Comparing Chordates

• A red-billed oxpecker perches on an impala

• These chordates have a mutually beneficial relationship

• Oxpeckers pick ticks and other external parasites off the impala, obtaining food while ridding their host of parasites

Comparing Chordates

Chordate Evolution

• Ever since the first chordates appeared more than 500 million years ago, they have been evolving

• During this continual process, chordates developed an incredible variety of adaptations

• Some of these traits—scales or hair, for example—are relatively simple

• Others—such as a four-chambered heart or an amniotic egg—are far more complex

• All these adaptations were tested and All these adaptations were tested and shaped by natural selectionshaped by natural selection

Chordate Origins

• Much of what scientists know about the origins of chordates comes from studying the embryos of living organisms

• Such studies suggest that the most ancient chordates were closely related to echinodermsechinoderms

• Do scientists know what these early chordates looked like?

• Surprisingly, the answer is yes

Chordate Origins• The variety of fossilized organisms

preserved in the rich Cambrian deposits of Canada's Burgess Shale includes a peculiar organism called Pikaia, shown in the figure at right

• When Pikaia was first discovered, it was thought to be a worm

• On closer inspection, scientists determined that Pikaia had a notochord—a flexible, supporting structure that is found only in chordates

• Pikaia also had paired serial muscles that were arranged in a manner similar to those of today's nonvertebrate chordates, such as lancelets

• On the basis of fossil evidence, On the basis of fossil evidence, scientists now classify scientists now classify PikaiaPikaia as an as an early chordateearly chordate

Chordate Origins

• To better understand the early evolution of chordates, biologists study a nonvertebrate chordate that is alive today—the tunicatetunicate

• The tadpolelike larvae of tunicates are the simplest living animals to have a notochord, a notochord, a dorsal hollow nerve cord, a tail that a dorsal hollow nerve cord, a tail that extends posterior to the anus, and extends posterior to the anus, and pharyngeal pouches—key features common pharyngeal pouches—key features common to all chordatesto all chordates

• Today, biologists are studying the genes that control the development of these features

Pikaia

• This is a reconstruction of Pikaia, a soft-bodied animal that lived during the Cambrian Period

• Which features did Pikaia have that were characteristic of chordates?

Pikaia

The Chordate Family Tree• The chordate family tree has its roots in

ancestors that vertebrates share with tunicates and lancelets

• The cladogram in the figure at right shows chordate phylogeny—how the different groups of living chordates are related to one another and to their invertebrate ancestors

• It also shows the evolution of distinctly vertebrate features, such as jaws and limbs

• Notice that the fishes—from hagfishes to lungfishes—include six different groups include six different groups with long and separate evolutionary with long and separate evolutionary historieshistories

• On the other hand, modern amphibians, reptiles, birds, and mammals share much share much more recent common ancestorsmore recent common ancestors

• Where do extinct groups, such as dinosaurs, fit into the chordate phylum?

• The answer may be found in the fossil record

Chordate Cladogram • The phylum Chordata includes both

vertebrates and nonvertebrate chordates

• All of these subphyla share a common invertebrate ancestor

• This cladogram shows the phylogenetic relationship of modern chordate groups to that common ancestor

• The different colored lines represent the traditional groupings of these animals, as listed in the key

• The red circles indicate some of the The red circles indicate some of the important chordate adaptationsimportant chordate adaptations

• Such adaptations are the results of evolutionary processes, including natural selection

Chordate Cladogram

Evolutionary Trends in Vertebrates

• The hard body structures of many vertebrates have left behind an excellent fossil record

• As a result, scientists know a great deal about vertebrates' evolutionary history

• In addition, scientists infer evolutionary trends by studying the characteristics of chordates living today

Adaptive Radiations  • The number of species within each chordate

group has changed over geologic time• Look at the figure Chordate Cladogram

again• The red circles in that figure represent

the origin of certain adaptive features• For example, one notable event in

chordate evolution was the development of jaws

• Another event was the development of paired appendages, including pectoral and pelvic fin or limb girdles

• Paired appendages allowed chordates, such as the salamander in the figure at right, to move more efficiently

• Over the course of evolution, the appearance of new adaptations—such as jaws and paired appendages—has launched adaptive radiations in chordate groups

• An adaptive radiation is the rapid An adaptive radiation is the rapid diversification of species as they adapt diversification of species as they adapt to new conditions to new conditions

