Evolution (chapter 5). Evolution “Nothing in biology makes sense, except in the light of evolution”…… –Theodosius Dobzhansky (1973) In the U.S. alone,

Evolution

(chapter 5)

Evolution• “Nothing in biology makes sense, except in the

light of evolution”……– Theodosius Dobzhansky (1973)

• In the U.S. alone, 25 - 40% of Americans either deny the theory of evolution of think that evidence for it is lacking.

• Main reason for denying this theory • Religious Beliefs

• Charles Darwin (1809 – 1882) - “on the origin of species by means of natural selection.”

• The work of Darwin is about adaptation of a species to the environment in which it exists.

• It does NOT explain the history of the universe or how the Earth formed.

Evolution

Evolution• Charles Darwin (1809 – 1882) - “on the origin of species

by means of natural selection.”

• Had two main hypotheses.– Branching descent – living species come from a species that lived

in earlier times. This explains common inheritance.

– Natural selection – explains that parents with genotypes that favor survival and reproduction leave more offspring than other parents. Therefore, these genetic traits become dominant in a given population.

• Both of these hypotheses have been tested thousands of times without being falsified.

Evolution explains the origin of a species

NOTThe origins of life

• How is the origin of species different from the origins of life?

Evolution

Origin of Life

• The Primitive Earth.– Theorized early primitive atmosphere consisted mostly

of:• water vapor, nitrogen, and carbon dioxide, with small

amounts of hydrogen and carbon monoxide.

• Little, if any, free oxygen

• At first the earth was very hot• Water existed as a gas

Origin of Life

Origin of Life

Origin of Life

Origin of Life

• Small Organic Molecules.– Primitive gases may have reacted with one another

and produced small organic compounds such as nucleotides and amino acids.

• Macromolecules.– RNA-first hypothesis.– Protein-first hypothesis.

RNA-first Hypothesis

• Only RNA is needed to progress towards the first cells

• RNA is both a substrate and an enzyme (ribozymes)– First cell had RNA genes (mRNA) – make proteins

• Some of these newly formed proteins were enzymes• Reverse transcriptase (found in retrovirus)

• Can make DNA from RNA

• DNA self replication can then occur

Protein-first hypothesis• Amino acids join together under dry heat

– Such as along shallow puddles on a rocky shore.– Heat from sun formed proteinoids (enzymes)

• These then could form DNA from nucleotides present in the ocean.

• This DNA can code for specific protein synthesis– Thus the cell could acquire of its enzymes, even the

ones which replicates DNA

Origin of Life

• Protocell.– A protocell, which could carry on metabolism but

not reproduce, may have formed when lipids and microspheres formed a lipid-protein membrane.

• A True Cell.– A true cell can reproduce.

• Modern cells replicate before cell division occurs.

Biological Evolution

• First true cells were prokaryotic.

– Eukaryotic cells evolved later, followed by the other kingdoms.

• Biological evolution is a change in life forms that has taken place in the past and will take place in the future.

–Adaptation is a characteristic that makes an organism able to survive and reproduce in an environment.

Evolution, based on the hypotheses of Darwin, has nothing to do with the

theory of the creation of the Earth.

Evolution• Charles Darwin (1809 – 1882) - “on the origin of species

by means of natural selection.”

• Had two main hypotheses.– Branching descent – living species come from a species that lived

in earlier times. This explains common inheritance.

– Natural selection – explains that parents with genotypes that favor survival and reproduction leave more offspring than other parents. Therefore, these genetic traits become dominant in a given population.

• Both of these hypotheses have been tested thousands of times without being falsified.

Galapagos Islands• On the Galapagos Islands

Galapagos Islands Darwin found a very limited assortment of animals

• No native mammals or amphibians were present.

• Found several species of large tortoises, a species of crab-eating lizards, and land birds

• These birds were made up of a dozen of closely related species

Galapagos Islands• Each cluster of these birds lived

on one or only a few of these islands.

• The tortoises also differed from island to island – even though the climate between islands was very similar.

• Hypothesized that each species cluster had arisen through a series of modifications from a single species that had originally colonized the islands.

Figure 5.2Galapagos IslandsDarwin concluded that the Galapagos had received its animal colonized from South America – the nearest continent from the islands.

Determined that geographical proximity is more important than climate or other environmental variables in influencing which species occurred in a particular place.

Natural Selection• Variation in population

• Variation inheritable

• Some individuals survive and reproduce better than others

• Survival and reproduction are tied to variation in traits among individuals (non-random)

• Therefore, these genetic traits become dominant in a given population.

• One species of organism deceptively resembles another.– The Viceroy is actually closely

related to butterfly A

• The Monarch is poisonous to Blue jays

• The Viceroy isn’t poisonous, but has the same colorings as the Monarch – so Blue jays do not eat the Viceroys.

Mimicry

Industrial Melanism• Powerful demonstration of

natural selection.

• Darker colored peppered moths were discovered in areas of the UK with high levels of pollution.– Known as melanic moths

• First found in 1890s at the height of the industrial revolution.

Remember genetic mutation?• Either a single nucleotide

substitution or an addition or a deletion in the genetic material controlling moth skin color occurred– Altered the open reading frame– May be caused by the pollution

itself

• The result was an ever increasing population of black peppered moths in certain areas.

Central Dogma of Molecular Biology

• DNA holds the code

• DNA makes RNA

• RNA makes Protein

• If pollution altered the genetic material of peppered moths, then the protein product (skin color) would be altered!

Industrial Melanism• Pollution blackened the trees and

killed the lichens the peppered moths feed on.– And affected the moths

– Lichen: symbiotic associations of a fungus with a photosynthetic partner.

• Moths were camouflaged to predators and population increased in polluted areas.

• As I said – A powerful demonstration of natural selection.

Industrial Melanism• Laws to control smokestack

emissions and other forms of pollutants are now being enforced.

• The lichens have returned to the previously polluted trees.

• Moths in these areas have returned to their original colorings – Again, natural selection

Figure 5.1The pattern of branching

descent.Species in the top row are descended from the ancestors below.

Red circle – the common ancestor to all other circles.

Red square - the common ancestor to all other squares.

Red hexagonal – ancestral to ALL species

In a classification – all squares in one group and circles in another

Branching Descent• Darwin explained this hierarchy as the natural result of

branching descent with modification.

• A process that produces the similarities and differences that biologists have used in classifying organisms.

Evidence for Branching Descent• Homologies: The construction of

family trees in based on shared similarities and gene sequences.

• Darwin noted the similarities of forearms in mammals.

• Darwin wondered on why mammals have the same bones in the same relative positions.

• All had a ratio of bones from five (fingers) to two (lower “arm” –radius and ulna) to one (upper “arm” -humerus).

Evidence for Branching Descent• Convergence – Similarities that

result from common ancestry should also be similar at a smaller level of detail.– Should grow from the same source tissue

• Convergence is an evolutionary phenomenon in which similar adaptations evolve independently in lineages not closely related.

• Bat wings are similar in structure to whale flippers, the human arm and the front legs of all mammals.

Evidence for Branching Descent• One frequent test of the hypothesis

of branching descent is to identify a group of organisms that share some particular character.

• Structures arose from modification of the same type of repeated part.

• Crustaceans mouthparts and legs are derived from a common set of leg-like appendages.– Vestigial structures – Structures

whose function have been lost in the course of evolution diminish in size.

– Ie – coccyx – human tail bone

Evidence for Branching Descent• Cephalopods - group of

mollusks that have beak-like jaws at the front of the mouth and a muscular part (foot) subdivided into a series of tentacles.

• When these feel threatened they release “ink” – hides the animal and propels it backwards – allows it to escape.

• All members of this group have the same escape mechanism – Therefore, same characteristics.

Post - Darwin• Steven Jay Gould : • Punctuated Equilibrium – Species remain static for long

periods of time and then changes abruptly.

• The new species begins as a small isolated population on the geographic periphery of the original species.

• The small size of the isolated population allows it to undergo rapid change

– Therefore producing a new species

• Once new species becomes successful, overcomes original species and the original species becomes extinct.

Hutchinson JR, et al. (2011) From flat foot to fat foot: Structure, ontogeny, function and evolution of elephant "sixth toes." Science, 334, 1699-1703

• This CT scan reveals the sixth "toe" - here seen in dark green towards the back on the elephant's foot

• For more than 300 years, the structure has puzzled researchers, but this study suggests that it helps to support

elephants' colossal weight.

• The structure was made of bone, although bone with a highly irregular and unusual arrangement.

• Closer examination also revealed that it showed a strong similarity with an unusual bone that is found in the front feet of

pandas.

Hutchinson JR, et al. (2011) From flat foot to fat foot: Structure, ontogeny, function and evolution of elephant "sixth toes." Science, 334, 1699-1703

• This bone - which is not quite an extra digit, but does the job of one - helps the panda to grip bamboo, and is called the panda's "thumb" or "sixth finger".

