Download Chapter 15 The biological diversity (variety of living things) on earth

Chapter 15
The biological diversity (variety of living things) on earth is staggering. Literally MILLIONS of species. But how
did this huge amount of diversity come into being? Science explains this via the process of evolution- the
change in the inherited characteristics of biological populations over successive generations.
In 1831, Charles Darwin began a 5 year voyage around the world, visiting many continents and remote islands.
Wherever the ship stopped, Darwin went ashore and collected plant and animal specimens. His observations
became the basis for the theory of evolution.
Darwin observed that animals were well adapted to the environments they were living in, but that even between
similar environments (like the grasslands of Australia and Argentina), the species were different. He wondered
why there were no rabbits in Australia, even though the habitat was perfect for them, and why there were no
kangaroos in England.
Darwin also collected fossils. Some of them resembled animals that were alive, and others looked unlike
anything he had ever seen. He wondered why these species disappeared, and how they were related to living
species.
The area that influenced Darwin the most were the Galapagos Islands. They were close together, but each had
very different climates and habitats. Tortoises lived on all of the islands, and you could tell which island a tortoise
came from by looking at the shape of the shell. He also collected birds from each of the islands. At first he
thought they were different species based on beak shape, but eventually learned they were just from different
islands.
Ideas on science were beginning to change around Darwin's time. Many people believed the earth was only a
few thousand years old. Scientists studying geology helped to show that the earth was actually millions of years
old, and changed slowly over time. This understanding of geological processes influenced Darwin- if the earth
could change over time, could life change as well?
There was no understanding of genetics in the early 1800s (Mendel didn't do his pea plant experiments until
1856). But a scientist named Lamarck did realize that organisms changed over time. He thought traits were
passed down via behaviors. So if you spent your life building muscles, your offspring would inherit big muscles.
Obviously this was wrong, but it did influence Darwin to believe that life could change characteristics over time.
A third major influence on Darwin's thinking was the idea of population growth. A scientist named Malthus wrote
a book saying that if the human population continued to grow unchecked, that eventually there wouldn't be
enough food or living space. Checks were war, famine, and disease. Darwin reasoned that this should apply to
all forms of life. Oysters produce millions of eggs each year- why weren't the oceans full of oysters? What factors
determined which offspring survives to reproduce?
It was over 25 years after his trip that Darwin published his thoughts in On the Origin of Species. He stated that
members of the same species can change based on one of two ways. The first is artificial selection- where
humans breed organisms with certain traits to get offspring with the wanted traits. Like breeding for the largest
hogs, fastest horses, or cows that produce the most milk.
The second is natural selection- over generations in nature, the traits that become most dominant are what
increases a species' fitness in its environment.
Darwin proposed that species must struggle for existence- competing for food, space, and other necessities. The
ability for an organism to survive and reproduce is based on its fitness. The fitness is a result of adaptationsany inherited characteristic that increases an organism's chance of survival. Successful adaptations enable
organisms to better survive and reproduce, allowing those adaptations to be passed down thru generations.
Survival of the fittest (another name for natural selection) is basically the idea that organisms with the best
adaptations survive. Those without those adaptations die. Over long periods, the different structural or habitat
adaptations causes a species to look different than its ancestor. This principle is known as descent with
modification.
Descent with modification implies that all living things are related to each other if you go back far enough. This is
known as common descent- that all living things are descended from a common ancestor- therefore a single
tree of life links all living things.
With these ideas in place, Darwin said that living things have been evolving on Earth for millions of years.
Evidence is in the fossil record, geographical distribution, homologous structures, and similarities in embryonic
development.
Fossil Record: The fossil record provides evidence that life has been evolving for millions of years. Often, extinct
organisms from the fossil record show similarities to living organisms, showing that they could be related.
Geographic Distribution: With similar, yet different species, being found in similar habitats on different continents,
Darwin proposed that they came from different ancestors, but because they were living in similar conditions,
natural selection caused them to develop similar adaptations.
Homologous Body Structures: The limbs of reptiles, birds, and mammals vary greatly in form and function, but
are constructed from the same basic bones. Similarities in body structure provide strong evidence of a common
ancestor.
Similarities in Embryology: Vertebrate embryos are very similar, providing evidence that they shared common
ancestry.
Chapter 16
Darwin formed his theory without knowing about genetics. In genetic terms, evolution is any change in the
relative frequency of alleles (traits) in a population. If the alleles for a particular trait start to increase in a
population's gene pool, we can say that population is evolving.
Mutations and gene shuffling are the two main sources for genetic variation that leads to evolution. Gene
shuffling is the way most changes come about- combining genes from two parents and producing an offspring
with its own unique traits. So basically, just the act of sexual reproduction is a major source of variation.
The number of phenotypes (expressed trait) for a given trait depends on how many genes control the trait. A
single trait gene is controlled by one gene. For example, a widow's peak is controlled by one gene. The
dominant allele is for a widow's peak to be present, but due to the frequency of the recessive allele, more people
do NOT have a widow's peak.
A polygenic trait is controlled by two or more genes and alleles. Height is a good example, and is why we see
such a range in height variations instead of just 7' tall or 4' tall.
Natural selection acting on single gene traits can lead to changes in allele frequencies and thus to evolution.
Mutations in brown lizards may produce red or black lizards. The red lizards would get seen and eaten by
predators quickly, not allowing that trait to become common. The black lizards may be able to heat up in the sun
faster, so they could catch more food and escape predators quickly. Over time, this trait would become more
common because the black lizards would survive to reproduce.
Natural selection acting on polygenic traits can affect the distribution of phenotypes in 3 ways:
-Directional selection: Favors one extreme phenotype over the mean or other extreme. This phenomena is
usually seen in environments that have changed over time. Changes in weather, climate, or food availability lead
to directional selection. Charles Darwin studied what was to become known as directional selection while he was
in the Galapagos Islands. The beak length of the Galapagos finches changed over time due to available food
sources. When there was a lack of insects to eat, finches with larger and deeper beaks survived because they
could crack seeds. Over time, as insects became more plentiful, directional selection favored finches with
smaller and longer beaks.
-Stabilizing selection: Favors the average individuals in a population. This process selects against the extreme
phenotypes and instead favors the majority of the population that is well adapted to the environment. Stabilizing
selection is often shown on a graph as a modified bell curve that is narrower and taller than the norm.
Diversity in a population is decreased due to stabilizing selection. However, this does not mean that all
individuals are exactly the same. Often, mutation rates in DNA within a stabilized population are actually a bit
statistically higher than those in other types of populations.
-Disruptive selection is a type of natural selection that selects against the average individual in a population. The
make up of this type of population would show phenotypes of both extremes, but have very few individuals in the
middle. Disruptive selection is the rarest of the three types of natural selection.
In rural areas, the peppered moths were almost all a very light color. However, these same moths were very dark
in color in industrial areas. Very few medium colored moths were seen in either location. It seems that the darker
colored moths survived predators in the industrial areas by blending in to the polluted surroundings. The lighter
moths were seen easily by predators in industrial areas and were eaten. The opposite happened in the rural
areas. The medium colored moths were easily seen in both locations.
So now that we understand under what circumstances evolution does occur, the question is "Are there
circumstances where evolution does NOT occur?" The Hardy-Weinberg principle says that 5 conditions must be
present for the allele frequencies to remain constant:
1- There must be random mating.
2- The population must be very large.
3- There can be no movement in or out of the population.
4- There can be no mutations.
5- There can be no natural selection.