1. Evolution by Natural Selection

2. The Role of Natural Selection in Evolution
Evolution-descent with modification
Most potent at individual level
Individuals do not evolve, only populations
Natural selection-processes where beneficial genes are passed on and harmful or less valuable genes are not as likely to be passed on

3. Theory of Natural Selection
Populations become genetically adapted to their surroundings over time
Four postulates of Natural Selection
There is variation among individuals in a population
Some of this variation is heritable
In each generation some individuals survive and reproduce better than others
Survival and reproduction are not random, but are tied to individual variations. Individuals with the most favorable variations are selected for.

4. Misconceptions “Survival of the Fittest” Struggle for life
Survival alone is not good enough
Survival and reproduction is the key
Struggle for life
Not necessarily open conflict
But finding the necessary resources for survival and reproduction

5. Genetic Variation
Spontaneous mutations
Genetic recombination

6. Genetic Diversity influences the process of natural selection
Resulting from mutation
Spontaneous mutation – changes in DNA that cannot be tied to any particular causative agent
Cosmic radiation, mutagenic chemicals
Naturally occurring genetic mutation rates are low (~1 in 100,000)
Possible outcomes when a gene is altered
Mutation so minor that it has no effect
Mutation harmful
Mutation is beneficial
In order for mutations to be passed to the next generation, they must happen in cells that will become sex cells

7. Resulting from sexual reproduction
Genetic Diversity
Resulting from sexual reproduction
Does not generate new genetic information
Genes recombine into new mixtures
Genetic recombination: each individual has unique set of genes, half donated by mother, half by father
Acquired characteristics do not influence natural selection not genetically determined gained during life of organism cannot be passed on to other generations

8. Processes that drive Natural Selection
Selecting agents – specific environmental factors that favor and influence the likelihood that certain characteristics will be passed on
Differential Survival
Some individuals possess characteristics enabling them to preferentially survive and pass on their genes
Differential Reproductive Rates
Some organism may be able to better utilize resources to produce more offspring
Differential Mate Selection
Some individuals possess characteristics enabling them to be more frequently chosen as mates
Size, aggressiveness, attractiveness to opposite sex

9. Differential Survival
Certain traits favor survival
Can be for finding shelter, food, disease resistance, etc.
Ex. Pesticide resistance

10. Differential Reproductive Rates
Two identical fields of clover
Cows act as the selecting agent by eating the taller plants first
Seeds from both fields were collected and grown under identical conditions
Tall plants rarely reproduced
Only short plants produced seeds
Cows selectively ate plants with tall gene
Seeds from ungrazed field produced tall, short but mostly medium sized plants
Seeds from grazed field produced mostly short plants

11. Differential Mate Selection
Also called Sexual Selection-differential reproductive success due to variation in obtaining mates

12. Hardy-Weinberg Concept
Conditions necessary for genetic makeup to remain constant
Mating must be completely random
Mutations must not occur
Migration of individual organisms into and out of the population must not occur
The population must be very large
All genes must have an equal chance of being passed onto the next generation (no natural selection)
Concept allows for comparisons
of genes within a population to determine if genetic changes are occurring
of genes of two different populations to determine similarities in genetic makeup
Concept is an argument for evolution because most of these conditions cannot be met

13. Processes That Drive Natural Selection
All the genes of all individuals in a population
Constant genetic makeup over several generations (unchanging gene pool) indicates that evolution is not taking place
A changing gene pool indicates evolution is taking place

14. Evidence for the Theory of Evolution
Species and populations are changing genetically through mutations, adaptations, environmental changes, selective breeding, extinctions
Evolution occurs by small steps
All species use same DNA code and amino acid building blocks
Difficult to eliminate a structure that becomes part of a process controlled by genes (example: appendix)
Closely related species have similar DNA and embryological development
Fossil records show changes in kinds of organisms.
Species appear and subsequently become extinct
New techniques and discoveries support evolution

15. Misconceptions about the Theory of Evolution
Evolution happened only in the past
Lots of evidence exists for current genetic changes
Evolution has a specific goal
Natural selection favors organisms that best fit the current environment
Random events can have major influence on natural selection
Changes in environment cause mutations that are needed for an organism to survive
Mutation are random events (not necessarily adaptive)
Individual organisms evolve
Individuals may adapt by changing behavior or physiology but they cannot change their genes
An existing species may have arisen from another present-day species
The species in question may have had a common ancestor

16. Species
Population of organisms whose members have the potential to interbreed naturally to produce fertile offspring but do not interbreed with other groups
Populations that demonstrate gene flow between them
Gene flow - movement of genes from one generation to the next or from one region to another
Smallest irreversible step in the evolutionary process
Two key ideas in these definitions
Species is a population of organisms (not individuals)
Offspring must be fertile
Identifying species
Biological species concept
Species can be identified by their inability to interbreed
Not a practical way to distinguish species
Morphological species concept
Method of using physical characteristics to identify species
Useful but not foolproof

17. Speciation Range – geographical area over which a species is found
Geographic isolation – portion of a species becomes reproductively isolated from the rest of the gene pool by geographic change
No gene flow occurring
Geographical barriers - geological features that keep portions of the species from exchanging genes
Speciation - process of generating new species
Over a long period of time, accumulated genetic differences may result in subspecies

18. How New Species Originate
Three step process
Geographic isolation occurs
Selective agents favor specific valuable genetic combinations
Genetic differences become so great that reproduction between two groups is impossible
Populations become separate genetically
Process has occurred only if gene flow between isolated populations does not occur even after barriers are removed
Separation of species is not enough to generate new species

22. Theory of Natural Selection as a Mechanism for Evolution
Proposed by Darwin and Wallace (1858)
Based on the following assumptions
All organism produce more offspring than can survive
No two organisms are exactly alike
Organisms are in a constant struggle for survival
Individuals that possess favorable characteristics for their environment have higher rate of survival and produce more offspring
Favorable characteristics become more common in the species and unfavorable characteristics are lost