Transcript Evolution10

Two main types of evolution



Evolution simply refers to change over time.
Biologists typically concentrate on studying
one of two levels of evolution.
Macroevolution: this refers to long-term
changes in species.


Ex: the evolution of multi-celled organisms from
single-celled organisms
Ex: the evolution of amphibian species from fish
species
And the second level…

Microevolution: this refers to short-term
changes in populations due to changes in the
environment.


Ex: the average beak depth of a finch species
changes after a major drought.
Ex: the percent of dark-colored moths in a
population increases as pollution increased during
the industrial revolution.
So who was this Darwin guy?



Charles Darwin is given much of the credit (and
blame) for the theory of evolution. In reality, Darwin
published his theory at the same time as another
scientist, Alfred Wallace.
Even before Darwin, scientists like Jean Baptiste de
Lamarck, were already proposing that species
change over time, and are not the same today as
they were at their “creation”.
Darwin remains so famous because his ideas on the
mechanisms of evolution, such as natural selection,
have continued to be a cornerstone of modern
evolutionary theory.
Evolution by Natural Selection


Natural Selection: the process by which a
population adapts to its environment due to
individuals with favorable traits successfully
reproducing more offspring than individuals
with less favorable traits.
It is called natural selection because the
environment is “selecting” for individuals who
are most fit in the current conditions. (The
less fit individuals die and/or do not
reproduce.)
Requirements for Natural Selection


Variation: individuals of a population are not
identical. Even though the individuals of a
population are of a single species, they
exhibit much phenotypic variation.
Limits on Population Growth: Populations
produce more offspring than can survive.
Therefore, there is a struggle for existence in
that resources are limited. Due to limited
resources not all offspring survive.
And the third requirement…

Differential Reproductive Success: The
individuals most adapted (i.e.: they have the
most favorable combination of traits) to the
environment are more likely to survive and
reproduce than individuals with a less
favorable combination of traits. Therefore,
not all traits are passed to future individuals
in equal proportions.
The Synthetic Theory of Evolution brings
Mendel and Darwin together


When Darwin proposed that populations evolve by
natural selection, he knew that phenotypic variation
was a crucial component of his hypothesis. In order
for natural selection to work, favorable traits
must be heritable. This means traits are passed
from parents to offspring.
Darwin had no way to explain how traits are
inherited, so his hypothesis could not be fully
supported.
An Example of Evolution by Natural
Selection


In the mid-1900’s, antibiotics were looked at
as miracle drugs that could combat and cure
any bacterial disease. Today, many bacterial
species are resistant to multiple antibiotics.
This is of great concern to the medical
community as well as the general public.
How does a bacteria species become
resistant to an antibiotic?
Common Misconceptions about evolution
and natural selection



Misconception: Individual
organisms adapt to their
environment.
“Survival of the fittest”
means the strongest
individual survives.
Traits an organism acquires
during its life can be passed
on to offspring.



Reality: Evolution can only
occur in populations, and
does not happen within the
lifetime of an individual.
Fitness is measured by an
organism’s ability to
produce viable offspring.
An individual who is most
fit produces the most
healthy offspring.
A weightlifter with big
muscles does not pass this
trait to his children.
Natural Selection is not the only
mechanism of evolution

Hardy-Weinberg Equilibrium predicts when
populations are not evolving (i.e.: when allele
frequencies are stable, and not changing). If a
population is changing genetically, then a biologist
can investigate if it may be due to one of the
following:





1. Nonrandom mating
2. Mutation
3. genetic drift
4. gene flow, and
5. Natural selection
1. Nonrandom Mating

Failure to choose mates at random from the
population.
Causes


Inbreeding within the same “neighborhood”.
Assortative mating (like with like).
Result


Increases the number of homozygous loci.
Does not in itself alter the overall gene
frequencies in the population.
2. Mutations

Inherited changes in a gene.
Result



May change gene frequencies (small
population).
Source of new alleles for selection.
Often lost by genetic drift.
3. Genetic Drift


Changes in the gene pool of a small
population by chance.
Types:


1. Bottleneck Effect
2. Founder's Effect
Bottleneck Effect


Loss of most of the population by disasters.
Surviving population may have a different
gene pool than the original population.
Result



Some alleles lost.
Other alleles are over-represented.
Genetic variation usually lost.
Importance


Reduction of population size may reduce
gene pool for evolution to work with.
Ex: Cheetahs
Founder's Effect


Genetic drift in a new colony that separates
from a parent population.
Ex: Old-Order Amish
Result


Genetic variation reduced.
Some alleles increase in frequency while
others are lost (as compared to the parent
population).
Importance

Very common in islands and other groups
that don't interbreed.
4. Gene Flow


Movement of genes in/out of a population.
Ex:


Immigration
Emigration
Result

Changes in gene frequencies.
5. Natural Selection


Differential success in survival and
reproduction.
Result - Shifts in gene frequencies.
Comment

As the Environment changes, so does
Natural Selection and Gene Frequencies.
Result

If the environment is "patchy", the population
may have many different local populations.
Can changes within a population lead to a
new species?



