microevolution

Process of evolution by natural selection

Variation in heritable traits within a population + differential survival and/or reproduction of specific = change in traits in a population over time

Microevolution

Change in allele frequencies in a population from one generation to the next

Example: population has a frequency of 80% blue eyes, 50% blue eyes in the next generation

*natural selection acts on phenotype, evolutionary change acts on genotype

Genetic variation

Important because variation is the raw material for evolution, and the environment ( and selection pressure) is unlikely to remain constant

No variation= no opportunity for evolution via natural selection

More variation= less likely that a change in environment will affect members of a population

Polymorphism

Two or more versions of a trait within a population

*an allele becomes fixed if the entire population is homozygous at that locus (example: all of the other alleles have been lost)

What creates genetic variation

Mutation- change structure in a gene, only way to generate novel alleles, new alleles means changes in a populations allele frequencies

Alteration of gene number or position- gene duplication, errors in meiosis

Sexual reproduction- combining existing alleles in new ways, meiosis/fertilization

Once genetic variation is created, what leads to changes in alleles in a population? (4 basic processes)

Mutations- occurrence of new mutation directly changes allele frequencies. Relatively low mutation rates in most organisms, mutation likely most important for creating variation than for directly changing allele frequencies

Genetic drift- change in allele frequencies between generations due to random events (chance). Tends to reduce genetic variation through the loss of alleles, can have positive, negative, or no affect on level of population adaptation, always present to some degree- especially in small populations

Two events that lead to genetic drift

Founder effect- few individual separated from source population, individuals become founders of new population. Allele frequencies of new population don’t match those of original population. Often founded in island/isolated populations

Bottleneck effect- large population suddenly reduced to small number due to random events (ex: environmental catastrophe), many individuals and alleles lost. Even after population returns to normal, the effects of the drift is evident in loss of alleles and reduced genetic variation

Once genetic variation is created, what ends to changes in allele CONTINUED

Gene flow- change in allele frequencies between generations due to input of individuals or gametes (ex: pollen) from other populations. Tends to reduce genetic variation among populations. May have positive, negative, or no affect on adaptation in the target population

Natural selection- three ways selection can alter frequencies.

*mutation, genetic drift, gene flow: positive, negative, or no effect. Natural selection: positive

Three ways natural selection can alter frequencies

Differential selection- favours individuals at the end of phenotypic range

Disruptive/diversifying selection- favours individuals on opposite ends of phenotypic range

Stabilizing selection- favours individuals at the middle

Why are we concerned about small populations

Population sizes of other organisms may be small. Because of habitat destruction, modification, degradation, hunting, intro of non-native species

Small populations experience greater affects of genetic drift. Bottleneck can introduce drift, even in “recovered” populations. If population stay small after bottleneck, drift continues to operate

In small populations, drift can actually become more important than selection.. population is out of luck. Lots of genetic variation, accumulation of deleterious mutations, increase reduction of population size and adaptive evolution shuts down. *under a drift, deleterious mutations increases

Sexual selection

Natural selection related to mating success

Intrasexual- acts on traits that affect success in competition with members of same sex for mates (male elk fighting with antlers for dominance)

Intersexual- acts on traits that affect success in being chosen for mating by the opposite sex (female choosing a male with large, bright feathers)

Sexual morphism

Difference in size, Color, shape, heavier, etc between the sexes ( male deer have antlers, female do not)

Sexual selections can operate in opposition to standard selection, since traits that confer mating success can also confer risks of tradeoffs

Ex: calling for mates increases reproductive success but also increases predation success

Why might natural selection fail to produce perfectly adapted organisms?

Natural selection works upon available variation, may vary in time and space, may be opposed by another evolutionary force, there are often tradeoffs, not all variation is subject to natural selection- neutral variation, evolution by natural selection takes time

Evolutionary trap

Previously adapted choice, based on a previously reliable cue, is no longer adaptive due to rapid environmental change