Processes of Evolution

Darwin’s four postulates: 1. Individuals within species are variable in traits

2. Some of these variations (traits) are passed on to offspring (that is, these traits are heritable) 3. In every generation, more offspring are produced than can survive due to limits of the environment 4. Individuals with “better” variations (traits) have greater survival and reproduction. They are naturally selected.

Natural Selection

• Natural selection for various traits among individuals of a population affects which individuals survive and reproduce in each generation

• Process results in adaptation to the environment (increases fitness)

Adaptation

• Some heritable aspect of form, function, or behavior that improves the odds for surviving and reproducing

• Environment specific

• Outcome of natural selection

Populations Evolve

• Biological evolution changes populations, not individuals

• Traits in a population vary among individuals

• Evolution: change in the frequency of traits

The Gene Pool

• All the genes in a population

• Genetic resource that is shared (in theory) by all members of population

Variation in Phenotype

• Each gene in gene pool may have two or more alleles

• Individuals inherit different allele combinations – leading to variation in phenotype

• Offspring inherit genes, not phenotypes

Variation in Populations

What Determines Alleles in a New Individual?

• Mutation

• Crossing over at meiosis I

• Independent assortment

• Fertilization

• Change in chromosome number or structure

Genetic Equilibrium

• Allele frequencies at a locus are not changing

• Population is not evolving

Five Conditions of Genetic Equilibrium

• No mutation

• Random mating

• Gene doesn’t affect survival or reproduction

• Large population

• No immigration/emigration

Microevolutionary Processes

• Drive a population away from genetic equilibrium

• Small-scale changes in allele frequencies brought about by – Natural selection – Gene flow – Genetic drift

Gene Mutations

• Infrequent but inevitable

• Each gene has own mutation rate

• Lethal mutations

• Neutral mutations

• Advantageous mutations

Results of Natural Selection

Three possible outcomes:

• A shift in the range of values for a given trait in some direction

• Stabilization of an existing range of values

• Disruption of an existing range of values

Types of Natural Selection

• Directional

• Stabilizing

• Disruptive

• Natural selection also drives maintenance of phenotypic (and genetic) diversity – Advantageous to have several forms

(morphs) – Sexual Selection – Balancing Selection

Directional Selection

Allele frequencies shift in consistent direction over time

Range of values at time 3

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Directional Selection

Pinpointing the Target of Selection

• Populations of rock pocket mice have fur that matches the rocks on which they live

– Black basalt: dark fur – Tawny granite: light fur

Pinpointing the Target of Selection

• DNA comparisons show that the two populations differ in Mclr gene sequence

Directional Selection

Drought in Galapagos caused selection of larger

beaks among ground finches

(Grant and Grant 2003)

Directional Selection

Pesticide Resistance

• Pesticides kill susceptible insects

• Resistant insects survive and reproduce

• If resistance has heritable basis, it becomes more common with each generation

Antibiotic Resistance

• Antibiotics first came into use in the 1940s

• Overuse has led to increase in resistant forms

• Most susceptible cells died out, while resistant forms multiplied

Stabilizing Selection

Intermediate forms are favored and extremes are eliminated

Range of values at time 1

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Human Birth Weight pe

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1 2 3 4 5 6 7 8 9 10 11 birth weight (pounds)

100 70 50 30 20

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percent mortality

human newborns rate of death

Disruptive Selection

• Happens when forms at both ends of the range of variation are favored

• Intermediate forms are selected against

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Disruptive selection • Individuals at both phenotypic

extremes are favored.

