What do fossils provide evidence of?

changing life forms

strata

layers of rock where fossils are deposited

• the older strata are deposited first

Charles Lyell

geologic processes that have shaped planet = uniform over time -> Earth must be older than previously thought (a few thousand years)

Jean Baptiste de Lamarck

proposed a mechanism for evolution based on use and disuse & inheritance of acquired characteristics

Use and disuse

• parts of the body use extensively become larger and stronger
• those not used deteriorate

Inheritance of acquired characteristics

characteristics acquired in lifetime -> passed onto next generation

(flawed theory)

What was the significance of Lamarck's findings?

recognized that species evolve; organisms and environment match through this gradual evolutionary change

inheritance of acquired characteristics - wrong

What was the significance of Lyell's findings?

an old Earth has time for evolution; a young Earth doesn't -> gave Darwin the "gift of time"

Example of inheritance of acquired characteristics

A weightlifter's child could be born with a more muscular anatomy

What was the impetus for the development of Darwin's evolution by natural selection?

Darwin's voyage on the HMS Beagle

Natural Selection

• Darwin's mechanism for evolution
• explains how adaptations arise

results in alleles being passed to the next generation in proportions different from their relative frequencies in the present generation

Adaptations

• heritable characteristics
• enhance an organism's ability to survive and reproduce in specific environments

Example of an adaptation

• Desert foxes have large ears, which radiate heat
• Arctic foxes have small ears, which conserve body heat

Darwin's theory of evolution: principle one

members of a population often vary in their inherited phenotypic traits

Darwin's theory of evolution: principle two

• a species can produce far more offspring than environment can support
• competition is inevitable

Darwin's theory of evolution: principle three

individuals with inherited traits that are better suited to local environment = more likely to survive and reproduce

"differential reproductive success"

Fitness

the reproductive success of an individual in a population

Darwin's theory of evolution: principle four

evolution -> unequal reproductive success of individuals -> accumulation of favorable traits in the population over generations

What does natural selection improve over time?

the match between organisms and their environment

If individuals move to a new environment, or their environment changes...

natural selection may result in adaptation to these new conditions

different genetic variations can be selected depending on...

the new environment

Individuals DO NOT EVOLVE...

populations evolve

• an individual cannot change its genetic makeup, but differential reproduce success overtime can change it.

Evolution

a change in the genetic makeup of a population over time

• change in allelic frequencies

Artificial selection

process by which species are modified by humans

Example of artificial selection

selective breeding for milk or meat production; development of dog breeds

Evidence for Evolution

• direct observations
• homology
• the fossil record
• biogeography

Direct observations of evolutionary change

• populations of organisms continue to evolve in real time

examples of direct observation of evolutionary change

• insect populations can become rapidly resistant to pesticides such as DDT
• bacterial populations become resistant to antibiotics, and viruses become resistant to other drugs

Homology

• similarity resulting from common ancestry

characteristics in related species can have an underlying similarity even though they have very different functions

Homologous structures

anatomical signs of evolution

examples of homologous structures

• forelimbs of mammals that are now used for a variety of purposes: flying in bats, swimming in whales

present and used in a common ancestor

Embryonic homologies

• comparison of early stages of animal development
• reveals many anatomical homologies in embryos, not visible in the adult organism

example of embryonic homologies

all vertebrate embryos have a post-anal tail and pharyngeal pouches

Vestigial organs

• remnants of structures that served important functions in the organism's ancestors
• structures of marginal, if any, importance to an organism

example of vestigial organs

• some snakes have remnants of pelvis and leg bones

Molecular homologies

• shared characteristics on the molecular level

examples of molecular homologies

• all life forms use the same genetic language of DNA and RNA
• amino acid sequences, coding for hemoglobin in primate species, shows great similarity (indicating a common ancestor)

Convergent evolution

• two species develop similarities as they adapted to similar environmental changes
• NOT because they evolved from common ancestor

explains why distantly related species can resemble one another

The likenesses that result from convergent evolution are considered...

analogous, not homologous

examples of convergent evolution

• torpedo shapes of penguin, dolphin, and shark = solution to movement through aqueous environment
• sugar gliders and flying squirrels occupy similar niches in their respective habitats

"similar problems have similar solutions"

paleontology

the study of fossils

Fossil record

• shows that evolutionary changes have occurred over time
• provide evidence of the origin of major new groups of organisms

succession of forms!

transitional fossils have been found that...

link ancient organisms to modern species

Biogeography

the geographic distribution of species

Species in a discrete geographic area...

tend to be more closely related to each other than species in distant geographic areas

example of biogeography (as evidence for evolution)

• In South America, desert animals are more closely related to local animals in other habitats than they are to desert animals of Asia

Continental drift & the breakup of Pangaea

• can explain the similarity of species on continents that are distant from each other today

Endemic species

found at a certain geographic location and nowhere else

example of endemic species

• Marine iguanas - found on Galápagos Islands and nowhere else

phenotypic variation often reflects...

genetic variation

examples of phenotypic variation

• having / not having attached ear lobes
• range in height

Mutations

• result in genetic variation on which natural selection can act

only source of new genes and new alleles

only mutations in cell lines that produce...

gametes can be passed to offspring

Point mutations

• changes in one nucleotide base in a gene
• can have a significant impact on phenotype

Example of a point mutation

sickle cell disease

Chromosomal mutations

• delete, disrupt, duplicate, or rearrange many loci at once
• usually harmful, but not always

gene duplications can...