Chordate Movement

• Amphibians were the first chordates to have four limbs

• Limbs allowed animals like this modern tiger salamander to crawl on land

• A rapid increase in the number and diversity of land vertebrates followed the evolution of four limbs

Chordate Movement

Convergent Evolution 

• Adaptive radiationsAdaptive radiations sometimes produce species that are similar in appearance and behavior, even though they are not closely related– This trend is called convergent evolutionThis trend is called convergent evolution

• Convergent evolution occurred many times Convergent evolution occurred many times during chordate evolution during chordate evolution when unrelated when unrelated species encountered similar ecological species encountered similar ecological conditions and evolved similar adaptationsconditions and evolved similar adaptations– For example, convergent evolution has produced

flying vertebrates as different as birds and bats

Chordate Diversity• Living chordates are extremely diverse, as

shown in the figure• Yet, the species of chordates that are

alive today are a small fraction of the total number of chordate species that have existed over time

• Today, vertebrates make up about 96 percent of all living chordate species and account for more than 50,000 species throughout the world

• The six living groups of chordates are the:

– Nonvertebrate chordatesNonvertebrate chordates– FishesFishes– AmphibiansAmphibians– ReptilesReptiles– BirdsBirds– MammalsMammals

• Of these, the largest group by far is Of these, the largest group by far is the fishes the fishes

Chordate Diversity• This pie chart shows the diversity

of chorates• The area of each sliceeach slice

represents the relative number of living species in each group of chordates

• The inner circleinner circle shows the six major chordate groups and gives the percentage of species contained in each

• The outer circleouter circle breaks down each major group and shows the number of known species

• Of the total number of fish species, what percentage is represented by the ray-finned fishes?

Chordate Diversity

Controlling Body Temperature

• On a spring morning, after a cold night, a tortoise lies on a rock basking in the sun

• Nearby, a snake slides out of its burrow beneath a rotting stump

• In a tree overhead, a young robin puffs up its downy feathers

• As you walk out of the water after an early swim, your skin gets goose bumps and you shiver

• All these activities are examples of the different ways that vertebrates control their body temperature

Body Temperature and Homeostasis

• Recall from Chapter 2 that many of the chemical reactions that are important in metabolism are influenced by temperature

• For this reason, essential life functions can be carried out most efficiently when an animal's internal body temperature is within a particular “operating range”

• For muscles to operate quickly and efficiently, for example, their temperature can neither be too low nor too high– If muscles are too cold, they may contract slowly, making it

difficult for the animal to respond quickly to events around it– If an animal gets too hot, on the other hand, its muscles may

tire easily and other body systems may not function properly

Body Temperature and Homeostasis

• Because most chordates are vertebrates, and mechanisms for controlling body temperature are well developed among vertebrates, this section will focus exclusively on that group

• The control of body temperature is important for maintaining homeostasis in vertebrates, particularly in habitats where temperature varies widely with time of day and with season

• Vertebrates have a variety of ways to control their body temperature

• All of these ways incorporate three important features:– Source of heat for the bodySource of heat for the body– Way to conserve that heatWay to conserve that heat– Method of eliminating excess heat when necessaryMethod of eliminating excess heat when necessary

• In terms of how they generate and control their body heat, vertebrates can be classified into two basic groups:– EctothermsEctotherms– EndothermsEndotherms

Ectothermy

• On cool, sunny mornings, lizards often bask in the sun

• This doesn't mean that they are lazy!• A lizard is an ectotherm, which means that its body

temperature is mainly determined by the temperature of its environment– Most reptiles, fishes, and amphibians are ectotherms—

animals whose body temperatures are controlled primarily by picking up heat from, or losing heat to, their environment

• Ectotherms often warm up by basking in the sun, and may cool down by seeking shelter in underground burrows

Ectothermy

• Ectotherms have relatively low rates of metabolism when they are resting

• Thus, their bodies do not generate much heat

• When active, an ectotherm's muscles generate heat, just as your muscles do

• However, because its body lacks effective insulation, the heat is lost to the environment fairly easily

