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IB Biology The Theory of Evolution

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Michelle Durham

on 21 December 2012

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Transcript of IB Biology The Theory of Evolution

Darwin's Theory of Evolution Darwin was a Naturalist, traveled on the HMS Beagle to collect specimens and find new species. observations 1. argentina and Australia has same grassland habitats but different speices of animals
2. some fossils resembled living organisms and others did not
3. GALAPAGOS island tortoises, iguanas and finches were his main interest questioned if animals living on different islands had once been part of the same species? IF SO....would turn peoples views upside down!! Current belief at the time:

earth only a few thousand years old
all aminals and plants exist as they did in the beginning doesnt explain: fossil record
geological data being generated at the time Charles Lyell was a geologist who determined that the EArth and its crust if continuously changing and the Earth is millions of years old. Darwin read Lyell while on the Beagle This explains how he found fossils of marine life on mountain tops, and theat this could not have happened in just a few thousand years For Darwin 1831 Jean-Baptiste Lamarck pre-dates Darwin by about 30 years attempts to explain that organisms change over time Lamarck's theory: organisms CHOOSE to use or not use structures
organisms gain/ lose traits during their lifetime
pass this on to offspring ex:
giraffe uses its neck to reach leaves on high branches
this use causes its neck to grow longer
offspring born with longer necks Inheritance of ACQUIRED traits did not know how traits were inherited
did not know that behavior cannot affect heritable traits
WAS the first to offer a hypothesis for evolution Another influence on Darwin: Thomas Malthus, economist Malthus if human population continues to grow unchecked, sooner or later we will insufficient resources famine, disease and war would affect human populations Darwin saw this concept in terms of plants and other animals besides humans human reproduce slowly and make few offspring, yet plants and smallrer aminals reporduce wuickly and make MANY offspring SO.... why are the continents not covered with maple trees and the oceans filled with fish and molluscs? obviously ALL offspring do not survive..... what determines which offspring survive and reproduce????? central question ot Darwins studies 1859 Darwin publishes "On the Origin of Species" Alfred Wallace another naturalist came to the same conclusion and his eminent publication pressed Darwin to publish the material he had been sitting on for 25 years The mechanism of change that Darwin presented was called NATURAL SELECTION Darwin NEVER used the term evolution HOW does Darwin's idea work???? member within a species vary form one another - can see this! 1 traits are passed from parent to offspring (did not now how, though, no idea about genes) Variations are Important humans were already using artificial selection in breeding crops and livestock! people thought that variations were unimportant, minor defects YET, Darwin attempted to compare nature to artificial selection Struggle for existence 2 competition between members of a species central to Darwin's idea 3 Survival of the Fittest ability to survive and reproduce in an environment is FITTNESS adaptation: any inheritied trait that increase chances for survival anatomical (structural), Physiological, or behavioral individuals with adaptations better suited for survival (higher fitness) will survive and reproduce more successfully survival of the Fittest called this process natural selection because of its similarities to artificial selection
only certain individuals get to reproduce
traits that are selected depend on fitness in the environment (not human choice) Descent with modification Each living species has descended, with changes, from other species over time Common Descent all species - living and extinct - were derived from common ancestors all living things are linked in a single "tree of life" Evidence for Evolution 1 Fossil record by comparing layers of rocks and fossils found in different layers, can confirm that life on earth has changed evidence of extinction, and relationships between old and current organisms

Evidence that complex plants and animals of today were preceded by simpler ones still contains gaps 2 Geographic distribution of living species animals livin under similar ecological conditions, have similar pressures of natural selection evolve similar features 3 homologous body structures different functions - same structure bird wing- whale fin - human arm - alligator leg different mature forms and functions, but develop from same embryonic tissue all 4 limbed animals have similar limb bones bones in birds wings are cmore closely alike to toher birds than to bat wings

