Chapters 16

Darwin’s Theory of Evolution

Chapter 16Darwin’s Theory of Evolution

Evolution- The process by which organisms have changed over

time.

Ideas that Led to Darwin’s Theory of Evolution

1. Jean-Baptiste Lamarck (1809)– Said organisms could

change in their lifetime by either using or not using various parts of their body

– Said these acquired changes can be passed on to offspring

– Example: A bird’s legs would grow longer if it waded in deeper water What would Lamarck say about me?

Why Lamarck’s Ideas Were Important

• He was the first to suggest that species can change

• He was one of the first to try to explain evolution scientifically

• He recognized that there was a link between an organism’s environment and its body structure

• His work got people like Darwin thinking about evolution.

Ideas that Led to Darwin’s Theory of Evolution

2. Thomas Malthus (1798)- Realized that more humans were being born than were dying- Reasoned that eventually there would not be enough living space- Thus, events must occur to control the population (famine, war, disease)

Why Malthus’s Ideas Were Important

• Organisms can produce thousands to millions of seeds/eggs a year and only a fraction survive

• Only the ones that survive can reproduce

• Darwin asked why certain offspring survive to reproduce and others do not

Ideas that Led to Darwin’s Theory of Evolution

3. Artificial Selection-- Humans choose certain organisms to breed in order to make offspring with a certain trait- Allowed Darwin to see that variations in traits can be passed from parent to offspring (he knew nothing about genetics)

Darwin’s Voyage on the Beagle

• 1831- Darwin sailed around the world on a boat called the Beagle

• He collected plants and animals to study

The Galapagos Islands- location of Darwin’s most famous findings

Darwin’s Ideas About Evolution

A. “The Struggle For Existence”- If more individuals are born than can survive, there must be competition for food, living space, etc. - He wanted to know which individuals would come out on top, who would win.

Overproduction Competition

Darwin’s Ideas About Evolution

B. “Variation and Adaptation”- Some variations in individuals are better suited for a certain environment. These are called adaptations.

- Adaptation- any heritable characteristic that increases an organism’s ability to survive in a certain environment

Darwin’s Ideas About Evolution

C. “Survival of the Fittest”- Fitness- how well an organism can survive and reproduce in its environment

- If an organism is well adapted to its environment and successfully reproduces it has “high fitness”

Darwin’s Ideas About Evolution

D. “Natural Selection”- The process by which organisms with variations most suited to their environment survive and leave more offspring.

NATURE SELECTS WHO SURVIVES- Well adapted individuals survive and reproduce- Over time, the traits of the well adapted individuals become the traits most commonly observed in the population

Example of Natural Selection

1. Grasshopper lays eggs 2. Yellow and green grasshoppers hatch

3.Predator eats the grasshoppers that are easy to see

4.Green grasshoppers are left to reproduce

After many generations, almost all grasshoppers will be __________.

Darwin’s Ideas About Evolution

E. “Common Descent”- To reproduce is to leave descendents- Darwin suggested that over many generations, successful species evolve into new species- Also, living species are descended, with modification, from a common ancestor

Evidence for Common Descent“Descent with Modification”

1. Homologous Structures- structures shared by related species and have been inherited by a common ancestor

• The same structure adapted in different ways, depending on the environment, to produce different species

• These structures have different functions. Common ancestor of alligator and chicken

Evidence for A Common Ancestor“Descent with Modification”

2. Analogous Structures- body parts that share common function, but not structure

Example: the wing of a bee and the wing of a bird- both are wings (same function)- structure is completely different

Evidence for A Common Ancestor“Descent with Modification”

3. Vestigial Structures- structures inherited from ancestors but have lost much or all of their function due to different needs of the descendents

Examples: The wings of an ostrich, the appendix in humans, the coccyx bone (remnant of a tail!), the formation of goose bumps (a vestigial reflex!) Coccyx

bone

Genetic Evidence for Evolution

• Darwin had no idea how heredity worked• We now know that genetic information

comes from DNA, mutations cause changes in DNA, crossing over causes variation in offspring, etc.

• All living cells use DNA and RNA to direct the production of proteins!!– Suggests that all organisms evolved from a

common ancestor with this genetic code

Genetic Evidence for Evolution

• Homologous Molecules- molecules with shared structure and/or function– Example: homologous proteins perform

similar functions in very different cells• “Cytochrome C”- a protein that functions in cellular

respiration. • Similar versions are found in cells from yeast to

humans

Genetic Evidence for Evolution

• Homologous Genes- genes with the same or similar sequences, controlling the same traits, in different organisms

Example: Hox genes- determine the head to tail development of embryos (which end is up)

• Minor changes in these genes produce variations among organisms• Are present in almost all multicellular organisms (from fruit flies to humans)