• 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!