bones in bat wings are more similar to arm bones in other mammals than birds

bones in bird wings are more similar to reptile forearms than mammals helps scientists group animals according to how recently they shared a common ancestor vestigal organs: organs or structures that are no longer in use 4 similarities in embryology embryos look similar during stages of development common cells common tissues common order of development species produce more offspring than can survive Chapter 16:
Evolution and Populations Darwin could not explain:
1. the source of variation within species
2. how traits are passed between generations Mendel's "Principles of Inheritance"
published in 1860's but not common recognized until after Darwin's death filled in the blanks! Together genetics, evolutionary theory and molecular biology explain HOW evolution occurs A population is a group of individuals of the same species
they interbreed and and share a common group of genes - GENE POOL gene pools contain at least 2 alleles for each trait the number of times an allele is present in a gene pool compared the other allele is called ~ relative frequency expressed in percents 40% B for brown fur
60% b for black fur sources of genetic variation: 1. mutation
2. meiosis mutations can affect physical, chemical or behavioral attributes
can affect fitness, even if not visually apparent
can be neutral, most often Meiosis independent assortment
crossing over
sexual reproduction only shuffles the genes into new combination, it does not change the frequency of an allele single-gene traits: traits controlled by one gene with at least 2 alleles. Can have as few and two different phenotypes polygenic traits: controlled by more than one gene, each with at least 2 alleles. Many possible combos and phenotypes Natural selection and single gene traits selective pressures can cause an allele to change frequencies if an allele increases fitness decreases fitness has no effect on fitness frequency frequency frequency does not change increases decreased genetic drift chance events cause allele frequencies to change natural disaster
disease founder effect a small group splits off from a larger population allele frequencies might be different between the groups CHANCE CHANCE genetic equilibrium allele frequencies remain unchanged= no evolution in order for evolution to occur, one or more factors must cause frequencies to change: chance events
changes in environment 5 conditions are required to maintain genetic equilibrium from generation to generation: 1. random mating
2. large pop size
3. no immigration/ emigration
4. no mutations
5. no natural selection equal opportunity to mate and pass on genes chance events wont effect large pops keep gene pools separate and isolated, no new or lost alleles can introduce new alleles equal chance of survival, no selective advantages Speciation if chance events and natural selection can lead to changes in allele frequencies, WHAT leads to the formation of NEW SPECIES???? populations that become reproductively isolated become a new species: Behavioral isolation differences in courtship or other behavior patterns Geographic isolation separated by geographic barriers such as rivers or mountains

if the two populations can interbreed when reintroduced, they are still the same species Temporal Isolation populations reproduce at different times Let's look at Darwin's Galapagos Finches Speciation of the Galapagos Finches occurred by founding a new pop; Geographical isolation; changes in the gene pool; reproductive isolation; and competition for resources Founders arrive 1st a few finches from South America find their way to the islands - population A Separation of Populations 2nd finches on different islands dont mix - they do not like to fly over open water. 3rd Changes in gene pool populations on each island become adapted to their ecological conditions - specifically food sources and climate

Natural selection causes different alleles for traits to be selected for - changes allele frequency 4th reproductive isolation populations A and B if birds were reunited - would not be attracted to each other:
beaks are different, different mating rituals even if they live in the same place again, their gene pools are still isolated, they wont mix 5th competition continued competition may create more species if species A and B end up on same island again, different conditions may lead to species B evolving into species C see page 410 DARWIN: Australia, New Zealand and Hawaii Isn't evolution just a theory?

Saying that evolution
is “just a theory”…

…is an attempt to convince the audience that evolution is only a guess that's open to debate. This is definitely not the case. The word “theory” means something special to scientists.

In everyday usage, “theory” often refers to a hunch or a speculation. When people say, “I have a theory about why that happened,” they are often drawing a conclusion based on partial or inconclusive evidence. Scientists have hunches, too, but they call them hypotheses, which are the starting point of all good science.

A scientific definition of theory is quite different from the everyday meaning of the word. A scientific theory refers to a comprehensive explanation of some aspect of nature that unifies a vast body of reliable knowledge. In other words, a theory is born when a substantial number of hypotheses point to the same conclusion. In science, a Theory:

•Explains a natural phenomenon.
•Predicts future occurrences or observations of the same kind.
•Can be tested through experiment or otherwise verified through empirical observation.
•Is Supported by a vast body of reliable knowledge. What are some notable scientific theories?

