Chapter 14

Macroevolution

Macroevolution and the Diversity of Life

– Microevolution• A generation-to-generation change in a

population’s frequencies of alleles.

– Macroevolution• Major changes in the history of life (fossil record)• Formation of new species which generates

biological diversity.

– The formation of new species: Speciation• In nonbranching evolution, a population

transforms but does not create a new species.• In branching evolution, one or more new

species branch from a parent species that may continue to exist.

What Is a Species?– Species is a Latin word meaning “kind” or

“appearance.”– The biological species concept defines a

species as• A population or group of populations whose

members have the potential to interbreed and produce fertile offspring.

– The biological species concept cannot be applied in all situations, for example, with fossils and asexual organisms.

Important exceptions:

Reproductive Barriers between Species

– Prezygotic barriers prevent mating between species.

– Examples of behavioral isolation:

Blue-Footed Boobies Courtship Ritual

Giraffe Courtship Ritual

Albatross Courtship Ritual

Reproductive Barriers between Species

– Postzygotic barriers

• Are mechanisms that operate should interspecies mating actually occur and form hybrid zygotes.

– Postzygotic barriers include• Hybrid inviability.• Hybrid sterility.

Mechanisms of Speciation

– A key event in the potential origin of a species occurs when a population is somehow severed from other populations of the parent species.

– The two modes of speciation are• Allopatric speciation.• Sympatric speciation.

Allopatric Speciation– Geologic processes

• Can fragment a population into two or more isolated populations.

• Can contribute to allopatric speciation.

– Speciation occurs only with the evolution of reproductive barriers between the isolated population and its parent population.

Sympatric Speciation– Sympatric speciation occurs if a genetic

change produces a reproductive barrier between mutants and the parent population.

Polyploidy, a mechanism of sympatric speciation, was first observed by Hugo de Vries.

– Polyploids• Can originate from

accidents during meiosis (nondisjunction).

• Can result from the hybridization of two parent species.

– Many domesticated plants are the result of sympatric speciation.

How Accidents During Meiosis Can Alter Chromosome Number

– In nondisjunction,• The members of a chromosome pair fail to separate

during anaphase.• Gametes with an incorrect number of chromosomes

are produced.

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Figure 8.20

– The result of nondisjunction

Down Syndrome: An Extra Chromosome 21

– Down Syndrome• Is a condition where an individual has an extra

chromosome 21.• Is also called trisomy 21.

– Polyploids• Can originate from

accidents during meiosis (nondisjunction).

• Can result from the hybridization of two parent species.

– Many domesticated plants are the result of sympatric speciation.

What Is the Tempo of Speciation?

– Traditional evolutionary trees diagram the descent of species as gradual divergence.

– Punctuated equilibrium• Is a contrasting model of evolution.• States that species most often diverge in spurts of

relatively sudden change.• Accounts for the relative rarity of transitional fossils.

The Evolution of Biological Novelty

– What accounts for the evolution of biological novelty?

Adaptation of Old Structures for New Functions

– Birds• Are derived from a lineage of earthbound

reptiles.• Developed from flightless ancestors, but how?

– An exaptation:• Is a structure that evolves in one context, but

becomes adapted for another function

• Is a type of evolutionary remodeling

– Exaptations can account for the gradual evolution of novel structures.

– Bird wings are modified forelimbs that were previously adapted for non-flight functions, such as:

• Thermal regulation• Courtship displays• Camouflage

– The first flights may have been only glides or extended hops as the animal pursued prey or fled from a predator.

Evo-Devo: Development and Evolutionary Novelty

– A subtle change in a species’ developmental program can have profound effects, changing the:

• Rate • Timing • Spatial pattern of development

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– Evo-devo, evolutionary developmental biology, is the study of the evolution of developmental processes in multicellular organisms.

– Homeotic genes are master control genes that regulate:• When structures develop • How structures develop• Where structures develop

– Mutations in homeotic genes can profoundly affect body form.

– Watch PBS movie “What Darwin Never Knew”

Earth History and Macroevolution

– Macroevolution• Major changes in the history of life (fossil

record)• Formation of new species which generates

biological diversity.

• Is closely tied to the history of the Earth.

Geologic Time and the Fossil Record

– The fossil record• Is an archive of macroevolution.

– Geologists have established a geologic time scale reflecting a consistent sequence of geologic periods.

– pg 281 in your text

– Radiometric dating• Is the most common

method for dating fossils.

• Has helped establish the geologic time scale.

Plate Tectonics and Macroevolution

– The continents are not locked in place.

• They drift about Earth’s surface on plates of crust floating on a flexible layer called the mantle.

– California’s infamous San Andreas fault

• Is at a border where two plates slide past each other.

– About 250 million years ago

• Plate movements formed the supercontinent Pangaea.

• Many extinctions occurred, allowing survivors to diversify.

– About 180 million years ago

• Pangaea began to break up, causing geographic isolation.

Mass Extinctions and Explosive Diversifications of Life

– The fossil record reveals an episodic history,

• With long, relatively stable periods punctuated by briefer intervals when the turnover in species composition was much more extensive.

