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EVOLUTION
Evolution can be defined as genetic change in a line of descent through successive
generations. Evolution is the processes of biological and organic change in organisms by
which descendants come to differ from their ancestors.
Shared ancestry is one of the core ideas in evolution. Taxonomists are scientists who
organize all living (and fossil) organisms to determine Phylogenetic relationships. For
example:
The Phylogenetic tree above looks at the relationships among invertebrates (animals
without backbones). Invertebrates are said to be monophyletic in that they all evolved
from a single common ancestor (ancestral colonial choanoflagellates). Chordates are
vertebrates (animals with backbones) like us. Our closest relative in the invertebrates are
Echinoderms (starfish etc.). You may ask what do we have in common with starfish?
We are both deuterostomes meaning our embryos have a common developmental pattern.
Taxonomy places organisms in a hierarchy of classification. Every organism has a
binomial scientific name. For example: humans are Homo sapiens. The first part of the
scientific name (Homo) is the genus (a group of closely related species). The last part of
the binomial name is the species. The classification system appears as follows:
Kingdom Animalia
Heterotrophs (get their food from digesting other organisms)
Phylum
Chordata
Animals with a notochord etc.
Class
Mammalia Animals that nurse their young
Order
Primates
Animals with nails instead of claws
Family
Homonidae Animals that walk upright
Genus
Homo
Animals that use tools
Species
sapiens
Modern humans
Charles Darwin
- 1831 – traveled the world on the HMS Beagle
- read and studied other scientist of his day – such as:
Thomas Malthus – who discovered that organisms tend to outgrow their
resources, so its members must compete for the resources\
remaining
Georges Cuvier – the father of paleontology would discovered that fossils are
relics of the past.
Charles Lyell - geologist who discovered the Earth is really very old.
Microevolution – cumulative genetic changes that give rise to new species.
Macroevolution – large-scale patterns, trends, and rates of change among groups of
species.
Evolution occurs in populations not in individuals. A population is a group of
individuals occupying a given area. Populations have a great deal of variation. Just look
at the differences in the individuals sitting around you. Individual humans have the same
morphological traits (2 arms, 2 legs etc.), their bodies function in the same way (for
example to digest food), and they have the same behavioral traits. However, the details
of the traits vary from individual to individual (for instance, height, hair color, skin color
etc.)
Gene Pool different alleles (i.e. red hair vs black hair) exist in a population. These genes
make up the populations gene pool. If a gene gains new prominence in a population
(like almost everybody has red hair) then it is said that microevolution has occurred.
Mutations occur at the level of DNA. One change of a nucleotide can result in major
changes in the phenotype (what the individual looks like). Microevolution leads to new
species as small changes accumulate in the population to produce new species. These
small changes over generations are referred to as natural selection. The main points of
natural selection are as follows:
A. The individuals of a population vary in their body form, functioning, and behavior.
B. Many variations can be passed from generation to generation.
C. Some traits are more advantageous than others in a particular environment.
D. Those traits that allow an individual to survive and pass on that trait to the
next generations or more fit.
E. A population is evolving when some forms of a trait become more or less common
in the population.
F. Over time, shifts in the makeup of gene pools have led to the diversity of life forms
on Earth.
Natural selection can change the gene frequency in a population. This is known as
Genetic drift. Genetic drift occurs most rapidly in small, isolated populations. This can
occur to several ways including:
1. The Founder effect. A small populations inhabits an area where that species
previously didn’t exist (like an island). Purely by chance some alleles are likely to be
absent in the new population.
2. The Bottleneck effect. A large population is reduced to a very small population
and some alleles disappear.
3. Gene flow can change when individuals enter or leave a population.
What is a "Species" - a species is a reproductively isolated group of organisms. In
other words, they share a common gene pool.
How fast does evolution occur? - evolution can occur gradually (by natural selection)
or in bursts. Species tend to remain constant for long periods of time and then quickly
evolve. This is referred to as "Punctuated Equilibrium". The driving force behind
punctuated equilibrium appears to be major environmental change.
Fossils – only the hard parts of animals and plants become fossilized. For example:
bones, teeth, shells, and seeds. The organism to be fossilized must be buried in sediments
or volcanic ash. With time, minerals replace to biological matter making fossils.
Fossilization is a rage process. Many fossils will not be uncovered because the layers
they are in are deep in the Earth. In addition, speciation occurs in a small area, making it
difficult to find transitional fossils.
Biogeography - the study of the world distribution of animals and plants asks the basic
question of why animals appear where they do. Species occur where they do because
they evolved there from ancestral species.
Comparative Morphology – reconstructing evolutionary history through comparison of
morphological characteristics.
Homologous structures have a common embryological development and evolutionary
history but may have different functions. For example: bats and humans have the same
bone structure in their arms but bats use their arms to fly and we use ours to swing and
grasp tools. In this case we would say bat arms and human arms are homologous.
Analogous structures have the same functions but different evolutionary origins. For
example: fly wings and bat wings are analogous because they have the same function but
they are not homologous because they come from different evolutionary origins.
Comparing different developmental patterns
The embryos of diverse organisms develop similarly and have nearly identical stages.
Differences in adults are often the result of mutations that altered the onset, rate, or time
of completion of certain developmental steps. For example: see the figure in your text
comparing the development of chimp versus human skulls. The differences in adults of
these two species comes about as the result of differences in growth patterns.
Comparing Biochemistry – DNA comparisons tell us how we are related to other
species. For example: Humans and chimps are better than 98% identical. In fact,
humans and chimps are closer to each other than crocodiles are to alligators, or horses are
to zebras.
Extinction – is a natural occurrence. Will over 90% of all species that have lived have
gone extinct. Mass extinction often occurs as the result of a catastrophic event such as an
asteroid collision. Mass extinction is followed by the sudden appearance of many new
species due to adaptive radiation where existing animals evolve to fill vacant niches.
Human Evolution – probably started when our ancestors became bipedal. The ability to
do this required changes to the shape of the spine, hips and legs. The structure of the
human hand allowed humans to become tool users. Changes in dentition allowed humans
to become omnivores and greatly expand their diet. Enlargement of the brain case
enabled humans to develop better behaviors for survival.
Early Earth - Earth began about 4.5 billion years ago. By four billion years ago the
Earth's crust had solidified. Oxygen was in short supply or totally absent. The
atmosphere was composed mainly of gaseous hydrogen (H2), nitrogen (N2), carbon
monoxide (CO), and carbon dioxide (CO2). The absence of oxygen allowed organic
molecules to develop and accumulate. Primitive cells eventually developed that used
energy from the sun to produce food, and oxygen was released as a byproduct.
Eventually enough oxygen accumulated for eukaryotic cells (like ours, that use oxygen)
to evolve.