The Evolution of Prokaryotes Scientists use fossils to study

evidence of early life on Earth. Fossil: the preserved or mineralized

remains or imprints of an organism that lived long ago.

The oldest fossils are 3.5 billion year old prokaryotes.

Some of the first prokaryotes were marine cyanobacteria. Cyanobacteria: photosynthetic

prokaryotes Helped release oxygen gas into oceans,

and eventually the air. Benaghhttp://www.mbari.org/staff/conn/botany/phytoplankton/phytoplankton_cyanobacteria.htm

http://www.dkimages.com/discover/Home/Plants/Fungi-Monera-Protista/Cyanobacteria/Cyanobacteria-2.html

Two Groups of Prokaryotes Two different groups or prokaryotes

evolved. 1. Eubacteria: prokaryotes that

contain a chemical called peptidoglycan in their cell walls.

2. Archaebacteria: prokaryotes that lack peptidoglycan in their cell walls and have unique lipids in their cell membranes.

Modern archaebacteria are thought to closely resemble early archaebacteria.

Chemical evidence indicates that archaebacteria and eubacteria diverged (separated)very early.

E.coli: an example

of Eubacteri

a: http://bioweb.uwlax.edu/bio203/s2008/moder_jus

t/

Methanosarcina: an

example of Archaebacteri

ahttp://

jinejeongsbi3u.wikispaces.com/Archaebacteria

Benagh

The Evolution of Eukaryotes Eukaryotes first appeared about 1.5 billion years ago. Eukaryotic cells are much larger and more complex than

prokaryotic cells, with a system of internal membranes (making organelles).

Most eukaryotes have mitochondria. Chloroplasts are found in protists and plants.

Use Chlorophyll: a green pigment present in most plants Protists were the first eukaryotic organisms—this is an

evolutionary advance first seen in protists! Benagh

http://cellssixthgrade.wikispaces.com/Chloroplasts http://www.cytochemistry.net/Cell-biology/mitoch1.htm

Three Types of DNA There can be up to three types of DNA in one

Eukaryotic cell:1. Eukaryotic nuclear DNA: inside the nucleus of the cell.2. Mitochondria DNA: DNA in the mitochondria.

Mitochondria are the size of prokaryotes. 3. Chloroplast DNA: DNA in the chloroplasts.

Chloroplasts are also the size of DNA. Benagh

http://microbewiki.kenyon.edu/index.php/Mitochondria

http://www.nature.com/scitable/topicpage/plant-cells-chloroplasts-and-cell-walls-14053956http://dbscience2.wikispaces.com/Kate

The origins of Mitochondria and Chloroplasts

Most biologists think that mitochondria and chloroplasts originated as described by the theory of endosymbiosis. Theory of Endosymbiosis: mitochondria are the descendants of

symbiotic, aerobic eubacteria and chloroplasts are the descendants of symbiotic, photosynthetic eubacteria Bacteria entered larger cells, and began to live inside the cell

performing either cellular respiration or photosynthesis.

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Support for the Theory of Endosymbiosis1. Size and Structure: Mitochondria and chloroplasts are the

same size as bacteria. Both are surrounded by two membranes. Contain cytochromes. Cytochromes: help with electron transport

2. Genetic Material: Mitochondria and chloroplasts have circular DNA similar to bacterial chromosomes. This DNA is different from the host cell nuclear DNA (DNA in the nucleus).

3. Ribosomes: Mitochondrial and chloroplast ribosomes are similar to bacterial ribosomes.

4. Reproduction: Like bacteria, chloroplasts and mitochondria reproduce by simple fission. This is independent of the host cell.

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Endosymbiosis and mutualism

The relationship between the large cell and the smaller contained cells in the theory of endosymbiosis is considered to be an example of mutualism. Mutualism: a relationship

between two species in which both species benefit. Benagh

http://www.pollinators.info/archives/tag/alfalfa-leafcutter-bees

Mitochondrial DNA and inheritance• Mitochondrial DNA (mtDNA)

can be useful in indicating close relatives, because the egg has mitochondria and the sperm does not.

