1. Natural selection can alter the distribution of traits, depending on which phenotypes are favored. Will always favor those traits with the highest reproductive success
directional selection
conditions favor individuals exhibiting one phenotypic extreme
occurs in environmental changes
disruptive selection
conditions favor individuals at both phenotypic extremes
stabilizing selection
acts against both extreme phenotypes, favors intermediate variants
reduces variation of a particular trait

2. Sexual Selection
individuals with certain characteristics are more likely to obtain mates
result in sexual dimorphism (differences between males and females)
often result in extreme ornamentation or behavioral displays in males, driven by female selection of mates (choosy females)
the benefit must outweigh the risk (benefit of passing on genes outweighs attracting predators with bright colors/displays)
birds (peacock)
may result in competition among males for access to females
Ungulate rutting

3. Speciation
process by which one species splits into two or more species
explains similarities and differences among organisms (common ancestry)
macroevolution: development of new groups of species
Birds, mammals, flowering plants

4. Species-Concepts (definitions)
Biological species concept
a species is a group of populations whose members have the potential to interbreed in nature and produce viable fertile offspring
“reproductively compatible”
emphasizes gene flow between populations
morphological species concept
characterizes a species by body shape and other structural features
subjective
ecological species concept
views a species in terms of its ecological niche
emphasizes adaptation to environments
phylogenetic species concept
smallest group of individuals that share a common ancestor

5. Reproductive Isolation
existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring
zygote = fertilized egg
prezygotic barriers
block fertilization from occurring by:
impeding mating
preventing mating from being completed successfully
hindering fertilization
postzygotic barriers
developmental errors lead to reduced survival of embryo
hybrids infertile, or don’t live long enough to reproduce
Zonkeys and Ligers!
FIGURE 24.3

6. How new species arise... Allopatric Speciation
gene flow interrupted by geographic barrier
geographic barrier is different for each population
Once gene pools have diverged, mutations arise and natural selection alters gene frequencies
this leads to reproductive isolation over time
evidence that speciation has occurred...
two populations brought back together and unable to reproduce, or prefer members of their own population

7. Sympatric Speciation
occurs in species that live in the same geographic area (less common)
caused by:
polyploidy (extra chromosomes--mistakes in cell division)
habitats (exploit habitat/resource not used by parent population)
sexual selection

8. Extinction Majority of organisms that ever lived are now extinct
5 historical mass extinctions
Permian
Largest extinction event…90% of species extinct
Effected mostly marine life. Recovery took ~30 million years.
Triassic-Jurassic
70% of species extinct. Dinosaurs evolved.
Cretaceous (K-T extinction)
resulted from volcanic eruptions and a warming atmosphere
75% of species extinct. Mammals and Birds evolved as a result.
Are we in a current 6th mass extinction??
Extinction rate is estimated x higher than background rate (yikes!)

9. Consequences of Extinctions
decrease ecosystem biodiversity drastically
millions of years for species to rebound
adaptive radiation: period of evolutionary change when groups of organisms form many new species as a result of their adaptations to fill vacant niches in an ecosystem
have occurred after all major extinctions
“Cambrian explosion”—530 million years ago, huge increase in animal life.
change ecosystem dynamics (disrupt balance in species interactions)

10. Evolution-Behavior Connections
Parental Care
Maximize reproductive success by caring for offspring
Maximize reproductive success by mating with several females…letting the females care for offspring
Brood Parasites
Certain species will lay their eggs in the nest of another and leave them to raise their young
Altruism
Reduces an individuals fitness, but increases the fitness of other individuals in the population
Kin selection: favors altruistic behavior by enhancing reproductive success of relatives
Bees, many rodents, elephants, chimpanzees…
Mate Choice/Sexual Dimorphism
Promiscuous (males showier)/monogamous (less differences between sexes)
Sneaky Males
Smaller males that sneak in to mate or fertilize eggs of the female
Some actually mimic females

11. Foraging Behavior Agnostic Behavior Mimicry Plant defenses
Natural selection favors behaviors that enhance efficiency of feeding (search, recognize, capture food)
Agnostic Behavior
Competition that results in access to a resource (food, mates)
Mimicry
Cryptic coloration (camouflage)
Aposematic coloration (warning coloration)
Across species, similar colors are used as warning (convergent evolution)
Batesian mimicry (harmless species mimics harmful one)
Mullerian mimicry (two unpalatable species mimic each other)
Plant defenses
Thorns, toxins, attracting other predatory animals to feed on herbivores
White-striped clover!