Evolution by way of Natural Selection

Introduction…

• Charles Darwin developed the Theory of Natural Selection by observing the natural world.

• He also took ideas other scientists had developed and used them to support his initial hypothesis.

Scientists whose work helped Darwin

• Malthus

• An English economist in the 1700s.

• He wrote a book in which he predicted that there would be a struggle for survival due to limited resources in the environment.

• Malthus was looking at human populations, but Darwin took the idea and applied it to all living things.

History of Evolution

• James Hutton & Charles Lyell • Hutton - rocks are in layers, which form

very slowly; proposed that the Earth is millions of years old.

• Lyell - geological features were formed by

processes that still occur today.

~ mountain building, volcanoes, erosion, etc.

Jean-Baptiste Lamarck

• He proposed that by selective use/disuse of organs, organisms acquired or lost certain traits during their lifetime.

• These traits could then be passed on to their offspring; which, over time, led to change in a species.

Lamarck’s Key Ideas

• 1.) Tendency toward perfection. • He believed that all organisms try to become

perfect by acquiring traits that help them live more successfully.

• Ex: In his view, the ancestors of birds had an urge to fly. Over many generations, they kept trying to fly, and their wings increased in size and eventually became good enough to allow flight.

Lamarck’s Key Ideas

• 2.) Use and Disuse • Organisms can change the size and shape of

parts of their bodies by using them in different ways.

• Ex: By trying to use their front limbs for flying, birds eventually were able to change those limbs into wings.

• The reverse is also true, according to Lamarck

Lamarck’s Key Ideas

• 3.) Inheritance of Acquired Traits • If an animal can change its body during its lifetime, its

offspring will also have these changes.

• If an animal was able to change its body to get a longer neck (say, by stretching it), its offspring will also have longer necks.

• However, by this reasoning, if you were to chop off your arm, your children should be born missing an arm because they should inherit this trait.

Evaluating Lamarck

• He was incorrect in many ways.

• He did not know how traits were inherited.

• He did not know that an organism’s behavior has no effect on its inheritable characteristics.

• But, he was one of the first to develop a scientific hypothesis for evolution and realize that organisms are adapted to their environments.

Darwin and Natural Selection

Who was Darwin?

• Charles Darwin

• Naturalist

• Scientist

• Wanted to be a priest

• Quit medical school to study the Bible and become a priest.

The Voyage…

• 1831

• 5 year trip on the HMS Beagle around the world to study life forms.

• Stopped in Brazil, Peru, Ecuador, the Galapagos Islands, New Zealand, Australia

His Observations

• Patterns of diversity.

• He saw similar ecosystems with very different looking animals.

• Ex: Australia and Argentina have very similar grasslands, but the animals in each are very different.

• No rabbits in Australia; no kangaroos in Argentina.

His Book

• On the Origin of Species • 1859

• Summarized the results of his voyage and later studies.

• Proposed a mechanism for evolution called natural selection

• Presented evidence demonstrating that evolution has been taking place for millions of years, and continues today in all living things.

Controversy

• His book caused an immediate sensation (think: The Da Vinci Code of its time.)

• Many people argued about his statements.

• Some people thought he was brilliant; others thought he was an idiot/heretic.

• But, WHAT did Darwin actually say?

Darwin’s Big Idea

Natural Selection!!!!

What is Natural Selection?

• It is a mechanism for change in populations that occurs when organisms with traits that allow them to survive better reproduce and pass those traits to their offspring.

• What happens to those organisms who don’t have the favorable traits?

• They are less likely to survive and reproduce

Natural Selection Summarized in 4 Easy Steps…

• 1.) In nature, there is a tendency for organisms to produce more offspring than can possibly survive.

• Ex: Fish

Natural Selection Summarized

• 2.) In any population, individuals will have slight differences in their phenotypes.

• Ex: Fish, zebras, snakes

Natural Selection Summarized

• 3.) Individuals with traits that make them more likely to survive in an environment will survive and pass on those traits to their offspring.

• Fish, zebras and snakes again…

• More of these individuals will survive as compared to those who do not have the helpful traits.

Natural Selection Summarized

• 4.) Eventually, the offspring of the survivors will make up a larger part of the population.

• Depending on environmental factors, after many generations, the population may look completely different from what it originally was.

Evolution by Natural Selection

• There is a struggle for existence:

• Members of each species compete for resources.

• Faster or more adept predators get more food; faster prey or those with good camouflage get away and reproduce.

• Adaptations allow organisms to survive.

Evolution by Natural Selection

• Survival of the fittest: • Those organisms best suited to their environment

will survive; those who aren’t will die. • Ex: Albinos in the wild

• Descent with Modification: • Species today look different from their ancestors.

• Each species today has descended, with changes, from other species over time.

• All living things are related to one another.

Adaptations

• Any trait that improves the chances of survival and reproduction is called an adaptation.

• Species develop adaptations through natural selection.

Types of Adaptations

• Structural adaptations: • Physical features that help in survival.

