NATURAL SELECTION AND VARIATIONPart 1

THE WORK OF CHARLES DARWIN

EVOLUTION AS A (SCIENTIFIC) THEORY

Evolution change in a population of organisms over time

Scientific Theory well-tested explanation based on evidence that unifies as broad range of observations and hypotheses

Law has a mathematical equation to explain it

Evolution is considered a theory as ALL evidence supports the idea that organisms have changed and continue to change, over time

CHARLES DARWIN

• Lived in England• 1809-1882

• 1831-1836: Beagle’s five-year voyage mapping the coastline of South America

• 1859: On the Origin of Species by Means of Natural Selection

• EVOLUTION explains how modern organisms evolved over long periods of time through descent from common ancestors

DARWIN’S VOYAGE

WHAT DARWIN KNEW• What did Darwin (and science) know?• The Earth is REALLY old

• 4.6 Billion Years• The Earth has changed over time

• Mountains and valleys formed• Rivers change course• Water erodes land

• Humans can change organisms through breeding

• Organisms have rapid Population Growth• Darwin realized most organisms don’t

survive and reproduce…• ???Which individuals survive and why???

WHAT DARWIN DIDN’T KNOW

•DNA and Genetics• Knew that traits were passed from parent to offspring• Didn’t know HOW

• Most of what you learned last semester!

Acquired Characteristics

List the characteristics that you believe you have always had. For example, you may have brown eyes or curly hair.

Inherited Characteristics

List your acquired characteristics. For example, you may have learned how to play a musical instrument.

• Jean Baptiste Lamarck – proposed theory of evolution based on growing fossil record

• Suggested that organisms could change during their lifetimes by selectively using or not using various parts of their bodies.

• Suggested that individuals could pass these acquired traits onto offspring, enabling species to change over time

• Published ideas in 1809 –year Darwin was born

parent mouseTail is removed

to simulate disuse of

characteristic during lifetime

offspring mouse

If Lamarck was correct, offspring would inherit

“acquired” trait of parent

= no tail

BUT offspring have tails

Lamarck was INCORRECT because:1. Organisms do not have an inborn drive to be perfect2. Evolution does not mean that over time species

becomes “better” somehow3. Evolution does not progress in predetermined

direction4. Traits acquired by organisms during lifetime cannot

be passed on to offspringLamarck gave us some good stuff too!• Lamarck was one of 1st to try to

explain evolution scientifically using natural processes

• Recognized link b/w org’s environment and body structures

THE FOSSIL RECORD

• Fossils: preserved remains or markings left by organisms that lived in the past

• Sand and silt is eroded from land and settles in the bottom bodies of water

• Layers pile up and form rock strata• Bodies get buried in layers

and become fossils• Fossils can also occur on land

under volcanic ash or dust

THE FOSSIL RECORD

• Younger strata are layer on top of older ones• Positions of fossils in rock strata can give us

relative age• Fossil Record: chronological collection of life’s

remains in the rock layers, recorded during the passage of time

• Fossils show that life on earth is changing

THE FOSSIL RECORD

• Oldest fossil found: 3.8 billion years old• Fossils of extinct species help us reconstruct the

past• Fossil evidence supports cellular and molecular

evidence of Earth’s history• Whale fossils from 40 billion years ago show that

whales had hind-limb bones• Supports theory

that whales evolved from ancestors with 4 limbs

• Darwin noticed that some fossils of extinct animals were similar to living species

• Glyptodont – giant armored animal found in area where current armadillo is found

• Armadillo = smaller version of glyptodont

SPECIES VARY OVER TIME

NATURAL SELECTION

STRUGGLE FOR EXISTENCE

• If more individuals are produced than can survive members of a population must compete to obtain food, living space, and other limited necessities of life

• Called: Struggle for Existence

• Which individuals come out on top of struggle?

VARIATION AND ADAPTATION

• Individuals have natural variations among heritable traits

• Some variants are better suited to life in their environment than others

• Predatory species that are faster, longer claws, sharper teeth catch more prey

• Prey species that are faster, better camouflaged avoid being caught

SOME VARIATIONS ARE FAVORABLE• Adaptation - any heritable

characteristic that increases an organism’s ability to survive and reproduce

• body parts or structures • bear’s claws• body color camouflage or mimicry

• physiological functions• a plant carrying out photosynthesis

• behaviors• avoidance strategies prey use• crane flapping wings

OverproductionOverproduction VariationVariation• Darwin realized that

species tend to produce excess offspring

• limit resources + excess offspring = competition• Not Everyone

Survives!

Differences among members of the same species

Most variation is heritable Siblings look more like

each other than they do other people

DARWIN’SOBSERVATIONS

DARWIN’SOBSERVATIONS

NATURALSELECTIONANDSPECIESFITNESS

• Overtime,naturalselectionresultsinchangesintheinheritedcharacteristicsofapopulation.

• Thesechangesincreaseaspeciesfitness• survivalrate

SUMMARYOFDARWIN’STHEORY

1. Organismsdiffer;variationisinherited2. Organismsproducemoreoffspringthan

survive3. Organismscompeteforresources4. Organismswithadvantagessurvivetopass

thoseadvantagestotheirchildren5. Speciesalivetodayaredescendedwith

modificationsfromcommonancestors

ARTIFICIAL SELECTION • Artificial Selection: selective breeding of

domesticated plants and animals to produce offspring with genetic traits that humans value

• nature provides variation, humans select variations that are useful.

