Lecture 6a:Macroevolution

Lecture 6a:Macroevolution

Macroevolution

Not really different than microevolution, but over a much larger scale

Changes to species, not changes to populations

Species- what are they anyway?

Biolgical Species Concept: members of the same species interbreed and have a shared gene pool, but are reproductively isolated from every other species

Speciation: the evolution of new species

Species

Two animals of the same species can reproduce and have fertile offspring

One species can consist of one or many populations

Appearance does not necessarily allow you to tell two species apart

Reproductive Barriers

As speciation occurs, reproductive barriers arise that prevent breeding from occurring between the different species

Barriers fall into two categories: prezygotic and postzygotic

Prezygotic BarriersHabitat Isolation: The different species inhabit different habitats

Temporal Isolation: Breeding season is at different times of the year

Behavioral Isolation: Pheromones, courtship rituals, songs or calls, etc are different

Mechanical Isolation: Genitalia are incompatible

Gamete Isolation: The gametes can not fuse to form a zygote

Postzygotic BarriersZygote mortality: A zygote is formed, but it does not survive

Hybrid sterility: The zygote develops into an adult, but it is sterile

F2 Fitness: The hybrids can reproduce, but the F2 generation can not

Types of SpeciationAllopatric Speciation

Occurs when populations become geographically isolated, and move further and further apart genetically from the original species

Ex. Ensatina salamanders, iguanas

Types of Speciation

Sympatric speciation

Population develops into two or more groups without geographic isolation

Polyploidy: increase in number of chromosomes to 3n or higher due to hybridization, sometimes followed by doubling of chromosomes- results in 3rd species

Types of Speciation

Adaptive Radiation

New species evolve from one ancestral species to fill different niches in the habitat

Ex. Galapagos finches, Hawaiian honeycreepers

Fossils

To study extinct species, especially ones from millions of years ago, we look to fossils:

The remains or traces of past life

Can take several forms- amber, footprints, petrification, actual remains

Geologic Time

See timeline

Cambrian explosion: all major groups of animals appeared

Number of species on Earth has continued to increase over time, even to the present day

Speciation We do not know how quickly species arise- there are two models:

Gradualistic Model- slow steady change over a long period of time

Punctuated Equilibrium: somewhat ‘sudden’ appearance of new species in fossil record

The transitional fossils are unlikely due to geographic isolation and small numbers

Mass Extinctions

Relatively sudden disappearances of large numbers of species

Have been several, of course dinos the best known example

Two main causes: Continental drift and meteorite impacts

Continental Drift

The continents on Earth are moving

Plate tectonics: The crust of Earth is floating on the molten mantle, the crust is in several pieces

As the continents move, the climate changes

Pangaea: its

formation 250 mya

was probably the cause

of the Permian

Extinction

Meteorites

Probably the cause of the dinosaur’s extinction

Caused massive cloud of dust that blocked the sun, lowering temperatures worldwide

Soot and iridium are found in Cretaceous clay, and a crater has been identified as well

SystematicsDKPCOFGS

As the category gets higher and higher, it gets more and more inclusive

Ideally, organisms are classified according to their evolutionary relationships, so taxonomy is in constant flux as we learn more and more

Phylogenetic Trees

Trees that show relatedness of different organisms

Indicate common ancestor, and lines of descent

Determined using comparative anatomy, embryology, molecular evidence

Anatomy

Remember, it matters where the characteristic in question arose from, not what it does now

Ex. Thorny devils vs. horny toads have Analagous structures

Ex. Vertebrate forelimbs are Homologous structures

Using DNA to determine relationshipsRemember: evolution occurs when mutations in DNA occur-- it can not occur without those random changes

Therefore, the more closely related animals are, the fewer differences there will be in their DNA

This allows new information about DNA to be included in our understanding of how life on Earth evolved