Speciation

• Process by which a daughter species evolves from a parent species

Genetic Divergence- when populations become reproductively isolated

Species-population of individuals who can interbreed under natural conditions producing fertile offspring

• Does evolution always result in a new species?

2 Concepts on Speciation

• Evolutionary species concept every species has its own evolutionary history, part of which is in the fossil record, and diagnostic traits

• Biological species concept reproductive isolation rather than trait differences define a species

Reproductive Isolating Mechanisms

• Any heritable feature of body form, functioning or behavior that prevents interbreeding between species

• Pre-zygotic and Post-zygotic

Pre-ZygoticMechanisms

• Behavioral• Temporal• Mechanical• Habitat isolation• Gametic mortality

Post-Zygotic Mechanisms

• Hybrid mortality lower in fitness, early death

• Hybrid sterility• Would natural selection favor pre- or

post-zygotic mechanisms?

Types of Speciation Processes

• Allopatric Speciation-species form due to physical barriers that prevents gene flow; mountain range, river, island

BARRIER

Species A becomes distinct from species B; cannot mate with each other

A B

Allopatric Speciation

• Salamanders in Central California

• Washington Sockeye salmon in lakes vs rivers

Allopatric Speciation: Adaptive Radiation

• A single ancestral species gives rise to a variety of species, each adapted to a specific environment

• Hawaiian honeycreepers• Occurred after dinosaur

extinction mammals diversified in only 10 million yrs!

Types of Speciation Processes

• Sympatric speciation-species forms within the home range of an existing species in absence of physical barrier

Species A-blue Species B-purple

Species A and B cannot mate

AB

Sympatric Speciation

• Polypoidy in plants• Autopoidy diploid plant

produces diploid gametes• Alloploidy hybrid plant

doubles it chromosome #

Models of Macroevolution

• Gradual Model-new species formed through gradual changes in traits over time; most consistent with fossil record

Models of Macroevolution

• Punctual Model-new species formed in a short amount of time; occurs through bottlenecks/founder effects, mutations, directional selection (natural selection)

Developmental Genes and Macroevolution

• Development of eyes is controlled by Pax6 gene– It did not matter if it was a

lens or compound eye

• Loss of gene results in failure of eye to develop in mice, humans

“…all animals share the same control switches for development…”

Developmental Genes and Macroevolution

• Development of overall shape is controlled by Hox genes– Control number and

appearance of repeated structures

• Differential expression in chickens vs snakes

“…animal diversity is due to variation in the expression of ancient genes…”

Evolution of the Modern Day Horse

• Does not fit a gradual model perfectly

• Trends observed– Increase in size– Toe reduction– Change in tooth

size/shape

• Modern horse evolved 4 mya adapted for living in open plains

“Macroevolution is not goal-oriented”

“…speciation, diversification, and extinction are common occurances…”

Darwin’s drawing in The Origin of Species