• Example: African seedcrackers (birds) have two food sources—hard seeds that require large beaks to crack, and smaller, softer seeds that smaller beaks are more suited to.

lower bill 12 mm wide

lower bill 15 mm wide

Fig. 12-15, 188

• Selection favors birds with very large or very small bill

• Birds with intermediate-sized bill are less effective feeders

Disruptive selection

Sexual Selection

• Selection favors certain secondary sexual characteristics

• Through nonrandom mating, alleles for preferred traits increase

• Leads to increased sexual dimorphism

Sexual Selection in Birds

(a) Intersexual selection: Sexual dimorphism in a finch species

(b) Intrasexual selection: Competing for mates

Figure 13.29

Balanced Polymorphism

• Polymorphism: “having many forms”

• Occurs when two or more alleles are maintained at frequencies greater than 1 percent

Polymorphism

Sickle-Cell Trait: Heterozygote Advantage

• Allele HbS causes sickle-cell anemia when heterozygous

• Heterozygotes are more resistant to malaria than homozygotes

less than 1 in 1,600

1 in 400–1,600

1 in 180–400

1 in 100–180

1 in 64–100

more than 1 in 64

Malaria case

Sickle-cell trait

Areas with high incidence of malaria

Frequencies of the sickle-cell allele

0–2.5%

10.0–12.5%

2.5–5.0%

5.0–7.5%

7.5–10.0%

>12.5%

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Figure 13.30

Genetic Drift

• Random change in allele frequencies brought about by chance

• Effect is most pronounced in small populations

• Sampling error: fewer times an event occurs, greater the variance in outcome

• Genetic drift occurs when chance

events determine which alleles are passed to the next generation.

• It is significant only for small populations.

• Genetic drift has four effects on small

populations:

• 1. It acts by chance alone, thus causing allele frequencies to fluctuate at random.

• Some may disappear, others may reach 100% frequency (fixation).

Genetic Drift: Small Populations • Frequency of b+ allele

Genetic Drift: Large Populations • Frequency of b+ allele

Mechanisms of Evolution • Genetic drift has four effects on small

populations:

• 1. It acts by chance alone, thus causing allele frequencies to fluctuate at random.

• Some may disappear, others may reach 100% frequency (fixation).

Mechanisms of Evolution • 2. Because some alleles are lost,

genetic variation of the population is reduced.

• 3. Frequency of harmful alleles can increase if the alleles have only mildly deleterious effects.

• 4. Differences between populations can increase. Chance events may lead to allele fixation in one population and loss from another population.

Mechanisms of Evolution • 2 and 3 can have dire consequences.

• Loss of genetic variation reduces the ability of the population to respond to changing environmental conditions.

• Increase of harmful alleles can reduce survival and reproduction.

• These effects are important for species that are near extinction.

Mechanisms of Evolution • Prairie chicken populations in Illinois

have been reduced by loss of habitat to farmland.

• In 1993, the population was <50. DNA from this population compared with museum specimens from the 1930s showed a decrease in genetic variation.

• 50% of eggs failed to hatch, suggesting fixation of harmful alleles.

Harmful Effects of Genetic Drift

Bottleneck

• A severe reduction in population size

• Causes pronounced drift

• Example – Elephant seal population hunted down to

just 20 individuals – Population rebounded to 30,000 – Electrophoresis revealed there is now no

allele variation at 24 genes: all are homozygous

Founder Effect

• Effect of drift when a small number of individuals starts a new population

• By chance, allele frequencies of founders may not be same as those in original population

• Effect is pronounced on isolated islands

phenotypes of mainland population

phenotype of island population

Fig. 12-19, p.191

Founder Effect • Albatross carries seed to island

Inbreeding

• Nonrandom mating between related individuals

• Leads to increased homozygosity

• Can lower fitness when deleterious recessive alleles are expressed

Gene Flow

• Physical flow of alleles into a population

• Tends to keep the gene pools of populations similar

• Counters the differences that arise from mutation, natural selection, and genetic drift

Gene Flow • Blue jay carries acorn between oak

populations

• Gene flow: Alleles move between

populations via movement of individuals or gametes.

• Gene flow has two effects:

• 1. Populations become more similar.

• 2. New alleles can be introduced into a population.

Gene Flow • In the 1960s, new alleles that provide

resistance to insecticides arose by mutation in mosquitoes in Africa or Asia.

• Mosquitos with the new alleles were blown by winds or transported by humans to new locations.

• The allele frequency increases rapidly in populations exposed to insecticides.

Trophy Hunting and Inadvertent Evolution • Humans have caused evolutionary

changes in many organisms.

• Red foxes with silver-tipped fur declined because of preferential hunting.

