Speciation and Extinction

Microevolution vs Macroevolution Microevolution consists of changes that evolve within a population; we are talking about the changes in 1 gene pool/allele frequencies; think Hardy-Weinberg Macroevolution refers to evolutionary changes above the species level; think new species evolving as a result of so many microevolution changes

Speciation A parent organisms evolving into one or more new species Can be slow and gradual or can occur in bursts followed by relatively quiet periods Called adaptive radiation in the extreme form (many new species arising around the same time)

Four different ways to define a species Biological Species Concept – states that a species is a group of organisms who are able to interbreed in nature and produce fertile offspring; they do not breed successfully with other species. This concept cannot be applied to fossils or asexual organisms and is therefore a limited definition The following concepts focus on the unity within a species rather than the separateness; can be applied to sexual and asexual organisms Morphological species concept Ecological Species Concept Phylogenetic Species Concept

Reproductive Isolation For the purpose of this class we will accept the biological definition of a species Takes place as a result of barriers that cause species to become unique Reproduction Isolation: barriers that prevent two species from producing fertile offspring Reduced hybrid viability -- weak offspring Reduced hybrid fertility -- sterile offspring Hybrid breakdown.

Pre-Zygotic Barriers (Before Fertilization) Does not allow fertilization to happen: Temporal: separate by time Habitat: different habitats (land vs. water) Behavior: different courtship rituals Mechanical: structures do not allow for fertilization to occur Gametes: sperm of one species may not be able to fertilize egg of another species

Post-Zygotic Barriers (After Fertilization) Sometimes the species are similar enough for sperm to fertilize egg; however, the offspring either dies or is unable to produce fertile offspring (genes are not passed to future generations) Hybrids: the offspring organisms of two different species; hybrids either die or are unable to reproduce (can’t pass along genes to future generations)

Speciation can occur in two ways: Allopatric speciation: geographic barrier separates the population (river, mountain range, great wall of China) Gene flow is interrupted Separated populations evolve independently through mutations, natural selection, and genetic drift Sympatric speciation: no geographic barrier Species overlap and could share a gene pool Polyploidy, appearance of new ecological niches, and sexual selection can all drive sympatric speciation Sympatric speciation can also result from the appearance of new ecological niches. For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees. Sexual selection can drive sympatric speciation. Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria.

Polyploidy Presence of extra sets of chromosomes due to accidents during cell division Autopolyploid – more than two chromosome sets derived from one species Allopolyploid – multiple chromosome sets derived from different species Common in plants (oats, cotton, potatoes, tobacco, and wheat) Not common in animals An autopolyploid is an individual with more than two chromosome sets, derived from one species. An allopolyploid is a species with multiple sets of chromosomes derived from different species. Polyploidy is common in plants. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids.

Sympatric Speciation via Polyploidy is Common in Plants Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Gametes produced are diploid.. Offspring with tetraploid karyotypes may be viable and fertile. Fig 24.10 Sympatric speciation by autopolyploidy in plants

Sympatric Speciation - Polyploidy --> Allopolyploid Species B 2n = 4 Unreduced gamete with 4 chromosomes Unreduced gamete with 7 chromosomes Hybrid with 7 chromosomes Meiotic error Normal gamete n = 3 Viable fertile hybrid (allopolyploid) 2n = 10 Figure 24.11 One mechanism for allopolyploid speciation in plants Normal gamete n = 3 Species A 2n = 6

Sexual Selection Natural selection driven by mate selection Can result in sexual dimorphism (different looking males vs females Two types: Intrasexual: selection within the same sex (males preventing other males from mating) Intersexual (aka mate choice): female choose male based on behavior and looks Many of these traits can be disadvantageous; however, because the sexual selective pressure is so high it wins out over environmental selection Peacock tail: Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival How do female preferences evolve? The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for.

Sexual Selection Figure 23.15 Sexual dimorphism and sexual selection

Extinction Mass extinction: dramatic increase in the rate of extinction around the same time period Evidence shows the Earth has experienced five mass extinction events – more than 50% of Earth’s species became extinct Causes for mass extinction: continental drift, volcanoes, comet, carbon dioxide level changes, gamma rays, sea level changes, climate change, methane hydrate, ocean anoxia, ocean circulations

(families per million years): Five Big Mass Extinctions 20 800 700 15 600 500 Number of families: (families per million years): Total extinction rate 10 400 300 5 200 100 Figure 25.14 Mass extinction and the diversity of life Era Period Paleozoic Mesozoic Cenozoic E O S D C P Tr J C P N 542 488 444 416 359 299 251 200 145 65.5 Time (millions of years ago)

The break-up of Pangaea lead to allopatric speciation. The current distribution of fossils reflects the movement of continental drift. Similarity of fossils in parts of South America and Africa supports the idea that these continents were formerly attached. The fossil record shows that most species that have ever lived are now extinct. The presence of iridium in sedimentary rocks suggests a meteorite impact about 65 million years ago. The Chicxulub crater off the coast of Mexico is evidence of a meteorite that dates to the same time.

Is a Sixth Mass Extinction Under Way? Consequences … Scientists estimate that the current rate of extinction is 100 to 1,000 times the typical background rate. Data suggest that a sixth human-caused mass extinction is likely to occur unless dramatic action is taken. Mass extinction can alter ecological communities and the niches available to organisms. It can take from 5 to 100 million years for diversity to recover following a mass extinction. Mass extinction can pave the way for adaptive radiations.

Adaptive Radiations - New Environmental Opportunities … Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities. Mammals underwent an adaptive radiation after the extinction of terrestrial dinosaurs. The disappearance of dinosaurs (except birds) allowed for the expansion of mammals in diversity and size. Other notable radiations include photosynthetic prokaryotes, large predators in the Cambrian, land plants, insects, and tetrapods.

Regional Adaptive Radiations Adaptive radiations can occur when organisms colonize new environments with little competition. The Hawaiian Islands are one of the world’s great showcases of adaptive radiation.

The Time Course of Speciation Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data. The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis (no change) punctuated by brief periods of rapid change. The punctuated equilibrium model contrasts with a Darwinian model of gradualism: slow continuous change over time in a species’ existence.

Patterns in Speciation Punctuated Equilibrium pattern Change / Time Gradualism pattern Figure 24.17 Two models for the tempo of speciation

You should now be able to: Define and discuss the limitations of the four species concepts. Describe and provide examples of prezygotic and postzygotic reproductive barriers. Distinguish between and provide examples of allopatric and sympatric speciation. Explain how polyploidy can cause reproductive isolation. Distinguish among the following sets of terms: intrasexual selection, intersexual selection, and sexual dimorphism. Briefly describe the Cambrian explosion. Describe the mass extinctions that ended the Permian and Cretaceous periods.