Forces of Evolution

POPULATION

= a group of organisms potentially capable of successful reproduction 

• Individuals of a population tend to choose mates from within the group 

• The largest reproductive population is the species 

Gene pool

 = the sum of all alleles carried by the members of a population

Evolution

• a change in allele frequencies from one generation to the next

What evolves

• Evolution is… A change in allele frequency of a population 

• An individual’s genotype and phenotype will never change 

• The genetic make-up of the population is what changes over time 

Mutation

• Alteration in the DNA 

• The source of variation in a population

• This is how potential new advantageous traits arise

  • Natural selection increases the frequency of positive mutations 

  • Decreases negative mutations

• Mutation is RANDOM

Genetic Drift 

• Changes due to chance 

  • Without strong selective pressure, populations will naturally change at random 

  • Emphasis on RANDOM

Genetic Drift : Founder Effect 

• Component of genetic drift 

• Can occur when a segment of the original population becomes isolated or cut off or migrates to a new area 

  • The migrating group represents a small sample of the original population 

Genetic Drift :Population bottlenecks 

• Component of genetic drift 

• Occurs when a population’s size is reduced for at least one generation 

• Reduction in gene variation means population may not adapt to new selection pressures

Gene flow

• New alleles introduced to the population 

• RANDOM 

• Important source for genetic variation 

• A→B (vice versa)

Nonrandom mating 

• in breeding

• sexual selection

In breeding 

• Mating between biologically related individuals 

• Raises the frequency of homozygous genotypes at all loci 

• Decreases the frequency of heterozygous genotypes at all loci (what we want)

Sexual selection

• Differential reproductive success among the members of the same sex within a given species 

  • Struggle between males to gain access to females 

  • Struggle by a female to choose the right mate 

• Selection is driven by the females 

• Evolutionary change through selection for certain male phenotypes 

  • Examples: larger canines, brighter coloration, larger body size 

• The sex with the more limited reproductive potential should be competed over by the sex with the greater reproductive potential 

• Intrasexual selection = competition between members of the same sec (male-male competition)

• Sexual dimorphism = phenotypic differences between sexes

  • Differences in size and shape between sexes

• Choosing males that offer a direct benefit to the female” 

  • Parental care

  • Protection 

  • Genetic qualities

Intrasexual selection

• competition between members of the same sec (male-male competition)

Sexual dimorphism

•  phenotypic differences between sexes

  • Differences in size and shape between sexes

Examples of natural selection at work

• Parasite destroyed male embryos of Hypolimnas

• Sex imbalance - 1% in butterfly population

• Within 10 generations (1 yr), males returned to around 40% of the butterfly population

• How did population rebound so quickly

Natural Selection: Directional Selection 

• A mode of natural selection in which a phenotype is favored, causing the allele frequency to shift over time in the direction of that phenotype

Natural Selection: Stabilizing Selection

• = mode of natural selection in which genetic diversity decreases in which genetic diversity decreases and the population mean stabilizes on a particular trait 

• Example: birth weight in humans 

Adaptation

• Trait that increases fitness

• Built by natural selection

• Being fit means being adapted to your environment… but environment changes

  • Process never stops

Adaptation “arms race”

• Everything is adapting 

• Predator/Prey coevolution 

• Examples: Cheetahs and gazelles 

• Each successive generation adapts to survive better than the previous one 

• The cheetah has evolved to become the fastest predator, while the gazelle has evolved to become faster and elusive