Reliance on theory is critical to every branch of science. Many scientific theories are so well established that no new evidence is likely to alter them substantially. We all accept that…

•the Earth orbits the Sun (heliocentric theory),
•living things are made of cells (cell theory),
•matter is composed of atoms (atomic theory), and
•the Earth's surface is divided into solid plates that move over geological time (theory of plate tectonics). Each theory has been barraged with scientific tests and still stands intact.

This is true of the Theory of Evolution, too, which is why scientists do not find it controversial. Any perceived controversy comes from people who do not understand the scientific process. Evolution is the change in genetic characteristics of a group of organisms over time. No more, no less. Because evolution is something we see, it is a fact that evolution has occurred. Drawing upon…

•the physical and behavior characteristics of species,
•their distributions,
•the evidence in the fossil record,
•and the DNA found in organisms,
we see that life changes through time. Scientists’ job is to explain what causes those changes to happen. An example of evolution observed on the grandest scale is the gradual changes we see in fossil organisms in layers of rock. A more notorious example would be the way bacteria become resistant to antibiotics, requiring the development of new antibiotics. “Evolution” is not the same as “the theory of evolution.” “Evolution” is the observation. “The Theory of Evolution” is an explanation for what we observe The Theory of Evolution is the scientific explanation for how evolution occurred, past and present. Since Darwin’s ground-breaking work was published in 1859 (that's 150 years ago!), biologists have amassed a tremendous body of reliable knowledge supporting the Theory of Evolution. In fact, the Theory of Evolution is so central to modern biology that the great biologist Theodosius Dobzhansky said “nothing in biology makes sense except in the light of evolution.” Contrary to common understanding, scientific theories do not “graduate” to laws…

Scientific laws are typically short, mathematical expressions representing how nature will behave under certain conditions. Most theories contain laws, and more importantly, they explain why laws work and what they mean.

Here Are Some examples of scientific laws

•Matter-Energy Equivalence: E=mc^2. This fits within the greater framework of the Theory of Relativity.
•Newton's Second Law: F=ma. This fits within the greater framework of Classical Mechanics (Newtonian Physics).
•Mendel's Laws of Inheritance: Yes, there are even laws contained within the Theory of Evolution! http://evolution.berkeley.edu/evosite/evo101/VA1BioSpeciesConcept.shtml http://www.techapps.net/interactives/pepperMoths.swf http://evolution.berkeley.edu/evolibrary/article/0_0_0/lines_01 http://www.pbs.org/wgbh/evolution/library/04/2/quicktime/l_042_02.html http://www.pbs.org/wgbh/evolution/library/04/2/l_042_02.html http://www.pbs.org/wgbh/evolution/library/11/2/e_s_3.html whales http://www.ucmp.berkeley.edu/education/explorations/tours/stories/middle/intro.html past lives http://evidenceforevolution.org/ Selective Breeding selecting and breeding individuals with desired traits plants and animals 5 for a trait with onle one allele- fixed trait Imagine a population of 500 flowers with two alleles C and C

C C is red
C C is white
C C is pink incomplete dominance R R R W W W if there are 320 red flowers, 160 pink and 20 white since they are diploid organisms there are 1000 genes C 800 copies
C 200 copies R W 320 X 2 and 160 X 1 20 X 2 and 160 X 1 use p to represent one allele and q to represent the other p = frequency of C

