CHAPTER 16 : DARWIN’S THEORY

Natural Selection and Evolution

Darwin’s Epic Journey

Charles Darwin was born in England on February 12, 1809. He grew up at a time when the scientific view of the natural world was shifting dramatically.

Geologists were suggesting that Earth was ancient and had changed over time, and biologists were suggesting that life on Earth had also changed.

The process of change over time is called evolution. Darwin developed a scientific theory of biological

evolution that explains how modern organisms evolved over long periods of time through descent from common ancestors.

Darwin’s Epic Journey Darwin was invited to sail on the HMS Beagle’s five-

year voyage mapping the coastline of South America. Left from England, sailed around South America, across

Pacific, around Australia and Africa, and back to England Darwin was the ship’s naturalist – he planned to

collect specimens of plants and animals on the voyage. Most famous for observations made at Galapagos Islands

No one knew it, but this would be one of the most important scientific voyages in history.

Darwin noted 3 patterns of biodiversity:

1. Species vary globally

2. Species vary locally

3. Species vary over time

16-1 Darwin’s Voyage of Discovery

Different, yet ecologically similar animal species inhabited separate, but ecologically

similar, habitats around the globe Ex) flightless birds: rheas, ostriches and

emus

1. Species vary globally

Native to AfricaLargest of 3Native to S.America Native to Australia

Slightly Larger than Rhea

Rhea Ostrich Emu

Different, yet related animal species occupied different habitats within a local area

(tortoise shell shape - Galapagos)

2. Species vary locally

3. Species Vary Over Time

Darwin also collected fossils, which are the preserved remains or traces of ancient organisms.

Darwin noticed that some fossils of extinct animals were similar to living species.

Lamarck’s Theory of Evolution

Inheritance of Acquired Traits Acquired traits could be inherited For example: If you spent your life lifting

weights, then your children would inherit large muscles.

Lamarck’s Theory of Evolution

Evaluating Lamarck’s Theory He was wrong in several

ways He didn’t know how

traits were inherited He did realize that

organisms adapt to their environment.

Artificial Selection

To find an explanation for change in nature, Darwin studied change produced by plant and animal breeders.

Breeders knew that individual organisms vary, and that some of this variation could be passed from parents to offspring and used to improve crops and livestock – selective breeding.

For example, farmers would select for breeding only trees that produced the largest fruit or cows that produced the most milk.

Over time, this selective breeding would produce trees with even bigger fruit and cows that gave even more milk.

Artificial Selection

Darwin called this selective breeding process artificial selection, a process in which nature provides the variations, and humans select those they find useful.

Darwin put artificial selection to the test by raising and breeding plants and fancy pigeon varieties.

BBC Darwin's Pigeons (2:18)

Artificial Selection Darwin had no idea how heredity worked or what caused

heritable variation, but he did know that variation occurs in wild species as well as in domesticated plants and animals.

Before Darwin, scientists thought variations among individuals in nature were simply minor defects.

Darwin recognized that natural variation was very important because it provided the raw material for evolution. Natural variation = differences among individuals of a species

When Darwin published his scientific explanation for evolution, it changed the way people understood the living world.

Population GrowthMalthus observed that babies

were being born faster than people were dying.

If population continued to grow their would be insufficient food and space.

In plants or animals most of the offspring die

Only those who survive can reproduce What determines which ones

will survive and reproduce?

The Struggle for Existence

After reading Malthus, Darwin realized that if more individuals are produced than can survive, members of a population must compete to obtain food, living space, and other limited necessities of life.

Darwin described this as the struggle for existence.

Variation and Adaptation

Darwin knew that individuals have natural variations among their heritable traits, and he hypothesized that some of those variants are better suited to life in their environment than others.

Any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment is called an adaptation.

Three types of adaptations that involve body parts or structures :

1. Structural – physical features of an organism

• Ex: long tongue to get food, sharp teeth, Camouflage

2. Behavioral – actions an organism takes

• Ex: migration, tracking prey, storing nuts, growing towards light

3. Physiological – functioning or biochemical processes

• Ex: venom, ink of octopus, protein in web, respiration rate, digestive enzyme, blood clotting

MimicryScarlet king snake exhibits mimicry—an adaptation in which

an organism copies, or mimics, a more dangerous organism. Although the scarlet king snake is harmless, it looks like the

poisonous eastern coral snake, so predators avoid it, too. Non poisonous snakes will also rattle their tale and flatten

their head to look poisonous to a predator.

Mimicry

Monarch butterfly - is toxic and very nasty to eat. Its bright orange coloration is

a warning to birds to leave it alone.

Non-toxic viceroy butterfly has developed colors and wing patterns that are very similar to those of the monarch most birds won’t take a

chance by taste-testing it

Camouflage

An adaptation that allows an organism to blend into its background and avoid predation.

Variations vs. Adaptations• Variations – differences that exist within a population

that may have no effect on fitness • Ex: length of your thumb

• Adaptations – a variation that all members of a population have inherited because that trait improves fitness• Ex: an opposable thumbs

Sources of variation• Sources of variation:

• Mutations – individual genes change • Ex: ATC AGC

• Events during meiosis – during meiosis, chromosomes get mixed up (crossing over) in making eggs & sperm

• Random fusion of gametes – which sperm fertilizes which egg – determined by chance

Survival of the Fittest

According to Darwin, differences in adaptations affect an individual’s fitness.

