Evolution by Natural Selection

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

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.

Misconceptions

• “Survival of the Fittest”– 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

Genetic Variation

• Spontaneous mutations• Genetic recombination

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

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 selectionnot genetically determined gained during life of organismcannot be passed on to other generations

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

Differential Survival• Certain traits favor survival

– Can be for finding shelter, food, disease resistance, etc.

• Ex. Pesticide resistance

Differential Reproductive Rates

Cows act as the selecting agent by

eating the taller plants first

Two identical fields of clover

Tall plants rarely reproduced

Only short plants produced seeds

Seeds from both fields were

collected and grown under

identical conditions

Seeds from ungrazed field

produced tall, short but mostly medium

sized plants

Seeds from grazed field produced

mostly short plants

Cows selectively ate plants with tall gene

Differential Mate Selection

• Also called Sexual Selection-differential reproductive success due to variation in obtaining mates

Hardy-Weinberg ConceptConditions necessary for genetic makeup to remain

constant

1. Mating must be completely random2. Mutations must not occur3. Migration of individual organisms into

and out of the population must not occur4. The population must be very large5. 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

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

Evidence for the Theory of Evolution

1. Species and populations are changing genetically through mutations, adaptations, environmental changes, selective breeding, extinctions

2. 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

3. Fossil records show changes in kinds of organisms. – Species appear and subsequently become extinct

4. New techniques and discoveries support evolution

Misconceptions about the Theory of Evolution1. Evolution happened only in the past

– Lots of evidence exists for current genetic changes

2. Evolution has a specific goal– Natural selection favors organisms that best fit the current

environment – Random events can have major influence on natural

selection

3. Changes in environment cause mutations that are needed for an organism to survive– Mutation are random events (not necessarily adaptive)

4. Individual organisms evolve– Individuals may adapt by changing behavior or physiology

but they cannot change their genes

5. An existing species may have arisen from another present-day species– The species in question may have had a common ancestor

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

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

How New Species Originate

Three step process1. Geographic isolation occurs2. Selective agents favor specific valuable genetic

combinations3. 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

Theory of Natural Selection as a Mechanism for Evolution

• Proposed by Darwin and Wallace (1858)• Based on the following assumptions1. All organism produce more offspring than can

survive2. No two organisms are exactly alike3. Organisms are in a constant struggle for survival4. Individuals that possess favorable characteristics

for their environment have higher rate of survival and produce more offspring

5. Favorable characteristics become more common in the species and unfavorable characteristics are lost