Biology evolution class 4-2-14 CH22 Darwin.pptx

Darwin’s Theory of Evolution

The Influence of Darwin’s Contemporaries

• Gradualism & geology (Hutton) – changes in the Earth can take effect through the cumulative effect of slow processes

• Uniformitarianism – (Lyell) the same geologic processes that exist today, existed in the past and occur at the same rate as in the past

Georges Cuvier & Paleontology• Cuvier (1769-1832) began the study

of fossils in strata (layers) of rock• Observations:

The older the rock layer the greater the variation of the fossils from contemporary animals

From one layer to the next certain species disappeared and new species emerged

• Conclusion: extinctions are a common part of life

Influence of Thomas Malthus • Malthus was an economic scholar

who wrote a paper titled The Principle of Population (1798)

• He stated that individuals in a population produce many offspring, but only a few survive the rest are eaten, diseased, frozen, starved, unmated or infertile

• He reasoned that this was beneficial for the stability and success of a population

Darwin’s Trip on the Beagle

• 1831- 1836 trip on the Beagle • Argentina – Darwin’s first significant

discovery in a region rich in fossils called Punta Alta

Darwin found fossils of 9 mammals unknown or little known to science

4 species of giant sloths; Megatherium was found to be the size of an elephant

Also found an extinct horse, giant rodent and the shell of a giant Armadillo

6 foot+ armadillo found in Florida; lived about 10,000 to 11,000 years ago

Eremotherium skeleton , National Museum ofNatural History,Washington, D.C.

Giant Sloth

• Several other fossils were also found in Argentina

• Importance of fossil findings: Darwin noticed that the plants and animals had developed adaptations and were similar to known species, but not identical

• Darwin also noticed that the living species he encountered resembled other species on the continent more closely than in similar climates on different continents

Galapagos Islands• The islands are an archipelago of

volcanic islands and are geologically young

• Importance: when volcanic islands form they are initially devoid of life

• Islands become colonized as organisms make their way from the nearest mainland to the island

What Darwin Learned on the Islands• He found species that didn’t exist

anywhere else; he reasoned that the animals became geographically isolated on the islands and change

• Darwin also discovered that each species changed from island to island; Darwin reasoned that this resulted from adaptation of each species to a given island’s characteristics

Above: GalapagosMockingbird

Right: San Cristobalmockingbird

Above: EspanolaMockingbird

The finches of the Galapagos Islands diversified after an initial colonization from the mainland to exploit different food sources on different islands.

Formulation of Darwin’s Theory• Observations from HMS Beagle trip• Question: Could a new species arise

from an ancestral form?• Hypothesis: Organisms that stray to

new habitats diversify and change• Experiments: not traditional

experiments; examined specimens and made inferences

Logic of Darwin’s Theory (as stated

by Ernst Mayr)

• Observation 1: For any species, population sizes would increase exponentially if all individuals that are born reproduced successfully

• Observation 2: Populations tend to remain stable in size

• Observation 3: Resources are limited• Inference 1: Overproduction of

individuals leads to competition for resources; only a fraction survive

• Observation 4: Members of a population vary; no 2 are exactly alike

• Observation 5: The variation is heritable

• Inference 2: Survival favors individuals who inherit traits giving them higher fitness and greater reproductive success

• Inference 3: The unequal ability of individuals to survive and reproduce leads to a gradual change in the population, with favorable characteristics accumulating over generations

Two Points of Darwin’s Theory1. Descent with modification (Darwin’s

term for evolution) – new species descend from ancestral species by the accumulation of modifications as POPULATIONS adapt to new environments

2. Natural Selection (mechanism of evolution) – individuals that are best suited for survival, live to reproduce and pass on their genes; DIFFERENTIAL REPRODUCTIVE SUCCESS

Population Variation

Six Evidences for Evolution1.Artificial Selection2.Anatomical Homologies3.Fossil Record4.Biogeography5.Embryological Homologies6.Molecular Homologies

