# Introduction to Paleoanthropology/Darwinian Thought



## Pre-Darwinian Thoughts on Evolution

Throughout the Middle Ages, there was one predominant component of the
European world view: stasis.

-   All aspects of nature were considered as fixed and change was
    unconceivable.
-   No new species had appeared, and none had disappeared or become
    extinct.
-   Strongly thought that if any new species were to appear, it would be
    through sexual intercourse through multiple different species.

The social and political context of the Middle Ages helps explain this
world view:

-   shaped by feudal society - hierarchical arrangement supporting a
    rigid class system that had changed little for centuries
-   shaped by a powerful religious system - life on Earth had been
    created by God exactly as it existed in the present (known as fixity
    of species).

This social and political context, and its world view, provided a
formidable obstacle to the development of evolutionary theory. In order
to formulate new evolutionary principles, scientists needed to:

-   overcome the concept of fixity of species
-   establish a theory of long geological time

From the 16th to the 18th century, along with renewed interest in
scientific knowledge, scholars focused on listing and describing all
kinds of forms of organic life. As attempts in this direction were made,
they became increasingly impressed with the amount of biological
diversity that confronted them.

These scholars included:

-   John Ray (1627-1705) - put some
    order into the diversity of animal and plant life, by creating the
    concepts of species and genus.
-   Carolus Linnaeus
    (1707-1778) - added two more categories (class and order) and
    created a complex system of classification (taxonomy) still used
    today; also innovated by including humans in his classification of
    animals.
-   Georges-Louis
    Leclerc
    (1707-1788) - innovated by suggesting the changing nature of
    species, through adaptation to local climatic and environmental
    conditions.
-   Jean-Baptiste Lamarck
    (1744-1829) - offered a comprehensive system to explain species
    changes; postulated that physical alterations of organic life would
    occur in relation to changing environmental circumstances, making
    species better suited for their new habitat; also postulated that
    new traits would be passed on to offspring (the theory known as
    inheritance of acquired characteristics).

Therefore, the principle of \"fixity of species\" that ruled during the
Middle Ages was no longer considered valid.

In the mid-19th century, Charles
Darwin offered a new theory which
pushed further the debate of evolutionary processes and marks a
fundamental step in their explanation by suggesting that evolution works
through natural selection.

## Charles Darwin (1809-1882)

Charles Darwin\'s life as a scientist began when he took a position as
naturalist aboard *HMS Beagle*, a ship charting the coastal waters of
South America. As the ship circled the globe over a five-year period
(1831-1836), Darwin puzzled over the diversity and distribution of life
he observed. Observations and collections of materials made during these
travels laid the foundation for his life\'s work studying the natural
world.

As an example, the *Beagle* stopped five weeks in the Galapagos
archipelago. There Darwin observed an unusual combination of species and
wondered how they ended up on this island.

Darwin\'s observations on the diversity of plants and animals and their
particular geographical distribution around the globe led him to
question the assumption that species were immutable, established by a
single act of creation. He reasoned that species, like the Earth itself,
were constantly changing. Life forms colonized new habitats and had to
survive in new conditions. Over generations, they underwent
transmutation into new forms. Many became extinct. The idea of evolution
slowly began to take shape in his mind.

In his 1859 publication *On the Origin of Species*, Darwin presented
some of the main principles that explained the diversity of plants and
animals around the globe: adaptation and natural selection. According to
him, species were mutable, not fixed; and they evolved from other
species through the mechanism of natural selection.

### Darwin\'s theory of natural selection

In 1838, Darwin, at 28, had been back from his voyage on the *Beagle*
for two years. He read Thomas Malthus\'s *Essay on Population*, which
stated that human populations invariably grow until they are limited by
starvation, poverty, and death, and realized that Malthus\'s logic could
also apply to the natural world. This realization led Darwin to develop
the principle of evolution by natural selection, which revolutionized
our understanding of the living world.

His theory was published for the first time in 1859 in *On the Origin of
Species by Means of Natural Selection, or the Preservation of Favoured
Races in the Struggle for Life*.

### Darwin\'s Postulates

The theory of adaptation and how species change through time follows
three postulates:

-   **Struggle for existence**: The ability of a population to expand is
    infinite, but the ability of any environment to support populations
    is always finite.

:   Example: Animals require food to grow and reproduce. When food is
    plentiful, animal populations grow until their numbers exceed the
    local food supply. Since resources are always finite, it follows
    that not all individuals in a population will be able to survive and
    reproduce.

-   **Variation in fitness**: Organisms in populations vary. Therefore,
    some individuals will possess traits that enable them to survive and
    reproduce more successfully (producing more offspring) than others
    in the same environment.

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-   **Inheritance of variation**: If the advantageous traits are
    inherited by offspring, then these traits will become more common in
    succeeding generations. Thus, traits that confer advantages in
    survival and reproduction are retained in the population, and traits
    that are disadvantageous disappear.

### Examples of adaptation by natural selection

During his voyage on the *HMS Beagle*, Darwin observed a curious pattern
of adaptations among several species of finches (now called Darwin\'s
finches) that live on the Galapagos Islands.

