Speciation

300 BIOLOGY

GB

2014

Achievement StandardEvolutionary processes involve the following biological ideas:

Role of mutation

Gene flow

Role of natural selection and genetic drift

Modes of speciation (sympatric, allopatric)

Reproductive isolating mechanisms that contribute to speciation (geographical, temporal, ecological, behavioral, structural barriers, polyploidy)

Patterns such as divergence, convergence, adaptive radiation, co-evolution, punctuated equilibrium, and gradualism.

Scientific evidence for evolution, which may include examples from New Zealand’s flora and fauna, will be selected from:

fossil evidence

Comparative anatomy (homologous and analogous structures)

Molecular biology (proteins and DNA analysis)

Biogeography.

The BasicsTerms: Genes – carry the genetic information required for cell growth, functioning and

replication

Alleles – alternative form of a gene (creates variation)

Gene Pool – all the alleles of a population

Gene Flow – movement of alleles in and out of a population

Speciation - evolution of new species, new species cannot reproduce with old species

Mutation – change in the base sequence in DNA, must occur in gametes to be passed on. Creates totally NEW phenotypes.

Bottleneck Effect – massive loss of alleles due to natural disaster, leaves a non representative population

Founder Effect – small non representative group migrate to a new habitat

Genetic Drift – random loss of alleles due to chance in a small population

Natural SelectionNatural Selection – those best suited to their environment mate and pass on their genes to the next generation increasing the number of these helpful genes in the gene pool, meanwhile those unsuited do not mate and those genes disappear from the gene pool.

Stabilizing selects against the two extremes and favors the middle (2)

Directional selects against one of the extremes (3)

Disruptive selects against the middle and favors the two extremes, this can lead to speciation (1)

Variation Differences between individuals in a species, caused by

differences in the DNA base sequence

The more differences in a species the greater chance that some of the species will survive changes within their environment

Variation is created during sexual reproduction, meiosis (crossing over, independent assortment, segregation) immigration and mutations

Variation must occur within the gametes in order to be passed on

Variation is reduced by natural selection, genetic drift (bottleneck effect, founder effect), emigration and natality.

Speciation One species evolves into two or more species that can no

longer interbred.

Usually occurs as a result of adaptation to new ecological niches and in response to the occurrence of new variations within the species that make an organism better able to survive and reproduce

Speciation

There are three ways in which new species can evolve:

1. Instant Speciation

Occurs within one generation as a result of polyploidy

more than two sets of chromosomes Usually occurs in plants

Speciation

2. Sympatric Speciation Occurs when a new species arises in the

SAME territory as the parent species Often occurs as a result of niche

differentiation (ie finches living in top of trees vs forest floor)

3. Allopatric Speciation Occurs when a new species evolves as a

result of being isolated from the parent species

NEW territory Often occurs when there is some kind of

geographical or environmental disturbance

Polyploidy and Aneuploidy Poly – many, polyploidy = more than one set of chromosomes

Aneu – one, aneuploidy = one chromosome is represented three times instead of the usual two (one from each parent)

Homologous Chromosomes usually separate during meiosis to create two gametes with haploid (n, half the original) chromosomes

Non disjunction – when homologous pairs of chromosomes fail to separate during meiosis the gametes can end up with: Having two copies of a single chromosome (aneuploidy)

Having a missing chromosome (aneuploidy)

Having 2 whole sets of chromosomes (polyploidy)

Having NO chromosomes (zygote is unlikely to form in this case)

Offspring formed from these gametes end up having: 3 or more of an individual chromosome – aneuploidy

3 or more sets of chromosomes – polyploidy

Polyploidy generally only occurs in plants and can be advantageous as often results in bigger better crops, or seedless crops (infertile plants have no seeds!)

