Evolution

19.1, 16.1-16.4, 17.1-17.4, 19.2

Fossils provide evidence about

extinct species Earth is more than 4.6 billion years old.

Relative dating - Uses index fossils to determine the

relative dates of rock layers.

Radiometric (absolute) dating – Uses the proportion of

radioactive : stable isotopes to calculate age.

Half-life – time required for half of the radioactive atoms in a sample

to decay, or break down into stable isotopes.

• Each radioactive element has a different half-life.

• This provides natural “clocks” that tick at different rates -

useful for dating rocks or fossils.

• carbon-14 – produced at a steady rate in the atmosphere,

decays into carbon-12 in 5730 years, used in absolute dating.

Theory of evolution

Theory – a well supported testable

explanation of phenomenon occurring in the

natural world.

Evolution – the process by which modern

organisms changed over time from ancient

common ancestors.

Microevolution – change in allele frequency in

populations over generations.

Macroevolution – large scale change, such as

the formation of new species.

3 patterns of biological diversity

Species – group of similar organisms that

can breed and produce fertile offspring.

1. Species vary globally – different yet

ecologically similar animals are found in

different yet similar environments.

2. Species vary locally – different yet related

species occupy different habitats in one area.

3. Species vary over time – fossils of extinct

species are similar to current species.

Remember…

The Earth is old and the process of change

exists today.

Traits acquired during an organism’s lifetime

are NOT passed to it’s offspring!

Most organisms don’t survive to reproduce!

Examples: sea turtles, insects, etc.

Artificial selection

Artificial selection – nature provides

variation (variety) in organisms’ traits, but

humans choose to breed those organisms

that have the most useful traits.

Example: humans breed cows that produce the

most milk.

Example: humans breed trees that create the

most fruit.

Summary: Theory of evolution

• Species are different due to variation in

their genes (variation results from random

mutation).

• Some individuals are better suited for

survival, and will leave more offspring

(natural selection or survival of the fittest).

Summary: Theory of evolution

• Over time, change within species leads to

the replacement of old species by new

species as less successful species

becomes extinct.

• There is clear evidence from fossils,

anatomy, physiology, DNA, and

embryology that the species now on Earth

have evolved from ancestors that are now

extinct.

Natural selection

Darwin proposed a mechanism for evolution that he called natural selection. Individuals whose characteristics are well-suited to their

environment survive and reproduce. • By surviving, these attributes can be passed onto their children,

causing an increase of these traits in the species population, thus causing a gradual change in the characteristics of the population.

Individuals whose characteristics are not well-suited to their environment die or leave few offspring.

Because natural selection favors a certain trait over

others, more individuals in the population carry the

genes for that trait.

AKA: survival of the fittest.

Parameters of evolution by NS

1. Struggle for existence – more organisms are produced

than can survive.

Competition – individuals or groups of organisms

compete for similar resources (territory, mates, food,

water, etc.) in the same environment.

2. Variation and adaptation – some variations are better

suited.

Adaptation – heritable characteristic that increases an

organism’s ability to survive and reproduce.

3. Survival of the fittest – individuals with adaptations that

are well suited to their environment survive and reproduce.

Fitness – measure of how well an organism

survives/reproduces.

Common descent

Common descent – all species (living and

extinct) descended from a common

ancestor.

Over many generations, adaptations caused

a successful species to evolve into a new

species.

The fossil record provides evidence for this

descent with modification.

Evidence for evolution includes:

1. Geographic distribution of species

2. Fossils

3. Anatomy (homologous structures)

4. Physiology (analogous structures)

5. Embryology

6. Universal genetic code

7. Biochemical homology

Geographic distribution of species

Species of animals on different continents

had similar structures and behaviors.

Darwin theorized that animals on each

continent were living under similar

ecological conditions and were exposed to

natural selection in a similar way.

Similar selection pressures caused

animals to evolve common features.

Fossils

A fossil is the preserved or mineralized

remains (bone, tooth, shell) or trace of an

organism that lived long ago.

Fossils show evidence that support the ancestry

between species.

Fossils trace the evolution of modern species

from extinct ancestors.

• Example: Fossils have shown that whales evolved

from four legged land mammals; 1990’s transitional

forms of whales found.

Anatomy

Evolutionary relationships can be viewed by

studying and comparing anatomy.

Scientists view the anatomy of limbs and see

common similarities.

Over course of evolution, vertebrates moved

into environments causing different survival

needs.

Anatomy

Homologous structures – parts of different organisms (that are often quite dissimilar) that developed from the same ancestral body parts.

Forelimbs of whales, bats, crocodiles, and chickens have similar anatomy but are modified for different functions – common ancestry.

Are similar in structure but differ in function!

Physiology

Analogous structures -- structures that are

similar in appearance and function but have

different origins and usually different internal

structures.

Examples: a bat’s wing and a moth’s wing--

both are wings and both are used for flight, but

a bat has bones and a moth does not.

Physiology

Vestigial structures – any body structure that

has reduced or no body function.

Examples: A human’s appendix; a whale’s pelvis.

As species adapt to environments the change in

form and behavior and continue to inherit these

structures as part of the body even though they

have no function.

Embryology

Embryology shows links between different

species.

Embryos of different species in early

development are indistinguishable from each

other.

Common cells and tissues develop in similar

patterns in all vertebrates.

Illustrates descent from a common ancestor.

Universal genetic code

Genetic code – mRNA codons specify

particular amino acids.

The genetic code of all organisms on Earth

(bacteria, yeast, fruit flies, humans) is the

same!

Example: the AUG codon always codes for the

amino acid methionine.

Biochemical homology

Similar DNA, RNA, and amino acid

sequences amongst species in same

taxonomic group.

Remember comparing your insulin gene DNA

and amino acid sequences to that of a cow?