Several traits of finches went through drastic changes in response to
changes in their environment. One example is beak depth:

-   There was huge variation in beak depth among finches on the island;
    it affected the birds\' survival and adaptation to local
    environmental changes.

:   

    :   During a drought, finches with deeper beaks were more likely to
        survive than finches with shallow beaks (which were at a
        disadvantage because it was harder for them to crack larger and
        harder seeds).

-   Parents and offsprings had similar beak depths.

Through natural selection, average morphology (an organism\'s size,
shape and composition) of the bird population changed so that birds
became better adapted to their environment.

### Benefits and disadvantages of evolution

#### Individual Selection

Adaptation results from competition among individuals, not between
entire populations or species.

Selection produces adaptations that benefit individuals. Such adaptation
may or may not benefit the population or species. In the case of
finches\' beak depth, selection probably does allow the population of
finches to compete more effectively with other populations of seed
predators. However, this need not be the case. Selection often leads to
changes in behavior or morphology that increase the reproductive success
of individuals but decrease the average reproductive success and
competitive ability of the group, population, and species.

:   

    :   Example of conflict between individual and group interests: All
        organisms in the population produce many more offspring than are
        necessary to maintain the species. A female monkey may, on
        average, produce 10 offspring during her lifetime. In a stable
        population, perhaps only two of these offspring will survive and
        reproduce. From the point of view of the species, the other
        eight are a waste of resources. The species as a whole might be
        more likely to survive if all females produced fewer offspring.

The idea that natural selection operates at the level of the individual
is a key element in understanding adaptation.

#### Directional Selection

Instead of a completely random selection of individuals whose traits
will be passed on to the next generation, there is selection by forces
of nature. In this process, the frequency of genetic variants for
harmful or maladaptive traits within the population is reduced while the
frequency of genetic variants for adaptive traits is increased.

Natural selection, as it acts to promote change in gene frequencies, is
referred to as directional selection.

#### Stabilizing Selection

**Finches\' beaks** (Example)

Large beaks have benefits as well as disadvantages. Birds with large
beaks are less likely to survive their juvenile period than birds with
small beaks, probably because they require more food to grow.

Evolutionary theory prediction:

-   Over time, selection will increase the average beak depth in a
    population until the costs of larger-than-average beak size exceed
    the benefits.
-   At this point, finches with average beak size in the population will
    be the most likely to survive and reproduce, and finches with deeper
    or shallower beaks than the new average will be at a disadvantage.

At this point, the population reaches equilibrium with regard to beak
size. The process that produces this equilibrium state is called
stabilizing selection.

Even though average characteristics of the beak in the population will
not change in this situation, selection is still going on. The point to
remember here is that populations do not remain static over the long
run; if so, it is because a population is consistently favored by
stabilizing selection.

### Rate of Evolutionary Change

In Darwin\'s day, the idea that natural selection could change a
chimpanzee into a human, much less that it might do so in just a few
million years (which is a brief moment in evolutionary time), was
unthinkable.

Today, most scientists believe that humans evolved from an apelike
creature in only 5 to 10 million years. In fact, some of the rates of
selective change observed in contemporary populations are far faster
than necessary for natural selection to produce the adaptations that we
observe.

:   

    :   The human brain has roughly doubled in the last 2 million years
        (rate of change of 0.00005% per year); that is 10,000 times
        slower than the rate of change observed among finches in the
        Galapagos Islands.

Therefore the real puzzle is why the change in the fossil record seem to
have been quite slow.

The fossil record is still very incomplete.

It is quite likely that some evolutionary changes in the past were
rapid, but the sparseness of the fossil record prevents us from
detecting them.

### Darwin\'s Difficulties

In *On the Origin of Species*, Darwin proposed that new species and
other major evolutionary changes arise by the accumulation of small
variations through natural selection.

This idea was not widely embraced by his contemporaries.

-   Many accepted the idea that new species arise through the
    transformation of existing species.
-   Many accepted the idea that natural selection is the most important
    cause of organic change.
-   But only a few endorsed Darwin\'s view that major changes occur
    through the accumulation of small variations.

Darwin\'s critics raised a major objection to his theory: The actions of
selection would inevitably deplete variation in populations and make it
impossible for natural selection to continue.

Yet Darwin couldn\'t convince his contemporaries that evolution occurred
through the accumulation of small variations because he could not
explain how variation is maintained, because he and his contemporaries
did not yet understand the mechanics of inheritance.

For most people at the time, including Darwin, many of the
characteristics of offspring were thought to be an average of the
characteristics of their parents. This phenomena was believed to be
caused by the action of blending inheritance, a model of inheritance
that assumes the mother and father each contribute a hereditary
substance that mixes, or \"blends\", to determine the characteristics of
the offspring.

The solution to these problems required an understanding of genetics,
which was not available for another half century. It was not until well
into the 20th century that geneticists came to understand how variation
is maintained, and Darwin\'s theory of evolution was generally accepted.