• Antibiotics are a strong source of directional selection, leading to evolution of antibiotic resistance in bacteria.

Hunting Resulted in the Decline of Silver Foxes

Connections in Nature: The Human Impact on Evolution

• Many human actions can alter the course of evolution.

• Pollutants and introduction of invasive species change aspects of the environment and alter selection pressures.

• Habitat fragmentation leaves isolated patches, which can affect evolutionary processes.

Evolutionary Effects of Habitat Fragmentation on a Hypothetical Species

Trophy Hunting and Inadvertent Evolution

• Bighorn sheep populations have been reduced by 90% by hunting, habitat loss, and introduction of domestic cattle.

• Hunting is now restricted in North America; permits to take a large “trophy ram” cost over $100,000.

Fighting over the Right to Mate

Trophy Hunting and Inadvertent Evolution

• Trophy hunting removes the largest and strongest males—the ones that would sire many healthy offspring.

• In one population, 10% of males were removed by hunting each year.

• The average size of males and their horns decreased over 30 years of study.

Trophy Hunting Decreases Ram Body and Horn Size

Trophy Hunting and Inadvertent Evolution

• This is also being observed in other species: – By targeting older, larger fish, commercial

cod fishing has selected for genes that result in maturation at earlier ages and smaller size. Fish that mature earlier can reproduce before they are caught, but small fish produce fewer eggs.

Trophy Hunting and Inadvertent Evolution

– African elephants are poached for ivory; the proportion of the population that have tusks is decreasing.

• The unintended effects of human harvesting on these animals illustrate how populations can change, or evolve, over time.

The Human Impact on Evolution

• Human actions can alter the mechanisms of evolution: natural selection, genetic drift, gene flow.

• We know with certainty that our actions cause major environmental changes; we can infer that they are also causing evolutionary changes.

Table 12-1, p.192

Evolutionary Patterns, Rates, and Trends

Microevolutionary Processes

• Small-scale changes in allele frequencies brought about by – Natural selection – Gene flow – Genetic drift

Crab Life Cycle

Larval and juvenile stages molt repeatedly

and grow in size egg

Macroevolution

• Major patterns and trends among lineages

• Rates of change in geologic time

Comparative Morphology

• Comparing body forms and structures of major lineages

• Guiding principle: – When it comes to introducing change in

morphology, evolution tends to follow the path of least resistance

Morphological Divergence

• Change from body form of a common ancestor

• Produces homologous structures

1

1

1

1

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early reptile

pterosaur

chicken

bat

porpoise

penguin

human

Morphological Convergence

• Individuals of different lineages evolve in similar ways under similar environmental pressures

• Produces analogous structures that serve similar functions

Morphological Convergence

Comparative Development

• Each animal or plant proceeds through a series of changes in form

• Similarities in these stages may be clues to evolutionary relationships

• Mutations that disrupt a key stage of development are selected against

Altering Developmental Programs

• Some mutations shift a step in a way that natural selection favors

• Small changes at key steps may bring about major differences

Trochophore larva

Molecular Evidence

• Biochemical traits shared by species show how closely they are related

• Can compare DNA, RNA, or proteins

Comparing Proteins

• Compare amino acid sequence of proteins produced by the same gene

• Human cytochrome c (a protein) – Identical amino acids in chimpanzee protein – Chicken protein differs by 18 amino acids – Yeast protein differs by 56

Sequence Conservation

• Cytochrome c functions in electron transport

• Some sequences are identical in wheat, yeast, and primates

Sequence Conservation

yeast wheat primate

Nucleic Acid Comparison

• Use single-stranded DNA or RNA

• Hybrid molecules are created, then heated

• The more heat required to break hybrid, the more closely related the species

Molecular Clock

• Assumption: “Ticks” (neutral mutations) occur at a constant rate

• Count the number of differences to estimate time of divergence

Biological Species Concept

“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”

