Download BIOLOGY 1021 Unit 3 Assignment

BIOLOGY 1021 Unit 3 ASSIGNMENT
Evolution by Natural Selection
All living things on Earth have structural and chemical systems in place to make them able to survive in the regions that they live.
These traits are known as adaptations. Adaptations allow living things to specialize for their particular environment and thus live
longer, better, and more fruitful lives. Thicker hides, longer necks, shorter limbs, different colors and patterns are all various types
of adaptations.
There have been many competing theories over the years regarding how adaptations come about. Modern science has built a
body of evidence to support the idea of evolution by natural selection. In this theory, adaptations have come about (and
continue to change) based on the conditions of the environment. Natural selection is the process where those organisms with the
traits best suited to their environment are the most likely to live to reproduce, and to produce more viable offspring. As this
happens each generation, the traits that benefit the organism in this environment become more and more common as they are
passed on to more viable offspring. This is known as a trait being selected. On the other hand, traits that are not helpful to an
organism are likely to become less and less common, as the individuals with those traits are less likely to reproduce, and are less
likely to produce as many offspring. This is known as a trait being selected against. Over many, many generations, favorable traits
are selected for, increasing their prevalence, while unfavorable traits are selected against, and become rarer (or disappear
altogether). Keep in mind that evolution is a slow process. It takes many, many generations for traits to change dramatically.
It is important to remember that not all evolutionary adaptations are towards bigger, stronger, more complex organisms.
Evolution does not have a goal in mind. There is no such thing as a “more evolved” creature, there only creatures that have
gained adaptations more recently. Natural selection selects for whatever traits help that organism live and reproduce in its current
environment. This may mean becoming leaner, smaller or more efficient, depending on the environment. This is why we see
related species with a variety of traits all around the world. Some have adapted to be large and powerful, others are small, quick,
and stealthy. Each environment around the world offers different opportunities and challenges. We can these environmental
traits that influence evolution selection pressures, as they will push organisms to adapt in one way or another.
Charles Darwin was one of the primary contributors to the concept of evolution by natural selection. During his travels, he noticed
many birds that were related to one another living on separate islands in the Galapagos. While these birds were once closely
related and looked the same, each one has adapted differently to each island, as the selection pressures on each island (food
sources) are different.
1)
What are adaptations and what is their purpose?
2)
What is natural selection and how does it create adaptations?
3)
What are selection pressures?
4)
What does it mean when a trait is “selected for” or “selected against”? What causes this?
5)
Is there a set “goal” for evolution? If not, what directs the direction of evolution?
Evidence for Evolution
Evolution is science, and like all science is based on evidence. There are several key pieces of evidence for evolution.
The first major piece of evidence is the fossil record. By examining fossils and comparing their structures with their age, we can
form a roadmap of how modern species came about. We can trace creatures that are now two separate species back to a
common ancestor. Common ancestors are a type of creature that over time had parts of its population evolve into entirely
separate species. For example, if you trace back far enough, there is a single ancestor species for all of modern day cats. Over
time, some of those common ancestors have evolved in one direction to become modern lions, while others have evolved into
modern cheetahs.
It is important to remember that these common ancestors are typically not still present (they are extinct). Therefore, the
statement “humans evolved from monkeys” is incorrect. The correct statement is “humans and monkeys come from a common
ancestor”. This common ancestor is neither a modern human nor a modern monkey – it was its own species that has since gone
extinct.
A chart showing the primate family. Humans, chimps, and bonobos split off from a common ancestor approximately 6 million years
ago. This ancestor is now extinct. This picture only depicts the species that are present today. There are many “dead ends” not
shown.
A diagram of the hominin group. This is a sub-group of primates to which humans (homo sapiens) are very closely related to. The
majority of hominins are extinct. Of those pictured, only chimpanzees and humans still remain. When a species cannot meet the
challenges of the selection pressures it faces, a species goes extinct.
The fossil record is not complete. This means that every common ancestor is not known. There are gaps that may never be filled if
no example of that particular species is ever found. Furthermore, not all species preserve well. It is very hard to find a fossil of
bacteria, for example. For this reason, fossil alone are backed up by other types of evidence. However, even without a physical
sample, common ancestors can be predicted using other pieces of evidence.