q = frequency of C R W = 800/ 1000 = 80% = 200/ 1000 = 20% For all loci the sum of all alleles must equal 100% or 1 Hardy - Weinberg Principle used to determine whether natural selection or other factors are causing evolution at a specific loci must determine what the genetic makeup would be in a population in NO evolution occurred at a loci if there are NO difference between the two THEN the real population is not evolving Example The allelic frequency and genotypes in a population will remain constant from generation to generation AS LONG AS only Mendelian segregation and recombination of alleles is occurring. HARDY-WEINBERG EQUILIBRIUM can use H-W principle to mathematically predict genotypic frequencies in future generatios Using flower example the probability that two C genes will randomly come together is p x p or p R 2 the probability that two C genes will randomly come together is q x q or q W 2 .8 X .8 = .64 or 64% .2 X .2 = .04 or 4% the probability that a C and C gene will randomly come together is p X q or pq W R .8 X .2 - .16 or 16% since heterozygotes can arise in two ways ( C egg and C sperm or C egg and C sperm) the chance of the heterozygote condition is pq + qp or 2 pq R R W W .16 + .16 = .32 or 32% for heterozygotes p + 2pq + q = 1 2 2 Conditions for H-W no mutations
random mating
NO natural selection
Extremely Large Pop size
No gene Flow genes can't be altered, duplicated or deleted preferential mating prevents random mixing of gametes selects for specific genotypes and alters allele frequency small pops subject to chance events! alleles entering or leaving change allele frequencies Applying the H-W principle PKU - phenylketonuria a recessive genetic disorder affects 1 in 10,000 newborns and is fatal if not treated To apply H-W: no new mutations of the PKU gene
no selection of mates based on the trait
ignore effects of differential survival
no genes leaving the gene pool we know that
the mutation rate is low
no inbreeding in US and selection is only against the homozygous phenotype
US is a large pop and the frequency of PKu outside US is same Therefore:
the frequency of individuals born with PKU will correspond to q 2 q = .0001 .01 = The frequency of the dominant alleles is then p = 1- q = 1 - .01 = .99 The frequency of carriers is then calculated: 2pq = 2 X .99 x .01 - .0198 or .02 2% 99% 1% This is an approximation, real numbers may be different Remember the 5 conditions! a deviation from any may be cause for evolution 1. mutation rare because mutations are generally neutral. BUT a mutation that drastically increases or decreases fitness can have a large effect on allele frequencies 2. non random mating can affect frequencies of homozygous and heterozygous genotypes, but by itself is minimal The mechanisms that alter the allele frequencies directly and cause most evolutionary change are natural selection
genetic drift (small pop size)
gene flow Natural Selection in genetic terms selection results in alleles being passed on to the next generation in proportions that differ from the current generation EX: Drosophila and the gene that confers resistance to DDT
0% frequency in fly strains pre-1930
37% frequency in fly strains post 1960 allele either arose thru mutation between 1930 and 1960 OR was already present but in virtually unmeasurable quantities EITHER WAY the rise in frequency of the allele most likele due to DDT introduction into environment An allele that confers insecticide resistance will increase in frequency in a population that is exposed tot he insecticide ADAPTIVE EVOLUTION Evolution that results in a better match between organisms and their environment GENETIC DRIFT Chance events that cause allele frequencies to fluctuate unpredictably from generation to generation small pops! 2 circumstances that result in genetic drift are: The Founder Effect and the Bottleneck effect the founder effect a few individuals become isolated from a larger population and establish a new population with a gene pool different from the original pop accounts for the high frequency of certain genetic disorders among isolated human populations retinitis pigmentosa 1 of 15 colonists that settled the island colony Tristan da Cunha carried the recessive allele for the disease

by 1960 4/ 240 has the disease

this is much higher frequency than in the original population 10 X's higher The Bottleneck Effect A sudden change in the environment may cause a drastic change in population size CHANCE alone may cause some alleles to be overrepresented in the survivors or some alleles to be completely absent all together small pops will continue to be subject to the effects of genetic drift until the pop size increases Even when a pop recovers in size it will most likely suffer from low levels of genetic diversity for a long time EX: greater Prairie chicken of Illinois numbered in the millions until prairies were turned into farmland. By 1993 only about 50 birds left low levels of genetic variation, only 50% of eggs hatch

