The Evolution of Populations Ch. 11

Genetic Variation in Populations 11.1

Gene pool —the combined alleles of all of the individuals in a population There are typically 2 or more alleles for a certain trait. (dominant or recessive) Allele frequency —the measure of how common a certain allele is in a population.

Allele Frequency If brown skin color became more advantageous, what would likely happen to the frequencies of alleles G and g in this gene pool?

Sources of Genetic Variation The 2 main sources of genetic variation are mutations & the genetic shuffling that results from sexual reproduction (recombination)

Natural Selection in Populations 11.2

Microevolution Microevolution: change in allele frequencies of a population over time Natural selection is one process that leads to microevolution

Natural selection acts on the distribution of traits. Remember: the range of most phenotypes seen in a population is the result of polygenic traits rather than single-gene traits

NS never acts directly on genes NS can only affect which individuals survive & reproduce, & which do not Also, remember that it is populations, not individual organisms, that can evolve over time

Natural selection acts on the distribution of traits. Normal distribution: phenotypes near the middle of the range tend to be most common, while extremes are less common Ex: height tall/short height less common  medium height more common

Natural selection acts on the distribution of traits. Natural selection can change the distribution of a trait in three ways: 1.) directional selection 2.) stabilizing selection 3.) disruptive selection

Directional Selection - occurs when individuals at 1 end of the curve have higher fitness than individuals in the middle or at the other end (large beaks or small beaks have higher fitness)

Stabilizing selection - occurs when individuals near the Stabilizing selection - occurs when individuals near the center of a curve have higher fitness than individuals at either end (medium beaks have higher fitness)

Disruptive selection - when individuals at the upper & lower ends of the curve have higher fitness than individuals near the middle (large and small = high fitness medium = low fitness)

Other Mechanisms of Evolution 11.3

Other Mechanisms of Evolution Other factors besides natural selection may lead to evolution Other mechanisms of evolution include: 1.) gene flow 2.) genetic drift 3.) sexual selection

Gene Flow Gene flow: the movement of alleles from one population to another Occurs when individuals move between populations Gene flow b/t neighboring species keeps their gene pools similar A lack of gene flow may cause neighboring populations to evolve into different species

Genetic Drift Genetic drift: changes in allele frequencies due to chance alone Causes a loss of genetic diversity Occurs in small populations Caused by 2 things: 1.) population bottleneck 2.) the founding of a small population

Bottleneck effect: Genetic Drift that occurs after a destructive event reduces the size of a population

Founder effect: Genetic Drift that occurs after a small number of individuals colonize a new area

Genetic drift has negative effects on a population. less likely to have some individuals that can adapt harmful alleles can become more common due to chance

Sexual Selection Sexual selection occurs when certain traits increase mating success

Evolution vs. Genetic Equilibrium 11.4 Genetic equilibrium - when allele frequencies remain constant, the population will not evolve

2.) No emigration or immigration 3.) No mutations 4.) Random mating 5 conditions are required to maintain genetic equilibrium and prevent evolution: 1.) Very large population 2.) No emigration or immigration 3.) No mutations 4.) Random mating 5.) No natural selection

Speciation Through Isolation 11.5

The Process of Speciation Speciation - formation of a new species Gene flow between 2 populations must stop in order for them to become new species

Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other Reproductive isolation - when members of 2 populations can’t interbreed & produce fertile offspring

Isolating Mechanisms 1.) Behavioral isolation - when 2 populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior Ex.) Birds with diff. mating songs

Isolating Mechanisms 2.) Geographic isolation - 2 populations are separated by geographic barriers like rivers, mountains, or bodies of water Ex.) Squirrels split by Colorado River

Isolating Mechanisms 3.) Temporal isolation - when 2 or more species reproduce at different times Ex.) Orchids releasing pollen on different days

Patterns in Evolution 11.6

Convergent Evolution Convergent evolution: evolution toward similar characteristics in unrelated species Ex: Dolphins (mammals) and sharks (fish) have evolved similar tail fins as each has adapted to similar environmental conditions

Divergent Evolution Divergent Evolution: closely related species evolve in different directions Ex: red fox & kit fox

Coevolution Coevolution: two or more species evolve in response to changes in each other

Extinction Extinction: the elimination of a species from Earth 2 types: 1.) background extinctions 2.) mass extinctions

Speciation often occurs in patterns. Punctuated equilibrium: episodes of speciation occur suddenly and are followed by long periods of little evolutionary activity Adaptive radiation: many species evolve from one species; descendent species usually adapt to a wide range of environments

Origin of Eukaryotic Cells Endosymbiotic theory - proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms