11: Evolution

Random processes:

• Mutation

  • A change in the structure of a gene caused by the alteration of base units in DNA

• Genetic drift

  • Random fluctuations in the gene variants of a population

  • Two types: Bottleneck and Founder effect
    • Bottleneck - Occurs when a population size is severely reduced and a small, random assortment of survivors is left (reducing genetic diversity, compared to the original population)

    • Founder effect - Occurs when a small subset of the population establishes a new colony (reducing genetic diversity, compared to the original population)

• Gene flow

  • The transfer of genetic material (interbreeding) from one population to another

    • Movement of alleles from one population to another

    • Involves at least two populations

    • Gene flow introduces new variation within a population

    • Barriers (mountain ranges, geographic distance) can prevent gene flow

      • Because of the common human preference for marrying within one’s own social and cultural group, ethnic and religious differences also act to limit gene flow

      • Social class and educational differences also can limit gene flow

    • If gene flow is prevented:

      • The different populations will adapt to particular environments (ecological niche creation)

      • Over time → leads to speciation

Non-random processes:

• Natural selection

  • Differential survival and reproduction of individuals due to differences in phenotype

  • Ex: salmon

    • Salmon often end up on dinner plates

    • When fishing, small salmon often thrown back in water

    • Average size has decreased by 30% in the last few decades!

• Artificial selection

  • Selectively breeding individuals together to develop particular traits in offspring; Sometimes genetic traits respond to indirect selection (some other trait they have in common)

  • Ex: Belyaev’s foxes

    • Can you breed “tame” foxes to produce a domesticated fox?

    • Dmitry Belyaev accomplished this after 30 years of selective breeding ~45,000 foxes

    • The friendly foxes that approached him were bred together

    • Foxes classified as:

      • Class 1: friendly, tail-wagging, vocalization

      • Class 2: let themselves be pet, no emotional response

      • Class 3: aggressive

    • Some behaviors may be influenced by hormones, and domestication changed some hormonal responses in animals

    • Shifted time of normal breeding season, and made some animals capable of reproducing twice a year

  • Other “side effects” (absent in wild animals but seen in domesticated dogs):

    • Loss of pigment in coat color (“star” mark on forehead)

    • Floppy ears

    • Rolled tails

    • Shortened tails and legs

• Sexual selection

  • Process by which mates are chosen

  • Individuals compete for access to mates

  • Example: Zebra finches

    • Researchers were examining finch populations and tagged finches with red tags so they could keep track of them

    • Female finches started having a preference for males with the red tags and chose to mate with them over males who didn’t have red tags

    • The gene frequencies of the overall population changed because birds with the red tags were mating and reproducing more often than other birds

    • The red tag did not provide any adaptive purpose or survival advantage; It just affected mate choice

Anagenesis

• Gradual evolution of species within a lineage (one taxon replaces another)

• Specialization in relation to habitat

• Loss of limbs in snakes and limbless lizards

• Degeneration of organs

• New species tends to replace old species

Cladogenesis

• Splitting of a parent species into multiple distinct species

• Rapid splitting of lineages due to new environment or extinction of competitors

  • Ex: Finches on Galapagos Islands

• Branching into different ecological niches

Monophyletic group (clade)

• Group of organisms classified in the same taxon that share a common ancestor (group includes all of the descendants of the common ancestor)

• Ex: mammals, primates

Paraphyletic group

• Group of organisms that consists of the last common ancestor, excluding a few descendants (those that have changed significantly are excluded)

• Ex: fish (mammals are descendants), reptiles (birds are descendants)

• Ex: Crocodiles are more similar to lizards than birds, in appearance (crocodiles share a more recent common ancestor with birds, however)

The order Primates, as a whole, is a monophyletic group

If you were only referencing nocturnal primates (like lorises and tarsiers), that would be a paraphyletic group

Homology

• Shared trait that was inherited from a common ancestor

• Examples:

  • Internal gestation and lactation in mammals

  • Opposable thumbs in primates

  • Tail of monkey and tailbone of chimp

Homoplasy

• Similar function, but evolved independently (not inherited from a common ancestor)

• Examples:

  • Wings in birds and bats

  • Body shape, fins, aquatic habitat in whales and sharks