Happy Monday! – 11/3
Given the information in the table below:
The closest evolutionary relationship most likely
exists between the
A human and the gorilla.
B horse and the zebra.
C human and the chimpanzee.
D chimpanzee and the gorilla.
Mechanisms of Evolution
What changes populations?
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“That mystery of mysteries…”
Darwin never actually tackled
how new species arose…
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So…what is a species?
 Biological species


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population whose members can interbreed &
produce viable, fertile offspring
reproductively compatible
Eastern Meadowlark
4 Forces of Evolutionary Change
Mutation
Gene Flow
Genetic Drift
Chemical
Changes to DNA
Migration
Small population
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Non-random mating
Selection
Sexual Selection
Natural Selection
1. Mutations
 Mutation – change or random error in
DNA sequence
chance
 radiation
 chemicals

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2. Gene Flow

When individual leaves or enters the
population
 Immigration: when an individual enters



the population
Emmigration: when an individual leaves
the population
Primarily affects small and isolated gene
pools
Impact is smaller in larger, less isolated
gene pools
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3. Genetic Drift
 Chance events changing frequency of
traits in a population

not adaptation to environmental conditions
 not selection

founder effect
 small group splinters off & starts a new colony
 it’s random who joins the group

bottleneck
 a disaster reduces population to
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small number & then population
recovers & expands again but
from a limited gene pool
 who survives disaster may be random
Founder effect
 When a new population is started
by only a small group of individuals
just by chance some rare alleles may
be at high frequency;
others may be missing
 skew the gene pool of
new population

 human populations that
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started from small group
of colonists
 example:
colonization of New World
Bottleneck effect
 When large population is drastically
reduced by a disaster
famine, natural disaster, loss of habitat…
 loss of variation by chance event

 alleles lost from gene pool
 not due to fitness
 narrows the gene pool
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4. Natural Selection
 Selection acts on any trait that affects
survival or reproduction
predation selection
 physiological selection
 sexual selection

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Predation Selection

act on both predator & prey




speed
behaviors
camouflage & mimicry
defenses (physical & chemical)
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Physiological Selection
 Acting on body functions




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disease resistance
physiology efficiency (using oxygen, food, water)
biochemical versatility
protection from injury
Sexual Selection
 Increases reproductive success
attractiveness to potential mate
 fertility of gametes
 successful rearing of offspring

Survival doesn’t matter
if you don’t reproduce!
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Sexual selection
ornamented males…
the traits
that get you mates
It’s FEMALE CHOICE, baby!
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Effects of Selection on Populations
 Changes in the average trait of a population
DIRECTIONAL
SELECTION
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
speciation?
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giraffe neck
horse size
human birth weight
rock pocket mice
Happy Tuesday! – 11/4
Cacti have thick stems and leaves that store water. These
structures enable cacti to conserve water while living in
dry desert conditions. Based on their functions, the
leaves and stems of cacti are examples of
A
B
C
D
vestigial structures.
structural adaptations.
behavioral adaptations.
homologous structures.
2 Types of Speciation:
1. Gradualism:
gradual divergence over long spans of time
 assume that big changes occur as the
accumulation of many small ones

2. Punctuated Equilibrium:
rapid bursts of change
 long periods of little or no change
 species undergo rapid change when they
1st bud from parent population

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Rate of Speciation
 Does speciation happen gradually or rapidly
Gradualism
Punctuated Equilibrium
gradual
accumulation of
small changes over
long time
rapid bursts of
change mixed with
long periods of
little or no change
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Charles Darwin
Charles Lyell
Stephen Jay Gould
Niles Eldredge
3 Patterns of Evolution:
 Different patterns of evolution occur in
different environments
Convergent evolution
 Divergent evolution (also called
adaptive radiation)
 Coevolution

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1. Convergent Evolution
 When unrelated species live in similar
environments in different parts of the
world
similar environmental pressures
 similar pressures of natural selection

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unrelated species
CONVERGENT EVOLUTION EXAMPLE
http://evolution.berkeley.edu/evolibrary/home.php
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Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.
2. Divergent Evolution

populations that were once similar change
and adapt to different living conditions
 Ex. Adaptive radiation
common
ancestor
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Adaptive Radiation Example
 When one species splits into many
species to fill open habitats.

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Darwin’s finches
24
3. Coevolution
 Two or more species reciprocally
affect each other’s evolution

predator-prey
 disease & host
competitive species
 mutualism

 pollinators & flowers
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