Transcript Concept 6.2 - practical ecology

6
Evolution and Ecology
Chapter 6 Evolution and Ecology
CONCEPT 6.1 Evolution can be viewed
as genetic change over time or as a
process of descent with modification.
CONCEPT 6.2 Natural selection, genetic
drift, and gene flow can cause allele
frequencies in a population to change
over time.
CONCEPT 6.3 Natural selection is the
only evolutionary mechanism that
consistently causes adaptive evolution.
Chapter 6 Evolution and Ecology
CONCEPT 6.4 Long-term patterns of
evolution are shaped by large-scale
processes such as speciation, mass
extinction, and adaptive radiation.
CONCEPT 6.5 Ecological interactions and
evolution exert a profound influence on
one another.
Trophy Hunting and Inadvertent Evolution: A Case Study
Bighorn sheep populations have been
reduced by 90% by hunting, habitat
loss, and introduction of domestic
cattle.
Hunting is now restricted in North
America; permits to take a large
“trophy ram” cost over $100,000.
Figure 6.1 Fighting over the Right to Mate
Trophy Hunting and Inadvertent Evolution: A Case Study
Trophy hunting removes the largest and
strongest males—the ones that would
sire many healthy offspring.
In one population, 10% of males were
removed by hunting each year.
The average size of males and their
horns decreased over 30 years of study.
Figure 6.2 Trophy Hunting Decreases Ram Body and Horn Size
Trophy Hunting and Inadvertent Evolution: A Case Study
This is also being observed in other
species:
• By targeting older, larger fish,
commercial cod fishing has selected
for genes that result in maturation at
earlier ages and smaller size.
Fish that mature earlier can
reproduce before they are caught, but
small fish produce fewer eggs.
Trophy Hunting and Inadvertent Evolution: A Case Study
• African elephants are poached for
ivory; the proportion of the population
that have tusks is decreasing.
The unintended effects of human
harvesting on these animals illustrate
how populations can change, or evolve,
over time.
Introduction
Humans have a large impact on the
environment—pollution, land use
change, climate change, etc.
We are just beginning to realize that we
also cause evolutionary change and the
consequences of this.
Ecology and evolution are strongly
interconnected.
CONCEPT 6.1
Evolution can be viewed as genetic change
over time or as a process of descent with
modification.
Concept 6.1
What Is Evolution?
Horn size in bighorn sheep is a heritable
trait. Trophy hunting selectively
eliminates rams with large horns, thus
favoring rams with genes for small
horns.
Trophy hunting is likely causing the
genetic makeup of bighorn sheep
populations to change, or evolve, over
time.
Concept 6.1
What Is Evolution?
Genes are made of DNA and specify
(encode) protein structure.
Genes can have two or more forms
called alleles.
Genotype is the genetic makeup of an
individual.
Concept 6.1
What Is Evolution?
A genotype is represented by letters, one
for each allele.
One allele is inherited from the mother,
one from the father.
Example: Two alleles, A and a;
individuals could be AA, Aa, or aa.
Concept 6.1
What Is Evolution?
Evolution is change in allele frequencies
(proportions) in a population over time.
For example, if the frequency of a in a
population is 0.4 or 40%, the frequency
of A is 0.6 or 60%.
If the frequency of a changed to 71%,
the population would have evolved at
that gene.
Concept 6.1
What Is Evolution?
Evolution can be defined more broadly
as descent with modification.
As a population accumulates differences
over time and a new species forms, it is
different from its ancestors.
But the new species has many of the
same characteristics as its ancestors
and resembles them.
Concept 6.1
What Is Evolution?
Charles Darwin used the phrase
“descent with modification.”
He proposed that populations change
over time through natural selection:
Individuals with certain heritable traits
survive and reproduce more
successfully than other individuals.
Concept 6.1
What Is Evolution?
If two populations experience different
environmental conditions, different
characteristics may be favored.
Natural selection causes the populations
to diverge genetically over time.
Figure 6.4 Natural Selection Can Result in Differences between Populations
Concept 6.1
What Is Evolution?
Natural selection acts as a sorting
process.
