• 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
• 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
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
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
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
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
• 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
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 100-1000x higher than background rate
(yikes!)
• 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)
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
• Foraging Behavior
▫ 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!