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How Populations Evolve
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
Individuals can adapt to their environment but they cannot evolve
Although natural selection acts on individuals, it is the population that evolves over time
as adaptive traits become more common in the group and other traits change or disappear
Thus, evolution refers to generation-to-generation changes in populations
Natural selection is more an editing process than a creative mechanism
Natural selection can only amplify or diminish heritable traits
Natural selection is not goal-directed; it does not lead to perfectly adapted organisms …
because things change
A trait that is favorable in one situation may be useless - or even detrimental - in
different circumstances
Darwin did not know about Mendel’s findings
He did not know how the variations that are the raw material for natural selection
arise in a population or how they are passed from parents to offspring
Mutation
New alleles originate by mutation providing the genetic variation that serves as raw
material for evolution
Sometimes, duplication of a gene or small pieces of DNA through errors in meiosis
can provide an important source of genetic variation and eventually lead to new
genes with novel functions
Populations Evolve
At the population level, biologists focus on the gene pool, which consists of all the
alleles, for all the genes, for all the members of the population
Gene Pools
Consider a wildflower population with two varieties of different colored
blooms
The allele for red flowers (R) is dominant to the allele for white flowers (r)
If these are the only two alleles affecting flower color and this population is
stable (not evolving) for flower color, then genotype frequencies can be
calculated from the allele frequencies
Let p = ƒ (R) and q = ƒ (r)
Then p + q = 1
The Hardy-Weinberg formula, p2 + 2pq + q2 = 1, can be used to calculate
the frequencies of genotypes in a gene pool from the frequencies of the
alleles
Public health scientists use the Hardy-Weinberg equation to calculate the carrier
frequency for certain inherited diseases
PKU
is a recessive disorder that prevents the breakdown of the amino acid
phenylalanine and
occurs in about one out of every 10,000 babies born in the United States
Microevolution
A nonevolving population is in genetic equilibrium, also known as Hardy-Weinberg
equilibrium, in which the gene pool remains constant from generation to generation
and the frequencies of alleles (p and q) and genotypes (p2, 2pq, and q2) are
unchanged
Evolution requires a change in allele frequency within a population
That change in allele frequency is evolution at its smallest scale microevolution
So processes that affect the allele frequencies in a gene pool are agents of
microevolution
Genetic drift
Genetic drift is the chance alteration of allele frequencies in a population
It has its greatest effects on small populations
Bottleneck effect
Certain catastrophic events can reduce the size of a population drastically
The small surviving population may not be representative of the original
population’s gene pool
The reduced gene pool is now the starting material as the population once
again increases
The bottleneck effect will usually reduce the genetic variability in a population
The reduction in individual variation will reduce adaptability and can
threaten the viability of the species
The founder effect
Genetic drift in a new colony can lead to allele frequencies that are drastically
different from the population that the founders left
Gene flow
Gene flow occurs from the movement of genes (alleles) from one population to
another
This is due to migration
The Hawaiian islands were formed through volcanic activity
Yet we find amazing examples of biodiversity
Like silverswords
Natural selection
Only natural selection consistently leads to adaptive evolution
Those individuals in a population that are better suited to the environment will be
more likely to survive and reproduce
Their contributions to the gene pool in subsequent generations will be greatly than
those that are less well suited
Evolutionary fitness is determined by the number of offspring an individual produces
The more offspring surviving and reproducing, the greater the impact on the gene
pool
Fitness only applies to a specific environment at a specific time
If the environmental conditions change, the relative fitness of an individual can
change as well
So “survival of the fittest” is misleading
It should really be “survival of the fit-enough”
So changes in allele frequencies can occur from generation to generation
Can we see phenotypic changes as well?
Peter and Rosemary Grant have studied the finches on the tiny Galapagos island of
Daphne Major
Every year, they capture each bird, take various physical measurements, and then
release the bird
In 1976, there were many more birds than there were in 1978
There were also some profound differences between the birds in 1976 and the
ones in 1978
That small difference allowed the finches to eat the tougher seeds that were
present after the drought
Natural selection does not create advantageous traits
It can only act upon the existing genetic variation
But it’s all about timing
Three General Outcomes of Natural Selection
When natural selection occurs, what are the possible results?
Results of natural selection
Directional selection
Disruptive selection
Stabilizing selection
Sexual selection
Sexual selection is a special form of natural selection
It occurs when differences in reproductive success are tied to differential success in
mating
Sexual dimorphism is a distinction in appearance between males and females not
directly associated with reproduction or survival
Does sexual selection always select for those that are the most fit?