evolution

Evolution

Thomas Malthus (1798)• Describes role of competition for

limited resources in human societies.• Not everyone will compete

successfully.

Social Theory

Historical Viewpoints• Historically, Earth and all organisms

have always existed in current state.– Creation by higher authority

• 18th-19th centuries, scientists began to realize that things may be much different than originally thought.

Darwin and Evolution

• Who was Charles Darwin?– English scientist born in 1809– Gave up medical studies, went to seminary.– Performed 30 years of observation on various

species– Wrote “On the Origin of Species by Means of

Natural Selection, or the Preservation of Favored Races in the Struggle for Life”

Darwin and Evolution

• Who was Charles Darwin?– Theory of Evolution: One type of organism

can gradually evolve into another.– Proposed natural selection as the mechanism

of evolution that produced the diversity of life on Earth.

Evidence Used by Darwin

• Fossils (armadillo)• Geographical distribution• Oceanic island observations


• Fossils:– Extinct species resemble current species.– Rock strata studies: Progressive changes in

characteristics can be seen in fossils found in varying layers.


• Geographical Distribution:– Lands with similar climates have unrelated

plant and animal life.– Diversity is not entirely influenced by climate

and the environment.

Evolutionary Thinking Before Darwin

• Jean-Baptiste de Lamarck: Evolution (1809)– French naturalist– Organisms changed forms over generations through

the inheritance of acquired characteristics– He believed that, over time, an animal would acquire

enough changes that one species would diverge into two

• Falsifiable: Animals don’t pass traits in accordance to behavior…how?


• Charles Lyell: Geologist– “Principles of Geology” (1830)– Geological forces still operating could account for the

changes geologists could see in the Earth’s surface.– Earth had not been placed into final form at the

moment of creation, but was undergoing steady change.

– If it is possible for the Earth to evolve, why not the living organisms?


• LaMarck:– Got this idea correct:

• Organisms can evolve…one kind of organism can be ancestral to a different kind of organism.


• Georges Cuvier: Extinction– French scientist– Examined fossils in rock formations and found

conclusive evidence of the extinction of species.


• Alfred Russell Wallace: Natural selection– English naturalist– Two years after Darwin had began his work…

half a world away.– Collected bird and butterfly specimens from

South America and Southeast Asia– Natural selection was the force that shapes

evolution.

Natural Processes Underlying Evolution

• Genetic variation among members of a population.

• Inheritance of variations by offspring• Natural selection

What is Natural Selection?

• Organisms that possess superior physical, behavioral, or other attributes are more likely to survive than those that do not possess them.

• Survival allows the favorable attributes to be passed on to offspring.

• As the frequency of these attributes increases in a population, the population as a whole gradually changes (Survival of the fittest)

Evidence for Evolution

• Radiometric dating;• Fossil placement;• Comparative morphology and embryology:

– Homologous structures owing to inheritance from a common ancestor (forelimb structure in a diverse group of mammals, bats, whales, cats, gorilla…one large upper bone, joined to two intermediate bones, joined to five digits.)

• Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment

Evidence for Evolution

• Evidence from Molecular Biology– DNA (nucleotide bases)

• Experimental Evidence– Brighter male guppies attracted more females than

drab guppies, but they also attracted more predators.– Removed predators and bright colored male guppy

population grew…reintroduced predators and drab colored male guppy population increased.

Darwin and Natural Selection• Force that causes populations to

evolve.• Relies on variation in traits in any

population• Based on 3 main principles

1) Competition2) Survival of the Fittest3) Descent with Modification

1. Competition• Members of a species population are in

competition with each other for resources like food and space.

• “Struggle for existence”• Those individuals with traits that give

them an advantage make them better competitors.

2. Survival of the Fittest• Organism’s ability to survive in it’s

environment called it’s fitness• Greater fitness means more

reproductive success!• Random variation in traits in a

population– If trait greater fitness, call it an adaptation– Some harmful, less fitness

3. Descent with Modification• Alleles for adaptations are passed on

through generations and accumulate in a species’ gene pool over time.

• If enough difference, produces a new species.

• Ex. Hawaiian Honeycreepers– All Hawaiian honeycreepers have similarities in

skeletal and muscle structure that indicate they are closely related.

• Each of the Hawaiian honeycreeper species has a bill specialized for eating certain foods. Scientists suggest that all 23 honeycreeper species apparently arose from a single species that migrated to Hawaii.