• This means all your mtDNA came from your mother, and all her mtDNA came from her mother. If your mother had other children, they have the same mtDNA as you! Benagh

http://news.bbc.co.uk/2/shared/spl/hi/pop_ups/03/health_ivf_step_by_step/html/1.stm

http://evolution.berkeley.edu/evolibrary/news/071101_genealogy

Other Organelles The folding in the plasma (cell) membrane may have

been the forerunner of both the endoplasmic reticulum and nuclear envelope based on similar structure and biochemical analysis.

Part of cell specialization: a process where cells become modified to perform specific functions in an organism.

Benaghhttp://en.wikibooks.org/wiki/Structural_Biochemistry/Cell_Organelles/Endoplasmic_Reticulum#Smooth_Endoplasmic_Reticulum_.28SER.29http://picsbox.biz/key/rough%20endoplasmic%20reticulum%20function

Multicellularity• Protists were the first eukaryotes.

Protists make up a large varied group of both multicellular and unicellular organisms.

• Unicellular organisms are very successful, but each cell must carry out all the activities of the organism.

• Distinct types of cells in one body can have specialized functions (like in your immune system, for example).

• Almost every organism you can see without a microscope is multicellular.

• Fossils of the first multicellular organisms are about 700 million years old.

A singled celled protist

Multicellular protists—

brown algaeBenagh

http://bio.rutgers.edu/~gb101/lab6_protists/m6a.html

http://sopastrike.com/strike

Origins of Modern Organisms Most animal phyla that exist today probably originated

during a relatively short time during the Precambrian and early Cambrian periods. This rapid diversification is called the “Cambrian Explosion”

Many unusual marine animals appeared at this time. A very rich collection of fossils found in a formation in

Canada called the Burgess Shale. Benagh

Mass Extinctions The fossil record indicates that a

sudden change occurred at the end of the Ordovican period—a large percentage of organisms became extinct.

Extinction: the death of all members of a species.

Mass Extinction: an episode during which large numbers of species becomes extinct.

Mass extinctions can allow new species to adapt and fill niches previously occupied by the now-extinct species, and thus help drive evolution. Benagh http://www.all-creatures.org/hope/gw/01_mass_extinction.htm

Continental Drift Continental drift also played an

important role in evolution. Continental Drift: the

movement of Earth's land masses over Earth’s surface through geologic time. Resulted in present-day position of the continents. Helps to explain why there are a

large number of marsupials in both Australia and South America, because these continents were once connected.

Benaghhttp://www.divediscover.whoi.edu/history-ocean/continental.html

The Ozone Layer While the sun gives us the light energy Earth’s organisms need, it also

produces dangerous ultraviolet (UV) radiation. Early life lived in the sea, which protected it from dangerous UV

radiation. However, land organisms needed protection. This protection is provided in the upper atmosphere by the ozone layer

which blocks UV radiation. The Ozone (O3—regular oxygen is O2layer formed about 2.5 billion years

ago as cyanobacteria began adding oxygen to the earth’s atmosphere.

Benagh

Organisms on Land Plants and Fungi: may have been the first multi-cellular organisms on

land—also can have a mutualistic relationship. On land 430 million years ago.

Arthropods: organisms with hard outer skeletons, segmented bodies, and jointed limbs. Examples are insects, lobsters, and crabs. First animals on land.

Vertebrates Fishes: First vertebrates, about 530 million years ago. Before organisms

lived on land. Amphibians: First vertebrates to inhabit the land, but still need to live near

water. 370 million years ago. Reptiles: Next vertebrates to live on land, better adapted to live on land

than amphibians. Evolved from amphibians. Appeared 340 million years ago.