Evolution can be supported by biogeography. This is the study of how living things are spread around the world. By tracking
where different species of creates are located, we can observe how common ancestors to modern species migrated around the
world over time to where they are today. In the case of Darwin’s finches (see page 1), these related birds once had a common
ancestor that travelled to each of the islands they now live on. Over time, the geography and weather of the land changed,
preventing these birds from moving from one island to the next, allowing them to evolve separately, influenced by the varying
selection pressures of each island. By combining the fossil record with biogeography, we can trace the migration of species from
one region of the world to another, and how they evolved along the way.
Comparative anatomy allows us to see evidence of evolution by showing that modern species have taken similar body
“blueprints” from their common ancestors and adapted them to different selection pressures over time. As species split off from
one another, each may take the same original structures and repurpose them in drastically different ways as they adapt.
The anatomy of a human arm, cat’s leg, whale’s flipper and bat’s wing. Note that all four use a similar layout of bones, but the size
and proportions have changed to meet each species’ needs. The common ancestor to all these species contained these bones, and
as each modern species was evolving, they changed their anatomy to meet the selection pressures they encountered.
The most recent evidence for evolution comes from genetics. The adaptations of species are controlled by the genes. New traits
are the result of changes to the DNA of that species. Therefore, species that share common ancestors should have a certain
amount of their genes still very similar to one another. This has been found to be true – species that have only separated away
from each other recently have more of the genes in common with each other than those who have split off further into the past.
The more recently humans had a common ancestor with another species, the more their genes are similar. In the case of
chimpanzees, humans share 99.5% of gene sequences in common. Something much, much farther removed from us such as a fruit
fly only share 60% of gene sequences in common.
If this percentage of shared genes seems too high to you, it is important to remember that the vast majority of genes in the body
code for basic things such as the metabolism of a cell. In this way, it is expected that humans, chimps and mice would all share so
many genes as so much of their body’s most basic functions are the same. A human, chimp and mouse heart all still perform the
same jobs, even if their shape, power, size and appearance vary.
It is important to remember that these are similar genes, not literally identical genes. As humans have split off from these other
species, we have evolved and changed. Likewise, these other species have evolved and changed too. However, our genes are still
very, very similar and code for effectively the same things. This is similar to how within a species there is a slight variance in genes.
Furthermore, the fact that ALL life on earth uses the exact same mechanism for storing genes, and that the genetic code is the
same for all species is strong evidence for a common ancestor. The odds of two organisms independently developing the exact
same chemistry to operate their genes is nearly impossible.
6)
What is a common ancestor?
7)
Why is the statement “humans evolved from chimps” incorrect?
8)
How does biogeography provide evidence for evolution?
9)
How does comparative anatomy provide evidence for evolution?
10)
How do genetics provide evidence for evolution?
Microevolution and Selection Events
Evolution is a slow and gradual process. A dinosaur does not lay an egg and a modern bird pops out, at least not literally. These
dramatic changes where one species changes into entirely different species are known as macroevolution (macro = big).
Macroevolution occurs over very, very long time scales. However, they are not one single event. Macroevolution is caused by a
large number of very small changes occurring. These small changes are known as microevolution (micro = small).
Mircoevolution occurs gradually, bit by bit, one simple change to the DNA at a time. As such, it happens much more quickly and
more often than macroevolution. Humans have observed microevolution happening. Bacteria evolving to become more and more
resistant to antibiotics is an example of microevolution seen in our lifetimes.
Sometimes microevolution is caused by simple mutation – when the DNA of an organism is changed by chemicals, radiation, or
the cellular machinery making a “mistake”. However, occasionally, there is a major event that occurs that influences evolution.
We call these selection events as they act as a strong selection pressure to influence evolution.
One category of selection events is called genetic drift. In these events, only a small portion of the population is able to have
viable offspring for some reason. This causes a decrease in diversity, and only a small portion of the population’s genes are passed
down to the next generation. During these events, certain traits may disappear if none of the offspring carry these traits.
One type of genetic drift event is called the bottleneck effect. A disease, natural disaster or other major event will kill off a
significant portion of the population. The survivors will go on to reproduce, but can only pass on the traits they happen to have. If
all of the individuals with a certain trait died in the event, that trait is now gone from the species.