• Ex: thorns of a rose, camouflage, mimicry

• Physiological Adaptations: • Changes in an organism’ s metabolism

• Ex: antibiotic-resistant bacteria; pesticide- resistant insects

Evidence for Evolution

• Fossil Record: • Fossils were known (even in Darwin’s time) to be

the remains of ancient life.

• Also, it was known that different rock layers formed at different times in Earth’s history.

• Fossils that are in the different layers are of different ages; the ones in deeper layers existed before the ones in more shallow layers.

Evidence for Evolution

• Geographic distribution of living species

• Darwin’s travels

• Different animals on different continents descended from different ancestors.

• However, because animals on each continent were living in similar ecosystems and exposed to similar challenges and pressures, they developed similar body structures.

Evidence for Evolution

• Homologous body structures:

• Limbs of organisms, for example, have different forms and functions, but are made of the same basic bones.

• The same bones in our arms are in the wings of a bird.

�Evidence for Evolution

• Compared Embryology:

• Embryology is the study of embryos, the early stages of animal development.

• Early in development, the embryos of different animals are hard to tell apart.

Genes and Variations

How do we get variations in the gene pool?

Sources of Genetic Variation

• 1.) Mutations

• 2.) Gene shuffling • Due to sexual reproduction

• Remember: in meiosis, chromosomes/genes separate (Principle of Segregation)

• Crossing over also leads to variation.

• Think about a deck of cards…

Traits

• The number of phenotypes for a given trait depends on how many genes control the trait.

• Single-gene trait vs. Polygenic trait.

Natural Selection and Traits

• N.S. on single gene traits can lead to changes in allele frequencies, which leads to evolution.

• Allele Frequency: • The percentage of a population that has a

particular allele

• Example: If 25% of the population has blonde hair (bb), the allele frequency for the “b” allele is 25%

• Ex: Effect of color mutations on lizard survival.

Natural Selection on Polygenic Traits

• When traits are controlled by more than one gene, the effects of natural selection are more complex.

• Fitness is the key!

Selection

• Natural selection can effect the distributions of phenotypes in 3 ways:

• Directional selection

• Stabilizing selection

• Disruptive selection

Directional selection

• Individuals at one end of a curve are more fit than other individuals.

• This shifts the curve in one direction.

Stabilizing Selection

• Occurs when individuals near the middle of the curve are more fit.

• The center of the curve stays in the same place, but the curve becomes narrow.

Disruptive Selection

• Individuals at the ends of the curve are more fit than those in the middle.

• Creates a “double humped” curve.

Genetic Drift

• Natural selection is not the only source of evolutionary change.

• In large populations, probability can be used to predict genetic outcomes.

• In small populations, the laws of probability do not always work. • Ex: coin flip

How Does Genetic Drift Happen?

• In small populations, individuals that carry an allele for a trait may leave more offspring (descendants) than those who don’t have the allele, �just by chance.

• Over time, a series of these chance happenings can cause an allele to be more common.

Two methods of Genetic Drift:

• Founder effect

• Bottleneck effect

Founder Effect

* A change in the allele frequencies due to

the migration of a small group of organisms

to a new area.

• Ex: Huntington’s in Venezuela

• Ex: Type O blood in Native tribes in South America

• Ex: The Amish and Microcephaly in Pennsylvania

Founder Effect

Bottleneck effect

• Change in allele frequencies due to a catastrophic event (natural disaster, disease, habitat destruction, etc.)

Speciation

• The evolution of a new species.

• Geographical isolation can lead to this.

2 Ideas on how evolution occurs:

• Gradualism:

• Evolution occurs SLOWLY over a long time.

• Adaptations will increase in number steadily over time.

• Darwin’s idea.

• Punctuated Equilibrium:

• Speciation occurs in rapid bursts followed by periods of no change.

• This idea fits the fossil record better.

How do scientists determine the age of fossils?

• Radioisotope dating • Isotopes are atoms with different numbers

of neutrons: Carbon 12, Carbon 13, Carbon 14.

• Carbon 14 is the normal isotope used for dating fossils. • By measuring the half-life of the isotope,

scientists can determine the age.

Major “Ages” in the History of the Earth:

Plants respond to the environment, too.

• Plants do not have nervous systems, but they do have the ability to respond to the environment.

Ways plants adapt: • 1.) Geotropism:

• Response of seedlings to the force of gravity.

• Causes roots to grow down and stems to grow up.

• 2.)Phototropism: • Ability of plant to respond to light.

• 3.) Thigmotropism: • Ability of plant to respond to touch.

• Climbing plants use this to find their way up/around an object.

• Ex: How kudzu climbs up a tree

Geotropism

Phototropism

Thigmotropism

Plant Hormones

• Auxins: • Responsible for regulating phototropism by

causing cells to get longer.

• Gibberellins: • Cause plant to grow taller.

• Increase rate of bud formation and seed germination

• Abscisic acid: • Inhibit plant growth during times of stress.

Plant Adaptations

• Seeds of some plants will go dormant in unfavorable conditions.

• Roots and stems are modified into storage organs.

• Conifers have needles instead of leaves, which prevent evaporation of water.

• Conifers also have thick bark which insulates them in the winter.

• Flowers can be pollinated in multiple ways: wind, birds, insects.