Download Natural Selection

Natural Selection
Chapter 15
Natural Selection?
• On the following three slides, you will
read statements based on student ideas
about now natural selection works. Use
your clickers to vote for whether you
think that the statement IS or is NOT
what scientists mean by “natural
selection.”
How Selection Works
1. Variation Exists
• All populations vary
as the result of the
accumulation of
_____________
_______over many
generations.
2. Inheritance of traits
• Inheritable traits
(those coded for by
genes) are passed
directly to the
offspring from the
parents through
genetic information.
3. Differential Survival
• More offspring are
born than can
survive. Many
offspring die young.
Those with traits best
suited to the
environment are
more likely, though
not guaranteed, to
survive.
4. Differential Reproduction
• Some survivors fail
to reproduce.
Some have traits
that better insure
reproduction than
others.
5. Differential Inheritance
• Survivors that
reproduce pass
some of their
traits on to their
offspring. Those
with favorable
traits may pass
those favorable
traits on — or not.
Natural Selection in Cats
• Traits that ensure a
domestic cat’s
survival among
humans:
• Endearing
qualities: purring,
snuggling, being
playful, cuteness.
• Helpful qualities:
rodent control.
• When domestic cats are
abandoned:
• Most die within a week or
two from starvation,
accident, or predation. The
traits that help them in a
domestic setting are useless
in the wild.
• The few that survive have
the strongest feral instincts.
• Av. feral lifespan: 2-4 years
Selection’s Effects
Generation 1
Generation 2
Generation 3
As time passes...
• With each generation,
domestic traits are
selected against, while
feral traits are favored.
• After several
generations, even
kittens that are
captured young can be
challenging to tame.
Genes and Evolution
• Genes are the units of
heredity.
• Genes are ____________that
code for proteins, which result
in our set of traits.
• Genes are passed from
parent to offspring through the
________________.
• Different “versions”
of genes are _____.
• Dominant alleles are
expressed in the
_____________
(expressed trait)
even if only one
copy is inherited.
• Recessive alleles
are expressed only if
_____________are
inherited.
“Genotype” is a description of the
alleles for a given trait in an
individual: BB, Bb, or bb
Genetic basis of inheritance
• _________ - the genetic
composition of an organism
_____- the
specific form of the gene
• _____________ -
the outward expression of
genotype
• _____________-
the total of all alleles for
all genes in a population
BEY2 gene – controls
pigmentation in the eye
You’re probably familiar with
genetics in terms of following
traits passed from parent to
offspring and predicting the
outcome.
Blackfin shark genotypes in the Gulf of Mexico.
http://www.zoology.siu.edu/heist/blacktip.htm
But how do we track traits
passed from generation to
generation across an entire
population? How does
population genetics work?
The Gene Pool Concept
• The “gene pool” of a population is the
entire collection of alleles for a given trait
throughout a given population.
• The word for all genes for all traits in an
individual or population is genome.
Genetic change
Mutation, genetic
drift, and natural
selection will, over
time, change the
genetics of both
populations.
Genetic change
Causes of Genetic Change in a
Population
Hardy-Weinberg Equilibrium
• Allele ratios in a gene pool will NOT change
from generation to generation (that is, no
evolution) ONLY if ALL of these things are
true:
• ___________________
• ___________________
• ___________________
• ___________________
• ___________________
Mutations
• Small mutations appear randomly in
populations.
• The appearance of mutations changes
allele ratios by
• “breaking” functional alleles (as in genetic
disorders)
• adding new alleles
• Copy errors are rare,
but given that genes
are copied millions of
times in a lifetime,
errors can occur.
• __________ in the
environment can
increase the error rate.
• Small errors can create
______________.
Population size
• In large populations, random events have a
very small effect.
• In small populations, because fewer
individuals have any given trait, random
events can have a larger effect. Changes in
gene ratios caused by random events are
called “__________________.”
A population bottleneck is genetic drift.
Founder effect is genetic drift.
Migration
• Migration into and out of a population
can change gene ratios.
• Immigrants can bring in new mutations,
or a different ratio of alleles.
• Emigrants may take away a high
proportion of a certain allele.
• Small population are more affected than
large populations.
Migration can lead to
Founders Effect:
Surtsey, Iceland
Surfaced 14 Nov 1963
Moss: 1967
Puffin nests:
2004
Selection
• Selection may increase or
decrease the frequency of
certain alleles:
• Directional selection:
favors one end of a
range over another.
• Disruptive selection:
disfavors the midrange.
• Stabilizing selection:
favors the mid-range.
Natural selection:
Differential survival.
Peppered moth
Changes allele frequency.
Mating Behavior
• Mate choice among most organisms is
selective, not random.
• ________________may favor traits that
are in conflict with natural selection. For
example, bright-colored male guppies
attract more females, but are also more
visible to predators.
Sexual selection: competition to mate
Widow bird (text E1-2 pg 7)
We can measure phenotype
Number of males with different tail lengths in a population of redcollared widowbirds. Data based on Pryke and Andersson 2002.
Antibiotic Resistance
• Antibiotic resistance has been an
increasing problem since the 1970s.
• How does natural selection contribute to
the rise in antibiotic-resistant bacteria?
(Note it is bacteria that become resistant,
not people. Bacteria do not become
“immune” — they do not have immune
systems.)
Helicobacter pylori is a bacteria that causes
stomach ulcers.
Most H. pylori bacteria,
when encountering an
antibiotic, metabolize it
with an enzyme that
turns it to a toxin.
A few H. pylori bacteria
have a mutation that
interferes with the
production of the
enzyme. The mutant
bacteria aren’t affected
by antibiotics.
Evolution Happens
• Because perfect Hardy-Weinberg
equilibrium is never met with in nature,
all populations experience small shifts in
gene ratios with each generation.
• Gene ratio shifts may fluctuate with
cyclical changes in climate. Long-term
changes in habitat (such as global
climate change) can shift the gene ratios
far enough to bring about speciation.
Recap
• Natural Selection is a phenomenon that
can be studied directly.
• Natural Selection causes change in the
genetics of a population over
generations (evolution).
• Other factors that can change genetics
of a population include migration, sexual
selection, mutations, and effects of
random events in small populations.