Natural Selection
Introduction:
Populations of organisms in nature are constantly changing. Individuals within a
population will either be well adapted to their surroundings (their environment) or they
will not. If they are well adapted, they will be stronger and more fit, which will increase
their reproductive success rate. If an individual’s reproductive rate is increased, its
genes have a better chance of staying within the population’s gene pool. If an individual
is not well adapted to its environment, they will most likely die off, and their genes will
be removed from the population’s gene pool. This process of differential reproductive
success in populations is also known as survival of the fittest.
Charles Darwin, from England, went on a “Five Year Scientific Expedition on the HMS
Beagle” and published On the Origin of Species in 1859.
His Main Observations:
1. Numerous cases for evidence of “Descent with Modification”. They were: fossils
within the fossil record, the Galapagos Island finches and orchid structure and function
2. Artificial selection on domesticated plants and animals
3. Charles Lyell’s Geological Uniformitarianism. It states that processes that alter the
Earth are uniform through time, not through catastrophic events. Mountains raise an
inch a year due to plate tectonics.
Darwin’s postulates for evolution by natural selection:
Individuals vary.
At least some variation is heritable.
Some individuals leave more offspring than others.
The survival of individuals is not random, but depends on heritable trait variation.
Trait variation influences fitness variation.
B.7C: Analyze and evaluate how natural selection produces change in
populations, not individuals.
Supporting Standard
In a population of interbreeding organisms (a species)
Individuals are variable. Some variation is heritable. Most populations produce more
offspring than can survive. Offspring with favorable traits (for their specific and current
environment) are more likely to survive and reproduce. Traits are created by the
expression of different alleles. Single gene traits can lead to changes in allele
frequencies and therefore to evolution. A gene is loosely defined as a unit of heredity.
Genes are segments of a chromosome. Alleles are versions of genes found on
homologous chromosomes. Homologous chromosomes carry the same genes, but may
have different versions of a given gene called alleles.
Species are defined as populations that create fertile offspring – in nature.
Natural selection produced changes in populations – NOT individuals. Natural selection
= survival of the fittest. This works for both natural and artificial selection.
B.7D: Analyze and evaluate how the elements of natural selection,
including inherited variation, the potential of a population to produce
more offspring than can survive, and a finite supply of environmental
resources, result in differential reproductive success.
Supporting Standard
Survival of the Fittest
Brought about by natural selection. Those best suited for an environment will survive to
produce offspring, ensuring that their genes are passed to future generations. Those
organisms with better fitness for their environment will have higher survival rates and
higher reproductive rates, which will increase the frequency of their alleles within the
population. Over time, natural selection results in changes in the inherited
characteristics of a population. These changes increase a species’ fitness in that
organism's specific environment.
Inherited variation
Single gene traits can lead to changes in allele frequencies and therefore to evolution.
Production of more offspring than can survive. Many species will produce more
offspring than can survive in order to ensure their genes are passed to future
generations. Many young do not survive to adulthood. The more offspring that are
produced, the greater the chance that at least some offspring will survive to the next
generation. This is seen commonly in plants, insects, fish, and small mammals.
Organisms fight for resources: food, water, shelter, breeding grounds, and fertile land.
Those best suited to use those resources will survive.
Differential reproductive success
Certain individuals within a species will have higher reproductive success than others
within the same species. Due to: genetic factors, adaptive factors, and environmental
factors. Some variants leave more offspring than others. Organisms adapt to the
environment in which they live. Those organisms that are best suited for a particular
environment will increase their ability to reproduce. These offspring are more likely to
survive to pass on their traits to future generations. Those organisms that can produce
the most offspring will ensure that at least some offspring survive to pass on their
parent’s genes. Those organisms that are best able to use resources will be more fit
and will have a better change for survival and for reproductive success. Individuals not
suited for their environment will die off, and therefore NOT reproduce. Frequency of
alleles change due to variation within populations.
Genetic Drift:
In small populations, individuals that carry a particular allele may leave more
descendents than other individuals – just by chance = random. Over time, a series of
chance occurrences of this type can cause an allele to become common in a
population.
B.7E: Analyze and evaluate the relationship of natural selection to
adaptation and to the development of diversity in and among species.
Readiness Standard
Adaptation is any inherited characteristic that increases on organism’s chance for
survival. These characteristics can include: anatomical characteristics, structural
characteristics, and physiological processes like how plants perform photosynthesis and
animals take in oxygen.
Over time, giraffes with longer necks were better able to survive as they could reach
food sources that shorter necked giraffes could not. Polar bears with larger body mass
and larger paw pads survived better in a cold and snowy environment than smaller
bears with smaller paws.
These changes occur over generations to the population – not to individuals
Development of species:
Which species survive to reproduce and pass on traits - and which do not - depends on
how well an organism is suited to its environment and their fitness. Species traits can
change depending on their adaptations to their environment. If species traits become
incompatible, species may diverge through: Genetic drift
Change in a gene pool due to chance
Allele frequency change
Geographic isolation
Behavioral isolation
Temporal isolation
Diversity within species
There is variation and diversity within species. This can be: genetic or physiological.
Variation can be brought on by: mutation or breeding. Some of these variants will be
better adapted to their environment and will have more reproductive success. This
increased their gene pool in the population. Some of these variants will be more poorly
adapted to their environment and will die off. This removes their gene pool from the
population.
This process occurs as a result of natural selection and adaptation over many
generations. Changes in species traits occur over time. Changes in allele frequency
with each subsequent generation. When can’t interbreed, speciation occurs.
Speciation:
Speciation is the formation of a new species. It occurs when two populations that are
capable of interbreeding become isolated from each other due to isolating mechanisms.
This can occur due to three types of isolating mechanisms: behavioral isolation,
geographic isolation and temporal isolation. Behavioral isolation is due to differences in
reproductive strategies, courtship rituals, or population behavior. Geographic isolation is
due to separation by geographical barriers such as: rivers, mountains, bodies of water.
Temporal isolation is due to the reproductive cycle of two or more species – where they
reproduce at different times: of the day – flowers that open during the day vs. during the
night, of the year – mating seasons of different herbivores
Allopatric
A species is geographically isolated from others of the same species. This can be
caused by many things. For example a mountain range may rise, a land bridge may
sink, a river may form, or a new population will form if the isolated populations can no
longer interbreed.
Sympatric
A new species is formed without being geographically isolated. It can occur as a result
of: genetic mutations, a sudden change in environment and food sources, or due to
some type of reproductive barrier within the species.
Mutations can result in new species or stronger individuals. Not all mutations are bad.
Some mutations are beneficial for the species. They can result in traits that help the
organism adapt to their environment. Some mutations change individuals within a
population so that those populations can no longer interbreed.