Individuals with favored traits have more
offspring, and their alleles will increase
in frequency in the population.
The population will evolve, but
individuals do not evolve.
CONCEPT 6.2
Natural selection, genetic drift, and gene
flow can cause allele frequencies in a
population to change over time.
Concept 6.2
Mechanisms of Evolution
Phenotype: Observable characteristics
that are determined by the genotype.
Individuals differ from one another in part
because they have different alleles for
genes.
Figure 6.5 Individuals in Populations Differ in Their Phenotypes
Concept 6.2
Mechanisms of Evolution
Different alleles arise by mutation—a
change in DNA.
Mutations can result from copying errors
during cell division, mechanical
damage, exposure to chemicals
(mutagens) or high-energy radiation.
Concept 6.2
Mechanisms of Evolution
Formation of new alleles is critical to
evolution.
If mutation did not produce new alleles,
all members of a population would have
identical genotypes and evolution could
not occur.
Concept 6.2
Mechanisms of Evolution
Recombination also produces different
genotypes within a population.
Offspring have combinations of alleles
that differ from those of their parents.
Mutation provides the raw material on
which evolution is based;
recombination rearranges the raw
material into new combinations.
Concept 6.2
Mechanisms of Evolution
Mutations are actually very rare.
In a generation, one mutation would
occur in every 10,000 to 1,000,000
copies of a gene.
In one generation, mutation acting alone
causes virtually no change in allele
frequencies of a population.
Concept 6.2
Mechanisms of Evolution
Three types of natural selection:
1. Directional selection: Individuals at
one phenotypic extreme (e.g., large
size) are favored.
Example: Drought favored large beak
size in medium ground finches.
Figure 6.6 Three Types of Natural Selection (Part 1)
Concept 6.2
Mechanisms of Evolution
2. Stabilizing selection: Individuals
with an intermediate phenotype are
favored.
Example: Parasitic wasps select for
small gall size of Eurosta flies; while
birds select for large gall size.
Figure 6.6 Three Types of Natural Selection (Part 2)
Concept 6.2
Mechanisms of Evolution
3. Disruptive selection: Individuals at
both phenotypic extremes are favored.
Example: African seedcrackers (birds)
have two food sources—hard seeds
that require large beaks to crack, and
smaller, softer seeds that smaller
beaks are more suited to.
Figure 6.6 Three Types of Natural Selection (Part 3)
Concept 6.2
Mechanisms of Evolution
Natural selection can result in
populations in which all individuals
have the favored allele:
Andean geese have evolved a type of
hemoglobin with a very high affinity for
O2, an advantage at high altitudes.
The allele frequency for this trait is
100%.
Concept 6.2
Mechanisms of Evolution
Genetic drift occurs when chance
events determine which alleles are
passed to the next generation.
It is significant only for small populations.
Concept 6.2
Mechanisms of Evolution
Genetic drift has four effects on small
populations:
1. It acts by chance alone, thus causing
allele frequencies to fluctuate at
random.
Some may disappear, others may
reach 100% frequency (fixation).
Figure 6.7 Genetic Drift Causes Allele Frequencies to Fluctuate at Random
Concept 6.2
Mechanisms of Evolution
2. Because some alleles are lost, genetic
variation of the population is reduced.
3. Frequency of harmful alleles can
increase if the alleles have only mildly
deleterious effects.
4. Differences between populations can
increase. Chance events may lead to
allele fixation in one population and
loss from another population.
Concept 6.2
Mechanisms of Evolution
2 and 3 can have dire consequences.
Loss of genetic variation reduces the
ability of the population to respond to
changing environmental conditions.
Increase of harmful alleles can reduce
survival and reproduction.
These effects are important for species
that are near extinction.
Concept 6.2
Mechanisms of Evolution
Gene flow: Alleles move between
populations via movement of
individuals or gametes.
Gene flow has two effects:
1. Populations become more similar.
2. New alleles can be introduced into a
population.
CONCEPT 6.3
Natural selection is the only evolutionary
mechanism that consistently causes
adaptive evolution.
