Evolution

A History of evolutionary thought :

• Linneaus– Swedish botanist– Founder of taxonomy• System of naming and classifying organisms

– Grouped things into categories that would later be used by Darwin to show relationships

• George Cuvier

– Founded paleontology- the study of fossils

– Understood that extinction was part of life

– Believed in catastrophism- boundaries between layers were due to catastrophes (flood, drought) and ravaged regions were populated by migrating species

• James Hutton

– Geologist– Sought to explain the past by looking at what currently

happens. He said the processes that formed the earth are still working on it and observable today

– Believed in gradualism (opposite of catastrophism)-changes in the earth were due to slow and continuous processes.

• Charles Lyell

– Geologist, friend to Darwin– Theory of Uniformitarianism-geologic processes are so

constant that their rates and effects are balanced over time• Earth must be old to allow for these to take place• Very slow changes that persist over time can produce

substantial changes

• Jean Baptiste de Lamarck

– Published his theory in 1809– He said evolution is driven by an organism’s

innate desire for perfection

• Lamarck’s incorrect ideas:1. The Law of Use and Disuse: • Parts of body used become stronger with use, those

not used will deteriorate– Most famous example he used was giraffes that had to

stretch their necks to feed gradually became long necked.

2. Inheritance of acquired traits-

modifications acquired in the organism’s lifetime would be passed on to its offspring.

• Criticisms of his theory:

– No evidence that acquired traits are passed on– Organisms do not sense their needs and decide

to change

• Thomas Malthus-mathematician

– Studied populations– Showed that resources grow linearly– Populations grow geometrically – Predicted that as the human population increased we

would run out of food and other resources

• Charles Darwin

– Dropped out of medical college– Studied with Sedgewick, a geologist and Henslow, a

botanist– Consuming interest in nature– Signed on as naturalist on H.M.S. Beagle when 22 yrs.

old. The voyage lasted 5 years.

• Galapagos Islands-he found species that existed no where else but were similar to species in South America

• He recorded 13 species of finches-each has different beak. Each is adapted to different food

• He realized species change from gradual accumulation of adaptations to different environments.– A single species of finch was separated by a

channel of water and developed into 2 populations. Each was adapted to its own environment.

Darwin’s idea of Natural selection:• There is an unequal ability of individuals to

survive and reproduce• Variations in traits affect an individual’s ability

to get resources it needs• The environment will select which traits are

the most fit to survive

• Darwin wrote The Origin of Species• Published in 1859

• A contemporary, Alfred Wallace, wrote to Darwin and wanted to publish his own idea of natural selection.

• Darwin came out with his book the next year and received credit for proposing the idea that species change through natural selection as he had more evidence for it.

• Ideas on evolution in Darwin’s book:

– Descent with modifications-all organisms are related to each other by descent from an unknown ancestor in the long ago past

– Over millions of years organisms accumulated adaptations that served to make them fit for a specific way of life

– Evolution explains the diversity and unity in life– Organisms over reproduce (more eggs and sperm are

produced than needed, more offspring made than survive)

Evidence for Evolution

I. Geological evidence from the fossil record– The fossil record shows:

• Prokaryotes are the oldest fossils• There is a chronological order of appearance in the

vertebrates (fish amphibians reptiles birds mammals• Transitional forms link older fossils to newer ones

• Fossil evidence for evolution in horses

II. Biogeographic evidence-geographic distribution of a species– Modern species are where they are because

they evolved from ancestors that inhabited those regions.

– Example:• Sugar gliders and flying squirrels—even though they

look alike and live in similar environments the sugar gliders are related to marsupials and the squirrel is a placental mammal

III. Comparative anatomy-anatomical similarities between species in the same categories 1. Homologous structures-same structures have

different functions. Example forelimbs of mammals

2. Analogous structures-have the same function but are different structures

– Example-bird wing and butterfly wing

3. Vestigial organs-structures of marginal or no use to an organism• Whales lack hind limbs but have vestigial bones

there, human appendix, human ear muscles. • It is not efficient to provide food, oxygen and space

to an organ that no longer has a function

IV. Embryological evidence– Closely related organisms go through similar

stages in their embryonic development• Gills, tails

V. Molecular biology

– The more closely related 2 organisms are the more alike their biochemistry is• Shows that all forms of life are related to some

extent. Even bacteria and humans have some proteins in common.

VI. Artificial selection

– Deliberately selecting plants and animals to breed for specific traits produces changes in the population of that species. Process mimics natural evolution.

Natural selection:

• Darwin’s 5 observations:– Overproduction of offspring– Populations tend to remain stable in size– Environmental resources are limited– Individuals in a population vary in their

characteristics– These characteristics are inherited

• Your fitness is your ability to pass on your genes vs. someone else’s being chosen.

• What effects your fitness?– Your survival– Your longevity– Early maturity*You have to consider the whole organism—an individual

may have a weak trait but overall be more fit to survive than another.

