How Populations Evolve I
Diversity is one of the key aspects of biology
The diversity of living organisms is called biodiversity
Change is normal in biology
This makes sense since the planet is constantly changing
The Diversity of Life
For all of human history, people have named, described, and classified the
inhabitants of our natural world
Taxonomy is the branch of biology concerned with identifying, naming, and classifying
species
The Linnaean system includes a method of naming species and a hierarchical
classification of species into broader groups of organisms
In the Linnaean system, each species is given a two-part Latinized name, a
binomial
The first part of a binomial is the genus (plural, genera), a group of closely
related species
The second part of a binomial is used to distinguish species within a genus
Linnaeus also introduced a system for grouping species into a hierarchy of categories
Beyond the grouping of species within genera, taxonomy extends to progressively
broader categories of classification - family, orders, classes, phyla (singular, phylum),
kingdoms, and domains
Grouping organisms into broader categories is a way to provide structure for our
understanding of the world
However, the criteria used to define more inclusive groups such as families,
orders, and classes are ultimately arbitrary
Explaining the Diversity of Life
Before the 1800s, it was thought that the earth was only about 6000 years old
All of life must have come into being relatively recently and in the current forms
that were seen
However, the discoveries of fossils of creatures no longer alive confused this line
of thought
Lamarck and Evolutionary Adaptations
Naturalists compared fossil forms with living species and noted patterns of
similarities and differences
In the early 1800s, French naturalist Jean-Baptiste de Lamarck suggested that the
best explanation for these observations is that life changes, that it evolves
Lamarck explained evolution as the refinement of traits that equip organisms to
perform successfully in their environments
He proposed that by using or not using its body parts, an individual may
develop certain traits that it passes on to its offspring
Of course, Lamarck was wrong in his ideas but his progressive thinking set the
stage for other naturalists to follow
Charles Darwin
In 1831, Charles Darwin set sail on the H.M.S. Beagle, employed as the ship’s
naturalist and captain’s companion
Darwin’s thoughts were shaped by the ideas of many of his contemporaries
Charles Lyell noted the dynamic geological nature of Earth
Jean-Baptiste de Lamark explored the possibility of descent with modification
Cuvier noted the extinction of some species Earth and the appearance of others
within different time-frames
Darwin spent many years thinking about descent with modification before he
concluded that the driving force was natural selection
The key piece was an essay by Thomas Malthus on the limits of human
population growth
Alfred Russel Wallace
Another English naturalist, Alfred Russel Wallace also concluded that natural
selection is the principal process underlying evolution
In 1859, Darwin finally published On the Origin of Species by Means of Natural
Selection or the Preservation of Favoured Races in the Struggle for Life
He pulled together ideas about:
the vast diversity of organisms
their similarities and differences
their origins and relationships
their geographical distribution
their adaptations to the surrounding environments
Descent with modification
Darwin argued that there was clear evidence that modern species are descended
from earlier ancestral species
Used the term “descent with modification”, not “survival of the fittest”
He proposed that the descendants of the earliest organisms spread into various
ecological habitats over millions of years
They then accumulated different modifications or adaptations appropriate to these
diverse ways of life
In the Darwinian view, the history of life is like a tree
At each fork of this evolutionary tree is an ancestor common to all of the
evolutionary branches that extend from that fork
There are successful (surviving) and unsuccessful (extinct) branches of the
tree
Once a branch ends, there are no more descendants of that lineage
But “surviving” refers to an instant in time
Evidence of evolution
Evolution has left, and continues to leave observable signs
There is extensive evidence in the fossil record
There are also historical vestiges seen in modern life
Radiometric dating has provided accurate data for the age of the planet as well as
the times of various major geological events
Fossil record
Fossils are preserved remnants or impressions left behind by organisms
Most fossils are found in sedimentary rocks
Because of this, each rock strata contains a unique set of fossils that
represents a local sampling of organisms that lived when the sediment was
deposited
Fossils from the same evolutionary periods are consistently found together in the
same geologic strata
We have also found a consistency between the relative ages of fossils as
assigned by evolutionary theory and the absolute ages determined by radiometric
dating
Comparative anatomy
The comparison of body structures between different species
Similarities indicate common ancestry
Forelimbs of mammals
They have different functions
They have similar structures
Particularly the skeletal elements
The structural similarity due to common ancestry is called homology
This confirms that evolution is a constant remodeling process
Old structures are modified to provide new functions
Unfortunately, this descent with modification also leads to anatomical
imperfections
Consider the human spine and knee joint
The retina of the eye is essentially installed backwards
Goose bumps attempt to warm us by fluffing up our long-gone fur
Descent with modification also leads to vestigial characters - structures which
used to serve a purpose but no longer do so
Comparative embryology
The comparison of structures that are present during the early development of
different organisms
Related organisms often have similar stages of embryonic development
All vertebrate embryos have a stage in which gill pouches are present
Many organisms show embryonic structures that are gone by birth
Darwin considered embryology “... the strongest single class of facts in favor of
change of forms.”
Biogeography
This is the study of the geographical distribution of species, both surviving and
extinct
Discoveries in the field of biogeography were crucial to the development of
Darwin’s ideas
The animals and plants of the Galápagos Islands resembled species found on the
South American mainland more than those found on similar but distant islands
(Cape Verde Islands)
Biogeography can explain the prevalence of certain types of organisms in certain
places
Australia
There are a wide variety of marsupials but relatively few placental mammals
This is much different from anywhere else in the world
Introduced placental mammals have thrived in Australia
The early Australian marsupials evolved in isolation from other regions
where early placental mammals diversified
In Australia, the marsupials were just more successful
Madagascar
Lemurs are only found in Madagascar
Their closest relatives are in Africa
Biogeography can also explain the unique distribution of certain organisms
Like the lungfish
The past and current distribution of organisms indicates that life on Earth has
been shaped by major geological forces over vast periods of time
Molecular biology
DNA, RNA, proteins, macromolecules
The more closely related two species are, the more similar the genes and gene
products
The more distantly related, the more different the genes and gene products
Based on the degree of similarity, can determine how related two species are
Remember that the same genetic code is used for all living organisms
Evolutionary Trees
Homologous structures, both anatomical and molecular, can be used to determine
the branching sequence of an evolutionary tree
Some homologous characters, such as the genetic code, are shared by all
species because they date to the deep ancestral past
In contrast, traits that evolved more recently are shared by smaller groups of
organisms