinction The irrevocable loss of a species from Earth
Mass extinctions Extinctions of many lineages, followed by adaptive radiations Five catastrophic events in which the majority of species on
Earth disappearedK–T extinction, abbreviation of Cretaceous–Tertiary
extinction, also called K–Pg extinction or Cretaceous–Paleogene extinction a global extinction event responsible for eliminating approximately 80 percent of all species of animals at or very close to the boundary between the Cretaceous and Paleogene periods, about 65.5 million years ago.
EXTINCTION
http://www.britannica.com/EBchecked/topic/1314796/K-T-extinction
SO…
Exhausted Yet?We’ve covered a great deal today- The Parameters of Genetic Equilibrium- The Mechanisms of Microevolution- The Methods of Genetic Change- The Major Patterns of MacroevolutionDo you have to remember EVERYTHING?- Pretty much.Should you be aware of these terms for later investigations?- Yep. Review the list on the board and be prepared to use these next week.