Fitness shows as 3 types of selection:

1. Directional – Shifts frequency curve in one direction or the

other by favoring what was once rare

• Examples of directional selection:a. Pepper moths come in 2 forms-black and

white• Industrial areas had trees thick with soot. This

made the black moth more fit to survive. The country had whiter trees. The white moth was more fit for survival there.

b. Bacterial resistance to antibiotics• Each time penicillin is used

most but not all bacteria are killed. The survivors have a genetic resistance to penicillin that is passed on to their offspring. This creates new strains that we cannot fight.

c. Insect resistance to pesticides (same idea as bacterial resistance)

2. Stabilizing – Acts against extreme phenotypes, favors the

most intermediate individual– reduces variation

3. Disruptive

– Environmental condition vary so that both extremes are favored

Key ideas about Natural Selection • It cannot produce perfection. You can only alter

existing structures.• Adaptations are often compromises (seals move on

land and water but aren’t best adapted for either)• There is an element of chance. When the wind

blows seeds to an island it doesn’t just select the best seeds

Some types of adaptations

1. Protective coloration a. camouflage

2. Aposematic colorationbright colors serve as a warningexample-poison arrow frogsPredators tend to be more cautious when dealing with bright colors

3. Mimicry-one species bears a resemblance to another species

a. Batesian mimicry-a harmless or palatable species mimics a harmful or unpalatableexample-monarch and viceroy butterflies

• In order to work, models (actual poisonous members) must outnumber the mimics

b. Mullerian mimicry-harmful species mimics another harmful speciesexamples: insects with yellow and black stripes

large numbers send the message that this pattern is bad and to avoid anything with it

4. Deceptive coloration-fake eyes, false heads

5. Symbiotic adaptations-two organisms living together and/or evolving together

a. Commensalism- (+,0) One benefits and one is unaffected. Example-shark and remora. Shark is not helped or hurt. The remora gets a free ride and left

over food

b. Mutualism- (+,+)– Relationship is beneficial to both– Examples-cleaning animals

c. Parasitism (+, -)– One benefits and the other (host) is harmed– Endoparasite live inside hostlike a tapeworm– Ectoparasites live outside host, like a tick

Causes of Population Diversity

• Microevolution-small changes that occur from generation to generation1. Natural selection-survival of the fittest• The most favorable traits get passed on

2. Mutations-create new alleles• They are passed on if they occur in the gametes

3. Mating• May or may not be random• Nonrandom mating causes evolution by shifting

genotype frequencies

4. Gene flow-process of gaining or losing alleles in a population due to migration • Gene pool-all the genes available in a population

Gene flow

5. Genetic Drift:

• A change in allele frequency in the gene pool due to chance events. Two situations lead to genetic drift:a. Founder effect-a few individuals leave to colonize an

isolated place. The smaller the sample, the less the colonists will represent the gene pool of the population they left.

Founder effect

b. Bottleneck effect-disasters (earthquakes, floods, fires) reduce the population– Who survives is random– Final population does not resemble the original

Hardy-Weinberg theory

• The frequencies of alleles and genotypes in a population remain constant from generation to generation (provided that only Mendelian genetics are at work)

Conditions for Hardy-Weinberg to work:1. Must have a large population size2. They must be isolated from other populations (no

migration)3. No net mutations4. Have random mating5. No natural selection is occurring

The theory tells what to expect when a population is not evolving.

• Example-wildflowers– Population of 500 plants

• 480 are pink 20 are white AA or Aa aa

If we know that 320 plants are AA then 160 are Aa“A” makes up a total of 800 alleles or a frequency of .8“a” makes up 200 alleles or a frequency of .2

• AA = .8 x .8 = .64• Aa = (.8 x .2) + (.2 x .8) = .32• aa = .2 x .2 = .04

– These conditions are rarely met in nature. – Violation of the above criteria will change the

allele frequencies, therefore evolution is occurring.

H-W equation

• Variables: from example:p = dominant allele A = .8q = recessive allele a = .2

P2 = AA genotype .8 x .8q2 = aa genotype .2 x .22pq = Aa or aA (.8 x .2) + (.2 x .8)

• Put it all together:

p2 + 2pq + q2 = 1 .64 + .32 + .04 = 1

p + q = 1 .8 + .2 = 1

Speciation

• Definitions-– The word species is Latin for “kind”-it is a distinct

group of organisms capable of interbreeding– Speciation-evolution of a new species– Hybridization-interbreeding between species,

serves to break up gene combinations

• Reproductive barriers between species:

1. Prezygotic-prevent fertilization from occurringa. Habitat isolation-ecological-2 groups inhabit the

same area but breed in different parts. Wood frogs breed in temporary water, bull frogs in larger permanent water

• b. Temporal isolation-reproduce at different time of day, season or year. 2 species of sage in California, one blooms in spring, one in fall

c. Behavioral isolation-courtship behaviors-different species have different signals-auditory, visual, chemical

d. Mechanical isolation-anatomical differences-genital organs may not matche. Gamete isolation-mating may occur but gametes don’t join due to differences.

Consider the different types of fish that release egg and sperm into water simultaneously.

• Post-zygotic-prevents development of a fertile offspring even if fertilization occurs

a. Hybrid inviability-genes from parents of different species do not interact properly for normal development

b. Hybrid sterility-if an interspecies offspring develops successfully then it cannot breed (mule)

c. Hybrid breakdown-rare case of interspecies offspring that mate but F2 generation will have multiple defects

• If no barriers form between them, then mating may occur creating a new species

What mechanisms occur to create a new species?

1. Allopatric speciation-one species is separated by a physical barrier into 2 or more isolated populations.

2. Sympatric speciation-a new species appears within the same geographic area where the original is found.

• Most often occurs by polyploidy where the offspring gets extra sets of chromosomes (3n or 4n)

3. Adaptive radiation

• Evolution of many diverse species from a common ancestor when introduced to a new environment

The speed of speciation-2 ideas:

1. Gradualism-evolution of a new species is gradual and continuous over long spans of time.

2. Punctuated equilibrium-speciation occurs after a long period of no change (stasis) followed by rapid bursts of evolution

Phylogenic relationships

Phylogeny-evolutionary history of a species

Phylogenic tree- a diagram that shows probable evolutionary relationships between groups

Coevolution:

A change in one species causes a change in another species

Examples: cuckoo and warbler (most symbiotic relationships)

Convergent evolution:

two distantly or unrelated organisms come to resemble each other, usually resulting from similar environments