Genetic Drift The Bottleneck Effect If a population

Genetic Drift: The Bottleneck Effect If a population is quickly reduced by starvation, disease, or a natural catastrophe, the surviving population likely has only a fraction of the alleles that were present before the population declined. The gene pool will have lost its diversity, and allele frequencies will have changed. This is called the bottleneck effect. In this model, the parent population has equal numbers of green and yellow individuals and a few red. A chance catastrophe leaves mostly green survivors with a few yellow and no red. The next generation is mostly green, with a few yellow.

Speciation: How Species Form Biologists consider physiology, biochemistry, behaviour, and genetics when distinguishing one species from another. This includes the definition that the individuals can interbreed to produce viable, fertile offspring. The Grevy’s zebra (Equus grevyi) in (A) is classified as an endangered species, whereas the plains zebra (Equus quagga) in (B) is widespread in Africa. Speciation, or microevolution, is the formation of new species from existing species. Two populations become reproductively isolated over time if there is little or no gene flow between them. Without gene flow, the populations may become different species.

Reproductive Isolating Mechanisms Two types of reproductive isolating mechanisms prevent gene flow between populations. They are further subdivided as follows: Recall: The zygote is the cell produced when a sperm fertilizes an egg. Pre-zygotic changes occur before it is created and post-zygotic changes occur after fertilization.

Types of Speciation Once an isolating mechanism has prevented gene flow, populations must remain genetically isolated from each other for speciation to occur. There are two types of speciation: • Sympatric Speciation occurs when populations that live in the same habitat diverge genetically and become reproductively isolated. Ex. Darwin’s finches • Allopatric Speciation occurs when populations are separated by a geographical barrier and diverge genetically. Ex. Iguanas on the island of Anguilla

Sympatric Speciation More common in plants than animals, chromosomal changes in plants or non-random mating in animals alters gene flow. The result is reproductive incompatibility without geographical isolation. Polyploidy can lead to the formation of new species through sympatric speciation. Example: Polyploidy plant An error in cell division results in an extra set or sets of chromosomes in the offspring. Now reproductively and genetically isolated, the plant may self-fertilize to survive.

Allopatric Speciation occurs when a population is split into two or more isolated groups by a geographical barrier. Eventually, the gene pool of the split population becomes so distinct (due to natural selection, mutations, and gene flow) that the two groups are unable to interbreed when re-introduced. Allopatric speciation occurs after a geographical barrier prevents gene flow between populations that originally were part of a single species. Example: Finches may have been blown to an island that was uninhabited. They adapted to an unoccupied ecological niche and, over time, evolved into a new species.

Wildlife corridors Prevent isolation, increases gene flow and maintains genetic diversity

Adaptive Radiation The Galapagos Islands’ finches are also an example of adaptive radiation since the common ancestral species diversified into a variety of differently adapted species (13 species evolved from a single species). This speciation occurs everywhere but is easily studied on island chains. Example: Red crossbill birds Different-sized birds have beaks that can open different-sized cones.

Divergent and Convergent Evolution • Divergent evolution is a pattern of evolution in which species that were once similar to an ancestral species diverge, or become increasingly distinct. In other words, as populations adapt, they become less and less alike. • Convergent evolution is a pattern of evolution in which similar traits arise because different species have independently adapted to similar environmental conditions.

Divergent Evolution example: Northern Ontario forests are home to many rodents: A. deer mouse B. porcupine C. beaver D. squirrel Leads to two predictable outcomes: 1. Competition between species is minimized as new species fill specialized niches 2. Given enough time, new species continue to evolve until most available resources are used The result is an overall increase in biodiversity as a single species evolves to fill ecological niches.

Convergent Evolution: occurs when two different species evolve to occupy similar ecological niches Example: Sharks and Dolphins: both have streamlined bodies, sharks have a cartilaginous skeleton, while dolphins have a bony skeleton. Sharks have a vertical tail that use a side-to-side body motion whereas dolphins power their movements with up and down motions using their horizontal tail

Convergent Evolution: occurs when two different species evolve to occupy similar ecological niches Complex eyes have evolved independently in many animal groups including: a) insects b) arachnids c) mollusks d) vertebrates

Convergent Evolution Example: Wings on birds and bats Natural selection favoured flight in these species, but they do not share a common ancestor. Thus, their wings evolved differently from different structures. Bat wings and bird wings are analogous as flight structures: their structure and function have evolved by different routes.

The Speed of Evolutionary Change Two models have been proposed to explain the speed at which evolution occurs. • Gradualism • views evolutionary change as slow and steady, before and after a divergence • supported since Darwin’s time • Punctuated Equilibrium • views evolutionary history as long periods of stasis, or equilibrium, that are interrupted by periods of divergence • proposed in 1972 by Niles Eldredge and Stephen Jay Gould

The Speed of Evolutionary Change It is now accepted that both models of evolutionary change are at work. While many species have evolved rapidly during periods in Earth’s history, the fossil record also shows very gradual change for some species over extended periods of time. Two models have been proposed to explain the speed of evolution: (A) gradualism and (B) punctuated equilibrium.