Another genetic drift event is the founder effect. When a group of organisms colonize an area, their descendants that grow up
there can only have the traits that were brought there by their founding ancestors, regardless of how much diversity was in the
original population. For example, if a group of white birds migrated from Europe to Africa, all the subsequent African birds would
be white, regardless of if there were birds of many colors still back in Europe.
The founder effect can be seen in the European-descended people of Newfoundland and Labrador. Most European-descended
people here can tie their family history back to a small number of locations in England, Ireland, and Scotland. These small number
of people would pass their traits onto modern Newfoundlanders. However, not every trait seen in those European countries came
over – Newfoundlanders only inherited the traits of the people who came over and were successful enough to have families. As
such, the genetic makeup of Newfoundlanders is different from the whole of these European countries. This has resulted in some
traits that are normally common in Europe being relatively rare in Newfoundland (as few if any Europeans with these traits came
over). As well, the reverse is true – there are many traits that are rare in Europe, but common here as our founders happened to
have those traits and passed them on to future generations. For example, Bardet–Biedl syndrome occurs in about 1 in 17500
Newfoundlanders, but only 1 in 160 000 Europeans. This is because the Europeans who migrated to Newfoundland happened to
have the genes for these conditions at a higher rate than the rest of the European population.
The bottleneck effect. Originally, blue, white and yellow organisms were present. However, after the bottlenecking event, only blue
and white organisms remain. The yellow gene has been lost. As well, the white population used to be the majority but after the
event, there are more blue organisms left. Bottleneck events can change both the diversity of traits in the population as well as the
proportion to which these traits appear.
The founder effect. If only orange and red butterflies migrate to the new island, all of the butterflies there will be orange and red.
The original population contained white butterflies, but this trait will not show up in the new colony unless an organism with those
genes migrates there.
11) What is genetic drift?
12) What is a selection event?
13) Describe the bottleneck effect.
14) Describe the founder effect.
15) In what way are European-descended Newfoundlanders an example of genetic drift?
Macroevolution and Speciation
Macroevolution occurs over long periods of time. In many cases, one portion of a species will end up facing different selection
pressures and thus will end up with differing adaptations than the rest of that species. Given enough time, the two groups may
adapt to become so different that they cannot breed with one another anymore. This is known as speciation. Speciation creates
two species from a population of originally one species.
Speciation can occur by several mechanisms. In general, speciation requires two populations of the same species to become
isolated from each other. As long as these two populations can intermix and inter-breed, they will not full separate from each
other. For two groups to become so different that they cannot have viable offspring, long periods of isolation are needed.
The most basic type of isolation is habitat isolation. Here, populations become separated from one another by physical
geography. As the Earth’s continents move, populations that could once interact may become separated. For example, many
species in what is now North America and Europe were once part of the same species. As the tectonic plates moved, the two
continents separated. Eventually, this led to the populations being unable to intermix. Once isolated, each group could evolve in
differing ways, with no sharing of genes between the two. This can also occur when the climate changes drastically. If waters rise,
areas that were once connected may become separated by water. Likewise, in times of drought, liveable areas may shrink down
and become separated by desserts. Now these two areas that were once connected have been separated, and thus the creatures
living in them cannot interbreed.
Another method of isolation is genetic isolation. This occurs when a mutation arises in the population that makes certain
individuals incompatible. If a group of individuals cannot interbreed with another group, the two will begin to evolve in their own
directions even if they still live in the same environment. The definition of a species is a group of individuals that can interbreed
with one another – if this becomes impossible, then two species exist, not one.
Temporal isolation occurs when two groups within a species have differing timing in their mating processes. If one group of
individuals are ready to mate in the spring, and another in the fall, the two groups will not interbreed. Related to this is
behavioural isolation, where two groups develop different social or mating behaviours. If one group develops the need to
perform a mating ritual while another does not, these two groups will not interbreed.
A final type of isolation is mechanical isolation. This occurs when two groups within a species are no longer to mate due to
physical anatomical differences preventing mating. Even if the sperm and egg cells are compatible, if the reproductive organs of
two groups become incompatible, they will not be able to mate, and will thus evolve into two separate species.
16)
What is speciation?
17)
Describe the types of isolation that can cause speciation.