Ernst Mayr

Variable Morphology

grown in water grown on land

Genetic Divergence

• Gradual accumulation of differences in the gene pools of populations

• Natural selection, genetic drift, and mutation can contribute to divergence

• Gene flow counters divergence

Genetic Divergence

time A time B time C time D

daughter species

parent species

Reproductive Isolation

• Cornerstone of the biological species concept

• Speciation is the attainment of reproductive isolation

• Reproductive isolation arises as a by-product of genetic change

Reproductive Isolating Mechanisms

• Prevent pollination or mating

• Block fertilization or embryonic

development

• Cause offspring to be weak or sterile

Prezygotic Isolation

Mechanical isolation

Temporal isolation

Behavioral isolation

Ecological isolation

Gametic mortality

Mechanical Isolation

• Wasp and zebra orchid

Temporal Isolation

• Cicada

Behavioral Isolation

• Albatrosses

Postzygotic Mechanisms

Early death

Sterility

Low survival rates

Models for Speciation

• Allopatric speciation

• Sympatric speciation

• Parapatric speciation

Allopatric Speciation

• Speciation in geographically isolated populations

• Some sort of barrier arises and prevents gene flow

• Effectiveness of barrier varies with species

Allopatric Speciation

Extensive Divergence Prevents Inbreeding

• Species separated by geographic barriers will diverge genetically

• If divergence is great enough it will prevent inbreeding even if the barrier later disappears

Archipelagos

• Island chains some distance from continents – Galapagos Islands – Hawaiian Islands

• Colonization of islands followed by genetic divergence sets the stage for speciation

Speciation on an

Archipelago

1

2 3

4

1

2 3

4

1

2

A few individuals of a species on the mainland reach isolated island 1. Speciation follows genetic divergence in a new habitat.

Later in time, a few individuals of the new species colonize nearby island 2. In this new habitat, speciation follows genetic divergence.

Speciation may also follow colonization of islands 3 and 4. And it may follow invasion of island 1 by genetically different descendents of the ancestral species.

Hawaiian Islands

• Volcanic origins, variety of habitats

• Adaptive radiations: – Honeycreepers: in absence of other

bird species, they radiated to fill numerous niches

Fig. 13-18d13, p.209 Housefinch (Carpodacus)

Ancestral Type

Akepa (Loxops coccineus) Fig. 13-18d1, p.209

Speciation in Hawaiian Honeycreepers

Speciation without a Barrier

• Sympatric speciation – Species forms within the home range of the

parent species

• Parapatric speciation – Neighboring populations become distinct

species while maintaining contact along a common border

Sympatric Speciation in African Cichlids

• Studied fish species in two lakes – Species in each lake are most likely

descended from single ancestor

• No barriers within either lake

Sympatric Speciation in African Cichlids • Feeding preferences localize species in

different parts of lake

Speciation by Polyploidy

• Change in chromosome number (3n, 4n, etc.)

• Offspring with altered chromosome number cannot breed with parent population

• Common mechanism of speciation in flowering plants

Figure 18.9 Page 299

Possible Evolution of Wheat

Triticum monococcum (einkorn)

T. aestivum (one of the common bread wheats)

Unknown species of wild wheat

T. turgidum (wild emmer)

T. tauschii (a wild relative)

42AABBDD 14AA 14BB 14AB 28AABB 14DD X X

cross-fertilization, followed by a spontaneous chromosome doubling

Parapatric Speciation • Populations in contact along a common border

giant velvet worm

blind velvet worm

We’re All Related

• All species are related by descent

• Share genetic connections that extend back in time to the prototypical cell

Patterns of Change in a Lineage

• Cladogenesis – Branching pattern – Lineage splits, isolated populations diverge

• Anagenesis – No branching – Changes occur within single lineage – Gene flow throughout process

Evolutionary Trees

ancestral stock

species 1

species 2 species 3

Summarize information about relationships among groups

Gradual Model

• Species emerge through many small changes accumulating over time

• Fits well with evidence from certain lineages in fossil record

Punctuation Model

• Speciation model in which most changes in morphology are compressed into brief period near onset of divergence

• Supported by fossil evidence in some lineages

Adaptive Radiation

• Burst of divergence

• Single lineage gives rise to many new species

• New species fill vacant adaptive zone

• Adaptive zone is “way of life”

• Cenozoic radiation of mammals