On the Origin of Species• Published 1859• Summarized his ideas on causes of

speciation• Darwin's On the Origin of Species by

Means of Natural Selection, made several points that had major impact on nineteenth-century thought:

Hutton and Lyell (geologists)• James Hutton (1785) - examined

geologic features such as rock layers and erosion, concluded earth very old.

• Charles Lyell (1833) through observing current earth processes, believed that these same geologic processes shaped the earth as we currently see it.

Early Evolutionist• Jean-Baptiste Lamarck

(1809)• Recognized that species

change over time• Idea: Inheritance of

Acquired Traits

Evolution: Getting from There to Here

• According to Lamarck, individuals passed on to offspring body and behavior changes acquired during their lives

– for example, giraffes evolved long necks because ancestral giraffes tended to stretch their necks and this neck extension was passed on to subsequent generations

Figure 17.1(a) How did long necks evolve in giraffes?

17.1 Evolution: Getting from There to Here

• According to Darwin, the variation is not created by experience but already exists when selection acts on it– populations of ancestral giraffes contained

variation in neck length– individuals who were able to feed higher up

on the trees had more food and so were able to survive and reproduce better than their shorter-necked relatives

Figure 17.1 (b) How did long necks evolve in giraffes?

17.1 Evolution: Getting from There to Here

• There are two views concerning the rate of evolutionary change

– gradualism states that evolutionary change occurs extremely slowly

• such change would be nearly imperceptible from generation to generation, but would accumulate over the course of millions of years

– punctuated equilibrium states that species

experience long periods of little or no evolutionary change (termed stasis), interrupted by bursts of evolutionary change

Figure 17.2 Two views of the pace of macroevolution

17.2 The Evidence for Evolution

• There are many lines of evidence supporting Darwin’s theory of evolution– the fossil record comprises the most direct

evidence of macroevolution– fossils are the preserved remains, tracks, or

traces of once-living organisms• they are created when organisms or their traces

become buried in sediment• by dating the rocks in which the fossils occur, one

can get an accurate idea of how old the fossils are


• Fossils in rock represent a history of evolutionary change– fossils are treated as samples of data and are

dated independently of what the samples are like

– successive changes through time are a data statement

– thus, the statement that macroevolution has occurred is a factual observation

Fossil Record• Preserved remain of ancient

life in rock support change over time.

• Fossils found in lower levels of rock older than ones above. (relative age)

• Majority of species that have existed on this planet are extinct!

• Very hard for an organism to become a fossil.

• When there are remains of organic material (carbon) in a fossil, we can use carbon dating to approximate age.

• Radioactive isotopes decay at a constant rate– Half Life length of time for ½ of an isotope to

decay• Ex. Carbon-14 decays to Nitrogen-14• Half-life = 5,730 years• Carbon 14 can only be uses to date fossils less than

50,000 years old!

Figure 17.3 Testing the theory of evolution with fossil titanotheres


• The anatomical record also reflects evolutionary history– for example, all vertebrate embryos share a similar

set of developmental instructions and features

Figure 17.4 Embryos show our early evolutionary history

Comparative Embryology

• closely related organisms go through similar stages during their embryonic development

17.2 The Evidence for Evolution• Homologous structures are derived from the

same body part present in an ancestor– for example, the same bones might be put to different

uses in related species

• Analogous structures are similar-looking structures in unrelated lineages– these are the result of parallel evolutionary

adaptations to similar environments• this form of evolutionary change is referred to as convergent

evolution

Homologous versus Analogous Structures

Figure 17.5 Homology among vertebrate limbs

Figure 17.6 Convergent evolution: many paths to one goal

Homologous Structures (Comparative Anatomy)

• Scientists note similarities among physiology of organisms

• Morphologically similar structures that perform different functions are called homologous structures.

