Evolution

10.1 – Early Ideas About Evolution

Key Concept There were theories of biological and geologic

change before Darwin.

Early scientists proposed ideas about

evolution. Evolution is the biological change over time by which descendants come to differ from ancestors.

A species is a group of organisms that can reproduce and have fertile offspring.

Theories of geologic change set the stage for Darwin’s theory.

There were three theories of geologic change: Catastrophism: natural disasters such as floods and

volcanic eruptions have shaped landforms and caused species to become extinct.

Gradualism: changes in landforms resulted from slow changes over a long period of time

Uniformitarianism: the geologic processes that shape Earth are uniform through time

Uniformitarianism is the prevailing theory of geologic change.

10.2 – Darwin’s Observations

Key Concept: Darwin’s voyage provided insight on evolution.

Charles Darwin

Known as the father of evolution

Traveled around the world on the HMS Beagle

Observed geological phenomena and adaptations & variation in species

Published findings in his book Origin of Species 1800’s

Darwin observed differences among island

species. Variation: difference in a physical trait of an individual compared to others in the same group

Galapagos tortoises that live in areas with tall plants have long necks and long legs

Galapagos tortoises that live in areas with low plants have short necks and short legs

Galapagos finches (Darwin’s finches) that live in areas with hard-shelled nuts have strong beaks

Galapagos finches that live in areas with insects/fruit have long, thin beaks

Adaptation: feature that allows an organism to better survive in its environment Species are able to adapt to

their environment

Adaptations can lead to genetic change in a population

Darwin observed fossil and geologic evidence supporting an

ancient Earth. Darwin found fossils of extinct animals that resemble modern animals

Darwin found marine fossil shells high up in the Andes mountains

Glyptodon Modern armadillo

He saw land move from underwater to above sea level during an earthquake

Darwin extended his observations to the evolution of organisms (gradual change leads to great change over time)

10.3 – Theory of Natural Selection

Key Concept: HOW DOES EVOLUTION OCCUR? Darwin

proposed natural selection as a mechanism for evolution.

Several key insights led to Darwin’s idea for natural

selection. Natural selection: mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than do other individuals

Artificial selection: process by which humans change a species by breeding it for certain traits.

Heritability: ability of a trait to be passed down

http://www.greyhound-data.com/d?z=-vUZlp&d=wv%27s+hamlin&x=0&y=0

There is a struggle for survival due to overpopulation and limited resources

Darwin proposed that adaptations arose over many generations

Natural selection explains how evolution can occur. Variation: heritable differences that exist in every population

are the basis for natural selection

Overproduction: Having many offspring increases the chance of survival but also results in competition for resources

Adaptation: certain variation that allows an individual to survive & reproduce better than other individuals it competes against Fitness: ability to survive and reproduce

Descent with modification: Heritability of adaptations. More individuals will have the trait in every following generation, as long as the environmental conditions remain beneficial for the trait

Natural selection acts on existing variation.

Natural selection can act only on traits that already exist.

New alleles (leading to new phenotypes) are not made by natural selection – they occur by genetic mutations.

Structures take on new functions in addition to their original function.

wrist bone

five digits

10.4 – Evidence of Evolution

Key Concept: Evidence of common ancestry among species

comes from many sources.

Fossils & the Fossil Record

Shows how species changed their form/shape over time

Ways of dating fossils: Relative dating: estimates the age of fossils by comparing

fossil to others in the same layer of rock

Pro: can be used if there is no other way to tell the age of the fossil

Con: layers of rock can be shifted by natural events (earthquakes, mudslides, etc.) and this can mess up estimate

Radiometric dating: uses the decay of radioactive isotopes (carbon-14 changes into nitrogen-14)

Pro: can give an accurate age

Con: can’t give an age for really old fossils (if all isotopes have decayed)

Biogeography Island species most closely resemble nearest

mainland species

Populations can show variation from one island to another

Example: Darwin’s finches

Embryology Similar embryos,

diverse organisms

Identical larvae, diverse adult body forms

Gill slits and “tails”as embryos

Larva

Adult barnacleAdult crab

Homologous Structures

Similar in structure, different in function

Evidence of a common ancestor

Example: bones in the forelimbs of different animals (humans, cat legs, whale fins, bat wings)

Not to be confused with analogous structures – those that have similar functions but are not made of similar structures. Not evidence of a close evolutionary relationship. Example: bat wings, insect wings.

