Evolution of Populations

• Genetic variation= wide range of phenotypes• Why is variation in a population beneficial?

– Genetic variation in a population increases the chance that some individuals will survive

• Gene pool- all genes, including different alleles, that are present in a population

• Allele frequency- # of times that the allele occurs in a gene pool, compared to other alleles for same gene– Expressed as %– Nothing to do with allele being recessive or dominant

• What are the main sources of genetic variation in a population?– Mutations and gene shuffling from sexual reproduction

Sources of Genetic Variation

• Main sources of genetic variation= mutations and genetic shuffling from sexual reproduction

• Mutations- any change in sequence of DNA– May affect organism’s fitness or ability to survive and

reproduce– May have no effect on fitness

• Gene shuffling- occurs during production of gametes– Crossing over– Sexual reproduction- major source of variation within

many populations

Natural Selection in Populations• Natural selection acts on

distributions of traits– Traits not undergoing natural

selection have normal distribution

• Natural selection can change the distribution of a trait in one of three ways

• Microevolution- evolution within a population– Observable change in allele

frequencies– Can result from natural evolution

Natural Selection in Populations

• Natural selection can take one of three paths• Directional selection- individuals at one end of curve have

higher fitness than individuals in the middle or at end

Natural Selection in Populations

• Stabilizing selection- takes place when individuals near the center of curve have higher fitness than individuals at either end

Natural Selection in Populations

• Disruptive selection- individuals at the upper and lower ends of the curve have higher fitness than individuals near middle

Other Mechanisms of Evolution

• Natural selection- not only source of evolutionary change

• Gene flow- movement of alleles between populations– Individuals join new populations and reproduce– Keeps neighboring populations similar– Low gene flow increases chance that two

populations evolve into different species

Other Mechanisms of Evolution

• Genetic drift is a change in allele frequencies due to chance– Causes a loss of genetic diversity– Most common in small populations

• How do populations become small enough for genetic drift to occur?– Bottleneck effect- an event greatly reduces the size of

a population- overhunting– Founder effect- small number of individuals colonize a

new area- Old Order Amish community

Other Mechanisms of Evolution

Other Mechanisms of Evolution

• Sexual selection occurs when certain traits increase mating success

• Why does it happen?– Due to higher cost of reproduction for

females– Females are more limited in potential

offspring each cycle- they want a good return

• Two types of sexual selection– Intrasexual selection: competition

among males– Intersexual selection: males display

certain traits to female

Evolution Versus Genetic Equilibrium

• Hardy-Weinberg principle- allele frequencies in a pop will remain constant unless one or more factors cause those frequencies to change

• Genetic equilibrium- allele frequencies remain constant• Conditions required to maintain genetic equilibrium

– Random mating: no sexual selection– Population must be large: no genetic drift– No movement into or out of population: no gene flow– No mutations: no new allele added to gene pool– No natural selection: all traits aid equally in survival

• Real populations rarely meet all five conditions

Speciation Through Isolation

• Speciation- formation of new species• What must happen for a species to evolve into two

new species?– Gene pools of two populations must become separated

• Reproductive isolation- members of two pop cannot interbreed and produce fertile offspring– Behavioral isolation– Geographic isolation– Temporal isolation

• Behavioral isolation- two populations have differences in courtship rituals or other reproductive strategies that involve behavior– Eastern and western meadowlarks

• Geographic isolation- two pops are separated by geographic barriers, like rivers, mountains, bodies of water– Abert and Kaibab squirrel– Do not guarantee formation of new species– May separate some organisms but not others

• Temporal isolation- two or more species reproduce at different times– Orchids in rain forest

Patterns in Evolution• Evolution through natural selection is not random

– It can have direction, results cumulative

• Different species often must adapt to similar environments– Convergent evolution- evolution toward similar characteristics in

unrelated species– Divergent evolution- closely related species evolve in different

directions, become increasingly different

Patterns in Evolution

• Species can shape each other over time– Coevolution- two or

more species can evolve together

– Species evolve in response to changes in each other

Patterns in Evolution

• Species can become extinct– Extinction- the elimination

of a species from Earth– Background Extinction

• Caused by local changes in environment

– Mass extinction• More rare but more intense• At least five mass

extinctions in last 600 million years

Patterns in Evolution• Speciation often occurs in

patterns• Punctuated equilibrium-

episodes of speciation occur suddenly in geologic time– Followed by long periods of little

evolutionary change

• Adaptive radiation- many species evolve from one species– Ancestral species diversify many

descendent species– Usually adapted to wide range

of environment

• Why is understanding evolution important?– Drug resistance in bacteria and viruses– Pesticide resistance in insects

• Evolutionary theory helps us understand and respond to these changes in ways that improve human life