Transcript chapter 16 - Cloudfront.net

The Evolution of Populations
CHAPTER 16
Genetic Equilibrium
 Population Genetics:
the study of evolution
from a genetic viewpoint

Most popltns have
variation amongst their
traits


i.e. different heights, skin
color, etc.
If graphed, looks like a
bell
Effects of Natural Selection
 Natural Selection can cause a shift in traits within a
population

Depends on which traits are favorable in that environment


Those traits will increase over time
4 Different Effects:
Stabilizing Selection
 Directional Selection
 Disruptive Selection
 Sexual Selection

Stabilizing Selection
 Occurs when the average form of a trait is
considered the highest fitness level (best for that
environment)


i.e. height
Causes decrease in variation
Directional Selection
 Occurs when one extreme is considered highest
fitness level


i.e.: an anteater’s tongue length
Causes a shift in variation
Disruptive Selection
 Occurs when both extremes are considered highest
fitness level


i.e.: black/white limpets on barnacles
Causes splitting of phenotypes; may lead to formation of new
species
Sexual Selection
 Occurs when female
organisms select a
mate based on
phenotype

May require extreme
male traits

i.e. bright colors, unique
mating calls, etc.
Speciation
CHAPTER 16
Formation of Species
 Speciation: process
of forming new
species
Manchester
Results in many related Terrier
popltns
 May remain similar or
Dandie
become quite different
Terrier

Concepts of “Species”
 1) Morphological: species defined by appearance
 Does not account for individual variations
 2) Biological: species defined by reproduction
patterns

Does not apply to ancient organisms
 3) Modern: species are similar in appearance and
can interbreed to produce fertile offspring

Concept currently used in Biology
Isolating Mechanisms
 Speciation occurs when members of a species stop
interbreeding and split

Become isolated from each other
 2 Types Isolating Mechanisms:
 1) Geographic
 2) Reproductive
Geographic Isolation
 Geographic
Isolation: the
physical separation of
members of a popltn
i.e.: by canyons,
mountains, rivers, etc.
 Once physically
separated, will continue
to diverge until they can
no longer mate

Reproductive Isolation
 Reproductive
Isolation: members
of a popltn no longer
able to mate
successfully
i.e. due to disruptive
selection, changes in
mating habits, etc.
 Once separated, will
continue to diverge

Rates of Speciation
 Gradualism: some species evolve gradually & at
stable rate
 Although speciation may take millions of years, it is
not always gradual
Punctuated Equilibrium
 Punctuated
Equilibrium: when
species evolve in rapid
spurts of great change
Involves long periods of
no change between
“growth spurts”
 Usually due to natural
disasters

The Gene Pool
 Gene Pool: the total genetic information available
in a popltn

All the possible alleles available
 Allele Frequency: # certain allele
total # alleles in gene pool


Used to predict phenotypes of future offspring
Hardy-Weinberg Principle
 “Allele frequencies in a popltn will remain constant
unless acted upon by an outside force”

AKA: Evolution will not occur unless…
 5 conditions must be met for this to hold true….
Hardy-Weinberg Conditions
 1) No Mutations can occur
 BUT, mutations occur constantly (can’t control)
 2) Individuals may not enter or leave
 BUT, individuals immigrate & emigrate constantly
 3) Population size must be VERY large
 BUT, popltn size is limited by many factors

i.e.: space, food source, competition, etc.
Hardy-Weinberg Conditions (con’t)
 4) Mating must be
completely random

BUT, mate selection is
influenced by proximity,
physical charac.,
relation, etc.
Hardy-Weinberg Conditions (con’t)
 5) Natural Selection does not occur (all
considered “favorable”)


traits
BUT, “favorable” traits are determined by environment
4 Types Natural Selection:
Stabilizing
 Directional
 Disruptive
 Sexual