Endothermy 

• An endotherm is an animal whose body temperature is controlled from within– Birds and mammals are endotherms, which means they can

generate and retain heat inside their bodies• Endotherms have relatively high metabolic rates that

generate a significant amount of heat, even when they are resting– Birds conserve body heat primarily through insulating feathers,

such as down– Mammals have body fat and hair for insulation

• Mammals can get rid of excess heat by panting, as Mammals can get rid of excess heat by panting, as dogs do, or by sweating, as humans dodogs do, or by sweating, as humans do

Comparing Ectotherms and Endotherms

• In an absolute sense, neither endothermy nor ectothermy is superior

• Each strategy has advantages and disadvantages in different environments– For example, endotherms move around easily during cool nights

or in cold weather because they generate and conserve their own body heat

– That's how musk ox live in the tundra and killer whales swim through polar seas

– But the high metabolic rate that generates that heat requires a lot of fuel

• The amount of food needed to keep a single cow alive would be enough to feed ten cow-sized lizards!

Comparing Ectotherms and Endotherms

• Ectothermic animals, such as the gila monster, need much less food than similarly sized endotherms

• In environments where temperatures stay warm and fairly constant most of the time, ectothermy is a more energy-efficient strategy– But large ectotherms run into trouble in habitats where

temperatures get cold at night or stay cold for long periods, such as boreal forest biomes

– It takes a long time for a large animal to warm up in the sun after a cold night

• Most large lizards and amphibians live in warm areas such as tropical rain forest biomes

Evolution of Temperature Control

• There is little doubt that the first land vertebrates were ectotherms

• But there is some doubt as to when endothermy evolvedBut there is some doubt as to when endothermy evolved– Although modern reptiles are ectotherms, some biologists

hypothesize that at least some of the dinosaurs were endotherms– Others hypothesize that endothermy evolved a long time after the

appearance of the dinosaurs, so that all the dinosaurs were ectotherms

• Evidence suggests that endothermy has evolved more than Evidence suggests that endothermy has evolved more than one timeone time– It developed once along the evolutionary line of reptiles that led to It developed once along the evolutionary line of reptiles that led to

birds and once along the evolutionary line of reptiles that led to birds and once along the evolutionary line of reptiles that led to mammalsmammals

Form and Function in Chordates

• The nonvertebrate chordates that are alive today represent a simple and ancient stage in the development of chordate body systems– However, the fact that the organ systems

are simple does not mean they are inferior– After all, lancelets and tunicates have

survived to the present day, so their body systems are well equipped to perform the essential functions of life

Form and Function in Chordates

• Among vertebrates, organ systems exhibit a wider range of complexity than those of nonvertebrate chordates– Many adaptive radiations of vertebrates have

produced a variety of specialized organ systems that perform essential functions and maintain homeostasis

• The complexity of vertebrate organ systems can be seen in the different ways that vertebrates feed, breathe, respond, move, and reproduce

Feeding

• Feeding and digestion help maintain homeostasis by providing the body with a continuing supply of needed nutrients

• Most tunicates, and all lancelets, are filter feeders

• These chordates remove small organisms called plankton from the water that passes through their pharynx

• A few adult tunicates feed on deposited material from the surface of the sediments on which they dwell

Feeding

• The skulls and teeth of vertebrates are adapted for feeding on a much wider assortment of foods, ranging from insects to large mammals, and from leaves to fruits and seeds

• Some vertebrates—such as baleen whales, flamingoes, and manta rays—are filter feeders with sievelike mouth structures that enable them to strain small crustaceans and fish from the water

• The long bill of the hummingbird and the narrow snout of the honey possum are both adaptations that enable them to feed on nectar

• Other vertebrates, such as the crocodile are adapted to eating meat

• Many mammals have sharp canine teeth and incisors that they use to tear and slice their food

Crocodile Feeding

• The blunt, broad jaws and numerous peglike teeth of this crocodile help it catch large prey—such as zebra—even in thick vegetation

• How do the mouth structures of a filter-feeding vertebrate differ from those of a carnivore like this reptile?

