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THE DEFINITION
• Biological evolution, simply put, is descent
with modification. This definition
encompasses small-scale evolution (changes
in gene frequency in a population from one
generation to the next) and large-scale
evolution (the descent of different species
from a common ancestor over many
generations).
• Evolution is the process by which modern
organisms have descended from ancient
ancestors. Evolution is responsible for both
the remarkable similarities we see across all
life and the amazing diversity of that life —
but exactly how does it work?
HOW DOES EVOLUTION TAKE PLACE?
THERE ARE TENANTS THAT WE CALL THE
MECHANISMS OF EVOLUTION: BY
DEFINITION A MECHANISM IS A
NATURAL OR ESTABLISHED PROCESS BY
WHICH SOMETHING TAKES PLACE OR IS
BROUGHT ABOUT.
1. Descent
• Genetic differences that are heritable and
passed on to the next generation
• Evolution only occurs when there is a change
in gene frequency within a population over
time. These genetic differences are heritable
and can be passed on to the next generation
— which is what really matters in evolution:
long term change.
Beetles on a diet
• Imagine a year or two of drought in which
there are few plants that these beetles can
eat. All the beetles have the same chances of
survival and reproduction, but because of
food restrictions, the beetles in the population
are a little smaller than the preceding
generation of beetles.
Beetles of a different color
• Most of the beetles in the population (say
90%) have the genes for bright green
coloration and a few of them (10%) have a
gene that makes them more brown.
2. Mutation
3. Migration
4. Genetic drift
5. Genetic Variation
• There are three primary sources of genetic
variation, which we will learn more about:
mutation, gene flow and sex.
6. Genetic drift
• In each generation, some individuals may, just
by chance, leave behind a few more
descendants (and genes, of course!) than
other individuals. The genes of the next
generation will be the genes of the "lucky"
individuals, not necessarily the healthier or
"better" individuals. That, is genetic drift. It
happens to ALL populations — there's no
avoiding the occurence of chance.
7. Natural Selection
• There is variation in traits. For example, some
beetles are green and some are brown.
• There is differential reproduction. Since the
environment can't support unlimited population
growth, not all individuals get to reproduce to
their full potential. In this example, green beetles
tend to get eaten by birds and survive to
reproduce less often than brown beetles do.
• There is heredity. The surviving brown
beetles have brown baby beetles because
this trait has a genetic basis.
• End result: The more advantageous trait,
brown coloration, which allows the beetle
to have more offspring, becomes more
common in the population. If this process
continues, eventually, all individuals in
the population will be brown.
• In some cases, we can directly observe natural
selection. The shape of finches' beaks on the
Galapagos Islands has tracked weather
patterns: after droughts, the finch population
has deeper, stronger beaks that let them eat
tougher seeds.
Darwin’s Finches
• In other cases, human activity has led to
environmental changes that have caused
populations to evolve through natural
selection. A striking example is that of the
population of dark moths in the 19th century
in England, which rose and fell in parallel to
industrial pollution. These changes can often
be observed and documented.
• Biologists use the word fitness to describe
how good a particular genotype is at leaving
offspring in the next generation relative to
how good other genotypes are at it. So if
brown beetles consistently leave more
offspring than green beetles because of their
color, you'd say that the brown beetles had a
higher fitness.
Any questions