Teacher Notes

Teacher Notes
Natural Selection – TEK 7E The State of Texas says you should know…
“Analyze and evaluate the relationship of natural selection to adaptation
and to the development of diversity among species.”
In other words – What effect does natural selection have on the variation of
a population? Why is variation important? What are sources of variation?
How is variation maintained?
What you need to know about Natural Selection…
Natural Selection is known as survival of the fittest. The populations that
are best fit for an environment will survive and produce offspring – this
passes on the traits that made them a better fit.
Best fit does NOT always mean the strongest or fastest – it could mean an
animal that is best at hiding, or a plant that can survive even in stressful
If there is a plague or natural disaster what survives better – a population
that is all the same or a population with lots of diversity? Lots of diversity
Does natural selection increase or decrease the diversity of the population?
(variation = diversity = not all the same)
Diversity decreases
Why do we have natural selection if it decreases diversity? It makes our
gene pool (population) stronger (better fit).
How do we put diversity back into a population if natural selection
decreases diversity?
Mutations - Provides the raw material for new variations. This is the only source
of completely new variation in a gene pool, all others are simply rearranging
existing alleles. Mutations can be beneficial or harmful.
Sexual reproduction/recombination - Creates individuals with new
combinations of alleles.
Crossing over - Exchanges of DNA between non-sister chromatids of
homologous chromosomes, occurs during prophase I of meiosis.
Independent assortment - During metaphase I, creates daughters cells with
random combinations of chromosomes. Every time a cell divides, different
chromosomes will be pulled to the opposite ends of the cell.
Charles Darwin was a naturalist who traveled around the world in the
1800s and discovered many new species of plants and animals. He
noticed different islands had animals and plants that were similar but had
adaptations to help them survive in the different environments on each
island. When he got home he tried to figure out explanations for what he
saw on his journey around the world. He came up with 4 simple ideas:
More offspring are produced than can survive.
Resources are limited.
There is a struggle for existence.
There is variation among species. (diversity)
Darwin tried to understand WHY there was variation among species – for
instance why did populations of finches on four different islands have four
different kinds of beaks? This led to the ideas that adaptations helping
species to survive are passed on to offspring so that populations evolve
over time. This is also known as survival of the fittest or natural selection.
What did he NOT know about (this was in 1800s)?
chromosomes, DNA, genes, protein synthesis, replication….these were all
figured out later
Last thought – Populations EVOLVE, Organisms are SELECTED.