Natural SelectionThe Mechanism of Evolution

Natural Selection

• Darwin based his theory of natural selection on 3 sets of observations:• The Struggle for Existence• Natural variations within the same species• Environment’s role in evolution

1. The Struggle for Existence

• Drawing from Malthus’s ideas, Darwin recognized that all species produce excessive # of offspring.

• But due to limited resources in nature, only a small percentage of offspring survive in each generation

• Carrying capacity (K) of a population: max. # of individuals that a particular habitat can support

Carrying Capacity (K)

• Availability of resources (water, food, sunlight, shelter, space, oxygen)

• Build up of waste such as excrement or carbon dioxide

• Predation

• Disease

Do you think the Earth has a carrying capacity? What do you think this means?

2. Variation

• Variation refers to differences among the members of the same species.

• Individual variation is widespread in all species.

• Much of this is heritable i.e. it passes from one generation to the next

• Effects how successful an organism is.

Where is the variation coming from?

1. Sexual Reproduction

• Crossing over leads to genetic recombination or “gene mixing”

• Daughter cells are different from the original cell

• Fertilization: male gamete contributes ½ of the chromosomes and the female gamete contributes the other ½.

Where else…

2. Mutations

• may produce genes that lead to certain diseases (e.g: phenylketonuria; PKU)

• may lead to more advantageous traits

• Only few genes mutate and most are neither useful nor harmful

3. The Role of the Environment

• Darwin observed that a key factor in the survival of an organism was how well it was suited for the environment.

• The environment selected those individuals with variations that were best suited for that environment.

• E.g. Darwin would’ve said that in a population, the environment favored the individuals born with longer necks so they survived and reproduced and passed their characteristics to the next generation

Ecological Niche

• An organism’s ecological niche select whether or not it will survive and reproduce.

• An Ecological Niche is the sum total of a species’ use of the biotic and abiotic factors in its environment.

• Biotic Factors: Predators or prey

• Abiotic Factors: Water availability, weather, etc.

So…• Natural Selection is the process

by which individuals with inherited characteristics well suited for the environment leave more offspring on average than individuals with adaptations less suited for the environment

• When this happens over many generations, each new generation has a higher proportion of individuals with advantageous traits

• This would cause a population to change over time

Simulation!!

• Activity and discussion

• Darwin based his theory of natural selection on 3 sets of observations:• The Struggle for Existence• Natural variations within the same species• Environment’s role in evolution

Changes in Beak Shape

• Peter and Rosemary Grant (Princeton University) studied Daphne Major Island finches for 30 years

• They found that average beak and body size of the medium ground finch changes as El Niños come and go

• El Niño is a warming of the ocean current that passes along the coasts of Peru and Ecuador and affects global weather patterns

Changes in Beak Shape

• The medium ground finch prefers smaller seeds and due to their abundance in wet years, eat few large seeds

• During dry years due to El Niños, all seeds are in short supply (due to fewer plants) and large seeds are the bulk of the diet

• Only birds with large and tough beaks survive these years.

• With the return of the wet years, the average beak size diverges again

Antibiotic Resistance in Bacteria

Antibiotics: medication used to kill or inhibit the growth of bacteria

Steps leading to antibiotic resistance:

1. A person is infected with a bacterial infection2. Antibiotics are prescribed to kill the bacteria3. The infection is largely destroyed and the patient gets better4. One bacterium survives because of a random mutation 5. The bacterium begins to multiply, once again making the patient sick again6. The patient returns to the doctor and receives the same antibiotic7. The patient remains sick8. The doctor prescribes a different antibiotic

Resistance

1. Mutation

2. Plasmid Transfer

• The donation of genetic information to another in a ring of nucleotides (plasmid).

Antibiotic Resistance in Bacteria

• Tuberculosis (TB) is severe bacterial chest infection caused by Myobacterium tuberculosis.

• In 1940s, Streptomycin was developed to fight it and by 1970s it was almost wiped out

• Resurgence in 2006 in South Africa and by 2008 had spread to 49 countries. In 2009 1/3 of world population had TB. 5% new infections were resistant to antibiotics

• Why?

Antibiotic Resistance in Bacteria

• Bacteria have many variations. Some can be destroyed by antibiotics, others are resistant.

• TB is treated by a 6-9 month course of 4 different antibiotics. Why?

• Patients may stop the treatment prematurely and not take all the antibiotics needed to kill all bacteria.

Survival of the Fittest?• Fitness of an individual

is its ability to survive and reproduce in its specific environment.

• Natural selection does not make organisms “better”. It doesn’t have a goal.

• If the environment changes in some way, another adaptation might be favored.

Evidence for Evolution• Evolution has left its

marks on all aspects of life:• The Fossil Record• Geographical

distribution• Anatomy• Development of

species• Molecular Biology

The Fossil Record• Most fossils are sound in

sedimentary rocks where younger layers of rock, or strata, are deposited on top of older ones

• Fossil Record is the chronological collection of life’s remains in the rock layers recorded over time

• Paleontologists are scientists who study fossils

• Oldest fossils (Prokaryotes) were found in rocks 3.5 Billion years old

The Fossil Record: Whale Evolution

• Fossils of aquatic fauna are usually the best preserved

• Fossil evidence supports the hypothesis that whales evolved from land-dwelling ancestors with four limbs

The Fossil Record:Origins and Extinctions

• Fossil evidence also provides evidence of the rise and fall of some species.

• Dinosaur-like animals first appeared 230 Million years ago and went extinct some 65 Million years ago [Video]

• Drawback: Species without hard tissue (like bones or shells) rarely become fossilized and so its little help in establishing the evolutionary history of those organisms.

Geographic Distribution: Closely Related but Different

• Darwin’s study of finches showed that they were all descendents of the single ancestral species but today all finch species in the Galapagos has a beak adapted to eating a specific type of food

• Same for the tortoise species that live on those islands

Geographic Distribution: Distantly Related but Similar

• Similar environments will select for similar adaptations

• E.g. the flightless birds of grasslands look similar to each other but their body structure indicated that they descended from different types of birds

Geographical Distribution: Marsupials vs Placental

Mammals• Marsupials: Their fetus leave

the uterus early and completes further development in a pouched

• Placental mammals: The fetus is protected by a placenta and develops inside the uterus

• The reason Australia has so many marsupials is because they all evolved from marsupial ancestors on an island isolated from placental mammals in the distant past

Comparative Anatomy: Homologous Structures

• Similar structures in species that share a common ancestor are called homologous structures.

• The structures that originally functioned in one way in an ancestral species are now modified for new functions.

• E.g. limbs of various mammals

Comparative Anatomy: Vestigial Structures

• Vestigial Structures are remnants of structures that may have had important functions in an ancestral species but have no clear function in some modern descendents. E.g.• Whales have vestigial hip

bones• Humans have appendices

Comparative Anatomy:Analogous Structures

• Analogous structures are structures, in distantly related species, that are anatomically different but serve the same function. E.g. wings of different kinds

• The evolutionary idea of descent is based on structure not function

• Convergent evolution

Comparative Development

• Comparing the various stages of development of various organisms shows embryos of closely related species with similar stages in development.

• E.g. Vertebrate embryos are all show pharyngeal gill pouches, even in animals with no gills.

Molecular Biology• DNA provides a record of an

organism’s ancestry.

• The greater the number of differences in DNA, the less likely it is that they share a close common ancestry. E.g. siblings have very similar DNA

• Cytochrome c sequence in the mitochondrial DNA is found in all oxygen dependent organisms.

• This sequence is highly conserved and the amount of difference in sequence is used to build phylogenetic trees.

Artificial Selection• Artificial Selection is the

selective breeding of domesticated animals to produce offspring with genetic traits that humans value.

• Breeders breeding allow only those plants or animals with useful traits to reproduce.

• Artificial selection can produce a great deal of change in a species in a short time.

Artificial Selection• Farmers breed cows that excel

in milk production

• Not a random process because the farmers leave nothing to chance.

• In selective breeding, microevolution can occur at a very fast pace

• BUT also reduce the genetic variability within the population leading to genetic disorders

• E.g. Pedigree dogs