Hox genes determine embryonic head-to-tail

patterning and are conserved in almost all

multicellular animals.

Allele frequencies

Population – mating group of organisms of

the same species.

Gene pool – all genes (and their alleles)

present in a population.

Allele frequency - # of times allele occurs in

a population.

Changes as population evolves over time.

Natural selection operates on individuals, but

causes a change in the allele frequency.

Sources of genetic variation

The main source of genetic variations in

populations is mutations!!!!

These mutations occur randomly.

Not all mutations affect an organism’s fitness.

Only heritable mutations matter for evolution.

Neutral mutations don’t change phenotypes.

Other sources of variation include:

1. Genetic recombination during crossing over and

independent assortment in meiosis.

2. Lateral gene transfer (bacteria only)

• Bacteria swap plasmids between members of the same

generation, then pass them to their offspring.

NS and phenotype

An organism’s genotype and environmental

conditions makes up its phenotype.

Natural selection operates on variation in

organisms’ phenotypes.

Higher fitness = phenotypes better suited for the

environment.

Phenotypes for traits

Number of phenotypes for a trait depends

on how many genes control the trait.

• Single-gene trait – trait controlled by one gene. • Ex. Banded or un-banded shell. • NS on these traits leads to

changes in allele and pheno. frequencies.

• Polygenic trait – trait controlled by two or more genes. • Ex. Height in humans. • NS on these traits affects

fitness of phenotypes.

NS on polygenic traits leads to

selection in populations

When NS on polygenic traits affects the

fitness of phenotypes, it leads to selection:

1. Directional selection

2. Stabilizing selection

3. Disruptive selection

Types of selection in

populations Directional selection –

organisms at one end of the

curve have a higher fitness

than those in the middle or at

the other end.

Stabilizing selection –

organisms in the center have

highest fitness.

Disruptive selection –

organisms at the ends of curve

have highest fitness.

NS is not the only source of

changes in allele frequencies

Genetic drift – change in allele frequency

that occurs in small populations due to

random chance.

Genetic bottleneck – change in allele frequency

following a dramatic reduction in population

size.

Founder effect – change in allele frequency

following migration of a small subgroup out of

the population to start a new population.

Genetic equilibrium

Genetic equilibrium – a MODEL to explain

what would happen to a hypothetical, non-

evolving population.

Allele frequencies NOT changing.

Hardy-Weinberg principle – states that allele

frequencies in a population should remain

constant unless something causes them to

change.

Hardy-Weinberg

P = frequency of dominant allele.

Q = frequency of recessive allele.

Equation: p2 + 2pq + q2 = 1 AND p + q =1

Equation in words: (frequency of AA) +

(frequency of Aa) + (frequency of aa) =

100% AND (frequency of A) + (frequency of

a) = 100%

Hardy-Weinberg

Remember genetic equilibrium occurs in

large populations.

HW predicts that 5 conditions can disrupt

genetic equilibrium and cause evolution to

occur: 1. Nonrandom mating (sexual selection)

2. Small population size – leads to of genetic drift.

3. Migration (immigration or emmigration) – aka gene flow

into or out of a population.

4. Mutations ***

5. Natural selection – different fitness exists for different

alleles.

Macroevolution

Speciation – evolutionary process by which

new species arise.

Extinction – the end of a species.

When environments change, the process of

evolution enables some species to adapt to

new conditions and thrive while some

species fail to adapt and become extinct.

Speciation

Species – population of organisms that can

interbreed.

Speciation – evolution/formation of a new

species.

Niche - combination of an organism’s profession and place where it lives.

No 2 species can occupy the same niche in the same location for a long period of time!

The more efficient species will survive and reproduce driving the other to extinction.

Isolating mechanisms of speciation

Reproductive isolation occurs when 2 populations

can’t interbreed - causes speciation! Once reproductive isolation occurs, natural selection increases the

differences between the separated populations.

1. Behavioral isolation – different courtship.

2. Ecological/habitat isolation – can only mate in specific or

preferred habitats.

3. Mechanical isolation – no sperm is transferred.

4. Gametic isolation – no fertilization of egg occurs.

5. Temporal isolation – reproduce at different times.

Geographic isolation – population becomes

divided (isolated) by a physical barrier.

Rates of speciation

Gradualism – slow, steady

change leading to new

species.

Punctuated equilibrium –

brief periods of rapid

change leads to the

formation of new species. • Rapid change occurs when a

small population is isolated

from the rest of the population

or migrates.

Molecular evolution

Molecular clock – uses mutation rates in DNA to

estimate the time 2 species have been evolving

independently.

Based on neutral mutations – those not under

selection.

Genes evolve through:

Modification of existing genes.

Duplication of existing genes.

• Crossing over – during meiosis.

• Gene duplications – extra copies undergo mutations.

• Gene families – multiple copies of duplicated gene

make similar yet different proteins.

Patterns of evolution

Divergent evolution – single species or group of species

evolve over a short period of time into different forms living

in different ways due to a change in environment that

makes new resources available.

Aka adaptive radiations

Ex. Dinosaurs, Darwin’s finches.

Convergent evolution – similar structures are produced in

distantly related organisms.

Ex. Mammals that feed on ants/termites evolved

independently 5 times.

Coevolution – 2 species respond to changes in each other

over time.

Neither can survive without the other.

Summary…

1. Individual organisms differ from one

another and differences are inherited.

2. Organisms produce many offspring and

some do not survive.

3. Organisms compete for limited

resources.

4. Each organism is unique and has

different advantages and disadvantages

in its struggle to survive.

Summary…

5. Adaptations that are best suited for an

environment allow organisms to survive

and therefore reproduce.

6. Species change over time.

7. Species alive today have modifications

from ancient ancestors.

8. All organisms on Earth are united by

common descent.