Crocodile Feeding

Feeding

• The digestive systems of vertebrates have organs that are well adapted for different feeding habits

• Carnivores such as sharks typically have short digestive tracts that produce fast-acting, meat-digesting enzymes

• Herbivores such as cows, on the other hand, often have long intestines that harbor colonies of bacteria– These bacteria are helpful in digesting the tough These bacteria are helpful in digesting the tough

cellulose fibers in plant tissues cellulose fibers in plant tissues

Vertebrate Digestive Systems

• The digestive systems of vertebrates are adapted for a variety of feeding modes

• As you can see, these systems differ in their degree of complexity

Vertebrate Digestive Systems

Respiration

• Chordates typically have one of two basic structures for respiration, or gas exchange

• As a general rule, aquatic chordates—such as tunicates, fishes, and amphibian larvae—use gills for respirationgills for respiration

• Land vertebrates, including adult amphibians, reptiles, birds, and mammals, use lungslungs

• However, some animals “break the rules”:– For example, several fishes, such as lungfishes, For example, several fishes, such as lungfishes,

have both gills and lungshave both gills and lungs

Respiration

• Some chordates have respiratory structures in addition to gills and lungs– Many bony fishes, for example, have accessory

organs for respiration, such as simple air sacs, that are derived from the gut

– All lancelets and some sea snakes respire by the diffusion of oxygen across their body surfaces

• Recall that diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration

– Many adult amphibians use their moist skins and the linings of their mouths and pharynxes to respire by diffusion

Gills

• The figure below shows how gills function in chordates

• As water passes over the gill filaments, oxygen molecules diffuse into blood in tiny blood vessels called capillaries

• At the same time, carbon dioxide diffuses from blood into the water

Gill Function 

• Fishes and many other aquatic chordates use gills for respiration

Gill Function 

Lungs

• Although the structure of the lungs varies, the basic process of breathing is the same among land vertebrates

• Inhaling brings oxygen-rich air from outside the body through the trachea and into the lungs

• The oxygen diffuses into the blood inside the lung capillaries

• At the same time, carbon dioxide diffuses out of the capillaries into the air within the lungs

• Oxygen-poor air is then exhaled

Lungs • As you move from amphibians to

mammals, the surface area of the lungs increases

• Observe this trend in the figure at right• The typical amphibian lung is little more

than a sac with ridges• Reptilian lungs are often divided into a

series of large and small chambers that increase the surface area available for gas exchange

• In mammals, the lungs branch extensively, and their entire volume is filled with thousands of bubblelike structures called alveoli (singular: alveoli (singular: alveolus)alveolus)

– Alveoli provide an enormous surface area Alveoli provide an enormous surface area for gas exchangefor gas exchange

• This lung structure enables mammals to This lung structure enables mammals to take in the large amounts of oxygen take in the large amounts of oxygen required by their endothermic required by their endothermic metabolismmetabolism

– However, because air must move in and out through the same passageways, there is always stale, oxygen-poor air trapped in the lungs of mammals and most other vertebrates

Lungs

• In contrast, in the lungs of birds, air air flows in only one directionflows in only one direction– A system of tubes in a bird's lungs, plus

air sacs, enables this one-way air flow

• Thus, gas exchange surfaces are Thus, gas exchange surfaces are constantly in contact with fresh air that constantly in contact with fresh air that contains a lot of oxygencontains a lot of oxygen– This supply of oxygen enables birds to fly at

high altitudes, where there is less oxygen in the atmosphere than at lower altitudes

Vertebrate Respiration

• Unlike most aquatic chordates, land vertebrates—like salamanders, lizards, birds, and primates—use lungs to breathe

• A few aquatic chordates, such as sea turtles and marine mammals, use lungs as well

Vertebrate Respiration

Circulation

• Circulatory systems maintain homeostasis by transporting materials throughout animals' bodies

• The first chordates, like tunicates and lancelets of today, probably had simple circulatory systems

• Tunicates have short, tubelike hearts with a simple pump but no true chambers

• Lancelets have a fairly well-developed circulatory system but no specialized heart

Single- and Double-Loop Circulation 

• As chordates evolved, more complex organ systems and more efficient channels for internal transport developed

• The figure at right shows the main transport systems in vertebrates

• Those that use gills for respiration have a single-have a single-loop circulatory systemloop circulatory system

• In this system, blood travels blood travels from the heart to the gills, from the heart to the gills, then to the rest of the body, then to the rest of the body, and back to the heart in one and back to the heart in one circuitcircuit

Vertebrate Circulatory Systems

• Most vertebrates that use gills for respiration have a single-loop circulatory system that forces blood around the body in one direction

• Vertebrates that use lungs have a double-loop system

• The hearts of fishes have two chamberstwo chambers

• Amphibians and most reptiles have three-three-chambered heartschambered hearts

• Crocodilians, birds, and mammals have hearts with hearts with four separate chambersfour separate chambers