genetic drift may have led to an increase in frequency of harmful alleles To test: collected and compared DNA from Prairie chickens DNA from 15 museum specimens
10 were from the 1930's when the pop numbers over 25,000
5 were from the 1960's when the pop numbered about 1000 created a baseline estimate of amount of genetic variation was present in the population BEFORE the pop decreased so drastically results: the 1993 Greater Prairie Chicken had lost 9 alleles that were present in the museum specimen

fewer alleles per locus than the museum specimen introduces 271 new birds from neighboring states: egg hatching increased to 90% http://nhscience.lonestar.edu/biol/hwe.html http://www.phschool.com/science/biology_place/labbench/lab8/concepts.html hardy-weinberg LAB BENCH Effects of Genetic Drift: significant in small pops 1 chance events occur in ALL pops, but impact allele frequencies in small pops drastically Can cause Allele Frequencies to change at Random unpredictable change from year to year without the consistency of natural selection 2 Can lead to loss of Genetic Variation since evolution depends on variation this can directly affect a populations to adapt to a changing environment 3 Can cause harmful Alleles for become fixed when alleles are lost, others become "fixed" harmful alleles that become fixed can threaten a pop's survival 4 Gene Flow The transfer of alleles into or out of a population due to the movement of fertile individuals or gametes gene flow can transfer alleles into a pop that increase fitness - insecticide resistance in mosquitoes.
natural selection can then cause frequencies of the new allele to increase Natural Selection and adaptive evolution Natural selection is NOT random! the relative fitness of an organism's phenotype determines the role of natural selection relative fitness: the contribution that an individual makes to the gene pool of the next generation RELATIVE to the contributions of other individuals Remember: selection acts directly on the PHENOTYPE and only indirectly on the underlying genotype. Directional, disruptive and stabilizing selection directional natural selection can affect allelic frequencies in 3 ways depending on which phenotype is favored an extreme phenotype is favored shifts the overall make up of the population in one direction common when environments change or migrate to new habitat Disruptive both extremes are favored intermediates have decreased fitness stabilizing favors intermediates and acts AGAINST both extremes regardless of mode - NS favors phenotypes with higher reproductive success adaptive evolution is a continuous, dynamic process adaptations arise nat sel increases frequency of favorable alleles as frequencies change, match between species and environment improves physical and biological components of environment change! traits favored by nat sel change neither genetic drift or gene flow CONSISTENTLY lead to adaptive evolution genetic drift can cause the frequency of a beneficial allele to increase (or decrease)
can cause the frequency of a negative allele to decrease (or increase) gene flow has the capability of introducing EITHER advantageous or disadvantageous alleles to a population what prevents nat selection from wiping out unfavorable alleles and ultimately reducing genetic variation? diploidy~ the presence of recessive alleles in heterozygotes remember nat sel favors PHENOTYPES! keeps alleles around that may be favored if the environment changes, even if they are not favored under the current conditions heterozygote advantage when the advantage is defined in terms of the genotype (typically it is the phenotype) when the phenotype of the heterozygote is intermediate to the phenotypes of the homozygotes ex: sickle cell anemia nat sel maintains two or more alleles at a loci frequency-dependent selection fitness depends on how common a phenotype is in the environment see example of scale-eating fish batsian mimicry:
nonpoisonous butterflies are colored like poisonous ones (increases fitness)

