Natural Selection

Artificial Selection

Human-directed, intentional breeding based on certain characteristics.

Plant and animal breeders have long understood this idea

3 Conditions of natural selection

• variation for a trait 

• heritability

• differential reproductive success

Variation for a trait

• variation must be heritable

• differential reproductive success

  • more offspring produced that can survive 

  • competition for resources

  • some are better competitors, survive, & reproduce

Heritable traits

Allow for greater reproductive success become more common in a population over time

differential reproductive success

• more offspring produced than can survive

• competition for resources

• some are better competitors, survive, & reproduce

fitness

A measure of relative reproductive success

result of natural selection

a population that is well-adapted to its current, local environment

evolution by natural selection

• does not lead to a perfect organism

• evolution is NOT progressive

• in each generation, the alleles that code for traits with better fitness tend to increase in frequency

  • “good enough” individuals survive

• with environment changes, which alleles are favored may also change

  • traits may longer be adaptive

galapagos finches beaks

• in dry years, finches had to eat large, hard seeds and bigger beaks were selected for

• in wet years, finches could eat small, soft seeds and smaller beaks were selected for

Theory of evolution by natural selection

• was revolutionary

  • overturned the idea that species were static and unchanging

  • replaced typological thinking with population thinking

  • it was scientific

    • proposed a mechanism that could account for change through time

    • its predictions could be tested through observations observation and experimentation

do not confuse evolution with natural selection

evolution- the pattern of genetic change in populations throughout time

natural selection- a process (mechanism) that explains how change happens

there are 4 other processes

• sexual selection

• genetic drift

• gene flow

• mutation

four modes of natural selection

• intermediate phenotype

  • stabilizing selection

• one extreme phenotype

  • directional selection

• both extremes in phenotype

  • disruptive selection

• no phenotype has a distinct advantage

  • balancing selection

Stabilizing selection

the intermediate phenotype is favored; neither extreme are favored

think birth weight- 3 lbs not favored, 12 lbs not favored, but 7 is favored

directional selection

one extreme phenotype is most fit

ex: average beak depth increased in ground finches during the 1977 drought

disruptive selective

extreme phenotypes are favored; intermediate phenotype selected against

ex: whitefish with low or high numbers of gill rakers are most likely to survive

balancing selection

no single phenotype is favored & all populations of a species at all times

ex: guppies with rare color patterns are favored

Sexual Selection

some traits confer greater fitness because they are attractive to the opposite sex

Fundamental asymmetry of sex

bateman-trivers thypothesis

Why extravagant traits were found in peacock males, but not females

• has observed pattern and hypothesized process

• pattern- traits that attract opposite sex are more elaborate in males

• process- eggs are more energetically costly to create than sperm

the fundamental asymmetry of sex

predictions:

• if eggs are more energetically costly to create than sperm, the females, but not males should be more choosy about mates since they invest a lot in each egg

• males will compete with each other for mates

• alleles that increase males attractiveness or success in male-male competition should increase in population

• sexual selection should act more strongly on males than on females

sexual dimorphism

males and females have different morphologies

intersexual selection: mate choice

hypothesis: organisms select traits that are “honest signals” of health and genetic quality

3 key observations:

• existence of colorful feathers or beaks due to presence of pigments called carotenoids

• carotenoids protect tissues and help the immune system fight disease

• animals must eat plants in order to obtain carotenoids

Take home on sexual selection

Selection can favor certain phenotypes

• many twists on sexual selection occur in diverse species

• multiple forms of sexual selection and/or ecological selection can occur at the same time in the same population

• alleles responsible for adaptive morphological, physiological, and/or behavioral phenotypes increase in frequency over time

How can selection change

• selection can change over time and space:

  • it occurs in context of ecology, highly complex and dynamic

• natural selection is the only evolutionary process that results in adaption

  • but not the only mechanism of evolution

genetic drift

• random chance

• change in allele frequencies in population due to change:

  • known in statistics as sampling error

• causes allele frequencies to drift up and down randomly over time

• drift occurs in every population, in every generation:

  • especially prevalent in small populations

Genetic drift- random

some alleles can be “lucky” while others can be “unlucky”- like flipping a coin to show the randomness

Genetic drift in large populations

losing an allele is a slim chance, but in small- an allele will drift to fixation quickly

key points about genetic drift

• genetic drift is random with respect to fitness

• genetic drift is most pronounced in small populations:

  • allele frequencies change much less in large population

  • changes in allele frequency are not adaptive

• over timme, genetic drift can lead to the random loss or fixation or alleles

  • results in a decrease in genetic variation over time

Kerr and Wrights experiment about genetic drift

• studied genetic drift in small populations of fruit flies, drosophila melanogaster

• focused on shape of body bristles:

  • variation due to a single gene with two alleles

• two alleles do not affect fitness of flies

• if changes in frequency of alleles occurred, it would not be due to natural selection

data about the kerr and wright experiment

in 70 of the 96 populations studied, one allele was lost due to genetic drift after only 16 generations

in the lab, genetic drift was found to:

• decrease genetic variation within populations

• increase genetic differences between populations