Biol 121- 2010.03.12- Evolution- Natural Selection (Ch. 24).docx

29 views3 pages
22 Apr 2012

For unlimited access to Class Notes, a Class+ subscription is required.

Biol 121 225
Freeman 491-500 Mar. 12, 10
Natural Selection, Ch. 24
Natural selection
-first proposed by
-used to explain
-four simple postulates/steps
-credit for ‘natural selection’ is given to Darwin
-Lamarck and others had already proposed evolution as a pattern in nature
before Darwin began his work but Darwin had the crucial insight of seeing
natural selection that could be used to explain pattern of descent with
1) Variation in a pop- Individual organisms that make up a pop vary in traits
they possess, like size or shape
2) Heritable traits involved - Some trait differences are heritable, so passed on
to offspring genetically
3) Variable fitness - In each generation, many more offspring are produced
than can possibly survive
-therefore only some individuals in the pop survive long enough to produce
offspring, and among individuals that produce offspring, some produce more
than others
4) Fitness due to heritable traits - Subset of individuals that survive best and
produce the most offspring is not a random sample of the pop
-individuals with certain heritable traits are more likely to survive + reprod
-natural selection occurs when indiv with certain char produce more offspring
than do indiv w/o those characteristics
The outcome of these four steps
-Evolution change in genetic char of a pop over time is the outcome of these
four steps
-variation among individuals is necessary for evolution
-populations must change over time for evolution to occur
Summing up the 4 steps in only 2
Evolution by natural selection occurs when
1) heritable variation leads to
2) differential success in survival and reproduction
Allele frequencies WRT evolution
-evolution can be defined as a change in allele frequencies in a pop over time
Example with light and dark winged
-two alleles for a gene for wing colour, one producing dark and other light
-predation by birds they eat dark-winged individuals much more readily
than they can find and eat light-winged individuals if have light-coloured bark
-therefore, predation by birds causes natural selection on wing colour leading
to differential success
-therefore, the allele for light coloured wings increases in the pop
Darwinian fitness
-ability of an individual to produce offspring (relative to that ability in other
indiv in the pop) this is a measurable quantity. How?
count how many offspring it produces relative to other individuals
-when some individuals have a heritable trait that increase their fitness in
their specific environment, that trait is naturally selected
-the pop evolves and adapts to the specific environment
e.g. wood frogs in North Am.
-short term change in an individual and not passed on to offspring
-wood frogs exposed to extreme cold in winter
-ice forms in their skin and their bodies produce some sort of natural
antifreeze, protecting their tissues
-this is change in individual’s phenotype that occur in response to changes in
environmental conditions
-this phenotypic change is not passed on to the offspring, therefore no evo.
Penicillin and resistance (WRT
-this is an example of antibiotic resistance
-penicillin antibiotic/drug that blocks cell wall formation in some bacteria
-resistant strains have a gene for lactamase in a plasmid this breaks down
-therefore, this trait is naturally selected b/c it increases fitness, and as the
Unlock document

This preview shows page 1 of the document.
Unlock all 3 pages and 3 million more documents.

Already have an account? Log in

Get access

$10 USD/m
Billed $120 USD annually
Homework Help
Class Notes
Textbook Notes
40 Verified Answers
Study Guides
1 Booster Class
$8 USD/m
Billed $96 USD annually
Homework Help
Class Notes
Textbook Notes
30 Verified Answers
Study Guides
1 Booster Class