Concept 6.3
Adaptive Evolution
Adaptations are features of organisms
that improve their ability to survive and
reproduce.
Adaptations include morphological and
physiological features such as enzymes
that function at high temperatures.
Figure 6.11 Adaptive Evolution in Soapberry Bugs
Concept 6.3
Adaptive Evolution
Rapid adaptive evolution can happen on
a continental scale.
Clines are patterns of change in a
characteristic over a geographic region.
Figure 6.12 Rapid Adaptive Evolution on a Continental Scale
Concept 6.3
Adaptive Evolution
Adaptive evolution is driven by
ecological interactions—organisms
interacting with one another and with
their environment.
Ecology is a basis for understanding
natural selection.
CONCEPT 6.4
Long-term patterns of evolution are shaped
by large-scale processes such as
speciation, mass extinction, and adaptive
radiation.
Concept 6.4
The Evolutionary History of Life
Species: Group of organisms whose
members have similar characteristics
and can interbreed.
Speciation: The process by which one
species splits into two or more species.
Concept 6.4
The Evolutionary History of Life
Speciation most commonly occurs when
a barrier prevents gene flow between
two or more populations of a species.
Barriers can be geographic or ecological.
The populations then diverge
genetically over time.
Figure 6.14 Speciation by Genetic Divergence
Concept 6.4
The Evolutionary History of Life
The key step in speciation occurs when
a population accumulates so many
genetic differences that they cannot
produce viable, fertile offspring if they
mate with the parental species.
Concept 6.4
The Evolutionary History of Life
Repeated speciation events increases
the number of species in a group, but
some species are also lost to
extinction.
An evolutionary tree is a branching
diagram that represents the
evolutionary history of a group.
Figure 6.16 An Evolutionary Tree of the Pinnipeds (Part 1)
Figure 6.18 The “Big Five” Mass Extinctions
Concept 6.4
The Evolutionary History of Life
Each mass extinction was followed by an
adaptive radiation: Increased diversity
of the surviving groups.
Mass extinctions remove competitor
groups, allowing survivors to expand
into new habitats or new ways of life.
Concept 6.4
The Evolutionary History of Life
Adaptive radiations can also occur when
a group evolves major new
adaptations.
Example: Stems that provide support
and waxy cuticles that prevent drying
allowed adaptive radiation of terrestrial
plants.
Concept 6.4
The Evolutionary History of Life
Biological communities are devastated
by mass extinction events.
It takes millions of years for diversity to
increase to the levels seen prior to the
mass extinction.
This has great implications if human
activities cause a sixth mass extinction.
Concept 6.4
The Evolutionary History of Life
Coevolution: Reciprocal evolutionary
change in interacting species.
CONCEPT 6.5
Ecological interactions and evolution exert
a profound influence on one another.
Concept 6.5
Joint Effects of Ecology and Evolution
The sunflower Helianthus anomalus
arose as a hybrid between two other
species.
The hybrid has new gene combinations
that allow it to grow in a different
environment than the parent species.
In the new environment, the hybrid
became a new species.
Figure 6.21 A Hybrid That Lives in a New Environment
Concept 6.5
Joint Effects of Ecology and Evolution
Hybridization resulted in an ecological
shift that illustrates how evolution
influences ecology.
But life under different ecological
conditions provided the selection
pressures that molded the hybrid into a
new species, showing how ecology
influences evolution.
Concept 6.5
Joint Effects of Ecology and Evolution
Evolution can result from a range of
ecological interactions, including
predation, competition, herbivory,
parasitism, and mutualism.
Speciation is often caused by ecological
factors.
Concept 6.5
Joint Effects of Ecology and Evolution
Evolution can alter ecological interactions.
If a predator evolves a new way to
capture prey, they prey species may go
extinct, decline, migrate to other areas,
or evolve new ways to cope with the
more efficient predator.
Connections in Nature: The Human Impact on Evolution
When human actions drive a species to
extinction, the course of evolution is
altered.
Many scientists think humans are causing
a sixth mass extinction.
If so, our actions will greatly and
irreversibly change the evolutionary
history of life on Earth.