Analogous Structures• Serve similar function but have a much

different structure– Ex. Insect wing and bird wing– Ex. Whale fin and fish fin

http://biodidac.bio.uottawa.ca/Thumbnails/showimage.cfm?File_name=GENE009B&File_type=GIF


• Traces of our evolutionary past are also evident at the molecular level– organisms that are more distantly related

should have accumulated a greater number of evolutionary differences than two species that are more closely related

– the same pattern of divergence can be seen at the protein level

Figure 17.7 Molecules reflect evolutionary divergence


• Evolutionary changes appear to accumulate at a constant rate – this permits changes in an individual gene,

compared over a broad array of organisms, to be dated from the time of divergence

– this dating is referred to as a molecular clock– for example, changes have accumulated in

the cytochrome c gene at a constant rate

Figure 17.8 The molecular clock of cytochrome c

17.3 Evolution’s Critics• The theory of evolution by natural selection is

the subject of often-bitter public controversy– the controversy began soon after the publication of

The Origin of Species but, by the turn of the twentieth century, evolution was generally accepted by the world’s scientific community

– more recent criticism has come from the following sources

• the Fundamentalist Movement• the Scientific Creationist Movement• Local Action• Intelligent Design

17.3 Evolution’s Critics

• Critics have raised a variety of objections to Darwin’s theory of evolution by natural selection

• The objections have either been refuted by biologists or do not necessarily reject evolution

17.3 Evolution’s Critics• The irreducible complexity fallacy refers to

claims by proponents of intelligent design that the molecular machinery of the cell is irreducibly complex

• Yet natural selection has acted on the whole system: at every stage of evolution, parts that improve function are added, but the parts evolve together such that each one becomes essential– for example, the mammalian blood clotting system

has evolved in stages from much simpler systems

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule• Population genetics is the study of the

properties of genes in populations

• Gene pool is the sum of all of the genes in a population, including all alleles in all individuals

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule• Variation within populations puzzled many scientists

– why don’t dominant alleles drive recessive alleles out of populations?

• G.H. Hardy and W. Weinberg, in 1908, studied allele frequencies in a gene pool– in a large population in which there is random mating, and in the

absence of forces that change allele frequencies, the original genotype proportions remain constant from generation to generation

– because the proportions do not change, the genotypes are said to be in Hardy-Weinberg equilibrium

– If the allele frequencies are not changing, the population is not evolving

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule• Hardy and Weinberg arrived at their

conclusion by analyzing the frequencies of alleles in successive generations– frequency is the proportion of something

compared to the total– knowing the frequency of the phenotype, one

can calculate the frequency of the genotypes and alleles in the population

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule• By convention, the frequency of the more

common of two alleles is designated by the letter p and that of the less common allele by the letter q

• Because there are only two alleles, the sum of p and q must always equal 1

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule• In algebraic terms, the Hardy-Weinberg

equilibrium is written as an equation

p2 + 2pq + q = 1

Figure 17.11 Hardy-Weinberg equilibrium

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule• The Hardy-Weinberg equilibrium only works if the

following five assumptions are met

1. The size of the population is very large or effectively infinite.

2. Individuals can mate with one another at random.

3. There is no mutation.

4. There is no immigration or emigration.

5. All alleles are replaced equally from generation to generation (natural selection is not occurring).

17.5 Agents of Evolution

• Mutation is a change in a nucleotide sequence in DNA– mutation rates are generally too low to

significantly alter Hardy-Weinberg proportions– mutations must affect the DNA of the germ

cells or the mutation will not be passed on to offspring

– however, no matter how rare, mutation is the ultimate source of variation in a population


• Migration is the movement of individuals between populations – the movement of individuals can be a

powerful force upsetting the genetic stability of natural populations

• the magnitude of the effects of migration is based on two factors

– the proportion of migrants in the population– the difference in allele frequencies between the migrants

and the original population


• Genetic drift describes random changes in allele frequencies– in small populations, the frequencies of

particular alleles may be changed drastically by chance alone

– in extreme cases, individual alleles of a given gene may be

• all represented in few individuals• accidentally lost if individuals fail to reproduce or

die

17.5 Agents of Evolution• A series of small populations that are isolated from one another

may come to differ strongly as the result of genetic drift

– founder effect occurs when one of a few individuals migrate and become the founders of a new, isolated population at some distance from their place of origin

• the alleles that they carry will become a significant fraction of the new population’s genetic endowment

– bottleneck effect occurs when a population is drastically reduced in size

• the surviving individuals constitute a random genetic sample of the original population


• Nonrandom mating occurs when individuals with certain genotypes mate with one another either more or less commonly than would be expected by chance– sexual selection is choosing a mate often

based on physical characteristics– nonrandom mating alters genotype

frequencies but not allele frequencies


• Selection, according to Darwin, occurs if some individuals leave behind more progeny than others, and the likelihood that they will do so is affected by their individual characteristics

• in artificial selection, a breeder selects for the desired characteristics

• in natural selection, conditions in nature determine which kinds of individuals in a population are the most fit