Vestigial Organs/Structures

Remnants of organs or structures that had a function in an early ancestor but have lost their function over time

Evidence of a common ancestor

Examples: Human appendix & tailbone

Wings on flightless birds (ostrich, penguins)

Hindlimbs on whales, snakes

Molecular Biology Common (universal) genetic code (A, T, C, &

G)

Similarities in DNA, proteins, genes, & gene products

Two closely related organisms will have similar DNA sequences & proteins

DNA fingerprints will also be very close if the species are closely related

11.1 – Genetic Variation Within Populations

Key Concept: A population shares a common gene pool.

Genetic variation in a population increases the chance that some

individuals will survive. Genetic variation leads to phenotypic variation

Necessary for natural selection

Genetic variation is stored in a population’s gene pool Made up of all the alleles of all individuals in a

population

Allele combinations form when organisms have offspring

Allele frequency: a measure of how common a certain allele is in a population. Can be impacted by natural selection.

Genetic variation comes from several sources. Mutations

Can form a new allele

Passed to offspring if in a gamete

Recombination Usually occurs during meiosis

Parents’ alleles rearranged during gamete formation

11.2 – Natural Selection in Populations

Key Concept: Populations, not individuals, evolve.

Microevolution Evolution within a population

Observable change in allele frequencies

Can result from natural selection

Types:

Directional selection

Stabilizing selection

Disruptive selection

Directional Selection Favors phenotypes at one extreme

Stabilizing Selection

Favors the intermediate phenotype

Disruptive Selection Favors both extreme phenotypes

http://www.youtube.com/watch?v=aTftyFboC_M

11.3 – Other mechanisms of Evolution

Key Concept: Natural selection is not the only mechanism

through which populations evolve.

Gene Flow Movement of alleles between populations

Occurs when individualsjoin new populations and reproduce Their alleles become part of

gene pool

Keeps neighboring populations similar

Low gene flow increases the chance that two populations will evolve into different species

bald eagle migration

Genetic Drift Change in allele frequencies due to chance

Causes a loss of genetic diversity in a population

Common in small populations

Bottleneck Effect is genetic drift after a bottleneck event Occurs when an event

drastically reduces population size

Founder Effect is genetic drift that occurs after the start of a new population Occurs when a few individuals start a new

population

Sexual selection occurs when certain traits increase mating

success. Sexual selection Occurs due to higher cost of reproduction for females

Males produce sperm continuously

Females are more limited in potential offspring each cycle

Two types: Intrasexual selection: competition among males

Intersexual selection: males display certain traits to females

11.5: Speciation through Isolation

Key Concept: New species can arise when populations are isolated.

If gene flow stops between two populations, they are said to be isolated.

Adaptations, mutation, and genetic drift may change the gene pools of the populations, and over time the populations may become more and more genetically different.

Reproductive isolation: when members of different populations can no longer mate successfully with one another.

This is the final step before speciation (the rise of two or more species from one existing species)

Several kinds of barriers can prevent mating between populations, leading to reproductive isolation.

Behavioral isolation: differences in courtship or mating behaviors.

Geographic isolation: physical barriers that divide a population into two or more groups.

Temporal isolation: timing prevents reproduction between populations.

11.6 – Patterns in Evolution

Key Concept: Evolution occurs in patterns.

Species can become extinct.

Extinction: elimination of a species from Earth Background extinction

Mass extinction

Background Extinction

Occur randomly, but at a low rate

Usually affect only a few species in a small area

Can by caused by local changes in the environment

Mass Extinction Rare, but very intense

Can operate at a global level

Caused by a catastrophic event such as an ice age

At least 5 mass extinctions in the last 600 million years

Extinction Species go extinct because they lack the

variation needed to adapt