too many nonpoisonous ones and the fitness decreases (chance birds get a pos reaction instead of neg) why doesnt evolution create perfection? can only act on existing variations ~ cant just make new alleles arise! limited by historical restraints
~doesnt just scrap ancestral anatomy and build new
~adapts existing current structures to new situations adaptation are usually compromises between phenotypes ex: structural reinforcement or agility? human joints and prehensile hands involves CHANCE, NAT SEL and the ENVIRONMENT HELLO meteors and ice ages! unpredictable changes can majorly alter evolutionary processes! THE ORIGIN Of SPECIES speciation - the process by which one species splits into two or more species microevolution ~ changes in allelic frequencies over time macroevolution~ evolution above the species level connects micro and macro evol a new group of organisms such as flowering plants ~ results from a series of speciations occurs thru mechanisms such as genetic drift, gene flow and natural sel and mutation macro micro The Biological Species Concept individuals with the potential to interbreed in nature and produce viable, fertile offspring reproductively compatible what holds a gene pool of a species together? gene flow between different pops of a species absence of gene flow is key in the formation of a new speices Reproductive Isolation because species are defined as reproductively compatible, formation of new species hinges on reproductive isolation biological factors that impede members of two species from breeding and producing fertile, viable offspring prezygotic barriers barriers that block fertilization habitat isolation temporal isolation different habitats, may be same area geographically though. breeding times different behavioral isolation courtship rituals prevent mate recognition PREVENT MATING ATTEMPTS Mechanical Isolation morphological differences prevent successful mating Gametic Isolation sperm cannot fertilize egg after mating attempted postzygotic barriers barriers that develop after fertilization Reduced Hybrid Viability parents genes are not compatible and impair development of the hybrid Reduced Hybrid Fertility hybrids are sterile, cannot produce offspring with either parent species Hybrid Breakdown hybrids viable and fertile, but next generation offspring (when mating between hybrid and a parent species) are not. Problems with the Biological Species Concept species that reproduce asexually
extinct species/ fossils; can't evaluate reproductive isolation
focus in solely on no gene flow not natural selection
there are unexplainable exceptions ~grolar bear grizzly and polar bears, ecologically and morphologically distinct but produce viable offspring other species concepts: morphological species concept: based on structural features
can be applied to sexual and asexual
no need to understand gene flow
common practice for distinguishing among species
PURELY SUBJECTIVE ecological species concept in terms in niches
fits asexual and sexual species phylogenetic species concept grouped based on closest shared ancestor
includes morphology and molecular comparisons
difficult to measure the degree of differences Speciation two paths for speciation; dependent on how gene flow is disrupted ALLOPATRIC SPEICIATION geographically isolated subpopulations 1 geographic separation occurs splitting the gene pool depends on ability of organisms to move about! 2 new mutations may arise, natural selection and genetic drift affect allele frequencies differently 3 reproduction isolation can be a by-product of the selection or drift populations diverge genetically 4 divergent evolution canyons form, rivers diverge, lakes split... EX Andros Island Mosquitofish EVIDENCE for ALLOPARTIC speciation snapping shrimp genus Alpheus in the Isthmus of Panama

dusky salamander Desmognathus ochrophaeus regions that are highly subdivided by barriers usually have more diversity of species (Hawaii) The greater the geographic distribution the more likely reproductive isolation is to occur ** although geographic isolation prevents interbreeding between allopartic pops, PHYSICAL separation is not a biological reproductive barrier
~ biological reproductive barriers are intrinsic Sympatric Speciation reproductive isolation of species living in the SAME geographic area Polyploidy cell division that results in an extra set of chromosomes most common in plants, has occurred in animals autoploidy 2 or more sets of chromosomes ALL from the same species could be the result of a failure of cell division resulting in doubling of chromosome number; can produce fertile offspring by self pollination or mating with other tetraploids alloploidy members of two species produce STERILE offspring
offspring reproduce asexually
subsequent generations are not sterile offspring are fertile and can reproduce with other hybrid species but NOT with either parent species Habitat Differentiation Sympatric Speciation use of a habitat/ resource not used by the parent pop apple maggot fly originally pest of native HAwthorne tree
Europeans intro the apple tree
flys begin infesting new trees
apple mature earlier and faster
Nat Sel favors flies with rapid growth cycles
temporal reproductive barriers developed Sexual Selection Sympatric Speciation mate selection based on phenotype can be a main reproductive barrier in gene pools even if the pops live in the same geographical area see Echlid fish speciation in Lake Victoria Adaptive Radiation adaptive radiation - the development of many different forms from an originally homogeneous group of organisms as they fill different ecological niches The Madagascar lemurs are an example of very successful adaptive radiation. They have a wide range of adaptations that have evolved. Some became adapted for life in trees, other for life on the ground, some for the lush environment of the rainforest while others thrive in the desert. While most lemurs are diurnal there are now species that are nocturnal. Adaptive radiation has allowed for many different species of lemurs to evolve. Many other organisms such as Darwin ’s finches of the Galapagos and Hawaiian honeycreepers. Mammals also show evidence of adaptive radiation if the pentadactyl limb is considered. Convergent -v- Divergent Evolution speciation occurs and results in different species that avoid competition with one another and evolve in very different ways it is known as divergent evolution Red fox and Kit Fox Lives in farmland and forest
red color helps it blend Lives in deserts
sandy color helps it blend
larger ears allow it to stay cool structural similarities suggest a common ancestor Occurs when different organisms that live in similar environments become more alike in appearance and behavior.
The environment selects similar adaptations in unrelated species.
Organisms develop analogous structures (same function, but different origins).