Vertebrate Circulatory Systems

Single- and Double-Loop Circulation 

• Vertebrates that use lungs for respiration have a double-loop circulatory system

• The first loop carries blood between the heart and lungs– Oxygen-poor blood from the heart is pumped to the

lungs, while oxygen-rich blood from the lungs returns to the heart

• The second loop carries blood between the heart and the body– Oxygen-rich blood from the heart is pumped to the

body, while oxygen-poor blood from the body returns to the heart

Heart Chambers 

• Chordate hearts are adapted to the complexity of internal transport for each of the different groups

• During the course of chordate evolution, the heart developed chambers and partitions that help separate oxygen-rich and oxygen-poor blood traveling in the circulatory system– In vertebrates that use gills for respiration, such

as fishes and larval amphibians, the heart consists of two chambers: an atrium that receives blood from the body, and a ventricle that pumps blood to the gills and then on to the rest of the body

Heart Chambers 

• The hearts of most amphibians have three chambers: two atria and one ventricle

• The left atrium receives oxygen-rich blood from the lungs• The right atrium receives oxygen-poor blood from the body• Both atria empty into the ventricle

– There is some mixing of oxygen-rich and oxygen-poor blood in the ventricle

– However, the internal structure of the ventricle directs the However, the internal structure of the ventricle directs the flow of blood so that most oxygen-poor blood goes to the flow of blood so that most oxygen-poor blood goes to the lungs, and most oxygen-rich blood goes to the rest of the lungs, and most oxygen-rich blood goes to the rest of the bodybody

Heart Chambers 

• Most reptiles have a three-chambered heart– However, unlike amphibians, most reptiles

have a partial partition in their ventriclepartial partition in their ventricle• Because of this partition, there is even less

mixing of oxygen-rich and oxygen-poor blood than there is in amphibian hearts

Heart Chambers

• Birds, mammals, and crocodilians have hearts that are completely partitioned into four chambers– This type of heart is sometimes described

as a double pump• One pump moves blood through the lung loop One pump moves blood through the lung loop

and the other moves blood through the body and the other moves blood through the body looploop

– The two loops of the circulatory system are completely separated

– There is no mixing of oxygen-rich and oxygen-poor blood

Excretion

• Excretory systems eliminate nitrogenous wastes from the body– In nonvertebrate chordates and fishes,

gills and gill slitsgills and gill slits play an important role in excretion

– However, most vertebrates rely on kidneys—excretory organs composed of small excretory organs composed of small filtering tubes that remove wastes from the filtering tubes that remove wastes from the bloodblood

Excretion

• Nitrogenous wastes—formed from the Nitrogenous wastes—formed from the breakdown of proteins—are first produced in breakdown of proteins—are first produced in the form of ammoniathe form of ammonia

• Ammonia is a highly toxic compoundAmmonia is a highly toxic compound that must quickly be eliminated from the body or changed into a less poisonous form– In tunicates, ammonia leaves the body through the

outflow siphons• Other waste byproducts, such as uric acid, are stored

within the tunicate's body and released only when the animal dies

Excretion

• In vertebrates, excretion is carried out mostly by the kidneys

• Aquatic amphibians and most fishes also excrete ammonia directly from the gills into the surrounding water through simple diffusion

• In mammals, land amphibians, and cartilaginous fishes, ammonia is changed into ureaurea, a less-toxic compound, before it is excreted

• In most reptiles and birds, ammonia is changed into uric aciduric acid

• Besides filtering wastes, vertebrate kidneys help maintain homeostasis by regulating the amounts of water, salt, and other substances dissolved in body fluids

Response

• Compared with invertebrates, most chordates have elaborate systems that allow them to respond to stimuli in their environment– Nonvertebrate chordates have a relatively

simple nervous system with a mass of nerve cells that form a brain

– Vertebrates have a more complex brain with distinct regions, each with a different function

Response

• Nonvertebrate chordates do not have specialized sensory organs

• In tunicates, however, sensory cells in and on the siphons and other internal surfaces may help control the amount of water passing through the pharynx

• Lancelets—which have a more defined head region—have a small, hollow brain with a pair of eyespots that detect light

