Experience 1) Darwin's Theory: Natural Selection

Ideas that Supported Darwin’s Thinking

hypothesis: tentative explanation, further tested by observations or experiments

• possible explanation for a set of observations or answer to a question

theory: well-tested, highly reliable explanation of phenomena in natural world

• lets scientists make accurate predictions about new events

• evolutionary theory therefore has some good evidence to explain, i.e., emergence of new diseases

evolution: modern organisms descended w/ change from ancient ones

• Darwin contributed most to this theory, explaining unity + diversity of life

  • most ppl of his time thought earth is a few thousand years old, little change

  • BUT new scientists started to think differently

Hutton and Geological Change

James Hutton studied bent and folded mountains and rocks, proposing that underground forces pushed up layers, causing them to twist and eventually get worn out by rain, etc.

The process of rocks building up and squeezing into layers takes a very long time, introducing his theory of deep time— our planet is old and has a very, very long history.

Lyell’s Principles of Geology

Lyell determined that since laws of nature are constant, the processes that shaped Earth a long time ago are the same as today; scientists can explain the past by observing present processes.

Lyell’s principle was confirmed when Darwin observed marine animals clinging onto a rocky shoreline after an earthquake and later similar marine animals in high above mountains, proving earthquakes could’ve built mountains over a long time. Darwin then started to wonder if life could change just like the earth does.

Observations Aboard the Beagle

Darwin went on the Beagle, an ocean voyage, to explore and explain the diversity of life he observed. He focused on three patterns of diversity: global, local, and over time.

1) Species Vary Globally

Ecologically similar but different species lived in similar but separate regions around the world.

• rheas → South America, ostrich → Africa, emus → Australia

  • all flightless birds in grasslands of Southern hemisphere

2) Species Vary Locally

Within a local area, populations of species w/ varied traits lived in different habitats.

• one rheas type in warm, other rheas type in cold south

• identify which island tortoise is from based off shell → cs each island has diff conditions

  • ex: curved, open shells n long necks can reach higher for vegetation → dome-shaped, short necks have low-level vegetation

3) Species Vary Over Time

fossils: preserved remains or traces of an organism

Darwin collected living things and fossils, but nb back then knew how to interpret that record.

These fossils showed extinct animals, i.e. glyptodonts, were similar but later changed into modern organisms, i.e. armadillos, demonstrating how species change over long periods.

Other Influences on Darwin

People before and like Darwin had different ideas of how evolution worked.

Lamarck’s Hypothesis

Individuals have an inborn drive to be more perfect, so they can change their body structures by using them in new ways, thereby acquiring the traits to help them in their environment.

• Acquired Characteristics

  • traits altered by an individual organism during its lifetime

    • ex: water bird starts acquires long legs if it went into deeper water for food

      • legs stretch to stay above surface → longer

      • unused features → shrink

  • inheritance of acquired characteristics → acquired traits r passed down , evolving long leg species

• Evaluating Lamarck’s Hypothesis

  • wrong!

    • no drive to b perfect

    • u cant pass down acquired traits

  • he’s still cool tho cs he influenced Darwin n others

    • first to argue species aren’t fixed

    • first to draw a scientific explanation of natural processes y species change

    • recognized adaptations r from environments
      

Population Growth

Malthus stated that since people were being born faster than dying, if humans grow unchecked, there wouldn’t be enough food and space for evb.

• forces that counter growth → war, famine, disease

Influenced Darwin’s evolutionary theory.

Artificial Selection

Darwin learned from planters, animal experts, and farmers that individual organisms are different from each other, and some of these differences were caused by inheriting variations from parents.

They used this to their advantage through:

artificial selection: selective breeding of organisms to promote occurence of desirable traits in offspring

• nature provides inherited variants, humans select which ones they want

• i.e. sweeter fruits

Inherited variation occurs both in wild and domesticated species. Before Darwin, variations were considered unimportant and minor defects. Now, variations could be a natural mechanism driving evolution.

Evolution by Natural Selection

Darwin pieced together his theory of evolution after reading Malthus, reflecting on Lamarck’s work, and experimenting w artificial selection.

(three things must occur before NATURAL SELECTION)

1. The Struggle for Existence

Malthus’ statement that more humans were being born than dying, and Darwin realized this applied to all other organisms. But which organisms survived and reproduced, and why?

2. Variation and Adaptation

Darwin determined organisms w/ certain inherited varied traits were better adapted to their environment.

adaptation: heritable characteristic increasing an organism’s ability to survive and reproduce in its environment

• behavior or body part

3. Survival of the Fittest

Darwin, like Lamarck, recognized there should b a connection between the way an organism “makes a living” and the environment it lives in. Differences in adaptations affect:

fitness: organism’s ability to survive AND reproduce in an environment

• well-suited adaptations → higher fitness, bad adaptations → lower fitness,

  • differences = survival of the fittest

4. Natural Selection

natural selection: organisms most suited to their environment survive and reproduce successfully; survival of the fittest

occurs when

1. more organisms born than can survive

2. natural heritable variation affects fitness

3. fitness varies

Adaptations just have to be good enough for an organism to pass off its genes. Natural selection isn’t fixed or perfect. If environments change, formerly adaptive traits could now be useless, and different traits r now adaptive.

• more diverse adaptations in species in diff environments

• environment changes faster before organism adapts → species is extinct

Example:

1. more grasshoppers born > die

2. color is a varied, inheritable trait

3. green color grasshoppers have higher fitness in a grassy environment

4. green grasshoppers > yellow grasshoppers