Lecture: How Does Evolution Happen?
Existing species do not evolve from each other. Rather, they have evolved from a
common ancestor. Remember the definitions:
species - similar organisms that can interbreed in nature to produce fertile, viable
offspring.
population - all the members of a species living in a defined area
gene – a unit of inheritance that controls a physical trait
gene pool – all the genes of every member of the population
speciation - the generation of new, separate species from a single population
(Speciation is also known as MACROEVOLUTION.)
microevolution – genetic change within a population that gives rise to variation, but
does not generate new species
macroevoluion – genetic change that splits one species into two new species that can
no longer breed together to produce fertile, viable offspring
When this happens, the two new species are said to be REPRODUCTIVELY ISOLATED
from each other. Reproductive isolation is what defines separate species.
Types of Reproductive Isolation
Pre-zygotic: These prevent the formation of a fertilized egg (zygote) between two
species.
Ecological – two species habitats are not compatible for mating/reproduction
Temporal – two species mating seasons are not the same
Behavioral – two species mating rituals are not recognizable to each other
Post-zygotic – mating occurs between two related species, so a zygote is formed. But
something else interferes with future generations.
Hybrid inviability – the hybrid cannot survive (for genetic reasons)
Hybrid sterility – the hybrid is viable, but it cannot reproduce.
What can cause reproductive isolation (speciation)?
There are FIVE THINGS THAT CAN CAUSE POPULATIONS TO EVOLVE.
1. MUTATION – Without genetic variation, there is no raw material for change
2. MIGRATION – populations share new mutations by sharing members
3. NON-RANDOM MATING
a. positive assortative mating – similar individuals mate together more often
than expected. (more homozygosity!)
b. negative assortative mating – dissimilar individuals mate together more
often than expected. (more heterozygosity!)
4. GENETIC DRIFT – Genetic changes can happen due to random chance if a
population is very small, and doesn’t have a representative sampling of its source
population’s gene pool
5. NATURAL SELECTION – Individuals with a particular phenotype/genotype have a
reproductive advantage over other individuals who do not have that same
phenotype/genotype. (Such individuals are said to have a selective advantage over
other members of their species.)
What Causes Populations to Evolve?
A population will NOT evolve if:
there are no mutations
the population is infinitely large
mating in the population is completely random
there is no immigration into the population or emigration out of it
there is no natural selection
Criterion 1. MUTATION
Mutation is the raw material of evolution. No genetic variation, no evolution.
The phenotype (for a particular trait) that is most common in a particular wild
population is known as the wild type.
Any allele other than the wild type is said to be mutant.
1. Mutation is the only way new genetic material can arise in a population
2. The larger the population, the more likely mutations will occur
Criterion 2. SMALL POPULATION SIZE
Small populations almost always have lower genetic variety than large populations.
In a very small population, there is a much greater chance for a particular allele to be lost
simple because—BY RANDOM CHANCE--it doesn’t make it into the next generation.
When a population evolves only because of small size resulting in random changes in the
population’s genes, GENETIC DRIFT is occurring.
Two basic types of genetic drift:
Founder Effect: a small sample of breeding individuals from a large population colonizes a
new area and stops breeding with the original population.
Bottleneck Effect: a large population is almost wiped out (due to something like a
hurricane, volcanic eruption, pathogen invasion or other catastrophe) except for a few lucky
individual survivors. They form the (non-representative) new population.
INBREEDING occurs when matings occur between related individuals significantly more often
than they occur between unrelated individuals chosen at random from within the population.
OUTBREEDING is the opposite: individuals mate with non-relatives more often than would be
predicted by random chance.
Inbreeding greatly increases the chances of recessive alleles being inherited in
homozygous form, and hence, showing up in the phenotype.
If the recessive allele is deleterious (harmful), then the disadvantage is obvious.
Note that small, isolated populations (such as island populations separated from the mainland)
will eventually consist of members who are related to one another. This leads to inbreeding.
A SMALL, ISOLATED POPULATION WILL GENERALLY EVOLVE MUCH MORE RAPIDLY
THAN A LARGE POPULATION.
Criterion 3. NON-RANDOM MATING
In a population in which there is polymorphism for a particular trait
• POSITIVE ASSORTATIVE MATING is taking place if phenotypically similar organisms
mate together more often than would be predicted by random chance.
Homozygosity should increase.
•
NEGATIVE ASSORTATIVE MATING is taking place if phenotypically different
organisms mate together more often than would be predicted by random chance.
Heterozygosity should increase.
Non-random mating can increase the possibility of smaller groups in a population
becoming isolated from each other and changing more rapidly because of that.
Criterion 4. MIGRATION:
Loss or addition of alleles from immigration or emigration will change the allele frequency in
the population. Gene Flow is the process by which genes move between populations.
gene flow spreads novel alleles that have arisen via mutation
gene flow has a homogenizing (you know what "homogenized" means, right? Making
the entire batch more uniform!) effect if a recipient population is small relative to a
donor population.
• Lack of gene flow may eventually lead to speciation, but the rate at which this occurs
depends on the species
A hybrid zone is an area of limited hybridization between two closely related species that
have come into contact after having been separated long enough for some reproductive
isolation to have occurred.
•
•
Criterion 5. NATURAL SELECTION
No genotype confers a reproductive advantage over another genotype.