Response

• Vertebrates display a high degree of cephalization,cephalization, or concentration of sense organs and nerve cells at the front of the body– The head contains a well-developed brain, which is situated

on the anterior end of the spinal cord• The vertebrate brain is divided into several parts,

including the cerebrum, cerebellum, medulla cerebrum, cerebellum, medulla oblongata, optic lobes, and olfactory bulbsoblongata, optic lobes, and olfactory bulbs

• The medulla oblongata controls the functioning of many internal organs

• The optic lobes are involved in vision and the olfactory bulbs are involved in the sense of smell

Response• The figure shows how the size and complexity of the cerebrum and cerebellum

increase from fishes to mammals• The cerebrum is the “thinking” region of the brain

– It receives, interprets, and determines the response to sensory information– The cerebrum is also involved in learning, memory, and conscious thought

• In fishes, amphibians, and reptiles, the cerebrum is relatively smallIn fishes, amphibians, and reptiles, the cerebrum is relatively small• In birds and mammals, especially primates, the cerebrum is greatly enlarged and may contain In birds and mammals, especially primates, the cerebrum is greatly enlarged and may contain

folds that increase its surface areafolds that increase its surface area

• The cerebellum, which coordinates movement and controls balance, is also The cerebellum, which coordinates movement and controls balance, is also most developed in birds and mammals most developed in birds and mammals

Vertebrate Brains

• The size and complexity of the cerebrum and cerebellum increase as you move from fishes to mammals

• Each region of the vertebrate brain serves a different function

Vertebrate Brains

Movement

• Unlike most other chordates, nonvertebrate chordates lack bonesnonvertebrate chordates lack bones– They do, however, have muscles

• Lancelets and larval tunicates swim with a fishlike movement of their muscular tails

• Some adult tunicates use their siphons to swim by jet propulsion

• However, most adult tunicates lose their tails and attach to a hard surface on the ocean floor for life

Movement• The skeletal and muscular systems

support a vertebrate's body and make it possible to control movement

• Vertebrates are much more mobile than nonvertebrate chordates. With the exception of hagfishes, all vertebrates have an internal skeleton of bone—as shown in the figure—or, in the case of certain fishes, cartilage

• The skeleton includes a backbone made up of individual bones called individual bones called vertebraevertebrae

• In most vertebrates, tough yet flexible tissues called ligaments connect the vertebrae and allow the backbone to bend without falling apart

• Most vertebrates have fin girdles or limb girdles that support the fins or limbs

Vertebrate Skeleton 

• Like the skeletons of most vertebrates, this lizard's skeleton has two pairs of appendages

• Muscles and ligaments attach the appendages to the backbone and help control movement

Vertebrate Skeleton 

Movement

• In many fishes and snakes, the main body muscles are arranged in blocks on either side of the backbone– These muscle blocks contract in waves that make the

body bend back and forth, generating forward thrust• In many amphibians and reptiles, the limbs stick out

sideways from the body in a position resembling a push-up

• Most mammals stand with their legs straight under them, whether they walk on two legs or on four– In this position, the legs can support the body weight

efficiently

Reproduction

• Chordates are diverse in the ways they reproduce and develop

• Almost all chordates reproduce sexuallyAlmost all chordates reproduce sexually – Vertebrate evolution shows a general trend from

external to internal fertilization

• The eggs of most nonvertebrate chordates—and many fishes and amphibians—are fertilized externally

• The eggs of reptiles, birds, and mammals are fertilized internally

Reproduction

• After fertilization, the development of chordates can be can be oviparous, ovoviviparous, or viviparousoviparous, ovoviviparous, or viviparous

• In oviparous speciesoviparous species, which include most fishes and amphibians and all birds, the eggs develop outside the mother's body

• In ovoviviparous animalsovoviviparous animals, such as sharks, the eggs develop within the mother's body and the embryos receive nutrients from the yolk in the egg– The young of ovoviviparous species are born alive

• The developing embryos of viviparous speciesviviparous species—including most mammals—obtain nutrients directly from the mother's body– As with ovoviviparous species, the young of viviparous animals are born

alive

Reproduction

• Some vertebrates, such as most amphibians, produce many offspring but give them little or no care– This reproductive strategy is successful in

circumstances favoring populations that disperse and grow rapidly

• Mammals and birds, in contrast, usually care for their young but produce few of them– This helps young survive in crowded, competitive

environments