6 Evolution and Ecology. Chapter 6 Evolution and Ecology CONCEPT 6.1 Evolution can be viewed as...

6Evolution 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 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


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.


• 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


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


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


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


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


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


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.

Concept 6.2


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.

Concept 6.2


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


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


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


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


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


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


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 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


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


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


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


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


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


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


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


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.

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