Examples: - Bird wings/insect wings
- Shark fins/dolphin fins Convergent evolution Homologous structure are evidence for divergent evolution AKA adaptive radiation Pace of Evolution Does evolution occur in rapid bursts or gradually?

This question is difficult to answer because we can’t replay the past with a stopwatch in hand.

However, we can try to figure out what patterns we’d expect to observe in the fossil record if evolution did happen in bursts, or if evolution happened gradually. Then we can check these predictions against what we observe. If evolution is slow and steady, we’d expect to see the entire transition, from ancestor to descendent, displayed as transitional forms over a long period of time in the fossil record. In the previous example, the preservation of many transitional forms, through layers representing a length of time, gives a complete record of slow and steady evolution This slide shows just a few steps in the evolution of whales from land-dwelling mammals, highlighting the transition of the walking forelimb to the flipper What would we observe in the fossil record if evolution happens in “quick” jumps (perhaps fewer than 100,000 years for significant change)? If evolution happens in “quick” jumps, we’d expect to see big changes happen quickly in the fossil record, with little transition between ancestor and descendent. In this example, we see the descendent preserved in a layer directly after the ancestor, showing a big change in a short time, with no transitional forms. When evolution is rapid, transitional forms may not be preserved, even if fossils are laid down at regular intervals. We see many examples of this “quick” jumps pattern in the fossil record. Does a jump in the fossil record necessarily mean that evolution has happened in a “quick” jump? We expect to see a jump in the fossil record if evolution has occurred as a “quick” jump, but a jump in the fossil record can also be explained by irregular fossil preservation. This possibility can make it difficult to conclude that evolution has happened rapidly. GRADUALISM PUNCTUATED EQUILIBRIUM Supporters of the idea of punctuated equilibrium argue that speciation happens very suddenly and quickly following or in response to an environmental change. volcanic eruptions, major climate change or a meteorite impact.

The organisms would have to adopt new adaptations rather quickly to cope with the change in environment Supporters of the idea of gradualism argue that the succession of small changes in the phenotype of species is evident in fossil records. This shows that speciation is a slow and gradual process and they also argue that we do not see rapid evolution happening today so it must be a slow, gradual process Some changes have occurred very drastically at certain points in history for example when mammals took over dinosaur habitats.

However, fossil records show that most species live for millions of years with little or no change.

Critics of the idea of punctuated equilibrium argue that the jumps in evolution could have more to do with an incomplete fossil record. Polymorphism occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph. A transient polymorphism is one that is changing in frequency over time. In transient polymorphism, one form is gradually being replaced by another.

As the name implies, it represents a temporary situation as a by-product of directional natural selection The most famous example of transient polymorphism is the peppered moths in England If natural selection eliminates individuals with detrimental phenotypes from a population, then why do harmful mutant alleles persist in a gene pool? A detrimental allele can remain prevalent when heterozygotes have some other advantage over individuals who have two copies of the wild type allele When carriers have advantages that allow a detrimental allele to persist in a population
balanced polymorphism PKU spontaneous abortion
Tay Sachs Tuberculosis
Cystic Fibrosis Cholerea Sickle Cell Anemia
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