As a biologist, I see evolution's
workings every day. Moreover, to me, no feature of life is more wonderful, and more
confirming of a spiritual content of life, than evolution. If I had to choose the most
beautiful feature of life itself, I would choose the
fact that life evolves.WHAT IS EVOLUTION?
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Basically, biological evolution refers to genetic change in a population
of organisms.
Evolving populations usually change from simpler states to more complex ones. Thus, the
first living things were one-celled or even simpler, but today the most complex organisms
are composed of many millions of cells. The first living things did little more than
reproduce, but today living things can think and create. On Earth, surely the most
"highly evolved" organism is the human, because of the human brain, but other
plants and animals also have amazing adaptations and capabilities.
It is useful to recognize two basic forms of biological evolution: Microevolution
and macroevolutuion.
MICROEVOLUTION
Let's say we start out with a population of reddish
dragonflies such as the one at the right. Let's also say that the climate is slowly
warming up, year after year. It happens that Bergmann's Rule (see our Ecological Rules page) asserts that geographic races of a species possessing smaller body size are found in
the warmer parts of the range, and races of larger body size in cooler parts.
Reflecting this dynamic, over many generations the average size of individuals of the
reddish dragonfly species shown at the right become smaller, as they adapt to the growing
heat. This is an example of microevolution.
Microevolution refers to adaptive change within
a species.
EXAMPLES OF MICROEVOLUTION
from notes by Bonnie Dalzell
One example of "evolution as we watch" is the
rapid adaptation of pest species to resist pesticides and herbicides. Another is the
adaptation of weed grains that are harvested and replanted with desirable grains. Some
wild grasses grow in fields of cultivated grains and the ripening times and seed weight
have come to match the ripening time and seed weight of the desirable grains so that the
weeds are cultivated along with the cultivated grains.
Bacteria evolve resistance to our antibiotics even more rapidly but they can assimilate
genes from other bacteria by ingestion so their evolution takes a different course than
the evolution of sexually reproducing, multicellular species |
The mechanism enabling microevolution is natural selection.
Here's how a particular instance of natural selection worked:
In Europe there is a speckled moth called the Peppered Moth, Biston betularia.
Among its members are very light-colored ones, and very dark-colored ones. Before the mid
1800s, the pale-colored ones were by far the most common kind encountered. Since they were
less visible when resting on pale tree trunks, birds had a harder time seeing them, and
eating them than they did the dark ones. However, then came the Industrial Revolution with
its pollution, which covered tree trunks with soot making them darker. Now the
dark-colored moths became the most common form because now they blended in the best
with the dark, soot-covered tree trunks. In other words, with regard to the entire
species, the Peppered Moth gradually evolved toward the darker form, because the
darker form was better adapted for a landscape darkened with soot. This is an example of
how selection can alter the relative proportion of an existing trait in a population. In
other words, it demonstrates a mechanism for how microevolution
works.
MACROEVOLUTION
You know that at one time in geological
history dinosaurs existed (Daniel's hand is over a dinosaur footprint in Utah at the left)
but now they don't. You also know that dinosaurs existed long before humans appeared on
Earth. These facts relate to the macroevolution of species on Earth.
Macroevolution refers to the appearance and disappearance
(extinction) of many species through time.
What is the evidence for macroevolution? The evidence is abundant, and found in several
fields:
- The fossil record: As our Fossil Page explains, usually fossils are deposited with the
oldest ones at the lower levels of geological strata. As you dig deeper and deeper, you
can watch species come and go through geological history, with the older species becoming
simpler and simpler in structure. You might enjoy referring to our Geological Time Scale where you can see how
different kinds of organisms arose and went extinct throughout Earth's evolutionary
history.
- The molecular record: When you study the
fossil record and figure out when new species arose from their ancestors, you see that the
longer they have been separated in geological time, the more differences there are in
their DNA (genetic material) and protein.
- Homology: This interesting word refers
to the fact that related species share similar patterns of bodily structure. For instance,
all animals with backbones share the same pattern of bones, muscles, nerves, blood
circulation and organs. Moreover, the arrangement of these things becomes more and more
complex as one moves from the fishes to amphibians to reptiles and mammals. This suggests
that over time progressive change has taken place.
- Development: Developing embryos pass
through stages exhibiting traits of the species from which they arose. For example, human
embryos have gill slits like fish and later possess a tail. Human fetuses even
possess a fine fur (the lanugo) during the fifth month of development.
- Vestigial structures: Many species
retain structures that were important to the species from which they arose, but which no
longer are useful. For example, the human appendix is what remains of the time when our
ancestors needed a pouch off the large intestine for bulky food to be stored, perhaps
where bacteria could break it down more before being sent on through the intestine. Think
in terms of our primate ancestors eating roots, tough fruits and such.
- Convergent Evolution: Since a specific
form or manner of being may be the best for exploiting a certain ecological niche,
unrelated organisms in different parts of the world often evolve to look like one another.
For example, in Australia many marsupial mammals have evolved to look strikingly like very
unrelated placental mammals in other parts of the world. Australia's marsupial Tasmanian
Wolf looks much like North America's placental Wolf. Australia's Flying Phalanger looks
just like North America's Flying Squirrel. On and on it goes..
- Patterns of distribution: Charles Darwin
pointed out that organisms living on oceanic islands resemble most closely the forms
living on the nearest continent. That wouldn't make sense if all organisms were specially
made, instead of having evolved from common ancestors.
An interesting point about macroevolution is that its rate does not appear to have been
taken place at a constant rate throughout evolutionary history. Macroevolution has
proceeded in jerks -- there were moments in time when it sped up, others when it slowed
down. So far no general explanation for why this might be so has been accepted by all
scientists. More on the pace of evolution here.
In 2007, a study appearing in PLoS Biology showed that though sexual
reproduction has not occurred in a certain group of bdelloid rotifers for 40,000,000 years
(Females lay eggs that are genetic clones of their mothers; the species has no
males) the group has evolved into different species. Therefore, sexual reproduction
is not needed for evolution to take place and new species to arise. Read more about
rotifers, including the bdelloids, here.
Well, there's still a lot not understood about evolution.
About the only fact accepted by all serious, unbiased researchers is this: |