Transcript evolution - Fulton County Schools

Chapter 16 and Chapter 17
How Does Evolution Work?
 Individual organisms cannot
evolve. Populations of a particular
species evolve.
 Natural selection acts on the range
of phenotypes in a population.
 Microevolution occurs as the
frequency of alleles in a population
changes.
Evolution-What Happens?
 Macroevolution or Evolution
occurs when there is a change
in allele frequency which
produces a new species.
Definitions
 Gene pool: All alleles of the population’s
genes.
 Allelic frequency: % of a specific allele
in the gene pool.
 Example: Approximately 75% have
dominant allele for tongue rolling. 25%
non-rolling
 Genetic Equilibrium: This exists when
the frequency of alleles remains the same
over generations. The population is not
evolving.
When Does Evolution Occurs?
 Evolution results when there are Forces that
change allelic frequencies.
 Forces that cause Evolution:
1. Gene flow: Transport of genes by migrating
individuals.
2. Nonrandom Mating: Mating based on
preferences
Example: a female may choose a mate based on
male size, color, or ability to gather food.
Forces of Evolution Continued
3. Mutation: Change in DNA
4. Genetic Drift: chance event
changes allelic frequencies –
Greatly affect small populations
such as the animals of the
Galapagos Islands or Amish.
Causes of Genetic Drift
1. Mating over a long time
period in a small
population.
2. Little movement of males
or females into or out of
the population.
3 Types of Natural Selection
 Stabilizing selection – favors average
individuals
 Directional selection – favors one of the
extreme variations of a trait
 Disruptive selection – favors
individuals with both extremes of a trait
(eliminates intermediate phenotypes)
What is a Species?
 A population or group
of populations whose
members have the
ability to breed with
one another and
produce fertile
offspring
Evolution of Species (Speciation)
 Significant changes in the gene pool
can lead to evolution of a new species
over time.
 Speciation occurs when members of
similar populations no longer
interbreed to produce fertile offspring
within their natural environment.
Artificial Speciation
Diane Dodd’s fruit fly lab, 1989
Why Don’t the Populations
Interbreed?
 1. Geographic isolation – physical
barrier divides a population.
 2. Reproductive isolation – formerly
interbreeding organisms can no
longer mate to produce offspring..
 3. Change in niche -- Change in food
source. Example Finches
1. Geographic Isolation
A physical barrier that separates a
population into groups.
Can be
1. Mountains or Rivers
2. Islands with water in between
Darwin’s 13 finches on Galapagos
3. Valleys caused by lava flow
4. Roads/Highways
1. Geographical Isolation
2. Reproductive Isolation
Prevents closely related
species from interbreeding
Timing
Behavior
Habitat
Timing
Similar species have different breeding
seasons
Eastern Spotted
Skunk
Western Spotted
Skunk
Behavior
Similar species may have different
courtship or mating behaviors.
Ex: Eastern & Western meadowlarks
almost identical in color shape and
habitat, but difference in courtship
rituals differ different species
Habitat
Species remain reproductively
isolated because they are adapted
to different habitats.
Ex: Stickleback fish one is a
bottom feeder, one spends time in
the top open layers of lakes in
British Columbia, Canada
Patterns of Evolution
Divergent Evolution – evolution
where species diverge or become less
and less alike as they adapt to
different environments.
2. Adaptive Radiation-ancestral
species evolves into an array of
species to fit diverse habitats. This is
a type of divergent evolution
1.
Both the wooly mammoth, which
occupied parts of North America, and
the elephant, still found in Asia and
Africa are presumed to have evolved
from a common ancestor.
 Their geographical isolation and
environmental selection pressures
caused further evolution of the species.
 Each, in its own location, occupies(d) a
similar niche.

Patterns of Evolution
Continued
2. Convergent Evolution –
Unrelated species occupy similar
environments in different parts of
the world.
Similar pressures of natural
selection lead to similar
adaptations.
Example of Convergent
Evolution
A Hummingbird
Moth
A Humming Bird
Rhea
Emu
Ostrich
Speciation can occur quickly or slowly
 Gradualism – idea that species originate
through a gradual accumulation of
adaptations.
 Punctuated equilibrium – hypothesis
that speciation occurs relatively quickly,
in rapid bursts, with long periods of
genetic equilibrium in between.
Gradualism
 Gradual changes
in species over
time
 Evidence of many
intermediate
forms in fossil
records
Punctuated Equilibrium
 Scientists found
remains of
intermediate
forms
 Also saw that
populations
remained the
same over large
periods of time
then suddenly
changed
The End