• Example: a farmer breeds only his best livestock

ARTIFICIAL SELECTION

• Humans have been modifying species for thousands of years

• Darwin saw this being done with dogs• He observed that a species could be changed a lot in a

short period of time• Thought that the same thing could be happening in

nature at a slower pace

DIFFERENCES

Natural Selection Artificial Selection

• Traits that are more beneficial in the organisms environment become more common in the population

• Humans choose the traits that become more common in the population

PESTICIDES:NATURAL SELECTION IN ACTION

• Pesticides are used to kill insects in crops and homes• Insects evolve resistance to pesticides over time• High doses and more potent pesticides are needed to

be used • In the 1950’s only small amounts of the pesticide

malathion were used• Today insects are resistant to

very high concentrations of malathion

PESTICIDE RESISTANCE

• Natural selection does not make orgs “better”

• Adaptations don’t have to be perfect – just good enough to enable org to pass its genes to next generation

• doesn’t have to move in fixed direction – no one perfect way of doing something

• If local env changes traits that were once adaptive may no longer be useful and different traits may become adaptive

• If env changes faster than species can adapt = extinct

COMMON DESCENT• Natural selection depends on ability of

orgs to reproduce• Every org alive today is descended

from parents who survived and reproduced

• Living species are descended with modification from common ancestors (descent with modification)

• Implies life has been on Earth for very long time

• Used fossil record for evidence

• Darwin based explanation for diversity of life on idea that species change over time

• Implies that all organisms are related• Common ancestor shared by tigers, panthers,

cheetahs• Common ancestor shared by these felines and

horses, then bats• Farther back all mammals share ancestor

with birds, alligators, fish

• According to the principle of common descent, all species – living and extinct – are descended from common ancestors

EVIDENCE OF EVOLUTION

BIOGEOGRAPHY• Study of where organisms live now vs. where

their ancestors lived in the past• Patterns in the distribution of living and fossil

species tell us how modern orgs evolved from their ancestors

Two patterns: • closely related species differentiate in slightly

different climates• very distantly

related species develop similarities in similar environments

• Pangolins!

PLATES OF THE EARTH

CO

NTIN

ENTA

L DRIFT

CLOSELY RELATED BUT DIFFERENT • Galápagos species evolved from mainland

species• natural selection produced variations that

resulted in different, but closely related, island species

• Ex: variation in shell shape among the giant land tortoises

DISTANTLY RELATED BUT SIMILAR

• similar global habitats, home to species distantly related

• similarities among those animals similar selection pressures had caused different species to develop similar adaptations

• Differences in body structures among those animals provide evidence that they evolved from different ancestors

• Ex: similar ground-dwelling birds (rheas, ostriches, and emus) inhabit similar grasslands in Europe, Australia, and Africa

THE AGE OF EARTH • Evolution takes a long time if such complex life has

evolved, then Earth must be very old • Radioactive dating Earth is about 4.5 billion years

old plenty of time for evolution by natural selection

24 HOUR DAY ANALOGY

COMPARING ANATOMY • Evolution homologous structures adapted to different

purposes• Result of descent w/ modification from common

ancestor• All vertebrate limbs had the same basic bone structure

• Some were used for crawling, some for climbing, some for running, others for flying

• similarities and differences help determine how recently two species shared a common ancestor

HOMOLOGOUS STRUCTURES • Biologists study anatomical details, embryos, and

evolutionary pattern to see if they are homologous• Plants can share homologous stems, roots, and

flowersThese limbs evolved, with modifications, from the

front limbs of a common ancestor whose bones resembled those of an ancient fish.

ANALOGOUS STRUCTURES • Body parts that share common function,

not structure • Clue to common descent is common

structure, not common function• Ex: bird’s wing and a horse’s front limb have

different functions but similar structures (homologous)

• wing of a bee and the wing of bird, same function but different structure (analogous)

RECENT FOSSIL FINDS• Paleontologists have

discovered hundreds of fossils that show stages in the evolution of modern species

• Other recent fossil finds connection between dinosaurs and birds; fish and four-legged land animals

RECENT FOSSIL FINDS

Here is evolution of whales from ancient land mammals

VESTIGIAL STRUCTURES • Inherited from ancestors, but have lost

much or all of their original function due to different environmental pressures

• Presence does not affect organism’s fitness so natural selection does not act to eliminate it

• Ex: hipbones of baleen whales• Ex: legs of three-toed skink• Ex: wings of a flightless cormorant

EMBRYOLOGY • Early developmental stages of vertebrates look

very similar• Same groups of embryonic cells develop in

same order and in similar patterns to produce many homologous tissues and organs in vertebrates

• Further evidence that organisms have descended from a common ancestor

• Evolution offers the most logical explanation for these similarities

LIFE’S COMMON GENETIC CODE

• All living cells use info coded in DNA to carry info from one generation to the next

• DNA is nearly identical in almost all organisms, including bacteria, yeasts, plants, fungi, and animals

HOMOLOGOUS MOLECULES

• Homology not limited to physical structures• Homologous proteins – share extensive

structural and chemical similarities• Example: cytochrome c – cellular

respiration – found in almost all living cells• Homologous genes – makes sense b/c all

animals share same code• Example: Hox genes – determine head-

to-tail axis in embryonic development –found in almost all multicellular animals

• Living organisms evolved through descent with modification from a common ancestor

GENETICS AND MOLECULAR BIOLOGY

• Example: Hemoglobin Differences