Evidence #1: Artificial Selection• Artificial selection is the selective

breeding of plants, livestock and pets; has been done for generations

• Darwin set up a pigeon breeding loft at his home and began breeding pigeons in order to study the variation that existed in the pigeon population

Pigeon BreedingPigeon breeding was a common hobby in Darwin’s time with individuals seeking new and unique varieties

Pigeons breeders developed around 200 different varieties of pigeons from the rock pigeon

Artificial Selection in Plants

Darwin’s conclusion based on artificial selection:

Darwin reasoned that if humans could so dramatically alter a species in a few generations then it seemed logical that nature could dramatically alter a species over hundreds or thousands of generations

Evidence #2: Anatomical Homologies• Definition – similarities in structural

characteristics result from two organisms sharing a common ancestor

• Reason for the observed differences in the structural features: altered features are not novel, but are modifications and adaptations of a previously existing structure

Two Types of Anatomical Homologies

1.Homologous Structures – structural features in organisms that are a variation based on a common structural theme

2.Vestigial organs or structures – structures present in an animal that serve no functional purpose, but exist as a remnant from an ancestor

Example: appendix in humans or pelvis in snakes

Vestigial Organs• Example: snakes – have remnants

of legs and a pelvic girdle

Python Fossil With Pelvic Girdle and Working Legs

Arrector Pili Muscles

“Goose Bumps”

Homologous Structures

Evidence #5: Embryological Homologies

• Related organisms share certain embryological similarities during development that serve different functions in the adults

•H

aeckl’s

Em

bry

os

http://www.millerandlevine.com/km/evol/embryos/Haeckel.html

Evidence #6 Molecular Homologies• Genetic code is universal among

organisms• All living organisms share a lot of

similarities in their DNAHumans and chimps – there is only a 1%

difference in their genetic make-upHumans and bacteria actually have

many genes in commonHumans and bananas share 50% of their

DNA

Specific Types of Molecular Homologies

1.Atavisms

2.Pseudogenes

Atavisms• Atavisms – anatomical structures

that sometimes appear in an organism, but serve no functional purpose

• Example: humans still contain the same genes that mice use to produce a tail, but only occasionally are these genes expressed resulting in a baby born with a small, nonfunctional tail

Pseudogenes• Pseudogenes are genes that are

permanently silenced in the genome• Humans have 2,000 pseudogenes out of

our 20,000 – 30,000 genes• Example: Gene to make Vitamin C

Humans contain the gene that is used to make Vitamin C from glucose, but it is inactivated by a mutation

The exact same gene and mutation is also found in primates

Other mammals also contain the nonmutated version of the gene and produce Vitamin C

Where Embryological and Molecular Homologies

Overlap

HOX genes

Hox Genes

• Hox genes are a family of segmentation genes that code for transcription factor proteins that regulate embryonic development

• These genes are conserved across most species and contain a a 180 nucleotide sequence called the homeobox

Hox Genes Function• When Hox genes are translated

the homeobox sequence forms a region of the protein called the homeodomain

• The homeodomain allows the protein to bind to the DNA and initiate transcription of genes involved in embryonic development

Conservation ofHOX Genes

Why is the fact that HOX genes are found in many

species important to evolution?

Answer: It demonstrates that it is NOT the appearance of new genes that results in

increasing complexity, but it is the result of the duplication

of existing genes

How do these genes become duplicated?

Answer: Errors during crossing over in Prophase I of meiosis

Chromosomal Abnormalities 1. Deletion – only a portion or

fragment of the chromosome is lost

2. Duplication – a fragment deleted from one chromosome is attached to another chromosome resulting in extra copies of that chromosomal segment

Question: Do we see evidence that natural

selection is still occurring?

Evolution of Drug Resistant HIV

Examples of Natural Selection

Important Points about Natural Selection

1. Occurs through interactions between individual organisms & their environment; individuals DO NOT themselves evolve; populations evolve

2. Only amplifies or diminishes current variations in alleles; acquired characteristics do not get passed on to offspring

3. It is situational; environmental factors vary and may favor different adaptations at different times. A trait that is favorable in one situation may be detrimental in another situation

Descent with Modification