Mammals and Birds: After the extinction of the dinosaurs, small mammals and birds became the dominant land vertebrates. Radiated after the extinction of the dinosaurs (dominant reptiles) 65 million years ago. Benagh

CHAPter

13.1

The Theory of Evolution by Natural Selection

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Darwin, Mechanism for Evolution The idea that life changes has been around for a long time, but now

one really understood how that might happen. In 1859, Darwin published evidence that species evolved, and

proposed an explanation of how it might happen. Before working as a naturalist, Darwin studied to be a physician and a

minister. In 1831, Darwin took a position as a naturalist on the voyage of the

HMS Beagle. Benagh

Science Before Darwin’s Voyage In Darwin’s time most scientists thought each species

was a divine creation that did not change. However, scientists were trying to understand the

origin of fossils. Jean Baptiste Lamarck proposed that organisms

change over generations because in the lifetime of an individual, physical features increased in size because of use or decreased because of disuse (Use Disuse Theory OR Inheritance of Acquired Characters). These changes are then passed on offspring. This is NOT how it happens!!!

Benagh

Darwin’s Observations On his voyage, Darwin found evidence challenging the

belief that species do not change. Darwin read Charles Lyell’s book Principles of Geology

which proposed that the surface of Earth changed slowly over many years.

Darwin saw things that could be explained only by a process of gradual change.In South America, he found fossils of extinct armadillos which

were similar but not identical to modern armadillos in the area. Darwin visited the Galápagos Island and noticed that the

species on the islands were similar to those from South America, but they changed since they arrived.Darwin called this Descent with modification, or evolutionBenagh

Growth of Populations Population: all the individuals of a species

that live in a specific geographical area and that can interbreed.Evolution occurs on the level of the

population!!! Darwin finally understood how evolution

happens when he read an essay by the English economist Thomas Malthus.Malthus stated that unchecked populations

grow by geometric progression, but that food supplies only increase by an arithmetic progression, so food can limit the potential growth of populations

http://www.pbs.org/wgbh/evolution/library/02/5/quicktime/l_025_01.html

Benagh http://www.blupete.com/Literature/Biographies/Philosophy/Malthus.htm

Evolution by Natural Selection Darwin realized Malthus’ applied to all species. Every organism has the potential to produce many offspring

during its lifetime. In most cases, only a few survive to reproduce. Individuals that have physical or behavioral traits that better

suit their environment are more likely to survive and will reproduce more successfully than those without the traits.

Benagh

Evolution by Natural Selection

Darwin called this differential rate of reproduction Natural Selection.In time, the number of individuals

that carry inherited favorable characteristics will increase, and the population will change or evolve!

Organisms differ from place to place because their habitats are different, and each species has reacted to its own environment. Adaptation: An inherited trait that

has become common in a population because the trait provides a selective advantage. Benagh

http://goose.ycp.edu/~kkleiner/ecology/EvolEcologyimages.htm

Publication of Darwin’s Work

• Darwin waited a long time to publish his ideas about evolution and natural selection because he feared controversy.

• He finally decided to publish his ideas after he received a letter and essay from Alfred Russel Wallace who also described evolution by natural selection.

Benagh

Darwin’s Theory Darwin published “On

the Origin of Species by Means of Natural Selection” in 1859. Many people were deeply disturbed by his assertion (made in a later book) that humans are related to apes.

Darwin’s theory of Natural selection is supported by four major points. Benagh

Darwin’s Four Major Points1. Inherited variation exists within the genes of every population or

species (the result of random mutation and translation errors). Or: Not every organism is identical!

2. In a particular environment, some individuals of a population or species are better suited to survive (as a result of variation) and have more offspring (natural selection).

Or: Some organisms do better and have more babies!3. Over time, the traits that make certain individuals of a

populations able to survive and reproduce tend to spread in that population.

Or: Organisms that do better give their advantages to those babies they had!

4. There is overwhelming evidence from fossils and many other sources that living species evolved from organisms that are extinct. Benagh

Darwin’s Ideas Updated

Since Darwin’s work was published, his hypothesis (that natural selection explains how evolution happens) has been examined by biologists.

New discoveries have given scientists new insight into how natural selection brings about the evolution of species.

Benagh

Change Within Populations• Darwin’s ideas were based on the idea that in any

population, individuals that are best suited to survive will produce the most offspring. These traits will become common new generations.

