AP Biology Unit 7

Natural Selection

• Organisms more adapted to their environment are more likely to survive and pass on the genes that aided their success. This process causes species to change and diverge over time.

1. Variation

   1. There is a genetic variation within a population that can be inherited.

2. Competition

   1. Overproduction of offspring leads to competition for survival.

3. Adaptation

   1. Individuals with beneficial adaptions are more likely to survive to pass off their genes.

4. Selection

   1. Over many generations, there is change in allele frequency.

artificial selection - humans intentionally breed organisms with certain desired traits, leading to a loss of genetic variation in the new population

convergent evolution - two or more species develop similar features despite not sharing a recent common ancestor

divergent evolution - two species diverge from a common ancestor and develop different characteristics, creates diversity within a species

• Evolution does not happen with individuals, only in a population

Hardy-Weinburg

5 conditions for evolution to not happen (equilibrium):

• large population

• random mating

• no mutation(s)

• no gene flow (no migration)

• no natural selection (all phenotypes are equal)

p + q = 1

• p = frequency of dominant allele; q = frequency of recessive allele

p2 + 2pq + q2 = 1

• p2 = frequency of homozygous dominant individuals; 2pq = frequency of heterozygous individuals; q2 = frequency of homozygous recessive individuals

genetic drift - changing allele frequencies by chance

(ex. natural disasters)

• founder effect - few individuals found population, reduction in genomic variability

  • ex. Amish

• bottleneck effect - drastic reduction in population, only a few left to populate; less genetic variability, more likely to go extinct

  • ex. Northern elephant seals

gene flow - individuals migrating

• can introduce new genetic variation into a population or reduce genetic variation in a population

Modes of Selection

• directional selection - shifting to one extreme

  • for one extreme; against other extreme

    ex. longer-tailed lizards

• stabilizing selection - selection for an intermediate trait

  • for moderate traits; against both extremes

    ex. medium-tailed lizards

• disruptive selection - selection for 2 extremes

  • for both extreme traits; against moderate traits

    ex. long OR short-tailed lizards

• intrasexual selection - members of same-sex compete for mates

  ex. males competing with each other

• intersexual selection - mates are selective, have preferences

frequency-dependent = fitness depends on how common phenotype is

ex. right & left-mouthed fish, oscillate frequency (as right-mouthed fish decrease, left-mouthed fish increase, visa versa)

species = individuals that breed and make viable, fertile offspring

barriers that keep speciesseparate:

• prezygotic (before embryo)

  • habitat isolation - different locations

  • temporal isolation - different mating times

  • behavioral isolation - mismatched mating traits prevent mating between two populations/species

  • mechanical isolation - physical incompatibility, unable to transfer DNA

  • gametic isolation - egg and sperm don’t fuse

• postzygotic (after embryo)

  • reduced hybrid viability - offspring weak, don’t survive

  • reduced hybrid fertility - healthy but sterile offspring

  • hybrid breakdown - fragments in plants (first generation ok but then falls apart)

speciation - formation of distinct and new species through evolution

• allopatric - different geographic locations

• sympatric - same area but caused by various preferences (mating, food, etc.)

rates of evolution - can happen at different rates:

• gradualism - slow change over time

  ex. fossil records

• punctuated equilibrium - sudden change followed by a period of stability (stasis)

  • adapted radiation

vestigial structures - historical remnants of structures that promoted fitness in their ancestors (no longer useful traits)

homologous structures - similar physical features in organisms that share a common ancestor, but the features serve completely different functions

ex. the limbs of humans, cats, whales, and bats

analogous structures - features of different species that are similar in function but not necessarily in structure and which do not derive from a common ancestral feature

absolute dating - provides more specific origin dates and time ranges (ex. carbon dating)

relative dating - tells how old something is in relation to other objects, but cannot provide a year or specific date of use

fossils, carbon dating, embryos comparison, rocks, DNA sequences, protein sequences, and anatomical structures can be used to help trace ancestry/age.

A phylogenetic tree is a diagram that shows the evolutionary relationships between different groups of organisms.

• represents the evolutionary history of a group of organisms

• the most recent common ancestor at the base

• the most distantly related organisms at the tips of the branches

A cladogram is a diagram that shows the evolutionary relationships between different groups of organisms based on shared characteristics.

• based on the concept of a monophyletic group