Evolution

~ a unifying theme of biology ~

Introduction to Evolution• What does biological “evolution” mean?

– Change in organisms over time

• How does this relate to the “origin of life”?– It doesn’t…two different topics!– Much more evidence to support evolution– Remember: science deals with natural world,

not belief

Introduction to Evolution• How could “life” have started?

– Old Idea: • “spontaneous generation” (abiogenesis)…

nonliving matter makes living• Disproved by Redi and Pasteur

– New Idea:• “biogenesis”…life comes from life• Remember Virchow’s contribution to the

cell theory?!?!?!

Introduction to Evolution

• So, then where did the first cells come from?

– Early earth• Very hot; volcanoes;

violent storms/lightning

• So, then where did the first cells come from?

– Miller & Urey experiment• Under early earth conditions,

organic compounds form & group together into “protocells”

• But, when did we actually get cells? And how?– Evidence of first cells

• 3.4 bln yrs ago• Most likely anaerobic bacteria

(no O2 in air)

– First photosynthetic cells• 2.8 bln yrs ago• Increased O2 in atmosphere• Led to ozone layer that protects

from UV rays = more cells• Aerobic cells could now develop

Darwin’s Ride on the HMS Beagle

• In 1831, the boat left England to chart the poorly known coastlines of S. America.

• When the crew was charting the coastline, Darwin was ashore collecting specimens and making observations.– His observations: most of the plants and animals of

S. America were different from those of Europe. The animals of the S. American temperate regions were more similar to those in the tropical regions of S. America than they were to temperate regions of Europe.

– He also observed marine fossils

Darwin’s Idea

• How did Darwin form ideas about evolution?– Fossils showed organisms changed over time

Darwin’s Idea

• How did Darwin form ideas about evolution?– Geologists showed that earth was very old

Darwin’s Idea

• How did Darwin form ideas about evolution?– He saw diverse species (ex: finches on Galapagos

Islands, all with variations)

Darwin’s Idea

• How did Darwin form ideas about evolution?– Malthus noticed populations grow faster than their

food supplies (they produce too many offspring)

Darwin’s Ideas• What did Darwin

conclude about species?

– 1. Evolution explains the unity and diversity of life.

– 2. Natural selection is the cause of adaptive evolution.

Darwin’s Ideas• What did Darwin conclude about species?

– Four assumptions:

• Populations produce large #’s of offspring (more than can survive with given food)

• Individuals in population have variations• Certain variations are more useful (these will

survive better)• Over time, “good” genes prevail in the

population, while “bad” ones fade out

Our Current Understanding

• Our current definition of evolution is referred to as a change over time in the genetic composition of a population.

• Eventually, enough change will accumulate in a population that it will constitute a new species.

Main Points of Natural Selection

1. Natural selection occurs through individuals and their environment.

2. Individuals don’t evolve.3. A population is the smallest unit that can evolve.4. Evolution is measured only as changes in the

relative proportions of heritable variation in a population over multiple generations.

5. Natural selection only amplifies inheritable traits, not acquired characteristics.

6. Natural selection is always operating, but which traits are favored depends on the environment.

The End