• Scientists now know that genes are responsible for inherited traits. Certain forms of genes called alleles become more common.– In other words: natural selection causes the allele

frequency to change.• Mutations and sexual reproduction provide the

variation needed for natural selection. – Random gene mutation is essential to evolution!Benagh

Species Formation The environment varies—populations of the same species in

different areas tend to evolve differently. Reproductive Isolation: the condition in which two population

of the same species do not breed with one another because of geographic separation, a difference in mating periods, or other barrier to reproduction. As two isolated populations become more different over time, they

may eventually not be able to mate, making them different species.

Benagh

The Tempo of Evolution For decades, most biologists though of evolution as a gradual

process that occurs continuously. However, some scientists suggested that species may stay unchanged for long periods of time and evolution occurs in spurts. These models are called:

1. Gradualism: the model of evolution in which gradual change occurs over a long period of time leading to species formation.

2. Punctuated Equilibrium: the model of evolution in which periods of rapid change in a species are separated by periods of little or no change.

Benagh

Chapter 13.2

Evidence of Evolution

Benagh

The Fossil Record• Fossils offer the most direct

evidence that evolution takes place—fossils of animals show a pattern of development from ancestors to modern descendants.

• Fossils provide a record of Earth’s past life-forms.

• Evolution: Change over time. – Evolution can be observed in

the fossil record. Benagh

The Fossil Record Darwin predicted that

intermediate forms between organisms would be found.

Since this prediction, many of these intermediates have been discovered.Intermediates (in-betweens) of

fishes and amphibians, between reptiles and birds, and between reptiles and mammals have been found.

Archeopteryx is a transitional or intermediate (in-between) form between reptiles and birds. http://kasamaproject.org/2011/08/02/archaeopteryx-reclassifying-our-beautiful-old-friend/

Benagh

The Fossil Record • Darwin’s theory is generally accepted by scientists

as the best available explanation for biological diversity on Earth.

• Based on a large body of supporting evidence, most scientists agree on these major points:

1. Earth is about 4.5 billion years old. • Why is this important?

2. Organisms have inhabited Earth for most of its history.3. All organisms living today share a common ancestry

with earlier, simpler life-forms. • How do we know this? Benagh

Formation of Fossils The fossil record is incomplete. Fossils do not form in all environments.

Organisms have to die under just the right conditions in order to be fossilized.

The fossil record will never be complete because not all organisms are fossilized. However, the record presents strong evidence that evolution has taken place.

Paleontologists: scientists who study fossils. Paleontologists determine relative age of

organisms and determine the order of fossils from oldest to youngest.

Fossils help scientists understand the geological time scale for all of the above reasons; without fossil evidence, we would not have as much evidence of the time needed for evolution to occur. Benagh

Fossils and Populations• Fossils can provide important

information regarding population structure and biogeography.–Biogeography: a science that deals with

the geographical distribution of animals and plants. –Population: a group of organisms of the

same species that live in a specific geographical area and interbreed.

Benagh

Anatomy and Development Comparisons of anatomy of different types of organisms

often reveal basic similarities in body structures even though the function may differ between organisms.

Vestigial Structure: a structure in an organism that is reduced in size and function and that may have been complete and functional in the organism’s ancestors.

Similarities in bone structure can be seen in vertebrates, suggesting they have a relatively recent common ancestor

Homologous Structures: structures that share a common ancestry. Similar structure in two organisms can be found in the common ancestor of the organisms. Example: human arm, monkey arm

Analogous Structures: are features of different species that are similar in function but not necessarily in structure and which do not derive from a common ancestral feature (compare to homologous structures) and which evolved in response to a similar environmental challenge. Example: bird wing, insect wing

Evolutionary history of organisms is also seen in the development of embryos. The stages of embryonic development are similar in many species. Benagh

Anatomy and Development

Benagh

Homologous Structures

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Biological Molecules

• The picture of successive change seen in the fossil record allows scientists to make a prediction that can be tested.

• If species have changed as seen in fossils, then genes should also have changed.

• Changes in a gene’s nucleotides should build up over time. Benagh

Proteins• Amino acid sequences of

similar proteins were compared.

• If evolution has taken place, then species descended from a recent common ancestor should have fewer amino acid differences in proteins than do species that aren’t as closely related.