Fitness describes how well an organism can survive and reproduce in its environment. Individuals with adaptations that are well-suited to their

environment can survive and reproduce and are said to have high fitness.

Individuals with characteristics that are not well-suited to their environment either die without reproducing or leave few offspring and are said to have low fitness.

This difference in rates of survival and reproduction is called survival of the fittest. In evolutionary terms, survival means reproducing and

passing adaptations on to the next generation.

Natural Selection

Darwin named his mechanism for evolution natural selection because of its similarities to artificial selection.

Natural selection is the process by which organisms with variations most suited to their local environment survive and leave more offspring.

In natural selection, the environment — not a farmer or animal breeder — influences fitness.

Natural Selection

Well-adapted individuals survive and reproduce.

From generation to generation, populations continue to change as they become better adapted, or as their environment changes.

Natural selection acts only on inherited traits because those are the only characteristics that parents can pass on to their offspring.

Natural Selection

Natural selection does not make organisms “better.” Adaptations don’t have to be perfect—just good enough to enable an organism to pass its genes to the next generation.

Natural selection also doesn’t move in a fixed direction. There is no one, perfect way of doing something. Natural selection is simply a process that enables organisms to survive and reproduce in a local environment.

Natural Selection

If local environmental conditions change, some traits that were once adaptive may no longer be useful, and different traits may become adaptive.

If environmental conditions change faster than a species can adapt to those changes, the species may become extinct.

Common Descent

Natural selection depends on the ability of organisms to reproduce and leave descendants. Every organism alive today is descended from parents who survived and reproduced.

Darwin proposed that, over many generations, adaptation could cause successful species to evolve into new species.

He also proposed that living species are descended, with modification, from common ancestors—an idea called descent with modification.

According to the principle of common descent, all species—living and extinct—are descended from ancient common ancestors.

•Darwin published his findings in 1859 in a book entitled The Origin of Species by Means of Natural Selection.•He was motivated to publish his book in 1859 because Alfred Wallace had independently come up with the same conclusions

16-3 Darwin Presents His Case

The Age of Earth

Evolution takes a long time. If life has evolved, then Earth must be very old.

Hutton and Lyell argued that Earth was indeed very old, but technology in their day couldn’t determine just how old.

Geologists now use radioactivity to establish the age of certain rocks and fossils. Radioactive dating indicates that Earth is about 4.5 billion years old—plenty of time for evolution by natural selection to take place.

1. Fossils2. Anatomy • Analogous

structures• Homologous

structures• Vestigial structures

3. Embryology4. Biochemistry5. Direct observation

16.4 Evidence of Evolution

1. Fossil Evidence

• Fossils - the remains of past life• Ex: shells, bones, teeth,

imprints• Tell us age, habitat, diet, &

lifestyle of organisms.• Record is incomplete – many

organisms leave no fossils behind

2. Anatomical Evidence – Homologous Structures

Darwin proposed that animals with similar structures evolved from a common ancestor with a basic version of that structure.

Structures that are shared by related species and that have been inherited from a common ancestor are called homologous structures.

Homologous structures are similar in structure because they develop from same tissues early in development may or may not have different functions.

Bones of vertebrate forelimbs

Homologous Structures

2. Anatomical Evidence –Analogous Structures

The clue to common descent is common structure, not common function. A bird’s wing and a horse’s front limb have different functions but similar structures (homologous structures).

Body parts that share a common function, but not structure, are called analogous structures. Analogous structures are used for the same purpose but are not due to a common ancestor. The wing of a bee and the wing of a bird are analogous structures.

Analogous Structures

Vestigial Structures

Not all homologous structures have important functions.

Vestigial structures are inherited from ancestors, but have lost much or all of their original function due to different selection pressures acting on the descendant. The hipbones of bottlenose dolphins are

vestigial structures. In their ancestors, hipbones played a role in terrestrial locomotion. However, as the dolphin lineage adapted to life at sea, this function was lost.

The human tailbone and appendix are vestigial structures.

Cormorant – flightless bird

Vestigial Structures

3. Embryology

Similar patterns of embryological development provide further evidence that organisms have descended from a common ancestor.

the more similar the embryos are at certain stages of development, the more closely related they are thought to be.

Which one is a…Chicken, Fish, Human, Rabbit, Salamander,

Turtle??????

Embryology - the more similar the embryos are at certain stages of development, the more closely related they are thought to be

4. Biochemical Evidence

• Universal genetic code – organisms use the same triplet code and the same 20 amino acids in proteins

• All organisms have certain organic molecules in common. 1. Hemoglobin - carries oxygen in blood 2. Cytochrome c - protein for cell

respiration found in almost all living cells

3. Hox genes – control development

Significance of biochemical differences

5. Direct Observation

• Direct observation – we’ve seen evolution occur in cases like:• The Grant’s studies of

finches on Daphne Major in the Galapagos (35 yrs)

• Bacteria becoming resistant to antibiotics

• Insects that become resistant to pesticides