• Disruptive selection is a form of selection in which the two extremes in an array of phenotypes become more common in the population– selection acts to eliminate the intermediate

phenotypes– for example, beak size in African blackbellied

seedcracker finches is under disruptive selection because the available seeds are only large or small

Figure 17.14 (b) Examples of selection


• Directional selection is a form of selection that occurs when selection acts to eliminate one extreme from an array of phenotypes– for example, in Drosophila, files that fly toward

light can be selected against and those that avoid light selectively bred, producing a population of flies with a greater tendency to avoid light

Figure 17.14 (c) Examples of selection

17.6 Sickle-Cell Anemia

• Sickle-cell anemia is a hereditary disease affecting hemoglobin molecules in the blood– the disorder results from a single nucleotide

change in the gene encoding -hemoglobin• this causes the sixth amino acid in the chain to

change from glutamic acid (very polar) to valine (nonpolar)

• as a result, the hemoglobin molecules clump together and deform the red blood cell into a “sickle-shape”

Figure 17.16 Why the sickle-cell mutation causes hemoglobin to clump


• Persons homozygous for the sickle-cell genetic mutation frequently have a reduced lifespan– the sickled form of hemoglobin does not carry oxygen

atoms well– the red blood cells that are sickled do not flow

smoothly through capillaries

• Heterozygous individuals make enough functional hemoglobin to keep their red blood cells healthy

17.6 Sickle-Cell Anemia• The frequency of the sickle-cell allele is about

0.12 in Central Africa– one in 100 people is homozygous for the defective

allele and develops the fatal disorder– in contrast, sickle-cell anemia strikes only roughly two

African Americans out of every thousand

• If natural selection drives evolution, why has natural selection not acted against the defective allele in Africa and eliminated it from the population?


• The defective allele has not been eliminated from Central Africa because people who are heterozygous are much less susceptible to malaria– the payoff in survival of heterozygotes makes

up for the price in death of homozygotes• this is called heterozygote advantage• stabilizing selection occurs because malarial

resistance counterbalances lethal anemia

Figure 17.17 How stabilizing selection maintains sickle-cell anemia

17.8 The Biological Species Concept

• Speciation is the macroevolutionary process of forming new species from pre-existing species– it involves successive change

• first, local populations become increasingly specialized

• then, if they become different enough, natural selection may act to keep them that way

17.9 Isolating Mechanisms

• There are six different prezygotic reproductive isolating mechanisms– geographical isolation– ecological isolation– temporal isolation– behavioral isolation– mechanical isolation– prevention of gamete fusion


• Geographical isolation occurs in cases when species exist in different areas and are not able to interbreed

• Ecological isolation results from two species who occur in the same area but utilize different portions of the environment and are unlikely to hybridize

Figure 17.20 Lions and tigers are ecologically isolated


• Temporal isolation results from two species having different reproductive periods, or breeding seasons, that preclude hybridization

• Behavioral isolation refers to the often elaborate courtship and mating rituals of some groups of animals, which tend to keep these species distinct in nature even if they inhabit the same places


• Mechanical isolation results from structural differences that prevent mating between related species of animals and plants

• Prevention of gamete fusion blocks the union of gametes even following successful mating


• If hybrid matings do occur, and zygotes are produced, postzygotic factors may prevent those zygotes from developing into normal individuals– in hybrids, the genetic complements of two species

may be so different that they cannot function together normally in embryonic development

– even if hybrids survive the embryo stage, they may not develop normally

– finally, many hybrids are sterile

Figure 17.21 Postzygotic isolation in leopard frogs

Figure 17.22 Populations can become geographically isolated for a variety of reasons

Biochemical Similarities

• Almost all living things use DNA, ATP (energy molecule), similar enzymes, same codons for protein synthesis, same 20 amino acids etc.

• Remember, at the cell level we are very close to most other eukaryotic organisms!

Evolution of Organisms

• All inheritable physical traits that appear in an organisms can be looked at as changes in an organism’s genetics

• Darwin did not understand genetics and therefore could not explain how traits were passed down through generations.

• We now can apply our understanding of genetics and apply them to evolutionary theory.

Geographic Isolation• Often, when geographic barrier is removed, the two

population can no longer interbreed and now become two species!

• 2 species on the opposite side of the grand canyon

evolution

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origin of species

charles darwin

shapes evolution

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