• result in an expanded genome with new genes
• may accumulate mutations over generations and take on new functions

most genetic variations within a population result from...

the sexual recombination of alleles that already exist in a population

sexual reproduction...

shifts existing alleles and deals them at random to produce individual genotypes

three mechanisms for the shuffling of alleles

• crossing over (prophase I of meiosis)
• independent assortment of chromosomes during meiosis
• fertilization

Population

a group of Individuals of the same species that live in the same area and interbreed, producing fertile offspring

Population genetics

the study of how populations change genetically over time

Gene pool

all of the alleles at all loci in all the members of a population

How many alleles does for a particular gene does each Individual have (diploid species)

two (and the individual may be heterozygous or homozygous)

Fixed (pertaining to a gene)

all members of a population are homozygous for the same allele

• only one allele exists at that particular locus in the population

the greater number of fixed alleles...

the lower the species' genetic diversity

Hardy-Weinberg equation

used to describe a population that is not evolving

• states the frequencies of alleles and genes in a population's gene pool will remain constant unless acted upon by forces other than Mendelian segregation and recombination of alleles

Hardy-Weinberg Equilibrium

1) no change in allelic frequency due to mutation

2) random mating

3) no natural selection

4) extremely large population size

5) no migration

What is the Hardy-Weinberg equation good for?

provides an excellent null hypothesis (conditions are seldom met in natural populations)

• if Hardy Weinberg analysis shows change in allelic frequency, can figure out which condition isn't being met

three major factors that alter allele frequencies...

(and bring about the most evolutionary change)

• natural selection
• genetic drift
• gene flow

Differential Reproductive Success

• individuals with variations that are better suited to their environment tend to survive and produce more offspring than those with variations that are less suited

Adaptive evolution

organisms adapted to their environment

Genetic Drift

unpredictable fluctuation in allele frequencies from one generation to the next

• random, unadaptive change in allele frequencies

the smaller the population...

the greater the chance there is for genetic drift

two examples of genetic drift

• founder effect
• bottleneck effect

Founder effect

a few individuals become isolated from a larger population and establish a new population whose gene pool is not reflective of the source population

Example of the founder effect

a mat of vegetation washes up on the shore of a Pacific island, host to a small population of lizards

• its gene pool is randomly different than that of the source population

Bottleneck effect

a sudden change in the environment that drastically reduces the size of a population

• the few survivors that pass through the restrictive bottleneck may have a gene pool that no longer reflects that of the original population

Example of the bottleneck effect

• The population of California condors was reduced to nine individuals

Gene flow

occurs when a population gains or loses alleles by genetic additions or subtractions from the population (often by migration)

• results from the movement of fertile individuals or gametes

gene flow occurs when...

alleles between different populations are mixed, resulting in a reduction of genetic differences between the populations

gene flow tends to reduce the...

genetic differences between populations, making them more similar

converse of gene flow

isolated populations do not experience it, tend to adapt to their unique environments and may have significant genetic differences from the ancestral population

Relative fitness

the contribution an organism makes to the gene pool of the next-generation relative to the contributions of other members

fitness in the context of evolution is only measured...

by reproductive success

natural selection acts more directly on the...

phenotype

natural selection acts more indirectly on the...

genotype

Three ways in which natural selection can alter the frequency distribution of heritable traits

• directional selection
• disruptive selection
• stabilizing selection

Directional selection

• shifts the overall makeup of the population by favoring variants that are at one extreme of the distribution

Example of directional selection

• darker mice are favored because they live among dark rocks

Disruptive selection

• favors variants at both ends of the distribution

Example of disruptive selection

• mice that colonize a patchy habitat made up of light and dark rocks, mice of an intermediate color are at a disadvantage

Stabilizing selection

• removes extreme variants from the population and preserves intermediate types

Example of stabilizing selection

• environment consists of intermediate color rocks, both light and dark mice selected against.

Sexual selection

• individuals with certain inherited characteristics are more likely than other individuals to obtain mates

Sexual dimorphism

• a difference between the two sexes in secondary sex characteristics such as differences in size, color, ornamentation, and behavior

How is genetic variation preserved in a population? (Why doesn't natural selection eliminate all unfavorable alleles?)

• diploidy
• heterozygote advantage

Diploidy

• because most eukaryotes are diploid, recessive alleles are hidden from selection in heterozygotes

Heterozygote advantage

• occurs when two individuals who are heterozygous at a certain gene locus have an advantage for survival

Example of heterozygote advantage

• In sickle cell disease, individuals homozygous for normal hemoglobin are more susceptible to malaria, whereas homozygous recessive individuals suffer from complications of sickle-cell disease. Heterozygotes benefit from some protection from malaria and do not have sickle-cell disease (mutant allele remains relatively common)

Why does natural selection not produce perfect organisms?

• Selection can only edit existing variations
• evolution is limited by historical constraints
• adaptations are often compromises
• chance, natural selection, and the environment interact