Benagh

DNA Sequence This pattern does not hold true for all proteins. A

certain protein may evolve more rapidly in some groups than others.

Comparisons of proteins may not reflect evolutionary relationships supported by the fossil record and other evidence.

More accurate hypotheses about evolutionary histories are based on large numbers of gene sequences.

These evolutionary histories based on DNA sequences tend to be similar to those from the fossil record.

Benagh

Chapter 13.3

Examples of Evolution

Benagh

Natural Selection at Work How does evolution occur? Darwin’s theory asserts that natural

selection is the mechanism that drives evolution, meaning that natural selection is what makes it happen. If an organism has a trait that makes it

easier to survive and reproduce, that trait will become more common in the population.

The environment dictates the direction and amount of change. If the environment changes, the set of

characteristics that most help an individual reproduce may change. Example: Polar Bear’s furBenagh

Factors in Natural Selection1. All populations have genetic variation.

Populations have individuals who are slightly different from one another in genetic makeup.

2. The environment presents challenges to successful reproduction.

An organism that does not survive to reproduce cannot pass on its genes.

3. Individuals tend to produce more offspring than the environment can support.

Individuals of a population often compete with one another to survive.

4. Individuals that are better able to cope with the challenges presented by their environment tend to leave more offspring than those individuals less suited to the environmental do. Benagh

Examples of Natural Selection

Tuberculosis (TB) is caused by the bacterial species M. tuberculosis and kills more adults than any other infectious disease in the world.

Two effective antibiotics became available to fight this bacteria.

However, in the late 1980s, new strains of Tuberculosis that are resistant to the antibiotics appeared.

These resistant bacteria evolved through natural selection.

Benagh

Evolution of Antibiotic Resistance A mutation probably occurred in a single

Tuberculosis cell, which allowed that cell to survive exposure to normally deadly antibiotics.

Normal bacteria cells die from antibiotics while the mutated bacteria continue to survive and grow.

Eventually, there could be enough mutated bacteria to kill the host and spread to other individuals.

Benagh

Evolution in Darwin’s Finches Darwin collected 9 species of finches from three of the Galápagos Islands.

All of these finches were similar to each other except for their bills. Two crushed seeds, one picked at fruit, one ate insects, one drank the blood of

other birds, etc. All of the finches had beaks adapted for these various tasks. Darwin suggested that these species evolved from an original ancestral

species. This is known as adaptive radiation.

Adaptive Radiation: Diversification of a species or single ancestral type into several forms that are each adaptively specialized to specific environmental conditions.

Benagh

Evolution in Darwin’s Finches During dry years, plants produce few seeds. The difference between survival and starvation is the ability to eat the larger, tougher

seeds. After several dry years, the birds that had longer, more-massive beaks had better

success and produced more offspring. When wet seasons returned, smaller beaked birds were more common. The numbers of birds with different beak shapes are changed by natural selection in

response to the available food supply. Natural selection does not CAUSE change; rather it selects genes that are already

present and most adaptive to the environment. The trait must be present and provide advantage for survival and reproduction.

Benagh

Gene Pools Natural selection utilized the diversity in a species’ gene pool. Gene Pool: The total number of genes of every individual in an

interbreeding population.Gene pools contain variations in genes, relative gene frequencies, and

allele frequencies. Genetic recombination can influence the gene pool and variation.

Variations: A modification in structure, form, or function. Relative Frequency: the average number of occurrences of a

particular event in a large number of repeated trials. Allele Frequency: the frequency of an allele compared to other

alleles of the same gene in a population. Natural selection makes the most successful alleles (different

copies of genes) most common in a population. In this way, natural selection changes the POPULATION, not the

INDIVIDUALS! Benagh

Formation of New Species (Speciation) Species formation occurs in stages. A species molded by natural selection

has an improved “fit” to its environment. Divergence: The accumulation of

differences between groups. Divergent (split apart) Evolution: The

process by which an interbreeding population diverges (splits) into two or more descendant species, resulting in once similar or related species to become more and more different.

