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Natural Selection
Charles Darwin observed that the individual members of any species show a great
deal of variation in their characteristics. They show differences in size, color, strength,
behavior, and many other features. Some of these characteristics are passed on from
parents to offspring. For centuries, in fact, plant and animal breeders have bred organisms
to emphasize or increase certain prized characteristics. Plant breeders select plants with
high resistance to diseases and mate them, hoping to obtain offspring with even higher
resistance. Likewise, horse breeders might select mating pairs to try to increase speed,
stamina, or both.
Sometimes these techniques result in significant differences from one breeding group
to another. The figure of various types of kale illustrates six different types of vegetables
that humans eat. All six plants are the same species. Humans have bred them for different
agricultural uses and to suit different human tastes.
cauliflower
cabbage
kale
broccoli
brussels
sprouts
kohlrabi
Six types of kale. These kale plants differ primarily in the part of the plant that stores most of the starch.
Darwin wondered. He thought that if humans could change plants, then natural
pressures could cause changes in species and organisms. The process that occurs in nature
is similar to selective breeding. The difference is that natural selection, not humans,
determines which organisms survive. A new species can result from natural selection
occurring across millions of years. Selective breeding has not produced a new species.
Remember that a species is a group of organisms whose members closely resemble
each. That is because they share many key characteristics. For example, all humans are
one species because we share many fundamental characteristics. Organisms belonging to
different species usually vary in key characteristics.
One characteristic that separates species is the ability to mate and produce fertile
offspring. A horse and a donkey can mate and produce offspring called mules. But because
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horses and donkeys belong to two different (although very closely related) species, the
offspring mules are sterile.
How does the process of natural selection take place? This figure shows the logic
of evolution. Evolutionary biologist Ernst Mayr proposed this summary of Darwin’s
thinking. The summary is a way to understand the evidence and inferences that underlie
evolution. Let’s follow the summary and try to understand some of the thinking that led
Darwin to his conclusions.
,ISTOF-AYRSPOINTS
&ACT !LL SPECIES HAVE SUCH GREAT POTENTIAL TO PRODUCE LARGE NUMBERS OF
OFFSPRING THAT THEIR POPULATION SIZE WOULD INCREASE EXPONENTIALLY IF ALL
INDIVIDUALSTHATAREBORNWOULDREPRODUCESUCCESSFULLY
&ACT %XCEPT FOR SEASONAL FLUCTUATIONS MOST POPULATIONS ARE NORMALLY
STABLEINSIZE
&ACT .ATURAL RESOURCES ARE LIMITED AND IN A STABLE ENVIRONMENT THEY
REMAINRELATIVELYCONSTANT
)NFERENCE "ECAUSE MORE INDIVIDUALS ARE PRODUCED THAN THE AVAILABLE
RESOURCESCANSUPPORTANDTHEPOPULATIONSIZEREMAINSSTABLETHEREMUST
BEAFIERCESTRUGGLEFOREXISTENCEAMONGTHEINDIVIDUALSOFAPOPULATION 4HIS
RESULTSINTHESURVIVALOFONLYAPARTOFTENAVERY SMALLPARTOFTHEOFFSPRING
OFEACHGENERATION
&ACT .OTWOINDIVIDUALSINAPOPULATIONAREEXACTLYTHESAME RATHEREVERY
POPULATIONDISPLAYSANENORMOUSVARIETYOFCHARACTERISTICS
&ACT -UCHOFTHISVARIATIONCANBEINHERITED
)NFERENCE 3URVIVALINTHESTRUGGLEFOREXISTENCEISNOTRANDOMBUTDEPENDS
IN PART ON THE CHARACTERISTICS THAT THE SURVIVING INDIVIDUALS INHERITED 4HIS
UNEQUALSURVIVALISAPROCESSOFNATURALSELECTIONTHATFAVORSINDIVIDUALSWITH
CHARACTERISTICSTHATFITTHEMBESTINTHEIRENVIRONMENT
)NFERENCE /VERTHEGENERATIONSTHISPROCESSOFNATURALSELECTIONWILLLEAD
TOACONTINUINGGRADUALCHANGEINPOPULATIONSTHATISTOEVOLUTIONANDTHE
PRODUCTIONOFNEWSPECIES
The logic of the theory of evolution. Read each fact and determine how evidence and inference
are used to establish a coherent theory of evolution.
The first three facts in the logic of evolution describe the situation that inevitably exists
in any environment:
• Without limits to population growth, organisms reproduce very rapidly.
• Nevertheless, our observations tell us that the size of most populations in the wild
stays more or less the same.
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• Our observations also tell us that the resources available in any natural
environment are limited.
One important factor that influenced Darwin’s views was an essay written in 1798
by Thomas Malthus. Malthus was a member of the English clergy. He argued that much
of the human suffering we see is the result of human population to growth. Human
population sometimes grows beyond the resources available to support it. Overpopulation
seemed to Darwin to be a characteristic of every population. It led him to infer that
individuals within a population must face a struggle for survival, and that only a few
individuals need to survive to pass on their characteristics to the next generation. The rest
fail to develop; die of starvation, predation, or other causes before they reproduce; or do
not reproduce for other reasons. The loss of reproductive individuals is one factor that
causes wild populations to be relatively stable in size.
The remaining two facts suggested to Darwin how this struggle for survival plays out
among the members of a natural population:
• Individual organisms within a population are different in particular characteristics
from one another.
• Most, although not all, of this variation can be inherited from one organism to the
next.
Darwin thought about the struggle for existence. He reasoned that particular
characteristics can help organisms survive. These characteristics must be inherited.
Successful organisms inherit characteristics that best equip them to survive in a particular
environment. These organisms are most likely to reproduce. They are likely to leave
behind offspring with the same beneficial characteristics. We call these beneficial
characteristics adaptations. By contrast, some organisms inherit characteristics that make
it more difficult to survive. They do not tend to reproduce in an environment. As a result,
their particular characteristics would be less likely to be passed on to surviving offspring.
For example, suppose a population of rabbits lived in a moderately warm climate.
Within that population, a small proportion of the individuals have thick fur. Now suppose
these rabbits have thick fur because their genetic material is different from that of other
rabbits living in the same area. This difference might be passed on to their offspring.
Imagine first that the thicker fur is a disadvantage to the rabbits in this environment
because they overheat when coyotes chase them. As a consequence, these thick-furred
rabbits do not survive and reproduce as well as the rabbits with thin fur. You probably can
see, in this case, that thick fur would not be a common trait in this population.
Now imagine that over several decades the climate in this environment gets colder.
In the colder climate, the rabbits with the thicker fur now have an advantage. That
is, the thick fur would be an adaptation that helps them survive the periods of cold
weather. Thick-furred rabbits would be more likely to survive. They would reproduce
more successfully. As a result, they are likely to pass on their characteristics to the next
generation. As the figure of rabbit population shows, the number of rabbits with thick fur
increases in the population. This is the process of natural selection.
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Relative number of rabbits in population
Warm Climate
Thin fur Thick fur
Cold Climate
(40 years later)
Thin fur Thick fur
Characteristics of a population change due to natural selection. Why do the heights of
each bar change? What do the changes have to do with evolution?
There is a key difference between selective breeding and natural selection. In selective
breeding, humans choose how to allow organisms to reproduce. In natural selection,
interactions with the natural environment determine which individual organisms survive
and reproduce.
In natural environments, many pressures affect survival and reproduction. Changes
in the environment can alter the balance of life. That is, some characteristics are more
beneficial than others. Sometimes these changes are catastrophic, like the effect a volcanic
eruption might have on both local and very distant populations of organisms.
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