Evolution 4 Flashcards
state hardy weinberg principle
In a given population one can find out the frequency of occurrence of
alleles of a gene or a locus. This frequency is supposed to remain fixed
and even remain the same through generations.
This principle says that allele frequencies in a population are stable
and is constant from generation to generation. The gene pool (total genes
and their alleles in a population) remains a constant. This is called
genetic equilibrium. Sum total of all the allelic frequencies is 1.
hardy weinberg principle in numerical form
In a diploid, p and q
represent the frequency of allele A and allele a. The frequency of AA
individuals in a population is simply p2
. This is simply stated in another
ways, i.e., the probability that an allele A with a frequency of p appear on
both the chromosomes of a diploid individual is simply the product
of the probabilities, i.e., p2
. Similarly of aa is q2
, of Aa 2pq. Hence,
p
2+2pq+q2=1. This is a binomial expansion of (p+q)2
.
what is the indicator of evolution
When frequency
measured, differs from expected values, the difference (direction) indicates
the extent of evolutionary change. Disturbance in genetic equilibrium, or
Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a
population would then be interpreted as resulting in evolution.
five factors leading to change in genetic equilibrium
Five factors are known to affect Hardy-Weinberg equilibrium. These
are gene migration or gene flow, genetic drift, mutation, genetic
recombination and natural selection.
type of selection: the peak is in the middle
- stabilising selection (in
which more individuals acquire mean character value)
-eg: sickle cell anaemia( HbaHbs is mximum)
This is because carriers of sickel cell anemia gene hve 95% normal rbc while 5% sickel shaped rbc. the sicklle chapes rbc does not allow the growth of malarial sporozoan, Plasmodium falciparum. Hence it provides a survival advantage to heterozygous individuals.
type of selection: 2 end peaks
disruptive selection
(more individuals acquire peripheral character value at both
ends of the distribution curve
type of selection: one end peak ( either homozygous dom or rec)
directional selection
more individuals acquire value other than the mean character value
main features of geological timescale
- Chronological order of the history of organic evolution on Earth.
- Total age of earth 4.5 billion years.
- Eras further divided into periods and periods of recent era are divided into smaller
time spans called epochs. - Divided into - 6 Eras Periods Epochs
what are the 6 eras
- Azoic era- Life was absent
- Archaeozoic era- Era of invisible life (micro organisms)
- Proterozoic era- Origin of protozoans, sponges, coelentrates, annelids & molluscs.
- Paleozoic era
- Mesozoic era- Age of reptiles
- Cenozoic era- Age of mammals
Time before Paleozoic era is also called as Pre cambium era (origin of 1st life).
Paleozoic era is divided into 6 periods-
- Cambrian- Trilobites- Giant arthropods (dominant)- extinct
- Ordovician- 1st vertebrates (jawless fishes)- extinct
- Silurian- 1st life on land
- Devonian- Golden age of fish
- Carboniferous- Age of amphibians, Pteridophyte
- Permian- Gymnosperm appeared
Mesozoic era-
Golden age of reptiles and gymnosperms. Divided into 3 periods-
- Triassic- Origin of dinosaurs, Prototherians (egg laying mammal)
- Jurassic- Golden age of Dinosaurs, Archaeopteryx appeared
- Cretaceous- Angiosperms appeared, Dinosaurs extinct
First cellular forms of life – 2000 mya
*
*
Invertebrates were formed – 500 mya
Jawless fishes – 350 mya
* Sea weeds and few plants – 320 mya
We are told
that the first organisms that invaded land were plants. They were
widespread on land when animals invaded land. Fish with stout and strong
fins could move on land and go back to water. This was about 350 mya.
coelacanths
In
1938, a fish caught in South Africa happened to be a Coelacanth which was
thought to be extinct. These animals called lobefins evolved into the
first amphibians that lived on both land and water. There are no specimens
of these left with us. However, these were ancestors of modern day frogs
and salamanders.
The amphibians evolved into reptiles
They lay thickshelled eggs which do not dry up in sun unlike those of amphibians.
Again we only see their modern day descendents, the turtles, tortoises
and crocodiles. In the next 200 millions years or so, reptiles of different Giant ferns (pteridophytes) were
present but they all fell to form coal deposits slowly.
Some of these land
reptiles went back into water to evolve into fish like reptiles probably 200
mya (e.g. Ichthyosaurs).
The land reptiles were, of course, the dinosaurs.
The biggest of them, i.e., Tyrannosaurus rex was about 20 feet in height
and had huge fearsome dagger like teeth. About 65 mya, the dinosaurs
suddenly disappeared from the earth. We do not know the true reason.
Some say climatic changes killed them. Some say most of them evolved
into birds. The truth may live in between. Small sized reptiles of that era
still exist today.
mammals
The first mammals were like shrews. Their fossils are small sized.
Mammals were viviparous and protected their unborn young inside the
mother’s body. Mammals were more intelligent in sensing and avoiding
danger at least. When reptiles came down mammals took over this earth.
There were in South America mammals resembling horse, hippopotamus,
bear, rabbit, etc. Due to continental drift, when South America joined
North America, these animals were overridden by North American fauna.
Due to the same continental drift pouched mammals of Australia survived
because of lack of competition from any other mammal.
Lest we forget, some mammals live wholly in water. Whales, dolphins,
seals and sea cows are some examples.
