The secret of
evolution
What makes
populations change?
First, let’s
clear up how
evolution does
NOT happen.
“If humans evolved from monkeys,
then why are there still monkeys?”
Because we didn’t
evolve from monkeys.
 Mushrooms don't evolve into pine trees.
 Fish don't evolve into amphibians.
 Frogs don't evolve into reptiles.
 Reptiles don't evolve into birds.
 Birds don't evolve into mammals.
New species evolve from
common ancestors.
Not from each other.
Species & Speciation
• species
• similar organisms that can interbreed in
nature to produce fertile, viable offspring.
• population
• all the members of a species living in a
defined area
• speciation
• the generation of new, separate species
from a single population
Descent with Modification
 Microevolution
 Genetic change in a population without the
generation of new species
Descent with Modification
 Microevolution
 Natural populations
show this type of
variation, too.
Descent with Modification
 Macroevolution
 Genetic (and other) changes that split one species
into two new species that can no longer breed to
produce fertile, viable offspring.
 When two species cannot produce fertile, viable
offspring they are said to be reproductively
isolated from each other.
 Reproductive isolation defines different species.
Reproductive isolation
 ECOLOGICAL
 Two species’
habitats are
not compatible
 (pre-zygote)
Reproductive isolation
 TEMPORAL
 Two species’ mating times are different
 (pre-zygote)
Reproductive isolation
 BEHAVIORAL
 Two species’
mating
rituals not
compatible
 (pre-zygote)
Reproductive isolation
 HYBRID INVIABILITY
 Two species’ hybrid offspring can’t survive
 (post-zygote)
X
=
X
Reproductive isolation
 HYBRID STERILITY
 Two species’
hybrid offspring
can’t reproduce
 (post-zygote)
But How?
 1. MUTATION
 The raw material of
evolution
 Piglet with a
mutation causing
holoprosencephaly
But How?
 2. SMALL POPULATION SIZE
 Random changes will have stronger effect.
 This is known as GENETIC DRIFT
But How?
 3. MIGRATION
 Separate populations
share genes and
mutations.
 These effectively
become small
populations.
 And that means
genetic drift.
But How?
 4. NON-RANDOM
MATING
 POSITIVE
ASSORTATIVE
MATING
 Similar individuals pair
off more often than
expected.
 Small effective
population means…
 GENETIC DRIFT
But How?
 4. NON-RANDOM
MATING
 NEGATIVE
ASSORTATIVE
MATING
 Dissimilar individuals
pair off more often
than expected.
 Small effective
population size
means…
 GENETIC DRIFT
But How?
 5. NATURAL SELECTION
 Darwin’s theory
Natural Selection:
Any trait can be…
 Adaptive – increases chances of leaving genes to the next
generation.
 Maladaptive – decreases chances of leaving genes to the
next generation.
 Neutral – does not affect chances of leaving genes to the
next generation.
Let’s consider some common
misconceptions about evolution
 Misconception: Evolution is a theory about the origin of
life.
 Misconception: Evolution is a process by which organisms
become "better".
 Misconception: Evolution is the result of life changing by
random chance.
 Misconception: Organisms purposefully "try" to evolve
traits because they need them.
 Misconception: Evolution means "survival of the
fittest".
 Misconception: Evolution is "just a theory".
 Misconception: Even the scientific community can't
agree on how evolution works. Therefore it's a bogus
idea.
 Misconception: Gaps in the fossil record are a major
problem for evolutionary theory.
 Misconception: Evolutionary theory has failed to explain
everything about how life originated and how it evolved.
 Misconception: Evolution can't be studied scientifically
because it is not observable or testable.
 Misconception: Most scientists no longer believe Darwin's
ideas about natural selection.
 Misconception: Believing that evolution happened leads to
immoral behavior and mayhem.
 Misconception: Evolution by natural selection means that
social Darwinism is natural and can't be avoided in human
culture.
 Misconception: Evolution and religion are incompatible.
 Misconception: Teachers should teach "both sides" of the
argument and let students decide for themselves.
 Misconception: Evolution is like a religion. Therefore,
teaching it violates the First Amendment of the U.S.
Constitution.
Next Time: Natural Selection
 We will be predators.
 We will prey upon beans.