Aneuploidy in the sex chromosome in humans can result in:Turners Syndrome – XOKlinfelters Syndrome – XXY

Aneuploidy in an autosomal chromosome in humans can result in:Downs Syndrome – trisomy 21 – 3

number 21 chromosomeEdwards Syndrome – trisomy 18 – 3

number 18 chromosomes

Types of Polyploidy

Polyploidy can result in both sterile and fertile offspring

In order to be fertile an organism needs to have an EVEN number of chromosomes (so they can line up in homologous pairs and separate during meiosis

Autopolyploids – organisms with multiple sets of chromosomes from the SAME species Eg a potato produces gametes with polyploidy (more than one set of chromosomes) and

mates with another potato giving rise to a new potato with autopolyploidy (3 sets of chromosomes but all from the potato family)

Often occurs when plants self fertilize

If both gametes have undergone non disjunction then the offspring will be fertile as it will be have an even number of chromosomes -4n or tetraploid

If only one gamete has undergone non disjunction it will result in infertile offspring as there will be an uneven number of chromosomes – 3n or triploid

Allopolyploids – organisms with multiple sets of chromosomes from DIFFERENT species

Eg a wheat plant fertilizes a rye plant

If the offspring has an uneven number of chromosomes due to non disjunction having occurred in one of the gametes then the offspring will be infertile

If the offspring has an even number of chromosomes due to non disjunction occurring in both gametes then the offspring will be fertile

If the uneven numbered gamete manages to fuse with another normal gamete and a plant with even chromosomes arises then it will be a fertile hybrid – this usual occurs as a result of self fertilisation.

Hybrid – made from two different species

Isolating Mechanisms In order for a new species to arise it must not be able to reproduce

with the parent species. Prevention of reproduction can occur in several ways:

1. Pre Zygotic (before a zygote is formed) Geographical – separated by space, river, ocean, mountain, road etc

Temporal –reproducing at different times of year, active at different times of day

Ecological – live in different ecological niches

Behavioural – different courtship behaviours

Structural barriers – reproductive genitalia incompatible

Gamete incompatibility – pollen grains don’t grow pollen tubes

2. Post Zygotic (once zygote has formed) Polyploidy – multiple sets of chromosomes, in an uneven number so

that offspring are infertile

Hybrid Inviability – zygote is aborted as has chromosomal incompatibility

Hybrid Sterility – off spring survives but is sterile – mule

Hybrid breakdown – hybrid is fertile but its offspring are sterile

Evolution Evolution - the gradual change in species over long periods

of time resulting in establishment of a new species, (lots of speciation's occurring one after the other over millions of years!)

Variation of alleles exists within the population

The organisms are exposed to a selective pressure such as a changing environment

Those with favorable phenotypes are more likely to survive and reproduce while those with less favorable phenotypes have less chance of surviving and reproducing. “survival of the fittest”

Each generation will be better adapted to the current environment.

Patterns of EvolutionSpeciation or evolution can occur in a variety of ways:

Divergence (A)

Common ancestor, but no longer look the same

Humans and apes from a common primate ancestor

Convergence (B)

Unrelated ancestor, but look similar due to similar selction pressures due to living in similar environment

Whales and fish look the same as both live in water but NOT related

Parallel Evolution (C)

Unrelated ancestor, dissimilar environment but still look similar

Adaptive radiation

Co-evolution

Species which are unrelated but have a close ecological relationship exert selection pressures on each other

Predator /prey eg flowers grow to allow certain birds to pollinate, birds develop long beaks so can get pollen

Rate of Evolutionary Change Gradualism

Slow progressive change over time

Punctuated equilibrium Generally slow change but with periods of rapid

evolution Caused by rapid and extreme changes to the

environment Eg ice age, volcanic eruption

Evidence for Evolution Fossils – comparison to current day species

Comparative Anatomy

Homologous structures Same origins but different function

Forelimb bones of birds, humans, whales, bats

Analogous structures Different origins but similar functions

Wings of bats, birds, and moths

Embryology The more similar embryos are of different species the less time has

past since they diverged

Bio geographical – geographical origins of current species distributions

Biochemical – similarities between DNA and proteins

The End