Convergent (come together) Evolution: A kind of evolution wherein organisms evolve parts that have similar structures or functions in spite of their evolutionary ancestors being very dissimilar or unrelated.

Speciation: The process by which new species form.

The body structure of these organisms are examples of

convergent evolution. http://bio1152.nicerweb.com/Locked/media/ch40/

fast_swimmers.html

Benagh

Benagh

Forming Subspecies Separate populations of a single species often live in several different

kinds of environments. In each environment, natural selection produces organisms better

adapted to this environment. When environments are different enough, populations can become

very different. Over time, populations of the same species become subspecies. Eventually these subspecies may become so different that they can no

longer interbreed successfully and they are then separate species. Microevolution: Evolution involving small-scale changes, i.e. within

the species level, occurring over a short period of time that results in the formation of new subspecies.

Co-evolution can also play a role in speciation. Co-evolution: the process in which long-term, interdependence changes take

place in two species as a result of their interactions. Benagh

Maintaining New Species • Once subspecies become different enough, a

barrier to reproduce usually prevents different groups from breeding with each other.

• Barriers of reproduction:– Geographic Isolation– Reproductive Isolation – Physical Differences– Infertile Hybrid Offspring

Benagh

Chapter 15.1How Populations Grow

What is a Population?• Population: all the individuals of a

species that live together in one place at one time.– It can be difficult to estimate the

population size of wild populations (and sometimes even human populations!).

• Populations tend to grow because individuals tend to have multiple offspring over their lifetime.– Eventually, limited resources in an

environment limit the growth of a population. Benagh

Modeling Population Growth• Population Model: A hypothetical

population that attempts to exhibit the key of a real population.

Exponential Growth Curve:

Also called a j-curve

Also called a s-curve

Logistic Growth Model:

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Three Key Features of Populations1. Population Size: The number of individuals in a population.– Can affect a population’s ability to survive. Small populations are

more likely to become extinct.2. Population Density: the number of individuals that live in a

given area– If individuals in a population are spaced out, they will not encounter

each other as often and will be less likely to reproduce.3. Population Dispersion: the way individuals of a population are

arranged in space. Clumped: individuals are bunched together in clusters.Even: location of individuals is at regular intervals.Random: location of each individual is self-determined or determined by chance.

Benagh

Growth Rate and Population Size• Exponential Growth Curve: a

curve in which the rate of population growth stays the same, and as a result the population size increase steadily. – However, populations do not

usually grow unchecked (checked by predators, disease, resource availability). Eventually populations reach carrying capacity.

• Carrying Capacity (K): The population size that an environment can sustain.

Exponential Growth Curve:

Also called a j-curve

Also called a s-curve

Logistic Growth Model:

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Resources and Population Size• As a population grows, limited

resources eventually become depleted and population growth slows.

• The Logistic Model: A population model in which exponential growth is limited by a density-dependent factor.– Density-Dependent factor:

limited resources that become depleted when the population is larger.

Exponential Growth Curve:

Also called a j-curve

Also called an s-curve

Logistic Growth Model:

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Growth Patterns in Real Populations

• Exponential Growth Patterns are best to describe faster growing organisms such as:– Many plants– Insects

• Logistic Growth Model is best to describe slower growing organisms such as:– Bears– Elephants– Humans

• Density-Independent Factors: environmental conditions– Weather– Climate

Exponential Growth Curve:

Also called a j-curve

Also called a j-curve

Logistic Growth Model:

Also called an s-curve

Rapidly Growing Populations• r-strategists: grow exponentially

when environmental conditions allow them to reproduce.– Results in temporarily large populations.

• When environmental conditions are good, the population grows rapidly. When conditions are poor, the population size drops quickly.

• Generally r-strategists:– Have a short life span– Reproduce early– Many small offspring– Offspring mature with little parental

care Benagh

Slowly Growing Populations

• K-strategists: organisms that grow slowly with small population sizes and a population density usually near the carrying capacity (K) of their environment.

• Generally K-strategists:– Have a long life– Mature slowly– Have few young– Provide extensive care for young

Benagh

Chapter 15.2

How Populations Evolve

The Change of Population Allele Frequencies

• Scientists now understand genetics much more than in Darwin’s time.