So far we have analyzed evolution within a
population, which consists of organisms of a single
species. One of today’s most compelling questions
in biology is how speciation occurs.
Speciation refers to the process by which a new
species evolves from a pre-existing species.
It has been difficult for biologists from all areas of
specialty (i.e.: botany and zoology) to agree on a
single definition for a species. This has complicated
experimental studies of speciation.
Macroevolution


Biologists have long been questioning how
the major taxonomic groups evolved to
produce the incredible diversity of species we
see today.
For example, when all living organisms were
single-celled there was much less diversity
than there is today. How is it that from these
single-celled ancestors, fish, amphibians,
birds, mammals, etc. evolved?
Species Definitions



Morphological species concept: defines a species as a
group of structurally similar organisms that differ from other
described species.
Biological species concept: defines a species as a
group of organisms able to interbreed to produce viable
offspring in nature.
Isolating Mechanisms are needed for new species
to arise.

Geographic isolation: If a population splits into 2
due to environmental changes, the two populations
may experience different evolutionary changes due
to different natural selection pressures.
Reproductive Isolating Mechanisms

This type of isolation prevents interbreeding
even when two populations live in the same
geographic region.


Prezygotic barriers: these barriers prevent
fertilization between gametes of two different
species. (ex: males of one population use a
different mating call to attract females than a
related species.)
Postzygotic barriers: these barriers prevent
hybrid offspring from either surviving or producing
offspring. (ex: an embryo fails to develop and
dies)
The fossil record provides a look at the
progression of life on Earth

As you examine the types of organisms found in
rock layers of different ages, some patterns emerge.



The earliest fossils (i.e.: the oldest fossils) are of unicellular
organisms only. Prokaryotic cells appear in the fossil
record earlier than eukaryotic cells.
Photosynthetic organisms appear in rock layers that are
younger than the time at which oxygen began to
accumulate in the atmosphere.
The first fish fossils appear before amphibian fossils,
amphibian fossils appear before reptile fossils, etc.
Fossil Record


Earliest - 3.5 billion years old.
Earth - 4.5 billion years old.
Example of transitional forms
Remember, the
fossil record does
not show simply a
progression of
species leading to a
modern species.
The fossil record
shows us the
evolution of life is
more like a
branching tree.
Horse’s leg bones
2. Homologies support Macroevolution

Morphological homologies: the theory of evolution
predicts that if a group of organisms share a common
ancestor that they would have similar structural
features even if they have different functions.
Patterns of Evolution

Divergent Evolution, defined: the process
of two or more related species becoming
more and more dissimilar


These species will share homologous structures
Convergent Evolution, defined: the
process by which unrelated species become
more similar as they adapt to the same kind
of environment

Their similar structures are considered analogous.
Divergent Evolution
Adaptive
Radiation
Example of Convergent Evolution
Adaptive Radiation: a special case of
divergent evolution

Adaptive radiation, defined: evolutionary
pattern in which many related species evolve
from a single ancestral species.


This occurs when many new niches open up in an
environment.
Examples include: evolution of many mammal
species after extinction of the dinosaurs; evolution
of many island species following colonization of
recently formed islands.
3. Another example of morphological
homology

Vestigial Organs: these structures have
little or no importance to the organisms that
have them.
4. Embryology is the study of how an
organism develops from a zygote

Embryological development: By examining
the growth and development of embryos of
different species, similarities abound.


For example, all vertebrate embryos have
pharyngeal pouches. These structures become
very different in the adults. For example, in fish
they become gills, but in humans they form tubes
that connect the middle ear with the throat.
Also, all chordates have a notochord during
development but this is not present in adults.
5. Molecular biology supports the theory
of macroevolution



It is difficult to find anatomical or embryological similarities
that connect such distantly related organisms as plants,
animals, and bacteria. However, there are abundant
molecular similarities that tie these organisms together in
the evolution of life.
For example, all organisms use DNA, RNA, and ribosomes
to make proteins. The genetic code is universal except for
a few minor exceptions in bacteria. These diverse
organisms also share many similarities in biochemical
pathways such as respiration.
Also, by examining proteins such as enzymes, scientists
have found evolutionary relationships that were unexpected
based on morphology.