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

– Extinction is inevitable in a changing world and occurs all the time.

• However, extinction rates have not been steady.

– Extinctions typically eliminate various species of organisms

• And are followed by explosive diversifications of organisms.

The Process of Science: Did a Meteor Kill the Dinosaurs?

– Scientists believe that about 65 million years ago, at the end of the Cretaceous period,

• A meteor impact contributed to the extinction of the dinosaurs.

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

CLASSIFYING THE DIVERSITY OF LIFE

– Systematics focuses on:• Classifying organisms • Determining their evolutionary relationships

– Taxonomy is the:• Identification of species • Naming of species • Classification of species

Some Basics of Taxonomy

– Scientific names ease communication by:• Unambiguously identifying organisms • Making it easier to recognize the discovery of a new species

– Carolus Linnaeus (1707–1778) proposed the current taxonomic system based upon:

• A two-part name for each species • A hierarchical classification of species into broader groups of

organisms

Naming Species

– Each species is assigned a two-part name or binomial, consisting of:

• The genus • A name unique for each species

– The scientific name for humans is Homo sapiens, a two part name, italicized and latinized, and with the first letter of the genus capitalized.

Hierarchical Classification– Species that are closely related are placed into the same genus.

Jaguar (Panthera onca)

Lion (Panthera leo)Tiger (Panthera tigris)

Leopard (Panthera pardus)

– The taxonomic hierarchy extends to progressively broader categories of classification

Leopard (Panthera pardus)

SpeciesPantherapardus

GenusPanthera

FamilyFelidae

OrderCarnivora

ClassMammalia

PhylumChordata

KingdomAnimalia

DomainEukarya

Dumb Kids Play Chess On Full Guy’s Stomach

Classification and Phylogeny

– The goal of systematics is to reflect evolutionary relationships.

– Biologists use phylogenetic trees to:• Depict hypotheses about the evolutionary history of species • Reflect the hierarchical classification of groups nested within

more inclusive groups

Panthera pardus

(leopard)

SpeciesGenus

Felidae

Order

Carnivora

Family

Canis

Lutra

Panthera

Mephitis

Canidae

Mustelidae

Canis lupus(wolf)

Canis latrans

(coyote)

Lutralutra

(Europeanotter)

Mephitis mephitis

(striped skunk)

Figure 14.21

Sorting Homology from Analogy

– Homologous structures:• Reflect variations of a common ancestral plan • Are the best sources of information used to

– Develop phylogenetic trees – Classify organisms according to their evolutionary history

– Analogous Structures emerge through Convergent evolution:

• Involves superficially similar structures in unrelated organisms

• Is based on natural selection

– Similarity due to convergence:• Is called analogy, not homology• Can obscure homologies

Molecular Biology as a Tool in Systematics

– Molecular systematics:• Compares DNA and amino acid sequences between organisms • Can reveal evolutionary relationships

– Some fossils are preserved in such a way that DNA fragments can be extracted for comparison with living organisms.

The Cladistic Revolution

– Cladistics is the scientific search for clades.– A clade:

• Consists of an ancestral species and all its descendants

• Forms a distinct branch in the tree of life

– This is what Neil Shubin did in our reading selection from his book “Your Inner Fish” (2009).

Human Family Treefrom Shubin’s “Your Inner Fish” (2009)

Cladistics in action

Hair, mammaryglands

Long gestation

Gestation

Duck-billedplatypus

Iguana Outgroup(reptile)

Ingroup(mammals)

Beaver

Kangaroo

Figure 14.23

– Cladistics has changed the traditional classification of some organisms

Lizardsand snakes

Crocodilians

Saurischiandinosaurs

Ornithischiandinosaurs

Pterosaurs

Birds

Commonancestor ofcrocodilians,dinosaurs,and birds

Classification: A Work in Progress

– Linnaeus:• Divided all known forms of life between the plant and animal

kingdoms• Prevailed with his two-kingdom system for over 200 years

– In the mid-1900s, the two-kingdom system was replaced by a five-kingdom system that:

• Placed all prokaryotes in one kingdom• Divided the eukaryotes among four other kingdoms

– In the late 20th century, molecular studies and cladistics led to the development of a three-domain system

KingdomAnimalia

Domain Archaea Earliest

organisms

Domain Bacteria

Domain EukaryaKingdomFungi

KingdomPlantae

The protists(multiplekingdoms)

Evolution Connection: Rise of the Mammals

– Mass extinctions:• Have repeatedly occurred throughout Earth’s history • Were followed by a period of great evolutionary change

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– Fossil evidence indicates that:• Mammals first appeared about 180 million years ago • The number of mammalian species

– Remained steady and low in number until about 65 million years ago and then

– Greatly increased after most of the dinosaurs became extinct

American black bear

Eutherians (5,010 species)

Millions of years ago

Monotremes (5 species)

Marsupials (324 species)

Ancestral mammal

Reptilian ancestor

Extinction of dinosaurs

250 200 150 100 5065 0

Figure 14.26