• In order to understand out populations change in response to evolutionary forces, you need to understand how populations behave in the ABSENCE of these forces!

Benagh

Allele Frequencies • Allele Frequency: the frequency of an allele compared

to other alleles of the same gene in a population. – Biologists began to study how allele frequency changed in

populations and wondered if dominant alleles (usually more common than recessive) would spontaneously replace recessive alleles in populations.

• Hardy and Weinberg demonstrated that dominant alleles do not automatically replace recessive alleles.– They showed that the frequency of alleles in a population

does not change. – Also, the ratio of heterozygous individuals to homozygous

individuals does not change unless the population is acted on by something that favors a particular allele.

The Hardy-Weinberg Principle• Hardy-Weinberg Principle: allele frequencies in a population do not change

unless evolutionary forces act on the population. • Hardy-Weinberg Equation: p2+2pq+q2=1• When no evolutionary forces are acting on a population, it is in:

– Genetic Equilibrium: relative allele frequencies remain the same.

• There are five principal evolutionary forces that can cause genotype ratios to change:1. Mutation2. Gene Flow3. Nonrandom Mating4. Genetic Drift5. Natural Selection Benagh

Five Principle Evolutionary Forces (Cause Genetic Change in a Population)• Mutation: source of variation and makes evolution possible.• Gene Flow: the movement of alleles into or out of a population.

Occurs because new individuals (immigrants) add alleles and Departing individuals (emigrants) take alleles away.

• Nonrandom Mating: when individuals prefer to mate with others that live nearby, or are of their own phenotype, or based on certain traits.

• Genetic Drift: the random change in allele frequency in a population.– Often more pronounced in small populations.

• Natural Selection: Causes deviations from Hardy-Weinberg by directly changing allele frequencies, since some alleles are being selected for and will spread in the population. Benagh

Evolutionary Forces • Natural selection causes deviations from Hardy-

Weinberg proportions by changing the frequencies of alleles. In other words, if mutation, gene flow, non-random mating,

genetic drift, or natural selection are occurring, then evolutionary forces are acting on a population!

• Fitness: A relative measure of reproductive success of an organism in passing its genes to the next generation.– The more fit an organism is, the more offspring it will

have.

Benagh

How Selection Acts • Selection cannot operate against rare or

recessive alleles (in heterozygotes) even if they are unfavorable, because selection can only operate against expressed traits.• Homozygous recessive individuals could

be removed from a population, but carriers (heterozygotes could not).

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Why Genes Persist• Genetic conditions are not eliminated by natural

selection because very few of the individuals bearing the alleles express the recessive phenotype.

• However, in the case of incest (which closely related individuals have children), more individuals will be likely to show the recessive phenotype because there is more chance of two carriers mating (if they got their genes from the same people, it is more likely that they both have the recessive traits). – Inbreeding is likely to be particularly pronounced in small

populations because there is more chance for two carriers of a recessive trait to mate with one another.

Benagh

Why Genes Persist

http://biology.clc.uc.edu/courses/bio105/sex-link.htm

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Natural Selection and the Distribution of Traits

• Polygenic Trait: A trait influenced by several genes.–Because genes can have

many alleles, polygenic traits tend to exhibit a range of phenotypes clustered around an average value.

Human height is an example of a polygenic trait.http://www.sbcoachescollege.com/wp-content/uploads/2010/08/manute-bol-dead-at-age-47.jpgBenagh

Directional Selection• Directional Selection: the frequency of a particular

trait moves in one direction in a range.– When selection eliminates one extreme in a populations,

the alleles for this extreme become less common!– Has a role in the evolution of single-gene traits such as

insect pesticide resistance. Benagh

Stabilizing Selection• Stabilizing Selection: the distribution becomes

narrower, tending to “stabilize” the average by increasing the proportion of similar individuals. – When selection reduces both extremes from a range of

phenotypes. Benagh

Disruptive Selection• Disruptive

Selection: the distribution of alleles favors the extremes, leading to two distinct groups.

http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec09.htmlBenagh