Darwin’s four tenets of Evolution by Natural Selection:
1.
2.
3.
4.
Organisms are capable of producing huge numbers of offspring (OVERPRODUCTION)
Those offspring are physically variable, and the variation is heritable (VARIABILITY)
Those offspring must compete for limited resources (COMPETITION)
Those whose phenotypic characters allow them to best exploit those limited resources
will leave the most genes to succeeding populations. (DIFFERENTIAL
REPRODUCTION)
An individual's DARWINIAN (EVOLUTIONARY) FITNESS is a measure of the proportion of
genes it contributes to succeeding generations. Evolutionary fitness is defined by the
environment. A phenotype that confers fitness today in a particular environment could be a
liability if the environment changes tomorrow!
In any given situation, any trait can be
•
•
•
adaptive – increases the reproductive success of the organism having it
maladaptive – decreases the reproductive success of the organism having it
neutral – does not affect the reproductive success of the organism having it
Common Misconceptions About Evolution
· Misconception: Evolution is a theory about the origin of life.
WRONG! Evolution is a PROCESS, and theories about how it happens consider only
how life changes, not how it got here in the first place.
· Misconception: Evolution is process by which organisms become “better”.
WRONG! Evolution is a PROCESS, and it has no value system. Organisms do not
become “better”. They only change to better match the environment in which they live.
· Misconception: Evolution is the result of life changing by random chance.
WRONG! Although random processes do contribute to evolution (mutation, genetic
drift), natural selection is not random. Better suited organisms leave more genes!
· Misconception: Organisms purposefully “try” to evolve traits they need.
WRONG! Organisms cannot evolve things because they need them. Random mutations
must provide the raw material of change, and those mutations are random.
· Misconception: Evolution means “survival of the fittest”.
WRONG! In the language of evolution, “fitness” does not refer to how fast, strong, or
bit an organism is. It refers ONLY to differential reproduction. The individual that
leaves the most genes to the next generation has the highest (Darwinian) fitness. You
may be very physically fit, but leave no offspring. Or you could be a total wimp and
have lots of babies. So “survival of the fittest” is NOT a good phrase to use in
describing natural selection.
· Misconception: Evolution is “just a theory”.
WRONG! Evolution is a PROCESS, not a theory. A theory is an explanation for
how/why something is observed. The theories of natural selection and genetic drift
explain how evolution happens. Evolution itself is not a theory.
· Misconception: Even the scientific community can’t agree on how evolution works.
Therefore, the entire idea is bogus.
WRONG! Science is not about arguing about being right. It’s about seeking facts. No
one in the scientific community argues that evolution happens. They disagree only
about HOW it happens. So they pose competing hypotheses and test them!
· Misconception: Gaps in the fossil record are a major problem for evolutionary theory.
WRONG! The fossil record is not the best evidence for evolution, though it contributes
pieces of the puzzle. Fossils are rare because physical conditions do not always foster
fossilization. So it’s expected that some might be missing. And there may be many still
hidden.
· Misconception: Evolution theory has failed to explain everything about how life
originated and evolved.
WRONG! The study of evolution is a work in progress. New discoveries are being made
constantly, and each one adds more information. Evolutionary theory is not a religion,
and hence it is not based on things that cannot be observed. Only observable facts
count, and we haven’t yet made every possible observation.
Common Misconceptions About Evolution, continued
· Misconception: Evolution cannot be studied scientifically because it is not observable
or testable.
WRONG! Evolution is observable and testable. Many experiments in the lab and field
have confirmed that evolution happens, and evolution has been verified in wild
populations many times.
· Misconception: Most scientists no longer believe Darwin’s original ideas.
WRONG! Even Darwin realized he did not have the whole picture. As more information
becomes available and new techniques invented to see into our cells and down into our
DNA, many of evolution’s secrets are revealed. This means that Darwin’s original ideas
are constantly being refined, but not that they have been rejected. In fact, his idea may
have been the most influential one in the history of biology.
· Misconception: Believing in evolution leads to immoral behavior and mayhem!
WRONG! Understanding evolution does not mean reverting to “wild type” behavior.
While we share many characteristics—both behavioral and physical—with other animals,
it is not likely that people, upon learning that they share a common ancestor with
flatworms, will begin to behave like flatworms.
· Misconception: Evolution and religion are incompatible.
WRONG! Religion and science (evolution) are very different. In science, only natural
causes are used to explain natural phenomena. Religion is based upon faith in things
that cannot be observed. You don’t have to choose one or the other if you want to
choose both.
· Misconception: Teachers should teach “both sides” of the argument and let students
decide for themselves.
WRONG! First, there is not a single religion, and so there are not just “two sides.”
Which creation myth should one choose in comparing evolution and creation? Biblical
creation? Scandinavian mythology? Native American? The choices are too many.
Second, comparing science to religion is comparing apples to jellyfish. They are not two
sides of the same thing, and should not be compared.
· Misconception: Evolution is like a religion, and teaching it violates the First
Amendment.
WRONG! Religion is based on faith in things that cannot be seen or verified. Evolution
is based on scientific observation, evidence, and inference, NOT faith. There is nothing
remotely religious about evolutionary biology. Hence, teaching the science of evolution
does not violate the Constitution’s ban on the establishment of a state-sponsored
religion.