Download Mechanisms of Evolution

Crash Course –
Natural Selection
Crash Course –
Speciation



Change in a population over time
Change in genetic make-up (allele frequencies)
Results in new species

A group of organisms of the same species
that share a geographical area and breed with
each other

Any trait or behavior that makes an organism
likely to survive in its environment
“Descent with Modification”
1.
Natural Selection = “survival of the fittest”
 organisms most suited for survival will reproduce and
pass genes for the beneficial trait
 Alleles for a trait that give an advantage increase
 Ex: giraffe neck length
2.
Genetic Drift = random changes in allele
frequencies due to chance
 not based on adaptations or fitness
 Ex: landslide
Migration = movement of individuals into and
out of a population
3.


Individuals bring their alleles with them
More likely to impact smaller populations
Bottleneck Effect = when an entire population is
reduced to just a few individuals
4.



Leads to decreased genetic variation
Population susceptible to extinction
EX: overhunting of cheetahs
Founder Effect = formation of a new population
when a few individuals leave a larger population
5.


New gene pool can be very different from the original
EX: island species

Formation of new, genetically distinct species from
populations of existing species

Isolating mechanisms prevent mating and
breeding between two species
 Geographical isolation = physical barriers
 Behavioral isolation = differences in courtship rituals
 Temporal isolation = differences in mating times
“Descent with Modification”
1.
Fossils = remains or traces of organisms that once
existed on Earth
 Support the ideas that life on Earth has changed and
that new species arise from existing species
 EX: fish existed before four-limbed creatures
Anatomy and Development
2.

Homologous structures = similarities in structure due
to evolution from common ancestor
▪

EX: limbs
Analogous structures = function similarly because
they are adaptations to similar conditions
▪

EX: wings
Vestigial structures = structures that have lost their
original function
▪
EX: leg bones in whales

Forelimbs of mammals

Wings built very differently


Leg bones in whales
Tail bone in humans
3.
Genes and Molecules (genetic codes)
 Genetic code is nearly identical in all living things
 The more similar two gene sequences are the more
closely they are related
 Can be used to create a lineage
“Descent with Modification”

Evolution toward similar characteristics in
unrelated species due to similar environments
 Analogous structures

When closely related species evolve in
different directions
 Homologous structures

Evolution of one species drives the evolution
of another
“Descent with Modification”


Most individuals fall in the middle with few
outliers
Ex: human height

The “middle” trait is selected for; more
individuals fall in the middle and even fewer
fall at either end

one of the extremes will be selected for and
become the new normal; direction of
distribution shifts

“middle” trait selected against so that both
extremes become more common
EVOLUTION
1.
Which of the following is a result of cheetahs
having gone through a population bottleneck?
A. The cheetah species is more likely to become extinct
Mutations occur more frequently in the cheetah genome
C. Individual cheetahs have different alleles for many genes
D. The cheetah population cannot increase past a certain size
B.
2.
Which is one way that natural selection differs
from genetic drift?
A. Natural selection causes the frequencies of alleles in a
population to change
B. Natural selection affects smaller populations more
often than larger populations
C. Natural selection depends on some traits helping
individuals in their environment
3.
A particular gene has two alleles, G and g. Each
allele has a frequency of 50% in a population.
Which is most likely to preserve these allele
frequencies?
A. The population doubles in size over several generations
A number of individuals with one allele migrate from the
population
C. One allele results in a phenotype that better enables organisms
to reproduce
B.
4.
Recently, bears have been found in the Arctic
Circle that are hybrids between grizzly bears and
polar bears. Which type of isolating mechanism
usually keeps these species from producing
offspring?
A. Mating and breeding in different habitats
Production of a nonviable embryo or fetus
C. Inability of sperm to reach the egg during mating
D. Failure of he fertilized egg to implant in the uterus
B.
5.
Under which of the following conditions would a
dominant allele that coded for a fatal disease
most likely remain in a population?
A. The allele is acted on by natural selection
B. The gene is expressed only in members of one sex
C. The gene is expressed in individuals later in life
D. The allele mutates to a nonfatal form during an
individual’s lifetime
6.
Imagine that DNA replication and cell division
were completely error-free and that cell DNA
were completely protected from substances that
cause mutation. How would natural selection be
affected?
A. It would become more efficient since it removes
mutations from the population
B. It would become ineffective since it depend on
variations within populations