Download Evolution by Natural Selection NOTES

Evolution by Natural Selection NOTES
EVOLUTION is the process by which species change over time, as they adapt to the environment in which they
exist.
The biological definition of a species states that a SPECIES is a group of organisms that can interbreed AND
produce fertile offspring. Thus, even though a horse and a donkey can interbreed to produce a mule, the horse
and mule are considered to be distinct species because mules are NOT fertile (meaning the mules cannot
reproduce).
A POPULATION is all the members of a single species inhabiting a defined area. For example, all the humans
(Homo sapiens) in Greenwich are a population. All of the house cats (Felus domesticus) in the United States are
another population. All the fish in the Atlantic Ocean would NOT be considered a population because there are
many different species of fish.
An ADAPTATION is any characteristic of a species that improves its ability to survive and reproduce in a given
environment. For example, the long teeth of a naked mole rat are an adaptation that allows them to survive
and reproduce successfully in an underground environment.
Darwin’s Theory of Evolution by NATURAL SELECTION explains the process by which species change over time.
The process of NATURAL SELECTION can be summarized as follows:
1.
Members of a population of a species show VARIATION in their INHERITED TRAITS. This variation is
generated by MUTATION and, for many species, SEXUAL REPRODUCTION.
2. Species OVERREPRODUCE: meaning many, many more individuals are produced than could possibly be
supported by the environment. MANY of these individuals, therefore, will die.
3. Individuals whose INHERITED TRAITS give them a higher probability to SURVIVE AND REPRODUCE IN A
GIVEN ENVIRONMENT, will leave more offspring. Those individuals without such advantageous inherited
traits are more likely to die and less likely to reproduce.
4. This unequal production of offspring will lead to the accumulation of FAVORABLE TRAITS in a population
over generations.
A factor of a GIVEN ENVIRONMENT that influences which INHERITED TRAITS will be FAVORABLE (traits that
increase the likelihood that an individual will survive and reproduce) is considered to be a SELECTIVE PRESSURE.
Such SELECTIVE PRESSURES may include climate (temperature, humidity, amount of sunlight, etc), food
availability, the presence of predators, the presence of other species competing for resources, etc.
Therefore, IF THE ENVIRONMENT CHANGES, the selective pressures change and the relative advantage
provided by a given variation in a given inherited trait may change. Other variations in that trait may become
more favorable and accumulate in the population over generations.
NATURAL SELECTION is sometimes explained as “survival of the fittest”. To an evolutionary biologist FITNESS
refers to an individual’s capacity to SURVICE AND REPRODUCE. Thus, the fittest individual is NOT necessarily
the fastest, strongest, smartest, etc. (although such traits can frequently provide an advantage to survival and
reproduction), by definition the “fittest” individual is the ONE WHO PRODUCES THE MOST OFFSPRING.