• Moles too have a bone masquerading as an extra digit, which helps them to dig.

• For elephants, the structure serves a simple purpose: it helps the hefty animals to stand up.

• Unlike pandas and moles, which only have the false digit in their front feet, elephants have the bone in all four of their feet.

Hutchinson JR, et al. (2011) From flat foot to fat foot: Structure, ontogeny, function and evolution of elephant "sixth toes." Science, 334, 1699-1703

• Their five conventional toes point forwards, giving elephants a tip-toed stance-

• BUT

– the extra "toe" points backwards into the heel pad, adding extra support and helping the heaviest land animal to hold up its bulk.

• To find out when and why this strange bone appeared, the researchers examined elephant fossils

• The first elephants appeared around 55 million years ago

Hutchinson JR, et al. (2011) From flat foot to fat foot: Structure, ontogeny, function and evolution of elephant "sixth toes." Science, 334, 1699-1703

• Early elephants had a different kind of foot, which seemed to be quite flat footed and didn't leave much room for this structure underneath.

• The structure seemed to evolved around 40 million years ago, and it seems to have evolved in concert with elephants getting bigger and more terrestrial and having upright feet, with a more tip-toed foot posture

• great example of how evolution tinkers and tweaks tissue to provide different functions - in this case to be co-opted to be used like a digit

Evolutionary diagram showing how elephant foot form

and posture evolved (copyright Julia Molnar, 2011)

• Underlying the diversity of life is a striking unity, especially at the lower levels of structure.

• Evolution accounts for this combination of unity and diversity– REMEMBER the universal

genetic language of DNA

Unifying theme in biology

Current definition:

Change in genetic composition of a population over time

Ancestral population

Divergent populations

Time

Envt

Fitness

Natural selection

Natural selection

Thousands to millions of years of natural selection

Ancestral canine

e

Natural environment “chooses” animals that reproduce

Artificial selectionBreeder chooses animals that will reproduce

Hundreds to thousands of years of selective breeding

Ancestral dog

Examples of natural selection and evolution in actiontoday?

Tuberculosis andAntibiotic resistance

Natural selection

Using Evolution to understand HIV AIDS

• Why do drug treatments not work in the long run?

• Why are some people resistant to HIV?

• Where did HIV come from?

• Will there be an AIDS vaccine?

• High mutation rate• High reproductive rate• Mutants that avoid AZT replicate most• Natural selection

= Variation

Why are some AIDS drugs ineffective now?

Why are some people resistant to HIV?

• Mutant forms of molecule that assists the virus entry into cells

• Primarily in European populations; none in Asian or African populations

HIV phylogeny

•Examine similarity in DNA sequence

•HIV was transmitted from chimps to humans 3 different times

Where did AIDS come from?

Why is it so difficult to design an AIDS vaccine?

• Natural selection– Mutation– Variation– Selection pressures

• Transmission from other hosts

Application of principles of natural selection

creationism • Bible-based creationists

– Use a biblical account of creation– Reject any scientific account that conflicts with scripture

• Intelligent-design creationists– Work within the framework of science to find evidence of

design in nature– State that biological systems are so complex that only an

intelligent (and benevolent) designer could have made them.

• Theistic evolutionists– Believe that God created the universe and all life– Evolution is in fact Gods creative process and part of the plan.

Misconceptions• Scientists disagree about whether evolution has

occurred

Scientists argue about HOW evolution occurs but they agree that evolution does occur.

MisconceptionsEvolution as a theory is incomplete or flawed.

Evolution is the only well-supported explanation for the diversity of life and there have been no credible challenges to evolution.

Modifications to some of Darwin’s original ideas have changed e.g., evolution occurs faster, genetic mutations are sources of variation

Misconceptions• If you accept evolution, you cannot be a religious

person

•Evolution and religion are not incompatible.

•Most Christian and Jewish religions have no problem with evolution.

•Many religions believe that a better understanding of the natural world enriches one’s faith.

•Not all scientists are atheists. I’m cool with

Theistic Evolution

Summary• Must understand the nature of science to understand

evolution.

• Evolution does occur and is observable.

• Natural selection can happen over short time periods.

• Study of evolution is important and applicable to medicine and many other disciplines.

Interested?

• Kenneth Miller. 1999. Finding Darwin’s God: a Scientist’s Search for Common Ground between God and Evolution

• Pennock, Tower of Babel (intel design)

The End.

Any Questions?